Volume 18 - Friction, Lubrication, and Wear Technology Part 20 ppsx

Fig. 8
Compressive yield strength levels typically obtained in the individual layers of a trilayer construction
bearing material
Corrosion Resistance. Bearing failure due to corrosion alone is rare. Corrosion usually interacts with mechanical and
thermal factors to produce failure by fatigue or seizure under conditions the bearing normally would be able to tolerate.
To a considerable extent, bearing corrosion can be avoided by use of oxidation inhibitors in commercial lubrication oils,
and by periodic oil changes. There are, however, many situations in which neither of these practices is dependable and
where bearing materials with inherently high corrosion resistance should be used.
Commercially pure lead is susceptible to corrosion by fatty acids. Lead-base and copper-lead bearing alloys can suffer
severe corrosion damage in acidic lubricating oils. Tin additions in excess of 5% provide effective protection against
this kind of corrosion, and for this reason tin is used extensively in lead-base bearing alloys. Both copper and lead are
attacked by acidic oils that contain sulfur. This is of particular concern with copper-lead and leaded bronze bearing alloys.
Effective protection can be obtained by employing layered construction, with a surface layer of either a lead alloy
containing tin or a tin alloy. As long as the corrosion-resistant surface layer is intact, the underlying copper-lead alloy will
not suffer damage by corrosion.
Tin and aluminum bearing alloys are substantially impervious to corrosion by the products of oil oxidation, and they are
used extensively in applications where the potential for lubricating oil corrosion is known to be high. Although
lubricating oil oxidation and contamination are the most common causes of bearing damage by corrosion, other sources of
bearing corrosion, such as seawater, animal and vegetable oils, and corrosive gas, should be recognized. Selection and
specification of a bearing material for a specific application should take into account the anticipated service conditions
under which the bearings will have to operate, and the potential for corrosion that these conditions may stimulate.
Heat and Temperature Effects. The reduced mechanical strength of bearing liner materials at elevated temperatures
is an important consideration in the selection of a bearing material for a given application. Fatigue strength, compressive
yield strength, and hardness decrease significantly with increased bearing operating temperature. As shown by the
softening curves in Fig. 9, lead-base and tin-base bearing alloys are most severely limited in this respect, and copper
alloys the least.

Fig. 9 Strength retention at elevated temperatures for selected bearing alloys. (a) Copper-
base alloys. (b)

Aluminum-base alloys. (c) Zinc-base alloys. (d) Lead-base alloys and tin-base alloys
Load Capacity. The load capacity of a bearing material is defined as the maximum unit pressure under which the
material can operate without excessive friction or wear damage. Load capacity ratings published as guides for machinery
designers generally represent upper limits, which may be safely employed only under very good conditions of lubricant
film integrity, counterface finish, mechanical alignment, and temperature control.
In cyclic-load service (for example, in crankshaft bearings), load capacity is primarily limited by fatigue strength. In
steady-load service, it depends more strongly on compressive yield strength, reflected in indentation hardness. In all
cases, the strength of the material at operating temperature will be the determining factor that governs the choice of
bearing material. Temperature and its control are therefore critically important to the successful operation of sliding
bearings.
Although useful to the designer as reference values, load capacity ratings must be recognized as imprecise and somewhat
judgmental approximations. They are not guaranteed or directly measurable material properties.
Bearing Material Systems
Because of the widely varying conditions under which bearings must operate, commercial bearing materials have evolved
as specialized engineering materials systems rather than as commodity products. They are used in relatively small
tonnages and are produced by a relatively small number of manufacturers. Much proprietary technology is involved in
alloy formulation and processing methods. Successful selection of a bearing material for a specific application often
requires close technical cooperation between the user and the bearing producer.
Single-Metal Systems
Most single-metal sliding bearing are made from either copper alloys or aluminum alloys. Some use is also made of cast
zinc-base alloys which serve as lower-cost substitutes for solid bronze. Commercially significant alloys that are used as
single-metal bearings are listed in Table 2.
Table 2 Single-metal bearing material systems
Bearing performance characteristics
(a)
Load
capacity
rating
(c)

Class

Material
Compatibility

Conformability

Embeddability

Fatigue

strength

Corrosion
resistance
(b)


MPa

ksi

Typical applications
1
Commercial bronze
(10% Zn)
F E F D B 28 4 Bushings, washers
Tin bronze High
lead (16-25% Pb)
D D D D E 21 3 Mill-machinery bearings, pump bearings, railroad-car bearings

Medium lead (4-
10% Pb)
E E E C D 28 4 Wrist pin bushings, pump bushings, electric-motor bushings, track-
roller bushings, farm-equipment gear bushings, mill-machinery
bearings, machine-tool bearings
Low lead (1-4% Pb)

F F F B B 34 5 Wrist pin bushings, mill-machinery bearings, machine-tool
bearings, earth-moving machinery bearings, farm-equipment gear
bushings
2
Unleaded F F F A B 34 5 Wrist pin bushings, mill-machinery bearings, machine-tool
bearings, railroad-car wheel bearings
3
Aluminum alloy,
low tin
D D D D A 28 4 Connecting-rod main bearings, bushings, mill-machinery bearings
Zinc alloy 12% Al E E F B E 28 4 Compressor bearings, pump bushings, mill-machinery bearings,
earth-moving machinery bearings
4
27% Al E F F A E 34 5 Compressor bearings, pump bushings, mill-machinery bearings,
earth-moving machinery bearings
Porous metal
Bronze
C C C D B 14 2 Electric motor bushings, home appliance bearings, agricultural
equipment bushings
Iron D D C D B 21 3 Electric motor bushings, home appliance bearings, agricultural
equipment bushings
5
Iron-bronze D D C D B 21 3 Electric motor bushings, home appliance bearings, agricultural

equipment bushings

(a)
Bearing performance characteristics rated on scale A through F, where A is highest (best) and F is lowest (poorest).
(b)
Corrosion resistance refers to corrosion by fatty acids of the kind that can form in petroleum-base oils.
(c)
Load capacity rating approximates maximum safe unit loading for operation with steel journal under cyclic loading and excellent lubrication.

Wide ranges of compositions and properties are available in the older copper group. Brasses and bronzes have been
widely used in bearing applications since the mid-1800s. Interest in the use of aluminum alloys was stimulated by World
War II metal shortages and greatly accelerated by the commercial introduction of aluminum-tin bearing alloys in 1946.
Since then, metal economics have dictated the use of aluminum alloy bearings, but brasses and bronzes continue to be
preferred by many designers of heavy and special-purpose machinery.
Single-metal systems do not exhibit outstandingly good surface properties, and their tolerance of boundary and thin-film
lubrication conditions is limited. As a result, the load capacity rating for a single-metal bearing usually is low relative to
the fatigue strength of the material from which it was made. Because of their metallurgical simplicity, these materials are
well suited for small-lot manufacturing from cast tubes or bars, using conventional machine shop processes.
Copper Alloys. Except for commercial bronze and low-lead tin bronze, copper alloys in single-metal systems are almost
always used in cast form. This provides thick bearing walls ( 3.20 mm, or 0.125 in.) that are strong enough so that the
bearing is retained in place when press fitted into the housing.
Commercial bronze and medium-lead tin bronze alloys C83420 and C83520 are used extensively in the form of wrought
strip for thin-wall bushings, which are made in large volumes by high-speed press forming. The relatively poor
compatibility of these alloys can be improved by embedding a graphite-resin paste in rolled or pressed-in indentations, so
that the running surface of the bushing consists of interspersed areas of graphite and bronze. Such bushings are widely
used in automotive engine starting motors.
The lead in leaded tin bronzes is present in the form of free lead that is dispersed throughout a copper-tin matrix so that
the bearing surface consists of interspersed areas of lead and bronze. In general, the best selection of materials from this
group for a given application will be the highest-lead composition that can be used without risking excessive wear, plastic
deformation, or fatigue damage.

Aluminum Alloys. Virtually all solid aluminum bearings used in the United States are made from alloys containing
from 5.5 to 7% Sn, plus smaller amounts of copper, nickel, silicon, and magnesium. Starting forms for bearing fabrication
include cast tubes as well as rolled plate and strip, which can be press formed into half-round shapes. As is the case with
solid bronze bearings, relatively thick bearing walls are employed in solid aluminum alloy bearings.
The tin in these alloys is present in the form of free tin that is dispersed throughout an aluminum matrix so that the
bearing surfaces consist of interspersed areas of aluminum and tin. Surface properties are enhanced by the free tin in
much the same way that those of bronze are improved by the presence of free lead.
The high thermal expansion of aluminum poses special problems in maintaining press fit and running clearances. Various
methods are employed for increasing yield strength (for example, heat treatment and cold work) to overcome plastic flow
and permanent deformation under service temperatures and loads.
Zinc Alloys. During the past 20 years, zinc-aluminum-copper casting alloys have been used to replace cast bronze alloys
in certain low-speed machinery bearing applications. This practice has advanced farthest in Europe, as an outgrowth of
World War II material substitution efforts.
These alloys do not contain any soft microconstituents that correspond to the lead used in bearing bronzes and to the tin in
cast aluminum bearing alloys. To a considerable degree, compatibility of the zinc-base alloys seems to derive from their
chemical behavior with hydrocarbon lubricants. Formation of a stable low shear strength film of zinc-base soap appears to
be an important factor.
Porous Metal Bushings. Oil-impregnated porous metal bushings can also be included in the single-metal systems
category. The materials used for these bushings include unleaded and leaded tin-bronze, bronze-graphite, iron-carbon,
iron-copper, and iron-bronze-graphite compositions. Oil content of these materials constitutes 8 to 30% of total volume.
Bimetal Systems
All bimetal systems employ a strong bearing back to which a softer, weaker, relatively thin layer of a bearing alloy is
metallurgically bonded. Low-carbon steel is by far the most widely used bearing-back material, although alloy steels,
bronzes, brasses, and (to a limited extent) aluminum alloys are also used. The bimetal bearing material systems currently
in significant commercial use are classified in Table 3.
Table 3 Bimetal bearing material systems
Bearing performance characteristics
(a)
Load
capacity

rating
(c)

Class

Backing layer Surface layer
Compatibility

Conformability

Embeddability

Fatigue

strength

Corrosion
resistance
(b)


MPa

ksi

Typical applications
Tin babbitt:
0.25-0.50 mm
(0.010-0.020 in.)
A A A F A 14 2

1
Steel
0.102 mm (0.004
in.)
A B B E A 17 2.5

Connecting-rod and main bearings, camshaft
bearings, electric-motor bushings, pump
bushings, thrust washers
Lead babbitt:
0.25-0.50 mm
(0.010-0.020 in.)
A A A F B 14 2
2
Steel
0.102 mm (0.004
in.)
A B B E B 17 2.5

Connecting-rod and main bearings, camshaft
bearings, transmission bushings, pump
bushings, thrust washers
Aluminum alloy:
High tin
B C C D A 41 6 Connecting-rod and main bearings, camshaft
bearings, transmission bushings, pump
bushings, thrust washers
Medium-tin
B C C C A 55 8 Connecting-rod and main bearings, camshaft
bearings, transmission bushings, pump

bushings, thrust washers
High-lead
B C C C A 55 8 Connecting-rod and main bearings, camshaft
bearings, transmission bushings, pump
bushings, trust washers
Low-tin
D D D C A 55 8 Camshaft bearings, transmission bushings,
thrust washers
3
Steel
Tin-free
D D D C A 55 8 Camshaft bearings, transmission bushings,
thrust washers
Copper alloy:
Copper-lead
C C C C F 38 5.5

Connecting-rod and main bearings, camshaft
bearings
High-lead bronze
D D D C E 45 6.5

Camshaft bearings, turbine bearings, pump
bushings, thrust washers
4
Steel
Medium-lead
bronze
E E E B D 55 8 Piston pin bushings, rocker-arm bushings, wear
plates, steering-knuckle bushings, guide

bushings, thrust washers
5
Medium-lead
bronze
Tin babbitt, 0.25-0.50
mm (0.010-0.020 in.)
A A A F B 14 2 Connecting-rod and main bearings, thrust
washers, railroad-car journal bearings, mill-
machinery bearings
6
Medium-lead
bronze
Lead babbitt: 0.25-
0.50 mm (0.010-
0.020 in.)
A A A F C 14 2 Connecting-rod and main bearings
7
Medium-lead-
bronze
Lead babbitt: 0.025
mm (0.001 in.)
A C B C C 48 7 Connecting-rod and main bearings
8
Aluminum
alloy, low tin
Lead babbitt, 0.025
mm (0.001 in.)
A C B D C 41 6 Connecting-rod and main bearings

(a)

Bearing performance characteristics rated on scale A through F, where A is highest (best) and F is lowest (poorest).
(b)
Corrosion resistance refers to corrosion by fatty acids of the kind that can form in petroleum-base oils.
(c)
Load capacity rating approximates maximum safe unit loading for operation with steel journal under cyclic loading and excellent lubrication.

The strengthening effect of a steel bearing back is illustrated clearly for classes 3 and 4 in Table 3; these ratings can be
compared with those for the aluminum and copper alloy single-metal systems in Table 2. When steel bearing backs are
employed, load-capacity ratings for both copper and aluminum alloys are sharply increased above those of the
corresponding single metals without degrading any other properties. Similarly, in classes 1, 2, 5, 6, and 7, the strong
bearing-back materials permit use of lead and tin alloys that have extremely good surface properties but that are so low in
strength that they can be used as single-metal bodies only under very light loads.
The strengthening effect of thin-layer construction on lead and tin alloys is illustrated in Table 3 (classes 1 and 2), where
a 50% increase in load capacity is achieved by reducing babbitt layer thickness. Although similar behavior has been
observed with aluminum and copper alloys, the thin-liner effects are less dramatic. Liner thicknesses employed with these
stronger alloys are established by metal economics and manufacturing process considerations, rather than by
strength/thickness relationships.
Deterioration in surface properties with increasing liner alloy fatigue strength is clearly seen by the comparison of classes
1 and 2 with classes 3 and 4, and by comparisons within classes 3 and 4 (Table 3). In practice, only those systems with
surface properties rated "D" or better are successful under boundary and thin-film lubrication conditions. This restricts the
use of bimetal materials in connecting-rod and main bearings to loads of 55 MPa ( 8 ksi).
Bronze-back bearings (see Table 3, classes 5, 6, and 7) do not exhibit combinations of performance characteristics
substantially different from those of steel-back bearings. The practical advantages of bronze as a bearing-back material lie
partly in the economics of small-lot manufacturing and partly in the relative ease with which worn bronze-back bearings
can be salvaged by rebabbitting and remachining. From the standpoint of performance, the advantage of bronze over steel
as a bearing-back material is the protection bronze affords against catastrophic bearing seizure in case of severe liner wear
or fatigue. Similar protection is provided by the aluminum alloy bearing back in class 8.
Although the surface properties of bronze bearing-back materials are not impressive, they are superior to those of steel,
and these "reserve" bearing properties can be of considerable practical importance in large expensive machinery used in
certain critical applications.

Trimetal Systems
Virtually all trimetal systems employ a steel bearing back, an intermediate layer of relatively high strength, and a tin alloy
or lead alloy surface layer. The systems in current commercial use are listed by classes in Table 4. Most of these systems
are derived from the bimetal systems of Table 3 (classes 3 and 4) by the addition of a lead-base or tin-base surface layer.
Table 4 Trimetal bearing material systems
Bearing performance characteristics
(a)
Load
capacity
rating
(c)

Class

Backing
layer
Intermediate layer
Surface layer Compatibility

Conformability

Embeddability

Fatigue

strength

Corrosion
resistance
(b)

MPa

ksi

Typical applications
1
Steel Medium-lead bronze Tin babbitt, 0.25-
0.50 mm (0.010-
0.020 in.)
A A A F B 14 2 Large connecting-rod and
main bearings, bushings
2
Steel High-lead bronze Tin babbitt, 0.25-
0.50 mm (0.010-
0.020 in.)
A A A F B 14 2 Large connecting-rod and
main bearings, bushings
3
Steel Copper-lead Lead babbitt,
0.075 mm (0.003
in.)
A B B E C 21 3 Connecting-rod and main
bearings, camshaft bearings
4
Steel Copper-lead Lead babbitt,
0.025 mm (0.001
in.)
A C C B C 59 8.5

Connecting-rod and main
bearings, bushings
5
Steel High-lead bronze Lead babbitt,
0.025 mm (0.001
in.)
A C C B C 83 12 Connecting-rod and main
bearings, thrust washers
6
Steel Medium-lead bronze Lead babbitt,
0.025 mm (0.001
in.)
A D D A C 83 12 Connecting-rod and main
bearings
7
Steel Aluminum, low tin Lead babbitt,
0.025 mm (0.001
in.)
A C C B B 55 8 Connecting-rod and main
bearings
8
Steel Aluminum, tin free, low
alloy
Lead babbitt,
0.025 mm (0.001
in.)
A C C B B 55 8 Connecting-rod and main
bearings
9

Steel Aluminum, tin free, low
alloy, precipitation
hardened
Lead babbitt,
0.025 mm (0.001
in.)
A C C B B 76 11 Connecting-rod and main
bearings
10
Steel Aluminum, tin free,
high alloy
Lead babbitt,
0.025 mm (0.001
in.)
A C C B B 76 11 Connecting-rod and main
bearings
11
Steel Silver Lead babbitt,
0.025 mm (0.001
in.)
A D D A B 83 12 Connecting-rod and main
bearings for aircraft
reciprocating engines

(a)
Bearing performance characteristics rated on scale A through F, where A is highest (best) and F is lowest (poorest).
(b)
Corrosion resistance refers to corrosion by fatty acids of the kind that can form in petroleum-base oils.
(c)
Load capacity rating approximates maximum safe unit loading for operation with steel journal under cyclic loading and excellent lubrication.

The strengthening effects of thin-layer construction are notable in those systems that incorporate electroplated lead alloy
surface layers 0.025 mm ( 0.001 in.) thick (Table 4, classes 4 through 11). Comparison of fatigue strength and load
capacity ratings of these systems with those of the corresponding bimetal systems in Table 3 shows that the thin lead alloy
surface layer upgrades not only surface properties but also fatigue strength. The increase in fatigue strength can be
attributed at least in part to the elimination of stress raisers, from which fatigue cracks can propagate.
Class 1 and class 2 trimetal systems comprise leaded bronze intermediate layers with relatively thick tin alloy surface
layers. These represent an evolution from bronze-back babbitt construction wherein steel has replaced most of the bronze.
This produces the expected economy and bearing-back yield strength, but retains the desirable "reserve" bearing
properties exhibited by bronze-back construction.
Class 11 trimetal systems, which have silver intermediate layers, are too costly for most commercial applications.
However, they provide an unequaled combination of high load capacity and corrosion resistance. They continue to have
limited use in radial piston engines for aircraft.
Trimetal systems with electroplated lead-base surface layers and copper or aluminum alloy intermediate layers provide
the best available combinations of cost, fatigue strength, and surface properties. Such bearings have high tolerances for
boundary and thin-film lubrication conditions, and thus can be used under higher loads than can any of the bimetal
systems. Although more costly than the corresponding steel-back bimetal systems, they are used in some high-volume
automotive applications as well as in larger mobile and stationary engines. A highly developed body of mechanical,
metallurgical, and chemical manufacturing technology has been established in the plain bearing industry, and this
technology permits mass production of precision trimetal bearings without a severe cost penalty.
Bearing Alloys
Sliding bearing alloys can be grouped as follows:
• Tin-base alloys
• Lead-base alloys
• Copper-base alloys
• Aluminum-base alloys
• Silver-base alloys
• Zinc-base alloys
• Additional metallic materials (gray cast irons and cemented carbides)
• Nonmetallic materials (nylon, PTFE, carbon-graphite, wood, rubber, and laminated phenolics)

Tin-Base Alloys
Tin-base bearing materials (babbitts) are alloys of tin, antimony, and copper that contain limited amounts of zinc,
aluminum, arsenic, bismuth, and iron. The compositions of tin-base bearing alloys, according to ASTM B 23, SAE, and
ISO specifications, are shown in Table 5.
Table 5 Designations and nominal composition of tin-base bearing alloys
Designation
(a)
Composition, %
UNS
SAE ISO ASTM
(B 23)
Sn

Sb

Cu

Other

Product form Applications
L13910

. . . . . . Alloy 1 91

4.5

4.5

. . . Cast on steel or bronze back Bimetal surface layer
L13870

. . . . . . Alloy
11
87

7 6 . . . Cast on steel; bronze or steel-
backed bronze
Bimetal and trimetal
surface layer
L13890

Alloy
12
SnSb8Cu4 Alloy 2 89

7.5

3.5

. . . Cast on steel; bronze or steel-
backed bronze
Bimetal and trimetal
surface layer
L13840

. . . . . . Alloy 3 84

8 8 . . . Cast on steel or bronze back Bimetal surface layer
. . .
. . . SnSb12Cu6Pb

. . . 80

12 6 2 Pb Cast on steel back Bimetal surface layer

(a)
UNS, unified numbering system; SAE, Society of Automotive Engineers; ISO, International Organization for
Standardization; ASTM, American Society for Testing and Materials

The presence of zinc in these bearing metals generally is not favored. Arsenic increases resistance to deformation at all
temperatures; zinc has a similar effect at 38 °C (100 °F) but causes little or no change at room temperature. Zinc has a
marked effect on the microstructures of some of these alloys. Small quantities of aluminum (even <1%) will modify their
microstructures. Bismuth is objectionable because, in combination with tin, it forms a eutectic that melts at 137 °C (279
°F). At temperatures above this eutectic, alloy strength is decreased appreciably.
Bulk mechanical properties of ASTM grades 1 to 3 are shown in Table 6. These properties have some value for initial
materials screening comparisons of alloys; but they are not reliable predictors of the performance of thin layers bonded to
a strong backing, which is the manner in which tin-base babbitts are usually used in modern bearing practice. Layer
thickness effects (Fig. 5) and temperature effects (Fig. 7) are more important practical considerations than the mechanical
property differences among the various alloy compositions.
Table 6 Typical mechanical properties of chill cast tin-base bearing alloys
Compressive yield strength
(a)


Designation
20 °C

(68 °F)
100 °C

(212 °F)
Ultimate tensile

strength, 20 °C

(68 °F)
Hardness, HB
UNS
SAE ISO ASTM

(B 23)
MPa

ksi MPa

ksi MPa ksi
Elongation,

20 °C

(68 °F), %
20 °C

(68 °F)

100 °C

(212 °F)

L13910

. . . . . . Alloy 1

30.3 4.40

18.3 2.65

64 9.3 2 17.0 8.0
L13890

Alloy 12

SnSb8Cu4

Alloy 2

42.1 6.10

20.7 3.00

77 11.2 . . . 24.5 12.0
L13840

. . . . . . Alloy 3

45.5 6.60

21.7 3.15

69

(b)
10.0
(b)
1
(b)
27.0 14.5

(a)
0.125% offset.
(b)
Values are for die-cast alloy specimens.

Compared with most other bearing materials, tin alloys have low resistance to fatigue, but their strength is sufficient to
warrant their use under low-load conditions. These alloys are commercially easy to bond and handle and they have
excellent antiseizure qualities. Their excellent corrosion-resistant properties make these alloys especially well-suited for
bearing applications in compressors, electric motors, and food-processing equipment.
Tin-base alloys vary in microstructure in accordance with their composition. Alloys that contain about 0.5 to 8% Cu and
<8% Sb are characterized by a solid-solution matrix in which needles of a copper-rich constituent and fine rounded
particles of precipitated SbSn are distributed. The proportion of the copper-rich constituent increases with copper content.
SAE 12 (ASTM grade 2) has a structure of this type in which the needles often assume a characteristic hexagonal starlike
pattern. Alloys that contain 0.5 to 8% Cu and >8% Sb exhibit primary cuboids of SbSn, in addition to needles of the
copper-rich constituent in the solid-solution matrix. In alloys with 8% Sb and 5 to 8% Cu, rapid cooling suppresses
formation of the SbSn cuboids. This is particularly true of alloys containing lower percentages of copper.
Lead-Base Alloys
Lead-base bearing materials (lead babbitts) are alloys of lead, tin, antimony, and in many cases, arsenic. Many such alloys
have been used for centuries as type metals, and were probably first employed as bearing materials because of the
properties they were known to possess. The advantage of arsenic additions have been generally recognized since 1938.
Nominal compositions of the most widely used lead babbitts according to ASTM, SAE, and ISO specifications are listed
in Table 7. Additional information on mechanical properties of some of these alloys is given in Table 8.

Table 7 Designations and nominal composition of lead-base bearing alloys
Designation Composition, %
UNS
SAE ISO ASTM

(B 23)
Pb Sb

Sn

As

Other

Product form Applications
L53346

Alloy
13
PbSb10Sn6 Alloy
13
84 10

6 . . .

. . . Cast on steel or steel-
backed bronze

Bimetal and trimetal
surface layer
L53585

Alloy
14
PbSb15Sn10 Alloy 7 75 15

10 . . .

. . . Cast on steel back Bimetal surface layer
L53620

Alloy
15
PbSb15SnAs Alloy
15
84 15

1 1 . . . Cast on steel back Bimetal surface layer
. . .
. . . PbSb14Sn9CuAs

. . . 77 14

9 0.5

1 Cu Cast on steel back Bimetal surface layer
L53565

. . . . . . Alloy 8 80 15

5 0.5

. . . Cast on steel back Bimetal surface layer
. . .
. . . . . .
(a)
87.5

9 3.5

. . .

. . . Cast on bronze back Bimetal surface layer

(a)
ASTM B 67

Table 8 Typical mechanical properties of chill cast lead-base bearing alloys
Compressive yield strength
(a)


Designation
20 °C

(68 °F)
100 °C

(212 °F)
Ultimate
tensile
strength, 20 °C

(68 °F)
Hardness, HB
UNS
SAE ISO ASTM
(B 23)
MPa

ksi MPa

ksi MPa ksi
Elongation,

20 °C

(68 °F), %
20 °C

(68 °F)

100 °C

(212 °F)

L53346

Alloy 13

PbSb10Sn6 Alloy 13

22.8 3.30

10.7 1.55

69 10.0 5 19 8.5
L53585

Alloy 14

PbSb15Sn10

Alloy 7 24.5 3.55

11.0 1.60

72 10.5 4 22.5 10.5
L53620

Alloy 15

PbSb15SnAs

Alloy 15

24.8 3.60

14.5 2.10

71 10.4 2 21 13
(a)
0.125% offset

Comments made in the section "Tin-Base Alloys" in this article concerning the significance of bulk mechanical properties
of tin-base babbitt alloys apply equally to those of lead-base alloys.
For many years, lead-base bearing alloys were considered to be only low-cost substitutes for tin alloys. However, the two
groups of alloys do not differ greatly in antiseizure characteristics, and when lead-base alloys are used with steel backs
and in thicknesses <0.75 mm (<0.03 in.), they have fatigue resistance that is equal to, if not better than, that of tin alloys.
Bearings of any of these alloys remain serviceable longest when they are 0.13 mm ( 0.005 in.) thick (See Fig. 5).
In the absence of arsenic, the microstructures of these alloys comprise cuboid primary crystals of SbSn or of antimony
embedded in a ternary mixture of Pb-Sb-SbSn in which lead forms the matrix. The number of these cuboids per unit
volume of alloy increases as antimony content increases. If antimony content is >15%, the total amount of the hard
constituents increases to such an extent that the alloys become too brittle to be useful as bearing materials.
Arsenic is added to lead babbitts to improve their mechanical properties, particularly at elevated temperatures. All lead
babbitts are subject to softening or loss of strength during prolonged exposure to the temperatures (95 to 150 °C, or 200 to
300 °F) at which they serve as bearings in internal combustion engines. Addition of arsenic minimizes such softening.
Under suitable casting conditions (see the section "Casting Processes" in this article), the arsenical lead babbitts (for
example, SAE 15 [ASTM grade 15]) develop remarkably fine and uniform structures. They also have better fatigue
strength than arsenic-free alloys.
Pouring temperature and rate of cooling markedly influence the microstructures and properties of lead alloys, particularly
when they are used in the form of heavy liners for railway journals. High pouring temperatures and low cooling rates,
which typically result from the use of overly hot mandrels, promote segregation and formation of a coarse structure. A
coarse structure may cause brittleness, low compressive strength, and low hardness. Therefore, low pouring temperatures
(325 to 345 °C, or 620 to 650 °F) usually are recommended. Because these alloys remain relatively fluid almost to the
point of complete solidification ( 240 °C, or 465 °F, for most compositions), they are easy to manipulate and can be
handled with no great loss of metal from drossing.
Lead-Base Electroplated Overlays. The improvement in fatigue life that can be achieved by decreasing babbitt

layer thickness has already been noted. Economically as well as mechanically, it is difficult to consistently achieve very
thin uniform babbitt layers bonded to bimetal shells by casting techniques. Therefore, the process of electroplating (see
the section "Electroplating Processes" in this article) a thin precision babbitt layer on a very accurately machined bimetal
shell was perfected. Specially designed plating racks allow the thickness of the plated babbitt layer to be regulated so
accurately that further machining is usually not required.
Electroplated tin alloys were found to be generally inferior to lead alloys, and only lead alloys are in commercial use as
electroplated bearing overlays. Table 9 lists the four most commonly used compositions. SAE alloy 192 is the most
frequently used. Tin in alloys 191, 192, and 193 and indium in alloy 194 impart corrosion resistance. Tin also increases
wear resistance. Both copper and indium enhance fatigue resistance.
Table 9 Designations and nominal composition of lead-base electroplated overlay alloys

Designation Composition, %
SAE
ISO Pb

Sn

Cu

In
Alloy 191

PbSn10 90

10 . . .

. . .

Alloy 192

PbSn10Cu2

88

10 2 . . .

Alloy 193

. . . 80

18 2 . . .

Alloy 194

PbIn7 93

. . .

. . .

7

When a tin-containing overlay is plated directly onto a copper-lead or bronze surface, the tin has a tendency to migrate to
the copper interface, forming a brittle copper-tin intermetallic compound. This decreases the corrosion resistance of the
overlay and causes embrittlement along the bond line. To avoid this deterioration, a thin continuous barrier layer,
preferably nickel 1.3 m ( 52 in.) thick, is plated onto the copper alloy surface just prior to plating of the overlay. In
addition to providing better surface behavior, overlays improve fatigue performance of some intermediate layers by
preventing cracking in this layer. Plated overlays generally range in thickness from 0.013 to 0.05 mm (0.0005 to 0.002
in.), with fatigue life increasing markedly as overlay thickness decreases. In order to take full advantage of the improved
fatigue life achieved with thin overlays, it is necessary to minimize assembly imperfections (such as misalignment) and to

maintain close tolerances on machined shafts and bearing bores. Engine components must be thoroughly cleaned before
assembly, and adequate air and lubricant filtration must be maintained if the overlay is to survive during the useful life of
the bearing. Under adverse wear conditions, however, premature removal of the overlay will not necessarily impair
operation of the bearing, because the exposed intermediate bearing alloy layer should continue to function satisfactorily.
Copper-Base Alloys
Copper-base bearing alloys comprise a large family of materials with a wide range of properties. They include
commercial bronze, copper-lead alloys, and leaded and unleaded tin bronzes. They are used alone in single-metal
bearings, as bearing backs with babbitt surface layers, as bimetal layers bonded to steel backs, and as intermediate layers
in steel-backed trimetal bearings (see Tables 2, 3, and 4).
The moderate strength and hardness of pure copper are readily increased by alloying, most commonly with tin (with
which copper forms a solid solution). Lead is present in cast copper-base bearing alloys as a nearly pure, discrete phase,
because its solid solubility in the matrix is practically nil. The lead phase, which is exposed on the running surface of a
bearing, constitutes a site vulnerable to corrosive attack under certain operating conditions.
The antifriction behavior of copper-base bearing alloys improves as lead content increases, although at the same time
strength is degraded because of increased interruption of the continuity of the copper alloy matrix by the soft weak lead.
Thus, through judicious control of tin content, lead content, and microstructure, a large family of bearing alloys has
evolved to suit a wide variety of bearing applications.
Table 10 gives specification numbers and nominal compositions of copper-base bearing alloys, as well as the forms in
which the alloys are used and general notations on typical product applications. The information in Table 10 should be
used in conjunction with the appropriate portions of Tables 2, 3, and 4 and with the brief descriptions that follow.
Table 10 Designations and nominal composition of copper-base bearing alloys
Designation Composition, % No.

UNS SAE ISO Other Cu Sn

Pb

Zn

Product form Applications

Commercial bronze
1
C83420

Alloy
795
. . . . . . 90 0.5

. . .

9.5

Wrought strip Solid bronze bushings and
washers
Unleaded tin bronzes
2
C52100

. . . CnSn8P . . . 92
(a)


8 . . .

. . .

Wrought strip Solid bronze bushings and
washers
3
C90300

. . . . . . . . . 88 8 . . .

4 Cast tubes Solid bronze bearings
4
C90500

. . . . . . . . . 88 10 . . .

2 Cast tubes Solid bronze bearings
5
C91100

. . . . . . . . . 84 16 . . .

. . .

Cast tubes Solid bronze bearings
6
C91300

. . . . . . . . . 81 19 . . .

. . .

Cast tubes Solid bronze bearings
Low-lead tin bronzes
7
C92200

. . . . . . . . . 88.5

6 1.5

4 Cast tubes Solid bronze bearings
8
C92300

. . . . . . . . . 87 8.5

0.5

4 Cast tubes Solid bronze bearings
9
C92700

. . . . . . . . . 87.5

10 2 0.5

Cast tubes Solid bronze bearings
Medium-lead tin bronzes
10
C83520

Alloy
791
. . . . . . 88 4 4 4 Wrought strip Solid bronze bushings and
washers
11

. . . . . . . . . F32/F62 88 4 4 4 Cast on steel back Bimetal bushings and
washers, trimetal
intermediate layer
12
C83600

. . . CuPb5Sn5Zn5 . . . 85 5 5 5 Cast tubes Solid bronze bearings,
bronze bearing backs
13
C93200

. . . CuSn7Pb7Zn3 . . . 83 7 7 3 Cast tubes Solid bronze bearings
14
. . . Alloy
793
. . . . . . 88 4 8 . . .

Cast on steel back Bimetal surface layer
15
. . . Alloy
793
. . . . . . 88 4 8 . . .

Sintered on steel back

Bimetal surface layer
16
C93700

. . . CuPb10Sn10 . . . 80 10 10 . . .

Cast tubes Solid bronze bearings,
bronze bearing backs
17
. . . Alloy
792
CuPb10Sn10(G)

. . . 80 10 10 . . .

Cast on steel back Bimetal surface layer,
trimetal intermediate layer
18
. . . Alloy
792
CuPb10Sn10(P)

. . . 80 10 10 . . .

Sintered on steel back

Bimetal surface layer
High-lead tin bronzes
19
C93800

. . . . . . . . . 78 7 15 . . .

Cast tubes Solid bronze bearings,
bronze bearing backs

20
. . . . . . . . . AMS
4825
74 10 16 . . .

Cast on steel back Bimetal surface layer
21
. . . Alloy
794
CuPb24Sn4(G) . . . 73.5

3.5

23 . . .

Cast on steel back Bimetal surface layer,
trimetal intermediate layer
22
. . . Alloy
794
CuPb24Sn4(P) . . . 73.5

3.5

23 . . .

Sintered on steel back

Bimetal surface layer,
trimetal intermediate layer

23
. . . . . . . . . F112 72.5

2.5

25 . . .

Cast on steel back Trimetal intermediate layer
24
C94300

. . . . . . . . . 70 5 25 . . .

Cast tubes Solid bronze bearings
Copper-lead alloys
25
. . . Alloy
49
CuPb24Sn(G) . . . 75 1 24 . . .

Cast on steel back Trimetal intermediate layer
26
. . . Alloy
49
CuPb24Sn(P) . . . 75 1 24 . . .

Sintered on steel back

Trimetal intermediate layer
27

. . . Alloy
48
CuPb30(P) . . . 70 . . .

30 . . .

Sintered on steel back

Bimetal surface layer,
trimetal intermediate layer
28
. . . Alloy
485
. . . . . . 48 1 51 . . .

Sintered on steel
back, infiltrated with
lead
Bimetal surface layer

(a)
Also 0.3 P

Commercial Bronze. Lead-free copper alloys are characterized by poor antifriction properties but fairly good load-
carrying ability. Wrought commercial bronze strip (SAE 795) with 10% Zn can be readily press formed into cylindrical
bushings and thrust washers. Strength can be increased by cold working this inexpensive material.
Unleaded Tin Bronze. The unleaded copper-tin alloys are known as phosphor bronzes because they are deoxidized
with phosphorus. They are used principally in cast form as shapes for specific applications, or as rods or tubes from which
solid bearings are machined. They have excellent strength and wear resistance, both of which improve with increasing tin
content, but poor surface properties. They are used for bridge turntables and trunnions in contact with high-strength steel,

and in other slow-moving applications.
Low-Lead Tin Bronzes. The inherently poor machinability of tin bronzes can be improved by adding small amounts
of lead. Such additions do not significantly improve surface properties, however, and applications for these alloys are
essentially the same as those for unleaded tin bronzes.
Medium-Lead Tin Bronzes. The only wrought strip material in this group of alloys is SAE 791, which is press formed
into solid bushings and thrust washers. C83600 is used in cast form as bearing backs in bimetal bearings. SAE 793 is a
low-tin, medium-lead alloy that is cast or sintered on a steel back and used as a surface layer for medium-load bimetal
bushings. SAE 792 is higher in tin and slightly higher in lead; it is cast or sintered on a steel back and used for heavy-duty
applications such as wrist pin bushings and heavy-duty thrust surfaces.
High-Lead Tin Bronzes. These contain medium-to-high amounts of tin, and relatively high lead contents to markedly
improve antifriction characteristics. SAE 794, widely used in bushings for rotating loads, has the same bronze matrix
composition as SAE 793 (4.5% Sn) but three times as much free lead. It is cast or sintered on a steel back and used for
somewhat higher speeds and lower loads than alloy 793. The bronze matrix of SAE 794 is much stronger than that of a
plain 75-25 copper-lead alloy. Alloy 794 can be used as the intermediate layer with a plated overlay in heavy-duty
trimetal bearing applications such as main and connecting-rod bearings in diesel truck engines. This construction provides
the highest load-carrying ability available in copper alloy trimetal bearings.
Copper-Lead Alloys. These are used extensively in automotive, aircraft, and general engineering applications. These
alloys are cast or sintered to a steel backing strip from which parts are blanked and formed into full-round or half-round
shapes depending on final application. Copper-lead alloys continuously cast on steel strip typically consist of copper
dendrites perpendicular and securely anchored to the steel back, with an interdendritic lead phase. In contrast, sintered
copper-lead alloys of similar composition are composed of more equiaxed copper grains with an intergranular lead phase.
High-lead alloy SAE 48 can be used bare on steel or cast iron journals. Tin content in this alloy is restricted to a minimum
value to maintain a soft copper matrix, which together with the high lead content improves the antifriction/antiseizure
properties of the alloy. Bare bimetal copper-lead bearings are used infrequently today because the lead phase, present as
nearly pure lead, is susceptible to attack by corrosive products that can form in the crankcase lubricant during extended
oil-change periods. Therefore, most copper-base alloys with lead contents >20%, including both SAE alloy 48 and alloy
49, are now used with plated overlays in trimetal bearings for automotive and diesel engines.
SAE 485 is a special sintered and infiltrated composite material, produced by methods described in the section "Powder
Metallurgy Processes" in this article. By these methods, it is possible to combine a very strong continuous copper alloy
matrix structure with a very high lead content, and to alloy the lead-rich constituent with sufficient tin to make it resistant

to corrosion. SAE 485 is used principally for bushing and bearing applications that involve alignment, shaft surface
finish, or unusual dirt contamination problems.
Mechanical Properties of Copper-Base Bearing Alloys. Table 11 shows the ranges of mechanical strength
properties that are exhibited by copper-base bearing alloys, according to alloy families and forms as listed in Table 10.
Indentation hardness tests provide the most generally useful indications of behavior under compressive loads, and are the
only standard strength tests that are applicable to all of the alloy forms. Conventional tensile and compression tests can be
performed only on solid alloy bodies, which represent a relatively small fraction of total copper-base bearing alloy
applications.

Table 11 Typical room-temperature mechanical properties of copper-base bearing alloys

Compressive yield

strength
(a)

Ultimate tensile

strength
Alloy family Product form
MPa ksi MPa ksi
Hardness, HB

Commercial bronze
Wrought strip

. . . . . . 310-440

45-64

78-115
Wrought strip

. . . . . . 400-580

58-84

80-160
Unleaded tin bronzes
Cast tubes 90-125 13-18 240-310

35-45

70-170
Low-lead tin bronzes
Cast tubes . . . . . . 275-290

40-42

65-77
Wrought strip

. . . . . . 310-440

45-64

78-115
Cast tubes 90-100 13-14 240-255

35-37

60-65
Medium-lead tin bronzes

Steel backed . . . . . . . . . . . . 50-130
Cast tubes 75-85 11-12 185-210

27-30

48-55
High-lead tin bronzes
Steel backed . . . . . . . . . . . . 55-90
Copper-lead alloys
Steel backed . . . . . . . . . . . . 30-80

(a)
0.1% offset

Test information of this kind is helpful in the material selection process as a supplement to information generated in
dynamic rig tests and in actual service. Except in certain solid-alloy bearings and bushing applications, alloy strength and
hardness values are rarely stated as absolute specification requirements.
Aluminum-Base Alloys
Successful commercial use of aluminum alloys in plain bearings dates back to about 1940, when low-tin aluminum alloy
castings were introduced to replace solid bronze bearings for heavy machinery. Production of steel-backed strip materials
by roll bonding (see the section "Roll Bonding Processes" in this article) became commercially successful about 1950,
permitting the development of practical bimetal and trimetal bearing material systems using aluminum alloys in place of
babbitts and copper alloys.
The ready availability of aluminum and its relatively stable cost have provided an incentive for continuing development
of its use in plain bearings. Aluminum single-metal, bimetal, and trimetal systems can now be used in the same load
ranges as babbitts, copper-lead alloys, and high-lead tin bronzes. Moreover, the outstanding corrosion resistance of

aluminum has become an increasingly important consideration in recent years, and has led to widespread use of
aluminum alloy materials (in the place of copper-lead alloys and leaded bronzes) in automotive engine bearings.
Designations and Compositions. Alloy designations and nominal compositions of the aluminum-base bearing
alloys in most extensive commercial use are listed in Table 12. In these alloys, additions of silicon, copper, nickel,
magnesium, and zinc function to strengthen the aluminum through solid-solution and precipitation mechanisms. Fatigue
resistance and the opposing properties of conformability and embeddability are largely controlled by these elements and
by the use of appropriate heat treatments. Tin and lead are instrumental in upgrading the inherently poor compatibility of
aluminum. Cadmium is also used as an alloy addition for this reason. Silicon has a beneficial effect on compatibility in
addition to its moderate strengthening effect. Although not well understood theoretically, this compatibility-enhancing
mechanism is of considerable practical value. Silicon is used effectively in many alloys for this reason (usually in
conjunction with tin, lead, or cadmium).





Table 12 Designations and nominal composition of aluminum-base bearing alloys
Designation Composition, %
No.

UNS SAE ISO Other

Al Si Cu

Ni Mg

Sn

Other
Product form Applications

High-tin alloys
1
A08081

Alloy
783
AlSn20Cu . . . 79 . . .

1 . . .

. . . 20 . . . Wrought strip,
bonded to steel
back
Bimetal surface
layer
2
. . . Alloy
786
. . . . . . 59.5

. . .

0.5

. . .

. . . 40 . . . Wrought strip,
bonded to steel
back
Bimetal surface

layer
High-lead alloys
3
. . . Alloy
787
. . . F-85 85 4 1 . . .

. . . 1.5

8.5 Pb Powder rolled
strip, bonded to
steel back
Bimetal surface
layer
4
. . . Alloy
787
. . . Al-6 88.5

4 0.5

. . .

. . . 1 6 Pb Wrought strip,
bonded to steel
back
Bimetal surface
layer
Intermediate-tin alloys
5

. . . Alloy
788
. . . SA-
151
82.5

3 1 . . .

. . . 12 1.5 Pb
0.2 Cr
Wrought strip,
bonded to steel
back
Bimetal surface
layer
6
. . . Alloy
788
. . . AS-
124
82 4 2 . . .

. . . 12 . . . Wrought strip,
bonded to steel
back
Bimetal surface
layer
7
. . . Alloy
788

. . . A-17-
X
83 2.5

0.7

. . .

. . . 12 2 Pb
0.2 Sb
Wrought strip,
bonded to steel
back
Bimetal surface
layer
8
. . . Alloy
788
. . . FA-
130
81 3 0.7

. . .

. . . 13 2 Pb
0.2 Sb
0.2 Sr
Wrought strip,
bonded to steel
back

Bimetal surface
layer
Low-tin alloys
9
. . . . . . AlSn6CuNi . . . 91.5

. . .

1 1 . . . 6.5

. . . Cast tubes Solid aluminum
alloy bearings
10
A08500

. . . . . . . . . 90.5

. . .

1 1 1 6.5

. . . Cast tubes Solid aluminum
alloy bearings
11
A08510

. . . . . . . . . 89.5

2.5

1 0.5

. . . 6.5

. . . Cast tubes Solid aluminum
alloy bearings
12
A08520

. . . . . . . . . 89.5

. . .

2 1.2

1 6.5

. . . Cast tubes Solid aluminum
alloy bearings
13
A08280

Alloy
780
. . . . . . 90.5

1.5

1 0.5

. . . 6.5

. . . Wrought strip
and plate
Solid aluminum
alloy bearings
14
. . . Alloy
770
AlSn6CuNi . . . 91.5

. . .

1 1 . . . 6.5

. . . Wrought strip,
bonded to steel
back
Bimetal surface
layer, trimetal
intermediate
layer
15
A08280

Alloy
780
. . . . . . 90.5

1.5

1 0.5

. . . 6.5

. . . Wrought strip,
bonded to steel
back
Bimetal surface
layer, trimetal
intermediate
layer
Tin-free alloys
16
A04002

Alloy
781
AlSi4Cd . . . 95 4 0.1

. . .

0.1 . . .

1 Cd Wrought strip,
bonded to steel
back
Bimetal surface
layer, trimetal
intermediate

layer
17
A04002

Alloy
781
AlSi4Cd F-154 95 4 0.1

. . .

0.1 . . .

1 Cd Wrought strip,
bonded to steel
back,
precipitation
hardened
Trimetal
intermediate
layer
18
. . . Alloy
782
AlCd3CuNi . . . 95 . . .

1 1 . . . . . .

3 Cd Wrought strip,
bonded to steel
back

Bimetal surface
layer, trimetal
intermediate
layer
19
. . . Alloy AlSi11Cu . . . 88 11 1 . . .

. . . . . .

. . . Wrought strip, Trimetal
784 bonded to steel
back
intermediate
layer
20
. . . Alloy
785
AlZn5Si2CuPb

. . . 91.5

1.5

1 . . .

. . . . . .

5 Zn 1
Pb
Wrought strip,

bonded to steel
back
Trimetal
intermediate
layer

Microstructural Features. The cast low-tin alloys (numbers 9 through 11 in Table 12) all display similar
microstructures consisting of equiaxed aluminum grains with NiAl
3
, free silicon (if present), and free tin precipitated in
the grain boundaries. Tin forms a nearly complete envelope around each aluminum grain. The copper and magnesium are
mostly or completely in solid solution in the aluminum and are not visible under the microscope. Microstructures of the
wrought low-tin, intermediate-tin, and high-tin alloys exhibit the expected effects of rolling and annealing, with the as-
cast aluminum grains replaced by new recrystallized grains and the insoluble phases (NiAl
3
, and silicon) uniformly
distributed throughout. The original continuous grain boundary envelope of free tin assumes a completely new
configuration, the tin now appearing as somewhat elongated, discontinuous "lakes." This characteristic structure, often
termed "reticular," results in much greater ductility than that of the cast alloys.
Microstructures of the lead-aluminum alloys (numbers 3 and 4 in Table 12) exhibit a recrystallized aluminum matrix with
a fine uniform dispersion of free silicon. The lead is present as thin stringers or ribbons of the lead-tin constituent,
elongated in the rolling direction. During recrystallization, this constituent does not coalesce into lakes as does free tin,
and the ribbon-like configuration persists in finished bearings. The effectiveness of the modest lead concentrations in
these alloys in imparting surface compatibility probably is related to the favorable orientation of the lead-tin ribbons
relative to the bearing surface.
The wrought tin-free alloys (numbers 16 through 20 in Table 12) display very simple microstructures, consisting of a
recrystallized aluminum matrix with the soluble strengthening additions (copper, zinc, and magnesium) in solid solution.
Insoluble phases (NiAl
3
, silicon, cadmium, and lead) are present in fine and uniform dispersion.

Mechanical Properties of Aluminum-Base Bearing Alloys. Conventional mechanical properties are somewhat
like microstructural features in that they are of more value in predicting the fabrication behavior of aluminum-base
bearing alloys than in predicting their bearing performance. With the exception of solid aluminum alloy bearings, in
which there is no steel back and where press-fit retention depends entirely on the strength of the aluminum alloy,
mechanical properties of finished bearings are rarely specified, usually for control purposes only. Consideration of some
of these properties (Table 13) does, however, contribute to an understanding of these alloys as a family of related
engineering materials, and of their relationship to the better-known structural aluminum alloys in addition to the copper-
base, tin-base, and lead-base bearing alloys discussed previously.
Table 13 Typical room-temperature mechanical properties of aluminum-base bearing alloys

Compressive yield

strength
(a)

Ultimate tensile

strength
Alloy family Product form
MPa ksi MPa ksi
Hardness, HB

High-tin alloys
Steel backed . . . . . . 100-130

15-19

25-40
High-lead alloys
Steel backed . . . . . . . . . . . . 40-50

Intermediate-tin alloys

Steel backed . . . . . . . . . . . . 50-60
Cast tubes 70-140 10-20 125-220

18-32

45-65
Wrought plate

80-140 12-20 140-170

20-25

40-55
Low-tin alloys
Steel backed . . . . . . . . . . . . 35-45
Tin-free alloys
Steel backed . . . . . . . . . . . . 35-65

(a)
0.2% offset

Product Applications. The majority of the current commercial applications of aluminum-base bearing alloys involve
steel-backed bimetal or steel-backed trimetal bearings. To determine the most cost-effective aluminum material for any
specific application, consideration should be given to the economic advantages of bimetal versus trimetal systems. The
higher cost of the high-tin and high-lead alloys usually is offset by eliminating the cost of the lead alloy overlay plate. The
cost effectiveness of the aluminum bimetal materials is clearly demonstrated by the fact that approximately 75% of the
passenger car engines built in the United States use high-lead aluminum alloy bimetals for main and connecting-rod
bearings. In Europe and Japan, intermediate and high-tin aluminum bimetals similarly dominate passenger car engine

bearing markets.
If the higher load capacity of a trimetal material is required, it then becomes important to select an aluminum liner alloy
that provides adequate but not excessive strength, so that conformability and embeddability are not sacrificed
unnecessarily. The tin-free alloy group (alloys 16 to 20 in Table 12) offers a wide range of strength properties, and the
most economical choice usually is found in this group.
Silver-Base Alloys
Use of silver in bearings is largely confined to unalloyed silver (AMS 4815) electroplated on steel shells, which then are
machined to very close dimensional tolerances and finally precision plated to size with a tin overlay of soft metal. The
overlay may be lead-tin, lead-tin-copper, lead-indium, or in some cases, pure lead. As a bearing material, plated silver is
invariably used with an overlay. Silver on steel with an overlay is regarded as the ultimate fatigue-resistant bearing
material.
Silver was widely used during and after World War II in aircraft applications, where its high cost could be justified. With
the phasing out of piston engines, however, the use of silver in bearings has greatly declined. Current applications are
specialized, chiefly in the aircraft and locomotive industries. In view of the rapidly rising cost of silver, any increase in
demand for this material would stimulate the search for a comparable less-expensive substitute.
Zinc-Base Alloys
The zinc-base alloys that have been used successfully for machinery bearings are standard zinc foundry alloys of the zinc-
aluminum-copper-magnesium high-performance type. Tubular shapes made by conventional sand, permanent mold, and
pressure die-casting methods are machined into bearings in the same way that solid bronze bearings are made. Most
applications have been direct substitutions for solid bronze bearings; the substitutions are made primarily to reduce costs.
Alloy designations, nominal compositions, and typical mechanical properties are shown in Tables 14 and 15 for the two
predominant alloys in the United States.
Table 14 Designations and nominal composition of zinc-base bearing alloys

Designation Composition, %
UNS
Trade

Zn

Al

Cu

Mg
Z35631

ZA-12

88 11

1 0.025

Z35831

ZA-27

71 27

2 0.015


Table 15 Typical room-temperature mechanical properties of zinc-base bearing alloy
Designation Compressive

yield
strength
(a)

Ultimate tensile

strength
UNS
Trade

Casting method
MPa ksi MPa ksi
Elongation, %

Hardness, HB

Sand cast 230 33 299 43 1.5 94
Permanent mold cast

234 34 328 48 2.2 110
Z35630

ZA-12

Die cast 269 39 404 59 5 125
Z35840

ZA-27

Sand cast 330 48 421 61 4.5 90
(a)
0.1% offset

The high compressive strength and hardness values of these materials suggest greater load capacities than those of solid
bronze and solid aluminum bearing materials (Tables 2 and 11). This is not the case in practice, however, largely because

of the high rate at which the zinc alloys soften with increasing temperature. Maximum recommended running
temperatures of 95 to 120 °C (205 to 250 °F) for the zinc-base alloys are approximately 100 °C (180 °F) below the
temperature limits for copper-base and aluminum-base bearing alloys.
Microstructurally, these alloys display a eutectic or peritectic aluminum-zinc matrix, surrounding zinc-rich or aluminum-
rich primary dendrites. Copper is in solid solution. Grain size varies greatly with casting method from coarse in sand
castings to finest in pressure die castings. Some experimental evidence associates the coarse sand cast structures with
superior bearing wear resistance.
Because of their low cost, the zinc-base alloys will probably continue to replace copper-base alloys in certain bearing
applications in construction, earth-moving, mining, and mill machinery markets. However, technical limitations with
respect to high-temperature strength and corrosion resistance will prevent any massive movement away from bronzes and
into zinc alloys.
Additional Metallic Bearing Materials
Additional commercially available metallic bearing materials include gray cast irons and cemented carbides.
Gray cast irons are standard materials for certain applications involving friction and wear (for example, brake drums,
piston rings, cylinder liners, and gears). Cast irons perform well in such applications, and thus should be given
consideration as bearing materials. Gray iron bearings have proved successful in refrigeration compressors where bearing
pressures are <4500 kPa (<650 psi) for main bearings and <5500 kPa (<800 psi) for connecting rod bearings. Normally,
the journals in refrigeration compressors are made either of steel (carburized and hardened to 55 to 60 HRC) or of
pearlitic malleable or ductile iron (hardened to 44 to 48 HRC and having a surface finish of 0.3 m, or 12 in., root-
mean-square, R
q
. Because of occasional dilution of the oil with liquid refrigerant and heavy foaming of the oil, lubrication
may become marginal for short periods of time. Fine-grain iron with uniformly distributed No. 6 (or finer) graphite flakes
usually performs well during these periods. The bearings are often phosphate coated to improve their seizure resistance.
This type of coating also creates a spongelike surface that promotes retention of oil.
For good wear resistance, gray cast iron should be pearlitic with randomly distributed graphite flakes. Cast irons have
been heat treated to obtain martensitic structures for use as cylinder liners, but the benefits of such heat treatment have not
been economically justifiable. Hardened cast iron has been used successfully as a material for the ways on machine tools.
Cemented Carbides. Extremely hard materials, including cemented tungsten carbides, titanium carbides, and other
combinations have been used successfully for various specialized bearing and seal applications. In terms of the bearing

performance characteristics listed in Table 1 these materials exhibit essentially zero conformability and embeddability,
but rank high in strength, hardness, corrosion resistance, and compatibility. Cemented carbides have been of greatest
interest in high-temperature aerospace applications, but have also been used to advantage in certain machinery and
machine tool applications.
Nonmetallic Bearing Materials
Nonmetallic bearing materials are widely used for a variety of applications. They have many inherent advantages over
metals, including better corrosion resistance, lighter weight, better resistance to mechanical shock, and the ability to
function with very marginal lubrication or with no lubricant present at all. The major disadvantages of most nonmetallics
are their high coefficients of thermal expansion and their low thermal conductivity characteristics. For many years,
carbon-graphites, wood, rubber, and laminated phenolics dominated the field of nonmetallic bearing materials. In the
early 1940s, development of nylon and polytetrafluoroethylene (PTFE, or Teflon) gave engineering designers two new
nonmetallics with very unique characteristics, particularly the ability to operate dry.
A wide variety of polymer composites is now being used very successfully in bearing applications. The addition of fiber
reinforcements and fillers such as solid lubricants and metal powders to the resin matrix can significantly improve the
physical, thermal, and tribological properties of these plastics.
Casting Processes
Single-Metal Systems. Except for porous metal oil-impregnated bushings, all the single-metal systems listed in Table
2 are commercially produced by casting, either with or without subsequent mechanical working. Plate, strip, and sheet
forms of commercial bronze, of low-lead and lead-free tin bronzes, and of aluminum-tin alloys are initially cast as ingots,
slabs, or bars by static and continuous casting methods similar to those used for other brass and aluminum mill products.
Subsequent rolling and annealing operations are also similar to those used for conventional mill products. Because of the
extreme hot shortness of leaded tin bronzes and aluminum-tin alloys, these alloys must be rolled either cold or at only
slightly elevated temperatures, with frequent intermediate annealing.
The recrystallized wrought structures of bronze and aluminum-tin bearing alloys are substantially different from the initial
cast structures, with respect to the configurations of the copper and aluminum phases and of the free-lead and free-tin
phases. The improvements in ductility and forming characteristics that result from these structural changes are of great
importance in subsequent bearing manufacturing operations. Bearing performance properties are not strongly affected by
these changes. Both the as-cast and wrought forms of these alloys are in commercial use and are equally acceptable in
bearing applications.
Tubular and cylindrical bronze, zinc, and aluminum-tin alloy shapes are produced by static, centrifugal, and continuous

casting methods, and subsequently are machined into bearings. The high-lead bronzes are used only in the as-cast
condition because of their low ductility and extreme hot shortness, which preclude any substantial amount of plastic
deformation of cast shapes. Cast aluminum-tin alloy tubes can withstand a limited amount of cold work, however, and in
some instances cold compression of 4 to 5% is employed to increase yield strength and improve press-fit retention in the
finished bearings.
Bimetal Systems. Specialized casting methods are widely employed for producing bimetal bearing materials in both
tubular and flat strip forms. Except for aluminum alloy systems (Table 3, classes 3 and 8), all bimetal systems in
commercial use can, at least in principle, be produced by casting methods, and systems that incorporate tin and lead
babbitt liners >0.1 mm (>0.004 in.) thick are universally produced by casting.
Babbitt Centrifugal Casting. Short tubular steel and bronze shapes (bearing shells) are commonly lined with tin or
lead alloys by various forms of centrifugal casting. In these processes, a machined steel or bronze shell is first pre-heated
and coated by immersion in molten tin or tin alloy. The prepared shell is then placed in a lathelike "spinner" and rotated at
a controlled speed about its axis. Molten babbitt is admitted through one end and is uniformly distributed around the
inside wall of the shell by centrifugal action. The molten layer then is cooled and solidified by spraying water against the
outside of the rotating bearing shell. When properly controlled, these processes produce fine-grain liner layers of
reasonably uniform thickness, completely bonded to the steel or bronze bearing-back material. Centrifugal casting
methods are especially well suited to large-diameter thickwall bearings, which are made in relatively small quantities, and
to full-round seamless bearings, which cannot be fabricated from flat strip.
Bronze Centrifugal Casting. Leaded tin bronzes also can be applied to the inner walls of steel shells by centrifugal
casting. Various methods of shell preparation are employed, including both molten-salt and controlled-atmosphere pre-
heating. Oxidation must be entirely absent from the inner wall of the steel shell for complete metallurgical bonding.
Centrifugal casting of bronzes is most successful with alloys containing >3% Sn and 20% Pb. Outside this composition
range, leaded tin bronze and copper-lead alloys are sensitive to lead segregation and consequent nonuniform
"centrifuged" microstructures. Within these composition limits and under well-controlled process conditions,
mechanically sound well-bonded bronze layers with reasonably uniform microstructures can be produced.
Bronze Gravity Casting. All copper-lead alloys and leaded bronzes containing 35% Pb can be successfully cast in
and bonded to steel shells by gravity casting methods, in which centrifugal forces are not a factor. In these processes, a
core usually is used to form an annular space inside the shell, into which molten bronze or copper-lead alloy is poured.
Several different processes of this kind are in commercial use, utilizing a variety of preheating methods, core materials,
pouring methods, and quenching procedures.

As in centrifugal casting, absence of oxides on the inner shell wall is necessary for complete bonding of the alloy layer to
the steel back. Liner microstructures produced by gravity shell casting methods generally are more uniform than those
obtained by centrifugal casting. For low-tin and high-lead compositions, gravity casting is preferred because of the
absence of centrifuging effects on the solidifying alloy.
Babbitt Strip Casting. Steel-backed tin alloy and lead alloy bearing strip materials are commonly produced by
continuous casting in specially designed process lines in which separate cleaning, etching, hot tinning, liner-alloy casting,
and quenching operations are carried out continuously on a moving steel bearing-back strip. One or more in-line
machining operations may also be incorporated so that the strip emerges with a closely controlled thickness, suitable for
bearing fabrication.
Bronze Strip and Slab Casting. The oldest commercial processes for producing steel-back copper-lead and leaded
bronze bearing strip also utilize continuous casting on a moving steel strip. Steel preheating, alloy casting, and quenching
operations are performed under a strongly reducing atmosphere to ensure freedom from oxidation. Some in-line
machining also can be done, but the cast strip usually is machined in a separate line for close control of thickness.
Additional cold rolling and annealing operations are also employed particularly with the low and medium lead-tin bronze
alloys, in which recrystallized structures are frequently preferred for their superior fabrication properties.
Strip casting of copper alloys is a difficult technology that requires close process control, a high level of operator skill,
and relatively expensive special-purpose equipment. It is used by only a few bearing manufacturers, but with considerable
commercial success. It is employed not only for thin-gage coiled materials but also for heavy-gage slabs with steel
thicknesses as great as 15 mm (0.60 in.).
Trimetal Systems. Trimetal materials with relatively thick surface layers (Table 4, classes 1, 2, and 3) are used mostly
in large bearings. These bearings are produced in relatively low volumes from steel shells initially lined by casting with
copper-lead alloys or bronze. After intermediate machining to remove excess liner alloy, such shells are commonly
relined with tin or lead babbitt by centrifugal casting. The methods used are essentially the same as those for casting in
bare steel or solid bronze shells.
Powder Metallurgy Processes
Single-Metal Systems. The only commercial use of powder metallurgy (P/M) methods for making single-metal
bearing materials is in the fabrication of copper-base and iron-base porous metal bushings, which are subsequently
impregnated with oil. The methods used are similar to those for making structural P/M shapes (that is, pressing in a closed
die and sintering under a reducing atmosphere). Bars, tubes, and finished parts are made in this way. Post-sinter coining
and repressing operations are frequently used to control final dimensions of finished parts.

Bimetal and Trimetal Systems. No powder metallurgy processes that use lead-base or tin-base bearing alloys are in
commercial use nor are there at present any commercially developed processes for lining bearing shells by means of P/M
methods. In the manufacture of steel-back copper-lead alloy and leaded bronze strip, however, P/M methods are
employed more extensively than any other method.
Continuous Sintering Process. A wide variety of steel-back copper alloy materials, including counterparts of all of
the cast copper-lead and leaded bronze bearing alloys (Table 3, class 4), can be produced by continuous sintering on a
steel backing strip. In these processes, prealloyed (PA) powder particles are spread uniformly onto moving steel strip. As
the strip passes through a furnace under a reducing atmosphere, the particles become sintered together, forming an open
grid bonded to the steel strip. After cooling, this bimetal is rolled to densify the liner alloy and then resintered to develop
complete interparticle and alloy/steel bonds. After resintering, the strip material may receive further rolling to attain
finish stock size, and sometimes to strain harden the alloy liner for increased strength.
Strip sintering technology makes possible the production of steel-core "sandwich" material, which is especially suitable
for applications requiring two bearing surfaces (such as in some thrust washers). In this instance, powder spreading,
sintering, cooling, and rolling are repeated on the opposite side, after which the strip is finally resintered. Sintered strip for
most automotive and truck bearing applications is processed in coils 5mm ( 0.2 in.) in overall thickness. Thick-wall
materials with steel layers up to about 16 mm ( in.) thick also can be processed into flat slab lengths.
Impregnation and Infiltration. Both bimetal and trimetal bearing materials also can be made by impregnation or
infiltration of a lower-melting lead alloy into a layer of sintered copper alloy powder. In impregnation, a bilayer strip
made from PA copper-lead alloy or leaded bronze powder is immersed in a bath of molten lead-tin alloy heated above the
melting point of lead. During immersion, some of the lead at the surface of the strip is replaced by the lead-tin alloy. In
infiltration, the copper alloy powder layer is free-sintered and not compacted after sintering. The open-grid sintered layer
is then infiltrated with material having a lower melting temperature than that of the grid alloy.
The infiltrant is usually molten lead or a lead alloy but it can be a nonmetallic material such as PTFE, which can be
introduced in paste or slurry form. A very useful class of self-lubricating trilayer structures is produced commercially in
this way; the PTFE-base infiltrant also forms a thin low shear strength surface film in these structures.
Powder Rolling. One very useful application of direct powder rolling that has been developed commercially in the
plain bearing industry is production of an aluminum-lead alloy strip for subsequent bonding to a steel back (see third item
under class 3 in Table 3). In this method, PA lead-aluminum powder and unalloyed aluminum powder are fed
simultaneously in separate streams to a powder rolling mill and continuously compacted into a bilayer aluminum strip.
After sintering, this strip is roll bonded to low-carbon steel, with the unalloyed aluminum bonding layer next to the steel.

This steel-back strip is used as a bimetal material for bearing applications where the unit loading is beyond the capability
of tin or lead babbitt bimetal material.
Roll Bonding Processes
Virtually all commercial manufacturer of bimetal aluminum alloy bearing strip materials (see Table 3, class 3) is currently
done by roll bonding the liner alloy to a steel backing strip. Both batch and continuous processes are employed, the latter
being favored for economical high-volume processing of lighter-gage material.
In all roll bonding processes, whether batch or continuous, very clean aluminum and steel surfaces are forced together
under intense pressure in a rolling mill, so that solid-phase bonding (cold welding) can occur between the two metals at
many sites in the interface. Heat, which may be applied simultaneously with pressure in hot rolling and subsequently in
postroll annealing, serves to develop complete diffusion bonding from the initial weld sites and to recrystallize the
aluminum alloys so that the final bimetallic strip product exhibits useful liner-alloy ductility and complete bonding.
Tin-aluminum alloys usually are not bonded directly to steel because of undesirable interactions between the free tin
constituent and the steel backing. A layer of electrolytic nickel plating on the steel surface is commonly used to alleviate
these effects with both low-tin and high-tin alloy compositions.
Another method commonly used with tin-aluminum alloys employs a tin-free aluminum interlayer. This is accomplished
by the use of Alclad tin-aluminum alloy strip. The tin-free cladding layer serves as the bonding surface and is present as a
distinct bond interlayer in the finished bimetal strip.
Direct roll bonding to steel is most commonly employed with tin-free aluminum alloys (fifth item under class 3, Table 3)
and with lead-aluminum strip materials.
Electroplating Processes
Plated Overlays. Lead alloy surface layers (overlays) whose thickness must be limited to <0.05 mm (<0.002 in.)
(Table 4, classes 4 to 12) are most commonly produced by electroplating the lead alloy on bimetallic bearings that have
previously been finish machined. Specially designed plating racks are used to ensure uniform distribution of plating
current over the bearing surface. With close control of current, critical dimensional tolerances often can be maintained so
precisely that no machining of the electrodeposited alloy surface is required. One manufacturer has commercialized a
process in which the lead alloy electroplating is applied continuously to precision-rolled bimetal strip. In this process, all
forming and machining operations are done after electroplating.
Electroplated lead babbitts comprise booth binary lead-tin and ternary lead-tin-copper compositions, all of which are
commercially codeposited from fluoborate electrolytes. To ensure against bond and plate defects, extreme care is
exercised in preparing the basis metal.

In addition to cleaning and etching treatments, preplating basis-metal preparation usually includes deposition of one more
very thin metallic interlayers. A thin layer of nickel is most frequently used over copper-lead alloys and bronzes to
prevent diffusion of tin from the plated surface layer into the copper basis metal. Copper is most often used over
aluminum alloys to ensure complete adhesion of the plated lead alloy layer, and nickel sometimes is plated over the
copper to prevent diffusion of tin from the lead alloy layer into the copper layer.
Binary lead-indium alloy overlays are also used with copper-lead and leaded bronze intermediate layers. These alloys are
produced by electroplating separate layers of pure lead and pure indium and subsequently diffusing the indium into the
lead in a low-temperature heat treatment operation. In this case, no diffusion barrier is required between the overlay and
the intermediate alloy layer.
Plated Silver Intermediate Layers. Pure silver and silver-lead alloy bearing liners are applied to steel shells by
electrodeposition from cyanide plating baths. Final machining usually is done after plating, leaving a substantially thick
layer (typically 0.25 to 0.38 mm, or 0.010 to 0.015 in.) of bonded silver liner material. Although as-plated thickness
tolerances are not critical, special racking and masking techniques are employed to restrict plating to the surfaces where it
is required and to eliminate local concentrations of high current density. If the structure of the plated layer is to be
uniform, and the bond strength of the liner uniformly high, the steel basis metal must be prepared very carefully and
plating-bath compositions and cleanness must be properly controlled. Although the basic principles involved in silver
plating of bearing liners are the same as for decorative silver plating, the unusually thick deposits involved (normally
>0.50 mm, or 0.020 in.) and the extremely high quality requirements for bond and plated-metal soundness have led to
development of several unique operating and control practices.
Bearing Material Selection
It must be emphasized that selection of a bearing material system for a specific application and of a mechanical design for
the bearing itself are closely interrelated processes. Neither process is entirely straightforward, neither can be approached
independently, and both require a good understanding of other interacting components of the machine system.
Although this article considers the principles involved in bearing operation, it has not attempted to present a detailed
discussion of mechanical design factors. The reader should therefore not expect to make final decisions on materials for
specific applications on the basis of this text alone.
Most manufacturers of plain bearings have experienced engineering staff personnel available to aid potential users with
both mechanical design and material selection. Because of the wide material selection offered by most of these
experienced specialized producers and their background of experience in practical applications, full advantage should be
taken of the engineering services such sources can provide.

Selected References
• Bearing and Bushing Alloys, SAE J459c, SAE Information Report, SAE Handbook 1990,
Part 1,
Society of Automotive Engineers, 1990
• Bearing and Bushing Alloys, SAE J460e, SAE Information Report, SAE Handbook 1990,
Part 1,
Society of Automotive Engineers, 1990
• E.R. Booser, Bearing Materials and Properties, Mach. Des., 10 Mar 1966, p 22-28
• K.G. Budinski, Surface Engineering for Wear Resistance, Prentice-Hall, 1988, p 15-42
• Bushing and Thrust Washer Design Manual, Clevite Engine Parts Div., Gould Inc., 1973, p 23-32
• T. Calayag and D. Ferres, "High Performance, High-Aluminum Zinc Alloys for Low-
Speed Bearings
and Bushings," Technical Paper Series Paper No. 820643, Society of Automotive Engineers, 1982
• "Custom Plain Bearing Products," Engine Parts Div., Gould Inc., 1986
•
G.J. Davies, G.S. Senior, and O. Beaurepaire, "The Development and Application of Polymer
Bearings," Technical Information Paper No. 3, Glacier-Vandervell, Inc. (Great Britain), 1990
• A.O. DeHart, Basic Bearing Types, Mach. Des., 10 Mar 1966, p 15-21
• "Fluid Film Bearing Products," Bushings and Bearings Div., JPI Transportation Products, Inc., 1990
• M.L. MacKay, L.J. Cawley, and G.R. Kingsbury, "A New Aluminum-
Lead Bearing Material for
Automotive Engine Service," Technical Paper Series Paper No. 760113, Society of Automotive
Engineers, 1976
• J. Masounave and G. Huard, Comparison between Continuously Cast and Sand Cast Zinc-
Aluminum
Alloys Used in Bearing Applications, Wear Resistance of Metals and Alloys,
conference proceedings,
ASM International, 1988, p 65-71
• I.D. Massey, N.A. MacQuarrie, D.R. Eastham, "Development of Crankshaft Bearing Material
s for

Highly Loaded Applications," Technical Information Paper No. 2, Glacier-Vandervell, Inc.
(Great
Britain), 1990
• S. Mohan, V. Agarwala, and S. Ray, Wear Characteristics of Rheocast and Stircast Al-Pb Metal-
Metal
Composites, Tribology of Composite Materials,
conference proceedings, ASM International, 1990, p
189-193