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606 Isayev

Copyright © 2000 Marcel Dekker, Inc.
Chapter 7.1
Material Handling and Storage Systems
William Wrennall
The Leawood Group Ltd., Leawood, Kansas
Herbert R. Tuttle
University of Kansas, Lawrence, Kansas
1.1 INTRODUCTION
Material handling and storage systems planning and
design are subsets of facilities planning and design.
Material ¯ow has both internal and external eects
on a site. There are in¯uences for the site plan and
the operations space plan. Conversely, the material
handling system impacts the facility plans, as illu-
stratedinFig.1.
In the facilities design process the material move-
ment determines the ¯ow paths. The material move-
ment origins and destinations are layout locations.
The storage locations and steps are eects of the opera-
tions strategy and thus the organization structure. A
lean manufacturing system may have material delivery
direct to point of use replenished daily and a pull sys-
tem cellular manufacturing process that produces to
order with a TAKT* time of 5 min. Such a system
could have inventory turns of 300 per year. A more
traditional system would have a receiving inspection
hold area, a raw material/purchased parts warehouse,
a single shift functional layout batch manufacturing
system, inspection and test with 90% yield, a separate
packing department, and a policy of one month's ®n-

ished goods inventory. The space plans for the tradi-
tional system should be very dierent from the lean
approach and so should the material handling and sto-
rage plans and systems. A ``pull '' system also indicates
unnecessary material in the system. If it does not pull it
should not be there.
Material handling determines the capacity of a man-
ufacturing plant. From the receiving dock to the ship-
ping platform the material ¯ow routes are the
circulation system. Flow restrictions can act as capa-
city limiters. The material handling and storage plan
determines handling and storage methods, unit loads
and containerization to support the operations and
business strategy.
The product volume plotÐthe plot of volume/quan-
tities of materials by product typically shows a nega-
tive exponential distribution, the important few and
the trivial many Pareto distribution. The plot can be
overlaid with the most suitable production mode as
illustrated in the product volume (PV)/mode curve,
Fig.2.
We have suggested the following modes:
1. Project
2. Cellular
3. Linked cellular
4. Line
5. Continuous.
607
* TAKT time is the rate at which your customer requires
product.

Copyright © 2000 Marcel Dekker, Inc.
These seven steps provide an understanding of the
material ¯ows in the facility. The calibrated ¯ows are
used to develop anity ratings. These initial steps are
also the basis for subsequent evaluation of layout
options and material handling system design.
Step 1. Classify Materials. Most manufacturing and
warehouse operations have a large variety of products
and materials. Situations with 20,000 or more distinct
items are not unusual. To analyze ¯ow or design a
material handling system around so many individual
items is not practical. Classi®cation reduces materials
to a manageable number of items so that the classes
then become the basis for determining ¯ow rates, con-
tainers, and handling equipment.
The initial classi®cations stratify materials for com-
mon handling methods and container design. Weight,
size, shape, ``stackability,'' and special features are
610 Wrennall and Tuttle
Figure 3 Material varieties.
Copyright © 2000 Marcel Dekker, Inc.
de®ningcriteria.Figure5showsaclassi®cationbased
on handling characteristics for a four-drawer cabinet.
In addition, similarities in product, process
sequence, and raw material are bases for grouping
items that move over the same routes.
Step 2. Identify Flow Units. Material ¯ow is measured
in units of material over a unit of time and the analyst
chooses appropriate units for both parameters. The
time unit is usually a matter of convenience and

depends largely on data availability. Typical examples
are cases per hour, tons per day, pallets per week.
Selection of the material ¯ow unit is more proble-
matic. Where only one type of material moves, the
selection is straightforward, for example, the bushel
for a grain mill. But few facilities have only a single
material or material type. A wide variety of size, shape,
weight, and other handling characteristics must be con-
sidered,asillustratedearlierinFig.3.Forexample,
integrated circuits are tiny, delicate, expensive, and
highly sensitive to electrostatic discharge (ESD), but
the operations that use integrated circuits also use
large metal cabinets. Between these extremes is a
wide range of diverse items to move.
Various items of the same size may have dierent
handling requirements and costs. A resistor and an
integrated circuit (IC) are very close in size. But resis-
tors are moved in bulk, in ordinary containers, and
without special precautions. The individual IC is sen-
sitive to ESD. It requires an enclosed, conductive and
expensive container. It may have a special tube or bag
to further protect it. Humans may touch it only if they
wear a grounded wrist strap and a conductive smock.
Individual items or materials are seldom handled
separately. Most items are in boxes, tote boxes, car-
tons, bundles, bales or other containers. These contain-
ers then are what need to be handled. But layout design
requires a standard unit of ¯ow. This is the equivalent
¯ow unit (EFU) which should have the following char-
acteristics:

Applicable to all materials and routes
Easily visualized by the users
Independent of the handling method.
The equivalent ¯ow unit should account for weight,
bulk, shape, fragility, value, special conditions and
other factors:
Weight is a common unit for most materials and is
usually available in a central database.
Bulk, or density, relates weight and size. Overall
dimensions determine bulk density.
Shape impacts handling diculty. Compact regular
shapes such as boxes stack and handle most
easily. Round and irregular shapes stack with
Material Handling and Storage Systems 611
Figure 4 Material ¯ow analysis.
Copyright © 2000 Marcel Dekker, Inc.
Tree trunks may be received and newsprint shipped.
Bulk liquids and gases may be received but pharma-
ceutical intravenous packs or bottles of tablets
are shipped.
Bauxite ore is received and aluminum ingots are
shipped.
Plywood is received, entertainment centers are
shipped.
Wood pulp and naphtha are received, chemicals,
textiles, and plastics are shipped.
What seems a minor change in the item sometimes
brings a dramatic change in the equivalent ¯ow units.
Figure 6 is a schematic ¯ow diagram that illustrates
changes in ¯ow intensity as the material is processed

for a four-drawer cabinet.
Figure7isariverdiagramillustratingmaterial¯ow
for all products in an entire plant. The diagram shows
how ¯ow intensity increases after the material is
painted and decreases after the parts are assembled.
Painted sheet metal parts are easily damaged and di-
cult to handle. Once assembled and packaged, the units
become protected, compact, and stackable and their
¯ow in equivalent ¯ow units decreases dramatically
for the same quantity and weight.
When a decision is made on an equivalent ¯ow unit,
convenience and familiarity often take precedence over
accuracy. The primary purpose of this analysis is to
rate ¯ow intensities into one of four categories. We
use the vowel letter rating system A, E, I, and O.
Accuracy of the order of Æ207 is therefore sucient.
For this level of accuracy, the following procedure is
used:
Review potential data sources.
Interview production and support personnel.
Material Handling and Storage Systems 613
Figure 6 Equivalent unit ¯ow analysis.
Copyright © 2000 Marcel Dekker, Inc.
represent the largest volumes and are representative of
others, data from the top 20±30% should be used.
Where groups of products have similar processes
and ¯ows, a representative item might portray an
entire group. When the product mix is very large and
diverse, random sampling may be appropriate. Figure
9 illustrates data selection guidelines

Process charts map the sequence of processes graph-
ically; routing sheets often have much the same infor-
mation in text form. With either source, each operation
must be examined to determine in which SPU that
operation will occur. This determines the route. From
the product volume analysis or other information, the
raw ¯ow is determined which is then converted to
equivalent¯owunits,asillustratedinFig.10.
This procedure is used directly if there are only a
few products and where processes and ¯ows are similar
and a single item represents a larger product group.
For large numbers of items, process charts with a
random sample are used.
Material Handling and Storage Systems 615
Figure 8 Equivalent ¯ow units.
Figure 9 Data selection guidelines.
Copyright © 2000 Marcel Dekker, Inc.
size, type, and class codes uses the same process and
follows the same route. Field 8 is the number of equiva-
lent ¯ow units per day for each route and size. These
subtotals are the basis for subsequent anity ratings,
stang, and for material-handling-system design.
Other possible ®elds might contain information on
the time required per trip, distance for each route and
speed of the equipment. From this the database man-
ager can derive the numbers and types of equipment
and containers required.
Step 6. Calibrate Flows. This step includes the calcu-
lation of material ¯ow from each route origin to each
destination. It also includes conversion of calculated

¯ows to a step-function calibration for use in layout
planning. The calibration scale can be alphabetical or
numerical. The vowel rating convention AEIO is used
here. The intensities of ¯ow distribution may indicate
the important few and trivial many. The calibrations
can be used for relative capacity of material-handling-
system selection.
Material Handling and Storage Systems 617
Figure 11 Material ¯ows from±to chart.
Copyright © 2000 Marcel Dekker, Inc.
For the calibration the ¯ow rates are ranked on a
barchart,asshowninFig.13.
The breakpoints are a matter of judgment and
should be made near natural breaks. Experience from
a range of projects suggests that the following propor-
tions are a useful guideline:
A 5±10%
E 10±20%
I 20±40%
O 40±80%
Transport work. Total material handling cost is
roughly proportional to the product of ¯ow intensity
and distance. In physics force multiplied by distance
de®nes work. For layout planning, material ¯ow inten-
sity I multiplied by distance D equals ``transport
work'' TW:
TW  DI
In an ideal layout all SPUs with anities would be
adjacent. Since an SPU occupies ®nite space, proximity
but not necessarily adjacency is possible. Placing two

particular SPUs together forces other SPUs farther
away. The theoretical optimum relative locations
occur with equal transport work on all routes where
total transport work is at the theoretical minimum.
Transport work, then, is a metric for evaluating the
layout. For evaluation, transport work is calculated
along every path on a layout and the summation
made. Layout options may be evaluated by comparing
their total transport work.
Transportworkisusefulinanotherway.InFig.14
distance is plotted on the horizontal axis and ¯ow
intensity on the vertical axis. Each route on the layout
plots as a point. As mentioned above, the ideal layout
would have constant (or iso-) transport work, such a
curve being a hyperbola. Routes with low intensity
have long distances; those with high intensity, short
distances. The product of distance and intensity for
either is then equal.
A ``good'' layout, from strictly a material ¯ow per-
spective, is one which has most or all points close to
the same hyperbolic isotransport work curve. Routes
which are signi®cantly distant from the hyperbola indi-
cate an anomaly in the layout.
618 Wrennall and Tuttle
Figure 12 Material ¯ow report.
Copyright © 2000 Marcel Dekker, Inc.
String
Three-dimensional
Distance±intensity plot
Animated.

Figure 15 is a schematic diagram. The blocks represent
locations on the layout and the arrows are material
move routes. In this example a single arrow represents
all materials. But dierent line styles or colors might
show dierent materials, or separate diagrams might
represent dierent material classes. Schematic dia-
grams are most useful in the early stages of a project
when they help the analyst and others to document,
visualize, and understand the material ¯ows.
Figure16isaquanti®edschematicdiagram.In
addition to routes it illustrates ¯ow intensity by the
thickness of shaded paths. The thicker the path, the
higher the ¯ow intensity. The quanti®ed schematic
may derive from the schematic as the project pro-
gresses and data become known.
620 Wrennall and Tuttle
Figure 15 Schematic ¯ow diagram.
Copyright © 2000 Marcel Dekker, Inc.
and ¯ow patterns. It can also assist in designing certain
types of handling systems, such as automatic guided
vehicles (AGVs).
Macrolevel ¯ow patterns. The facility layout
affects sequence and characteristics of material ¯ow.
Indeed, the ¯ow pattern dictates the shape or arrange-
mentwithinafacility.Figure22showsthebasic¯ow
patterns: straight-through ¯ow, L-shape, U-shape or
circular, and hybrids.
With straight-through or linear ¯ow, material enters
and exits at opposite ends of the site or building.
Flow deviates little from the shortest straight line

path. Material movement is progressive. Receiving
and shipping areas (entrances and exits) are physically
separate.
Straight-through ¯ow is simple and encourages
high material velocity. Operations are typically
sequential. This ¯ow pattern has been a hallmark of
622 Wrennall and Tuttle
Figure 17 Locational ¯ow diagram (shaded lines).
Figure 18 Locational ¯ow diagram (multiple lines). Figure 19 River diagram.
Copyright © 2000 Marcel Dekker, Inc.
mass production. With this type of ¯ow, material
tracking and handling are relatively simple. In fact,
Henry Ford and Charles Sorensen invented the
assembly line to solve a material ¯ow problem.
Straight-through ¯ow can also be vertical movement
in a high or multistory building. This ¯ow pattern
was used in some of the ®rst water-powered textile
factories in England.
L-shape ¯ow has a 908 directional change. This
pattern results from multiple material entry points
along the ¯ow path and a need for direct access. It is
a ¯ow pattern sometimes used in paint shops.
U-shape or circular ¯ow is an extension of the L-
shape ¯ow. The loop may be open or closed.
Materials return to their starting vicinity. These
patterns combine receiving and shipping docks with
shared personnel and handling equipment.
Conversely, one set of truck docks in a building can
create a U or circular ¯ow, for example, morning
receiving and afternoon shipping patterns.

The use of common receiving and shipping person-
nel is not conducive to good security. In pharmaceuti-
cal manufacturing regulations may require strict
separation of receiving and shipping facilities.
Incoming material handling, storage, and material
physical characteristic dierences may also require dif-
ferent personnel skills from those required at shipping.
Hybrids, such as X, Y, Z, or star, are combinations
or variations of the basic ¯ow patterns.
Flow complexity. Simple material ¯ow patterns
have fewer routes, fewer intersections and shorter dis-
tances. River and locational diagrams show ¯ow com-
plexity. These can be used to evaluate the relative
complexity inherent in various layouts.
Material Handling and Storage Systems 623
Figure 20 String diagram.
Figure 21 Three-dimensional material ¯ow diagram.
Copyright © 2000 Marcel Dekker, Inc.
convenient in that respect. However, the convenience
for the designer becomes a high-cost system that
encourages large lots and high inventories. It seldom
supports just-in-time and world-class manufacturing
strategies.
The macrolevel handling plan speci®es the route,
container and equipment for each move. It then accu-
mulates the total moves by equipment type and calcu-
lates equipment requirements. To prepare a handling
plan:
Material Handling and Storage Systems 625
Figure 23 Transport work material ¯ow evaluation.

(a)
(b)
Copyright © 2000 Marcel Dekker, Inc.
1. Assemble ¯ow analysis output:
a. Routes and intensities
b. Material ¯ow diagrams
c. Layout(s).
2. For each route and material class select:
a. Container
b. Route structure
c. Equipment.
3. Calculate equipment requirements.
4. Evaluate and select equipment.
1.3.1 Containers
Materials in industrial and commercial facilities move
in three basic forms: singles, bulk, and contained.
Singles are individual items handled and tracked as
such. Bulk materials, liquids, and gases assume the
form or shape of their container. Fine solids such as
¯owable powders are also bulk. In containerized hand-
ling, one or more items are in or on a box, pallet,
board, tank, bottle, or other contrivance. The con-
tainer restrains the items within and, for handling pur-
poses, the container then dominates.
Some materials take several forms. Nails and screws
can move on belt conveyors almost like powders. Later
in the process, handling may be individually or in con-
tainers. Containers oer several advantages:
Protecting the contents
Improving handling attributes

Improved use of cube
Standardizing unit loads
Assisting inventory control
Assisting security.
Pallet and palletlike containers have in the past been
the most widely used. In many industries they still
are.
``Tote pans'' and ``shop boxes'' have evolved into
sophisticated container families. They are versatile for
internal and external distribution and are an important
feature of kanban systems. Because of their wide use
they should be standardized and selected with great
care.
Just-in-time, cellular manufacturing and time-based
competition strategies require moving small lot sizes to
point of use, which calls for smaller containers.
For broad use in large plants, a family of inter-
modular units is important. The International
Organization for Standardization (ISO) and the
American National Standards Institute (ANSI) have
set size standards. The most popular families use
48 in: Â 40 in: and 48 in: Â32 in: pallet sizes. Figure
24 shows one system.
Larger-than-pallet containers are primarily for
international trade and ISO standardized unit have
been designed. There is, of course, a large variety of
nonstandard loads and containers.
The key to container selection is integration.
Container, route structure, and equipment are inti-
mately connected; they should ®t with and complement

each other. Other issues such as process equipment
and lot size also in¯uence container selection.
Unfortunately, most container selections occur by
default. Certain containers pre-exist and new products
or items get thrown into them. Existing route struc-
tures may also dictate container selection.
Manufacturing strategy should in¯uence container
selection. Conventional cost-based strategies indicate
large containers corresponding to large lot sizes; con-
temporary strategies emphasize variety and response
time. The smallest feasible container corresponding
to small process and movement lot sizes should be
selected.
626 Wrennall and Tuttle
Figure 24 Box con®gurations for standard pallets.
Copyright © 2000 Marcel Dekker, Inc.
1.3.2 Route Structure
Route structure in¯uences container and equipment
selection. It impacts costs, timing and other design
issues. Figure 25 shows the three basic route structures:
direct, channel, and terminal. In a direct system, mate-
rials move separately and directly from origin to desti-
nation. In a channel system which has a pre-established
route, loads move along it, often comingled with other
loads. In a terminal system, endpoints have been estab-
lished where the ¯ow is broken. Materials may be
sorted, consolidated, inspected, or transferred at
these terminals. In practice, many hybrids and varia-
tions of these basic route structures occur, as Fig. 26
shows.

1.3.2.1 Direct Systems
Direct systems using fork trucks are common. In
operation, a pallet of material needs moving to another
department; the foreman hails a fork truck driver who
moves it to the next department. An analogy for a
direct system is how taxis operate, taking their custo-
mers directly from one location to another without
®xed routes or schedules.
Direct systems are appropriate for high ¯ow inten-
sities and full loads. They also have the least transit
time and are appropriate when time is a key factor,
provided there is no queuing for transit requests.
1.3.2.2 Channel Systems
Channel systems use a predetermined path and
schedule. In manufacturing some automatic guided
vehicle systems work this way. Manned trailer trains
and industrial trucks ®t channel systems. They follow a
®xed route, often circular. At designated points they
stop to load and unload whatever is originating or
reaching a destination at that point. City bus systems
and subway systems use the channel structure.
Channel systems are compatible with just-in-time
(JIT) and world class manufacturing strategies. Many
JIT plants need to make frequent moves of small quan-
tities in tote boxes. They may use a channel system
with electric industrial trucks or golf carts. These
carts operate on a ®xed route, picking up material
and dropping o loads as required. Externally, over
the road trucks make several stops at dierent suppli-
ers to accumulate a full load for delivery.

Simultaneously, they return kanban signals and
empty containers for re®ll.
Lower ¯ow intensities, less-than-full loads and long
distances with load consolidation bene®t from channel
systems. Standardized loads also indicate the use of a
channel system.
1.3.2.3 Terminal Systems
In terminal systems loads move from origin to ultimate
destination through one or more terminals. At the
Material Handling and Storage Systems 627
Figure 25 Basic route structures.
Figure 26 Hybrid route structures.
Copyright © 2000 Marcel Dekker, Inc.
terminal material is transferred, consolidated,
inspected, stored, or sorted. The United States Postal
Service, Federal Express, and United Parcel Service all
use terminal systems.
A single central terminal can control material well.
Multiple terminals work well with long distances, low
intensities and many less-than-full loads. Airlines use
terminal systems for these reasons.
A warning about multistep distribution systems.
The characteristics of ultimate demand assume seaso-
nal characteristics and lead to misinterpretations in
production planning.
1.4 EQUIPMENT
There are hundreds of equipment types each with
varied capacities, features, options, and brands. The
designer chooses a type which ®ts the route, route
structure, containers, ¯ow intensity, and distance.

These design choices should be concurrent to assure
a mutual ®t.
Any material move has two associated costsÐ
terminal and travel. Terminal cost is the cost of
loading and unloading and does not vary with dis-
tance. Transport cost varies with distance, usually in
a direct relationship as Fig. 27 illustrates. Equipment is
suitable for either handling or transporting but seldom
both.
1.4.1 Using the Distance±Intensity Plot for
Selection
The distance±intensity (D-I) plot is useful for equip-
mentselection.Figure28isarepresentativeD-Iplot
with isotransport work curves. Each route on a lay-
out plots as a point on the chart. Routes which fall in
the lower-left area have low intensity and short dis-
tances. Typically these routes would use elementary,
low-cost handling equipment such as hand dollies or
manual methods. Routes in the upper left have short
distances but high intensity. These require equipment
with handling and manipulating capabilities, such as
robots, or short-travel equipment, such as conveyors.
Routes on the lower-right have long distances and
low intensities. Equipment with transport capabilities
like a tractor trailer train is needed here. Plots in the
middle area indicate combination equipment such as
the forklift truck. In the upper-right area, long dis-
tances and high intensities indicate the need for a
layout revision. If substantiated, long routes with
high intensities require expensive and sophisticated

equipment.
Figure29isfromarecentstudyonhandlingcosts.
In this study, representative costs for handling pallets
with several common devices were calculated. These
costs included direct, indirect, and capital.
Shaded areas on the diagram show regions where
each type of equipment dominates as the lowest cost.
This chart is generic and may not apply to a particular
situation; nor does the chart account for intangibles
such as ¯exibility, safety or strategic issues.
1.4.2 Using Material Flow Diagrams for
Equipment Selection
Locational, river and string diagrams also help with
equipment selection. Here, the ¯ow lines indicate dis-
tance, ¯ow intensity and ®xed and variable paths.
Figure30showshowtointerpretthisinformation.
628 Wrennall and Tuttle
Figure 27 Terminal/travel cost comparisons.
Copyright © 2000 Marcel Dekker, Inc.
632 Wrennall and Tuttle
Figure 31 Material handling selection guide.
Copyright © 2000 Marcel Dekker, Inc.
which are: three or four wheel; battery driven or inter-
nal combustion engine; rider, stand-up or walkie;
duplex, triplex or quad mast; and pneumatic, cush-
ioned, or solid tires.
The counterbalanced design puts a large force on
the front wheels and can cause ¯oor loading problems.
Lifting capacity diminishes with height and there is
some danger of overbalancing. Carton clamps, side

shifters and coil handlers are some of the available
attachments.
Reach trucks have small wheels near the forward
edge and on each side of the load, thus requiring less
counterbalancing. In operation, the forks or the entire
mast extends to pick up or set down a load. The truck
does not travel in this extended position. Some char-
acteristics of reach trucks as compared with counter-
balanced trucks are:
5%±15% slower
Have nontilting forks
Require better ¯oors
Use smaller batteries
Have poorer ergonomics
Are lighter weight
Work in narrower aisles.
Other forklift trucks include the following:
Pallet trucks are small, inexpensive machines which
pick up pallets resting on the ¯oor or on low
stacks. Both manual and battery-powered mod-
els are available. Pallet trucks cannot handle
double faced pallets and require almost perfect
¯oor conditions.
Stackers are small manual or electric machines simi-
lar to pallet trucks. Unlike pallet trucks, they can
elevate and thus stack their loads. Outriggers or
legs support the weight of the load. Outriggers
straddle the load; legs are underneath the forks.
Stackers are often used in maintenance or tool
changing.

Four-way trucks are useful for carrying long items
lengthwise through relatively narrow aisles. They
are variations of the reach truck with rotatable
wheels that allow them to travel in four direc-
tions.
Turret trucks have a mast that rotates on a track
without extending beyond the width of the
machine. These trucks can operate in an aisle
only 6 in. wider than the truck and access pallets
on both sides. Turret trucks are used for high rise
storage operations.
Side-loader trucks pick up and carry the load on the
side. The forks are at right angles to the travel
direction, which is useful for long, narrow loads
such as pipe or lumber. The side loader carries
such loads lengthwise down the aisle.
1.4.5 Conveyors
Conveyors are ®xed devices which move material con-
tinuously on a pre-established route. These systems
range from short, simple lengths of unpowered con-
veyor to vast networks of conveyor sections with
sophisticated controls.
Belt conveyors have a ¯exible belt which rides on
rollers or a ¯at bed. The belt may be cloth,
rubber, plastic, wire mesh, or other material.
Most articles can ride a belt conveyor up to 308
inclination.
With roller and skate-wheel conveyors, objects ride
on rollers or wheels. Any objects on the conveyor
should span at least three sets of rollers.

Movement can come from powered rollers,
gravity, or operators.
Chain conveyors carry or push objects with a chain.
Many varieties are available.
Overhead conveyors use an I-beam or other shape as
a monorail. Carriers roll along the monorail with
loads suspended underneath. A chain connects
the carriers and pulls them along. In a power-
and-free system, the chain and carriers are inde-
pendent. A disconnection mechanism stops the
carrier. Power-and-free systems oer more ¯ex-
ibility than standard monorails but at a much
higher cost. Recent designs of power and free
conveyors are inverted and ¯oor mounted.
1.4.6 Vibratory Machines
Ware [1] de®nes a vibratory machine as ``any unit
intentionally or purposely vibrated in order for it to
perform useful work. Vibration induces a material to
move instead of forcing it.''
The two distinct categories of vibratory machines
that are most often used in material handling systems
are those for accomplishing induced vertical ¯ow and
induced conveying.
1.4.7 Automatic Guided Vehicle Systems
Automatic guided vehicle systems (AGVS) use driver-
less vehicles to transport materials within an operation.
Material Handling and Storage Systems 633
Copyright © 2000 Marcel Dekker, Inc.
AGV size can vary from small, light-duty vehicles that
carry interoce mail to heavy-duty systems that trans-

port automobiles during assembly. Several types of
guidance are available with a range of sophistication
in logic and intelligence.
Most AGVs move along a predetermined track
system not unlike a model railroad. Optical tracking
systems use re¯ective tape or paint on the ¯oor to
de®ne the track. A photosensitive device on the
vehicle detects drift from the track and actuates the
steering mechanism for correction. Optical systems
are inexpensive and ¯exible. They are sensitive
to dirt, however, and many users consider them
unsatisfactory.
Electromagnetic guidance systems follow a mag-
netic ®eld generated by conductors laid in the ¯oor.
The frequency of this ®eld can vary in each track sec-
tion and thus identify the vehicle's location. Sensors on
the vehicle detect the ®eld, its location and perhaps the
frequency. The guidance system corrects the vehicles
track accordingly. Electromagnetic guidance systems
are somewhat expensive to install or relocate, but
AGV owners generally prefer electromagnetic guidance
systems for their reliability.
A newer type of guidance system optically reads
``targets'' placed high on walls and columns. The sys-
tem then computes vehicle position with triangulation.
In the future, guidance systems may use the satellite
navigation systems.
Figure 32 illustrates some of the vehicles available
for AGV systems. Tractor±trailer systems use a driver-
less tractor to tow one or more trailers, using manual

or automatic coupling. Such systems are best for large
loads and long distances. Some vehicles serve as assem-
bly stations in addition to moving loads.
Self-loading vehicles stop at ®xed stations and load
or unload containers. These are normally pallet-size
loads.
AGV forklift systems use vehicles similar to pallet
trucks. They can pick up a pallet, carry it to a new
location and lower it automatically. All or part of
the cycle may be automatic.
Special systems may use ®xtures to carry engines,
automobiles or other large products through a produc-
tion process.
At the lowest level of control vehicles follow a single
path in a single direction. They stop at predetermined
stations, at obstructions or when encountering
634 Wrennall and Tuttle
Figure 32 Automatic guided vehicles.
Copyright © 2000 Marcel Dekker, Inc.
another. Intelligent trucks have preprogrammed desti-
nations, locating their position by sensing the magnetic
frequencies. These vehicles can use multiple paths to
navigate to and stop at their assigned destination.
Centralized control systems use a computer to track
vehicles and control movement. Vehicles broadcast
their current location and the computer sends control
signals back to the vehicle controlling both movement
and route.
Towveyors were the precursors to AGVs. They are
powered by a cable or chain which moves continuously

in a ¯oor groove. A pin or grip mechanism connects
and disconnects the vehicle.
1.4.8 System Design and Documentation
When the ¯ow analysis is complete and a layout
selected, it is time to prepare the macrolevel material
handling plan.
Now that the handling for each route has been iden-
ti®ed, equipment requirements are estimated. In the
case of ®xed-path equipment, such as roller conveyors,
this estimation is simple and straightforward. Where
variable-path equipment is used on multiple routes,
estimate the total time required for each route and
material class as well as the eective equipment utiliza-
tion.InFig.33anexampleestimateisshownfora
bakery ingredients warehouse.
1.5 WAREHOUSING AND STORAGE
The most successful manufacturers and distributors
now recognize that inventory often camou¯ages some
form of waste. The causes of waste are in the structure
of the inventory systems. The ultimate goal is to
restructure and eliminate all storage of products.
Restructuring for minimum inventory is usually
more fruitful than pursuing better storage methods,
although compromises must be made and a require-
ment for some storage often exists.
1.5.1 Stores Activities
This section explains how to design optimum storage
systems for the inventory which remains after a suita-
ble restructuring eort.
Storage operations have two main functions: hold-

ing and handling. Holding refers to the stationing of
materials in de®ned storage positions. Handling is the
movement to and from the storage position. Ancillary
activities such as inspection, order picking, or receiving
are also part of handling.
Average turnover is the ratio of annual throughput
to average inventory over the same period.
Throughput and inventory may be in dollars, produc-
tion units, or storage units ($, pieces, pallets, cartons).
Turnover 
Annual throughput
Average inventory
The relative importance of holding and handling in a
particular situation guides the analysis. With high
turnover, handling dominates; with low turnover, hold-
ing dominates.
Handling-dominated warehouses call for detailed
analysis of procedures and material handling. These
warehouses use more sophisticated handling devices
such as automatic storage and retrieval systems
(ASRS) and automated conveyors.
Holding-dominated warehouses call for simple,
inexpensive, and ¯exible handling equipment. These
warehouses often require high-density storage meth-
ods, such as drive-through racking.
1.5.2 Storage Equipment
The types of storage equipment available are almost as
diverse as the types of containers and handling equip-
ment. The selection of both storage equipment and
containers is interrelated.

1.5.3 Analysis and Design of Storage Systems
The design of storage systems should co-ordinate with
the layout design of the total facility. Layout planning
has four phases: orientation, macrolayout, populated
layout, and implementation.
Orientation. Storage planning during this phase is at
a high level. In this phase the planners are
oriented to the entire scope of the project, for
example, the building size estimates, planning
assumptions, project stang, and policies and
strategies to be supported.
Macrolayout. This is the main planning phase where
the major storage area SPUs are determined. In
addition to determining storage space these SPUs
can include pick and pack areas, docks, and
receiving areas, although some of them may be
in a separate location. The designer re®nes esti-
mates of storage space and co-ordinates them
with other design and strategic decisions.
Material Handling and Storage Systems 635
Copyright © 2000 Marcel Dekker, Inc.
Information systems plan
Stang plan.
Populated layouts show the location of all racks, aisles,
doors, oces, and other features. The layout should
have sucient information to prepare architectural
and installation drawings.
The material handling plan for the storage opera-
tions is similar to that made for the macrolayout of any
facility. It shows all origin and destination points for

materials. It shows ¯ow rates, equipment, and contain-
ers used on each route. For many warehousing opera-
tions, the material handling plan is simple and can be
overlaid on a layout plan.
The equipment requirements summary lists the
types and numbers of storage and handling equipment.
It should also include a summary speci®cation for each
type of equipment.
The information systems plan speci®es the type of
information which is to be available, equipment
required and other data necessary to purchase equip-
ment and set up systems. It should include manual as
well as computer-supported systems.
Preparing a complete storage plan requires the fol-
lowing steps:
1. Acquire data/information.
2. Classify storage materials.
3. Calculate material and order ¯ows.
4. Calculate storage requirements.
5. Select equipment.
6. Plan the layout.
7. Specify information procedures and systems.
Step 1. Acquire Data/Information. Information re-
quired for the storage analysis covers products,
volumes, inventory, orders, and current and past
operations.
Products and volumes. Information on products
includes general orientation material on the
types of products to be stored and any special
characteristics. A detailed list or database should

be included with every item number, and pro-
ducts should be included by size, brand, or
other classi®cation. Volume information should
include historical sales (or throughput) volumes
for each line item or product group as well as
total sales. This is often the same product volume
information used for facility planning and mate-
rial handling analysis. A product pro®le showing
items or groups and their volumes on a ranked
bar chart is useful. Forecasts by product group
should be obtained or prepared.
Inventory. Historical inventory information may be
available when there is a similar existing opera-
tion. The information should include average
and peak inventory for each item or product
group over a meaningful period. When historical
information does not apply, policies or judgnent
must suface. A decision to keep ``two months on-
hand'' or ``maintain an average 10 turns'' can
help establish inventory requirements.
Orders. An order refers to any withdrawal request.
It may be a sales order, production order or ver-
bal request for incidental supplies. An order pro-
®le shows the average line items and line item
quantity per order. The pro®le may also include
weekly or seasonal order patterns and should
include forecast trends and changes. Identifying
urgency or delivery requirements may be neces-
sary in some cases.
Current and past operations. This information

includes stang, space usage, procedures, opera-
tion sequence, equipment, policies, and any other
pertinent information not included above.
Step 2. Classify Materials. The classi®cation of mate-
rials is similar to classi®cation activities used for mate-
rial ¯ow analysis. There may be slight differences,
however, since the primary concern here is storage
characteristics.Figure34showsoneclassi®cation
scheme. Categories to select from are function, desti-
nation, work-in-process, ®nished goods, high turnover
items and slow movers.
Step 3. Calculate Material and Order Flows. Material
¯ows for a storage operation are calculated in the same
way as for any other layout. Orders are an additional
parameter. Order ¯ows in a storage operation affect
the timing of an order and picking patterns.
Step 4. Calculate Storage Requirements. For each
storage class the storage space requirement must be
calculated. This may be done by using turnover rates,
existing data, or computer simulation. It is necessary in
this step to con®rm inventory policies and storage area
utilization levelsÐrandom storage with high space
utilization or dedicated locations with lower overall
space utilization.
A ``pull'' replenishment system with certi®ed ven-
dors requires less space for operating, janitorial, main-
tenance, and oce supplies.
Step 5. Select Equipment. In a warehouse operation
handling and storage equipment are interrelated and
should be selected together. Handling equipment types

were discussed previously. Storage equipment types are
discussed in Sec. 1.5.3.
Material Handling and Storage Systems 637
Copyright © 2000 Marcel Dekker, Inc.
Material Handling and Storage Systems 639
Figure 35 Storing space planning guide.
Figure 36 External material and information ¯ows.
Copyright © 2000 Marcel Dekker, Inc.
may be necessary. Since pallets are supported only on
their edges, pallet quality must be high. Limited FIFO
is possible if there is access to both sides.
1.5.5 Small Parts Storage
Small parts storage systems are either static or
dynamic. Static systems include shelving and drawers
in various con®gurations. Dynamic systems are verti-
cal carousels, horizontal carousels, mini-trieves and
movable-aisle systems.
Shelving is a basic inexpensive and ¯exible storage
method. It often does not use space eectively and is
costly for intensive picking operations.
Modular drawer systems oer denser storage than
shelving. They are more expensive than shelves and
more dicult for picking.
1.5.6 Automatic Storage and Retrieval Systems
Automatic storage and retrieval systems (ASRS) store
materials in a high-density con®guration. These sys-
tems use a stacker crane or other mechanical device
to carry each load to its location and place it in
storage. The same crane retrieves loads as required
and delivers them to an output station. A computer

system controls movements and tracks location. The
ASRS computer often is in communication with a pro-
duction control system such as MRP. Such systems
usually work with pallet-size loads. Mini-trieve systems
are similar in concept to automatic retrieval systems
but use smaller loads such as tote boxes.
1.6 CONCLUSION
Materials movement is a key consideration in facility
planning. The material ¯ow analysis is necessary for
proper facility design and is a prerequisite for the
design of material handling systems and storage
areas. It is also an important means of evaluating
design options.
It is important to select the material handling equip-
ment to ®t the determined material ¯ow system. Often
the ¯ow and handling are forced to ®t the material
handling methods you have been sold.
Even though we want to eliminate material handling
and storage waste product storage may be required for
aging, quarantine, or qualifying processes. In other
cases storage serves as a buer in an improperly
designed and maintained system.
Following the step-by-step procedures outlined in
this chapter will support the operating strategy by
reducing costs, time and material damage. This is
basic to achieving world class and being a time-based
competitor.
640 Wrennall and Tuttle
Figure 37 Internal material and information ¯ows.
Copyright © 2000 Marcel Dekker, Inc.