Tài liệu Measuring Your True Equipment Productivity docx

CHAPTER V
Measuring Your True Equipment Productivity


Unlocking The Hidden Factory

Within most plants around the world there lies a hidden factory. Occasionally you catch a
glimpse of it, when production is humming along and everything is going right and no
machine is down. You know it's there, just below the surface, the potential of what your
plant could be if everything would just continue to work as it should. You wish it could
be that way all the time, but somehow problems get in the way and it vanishes, screened
from your sight by the reality of everyday business.

TPM is the key that can unlock that hidden factory and bring perhaps another 25 to 30%
of capacity into your production areas. Here is how you calculate your current equipment
productivity and determine your improvement potential.


Equipment Productivity

True equipment productivity is measured by Total Effective Equipment Productivity
(TEEP).

This is the overall formula that includes Equipment Utilization (EU) and Overall
Equipment Effectiveness (OEE). Most of the current TPM literature discusses only OEE
and disregards the fact that a high level of equipment utilization is required to accomplish
a high degree of equipment productivity and a good Return on Assets (ROA). You can
improve your OEE at the expense of equipment utilization by doing all your set-ups and
PMs during planned downtime. If plant management is truly interested in getting good
asset and capacity utilization, the TEEP formula is of prime importance (Figure 7).

Total Effective Equipment Productivity (TEEP), with the emphasis on "effective
productivity", includes planned downtime and is a combined measure of equipment
utilization and overall equipment effectiveness.

Overall Equipment Effectiveness (OEE) is the traditional and most widely used TPM
measure. It reflects how the equipment is performing overall while it is being operated.
As a matter of fact, it is not an exact measure of the equipment effectiveness, since set-
ups or changeovers and resulting adjustments are included. This does not have much to
do with the equipment's performance itself, but reflects the overall equipment's
effectiveness while the equipment is being run.


The Three Major TPM Formulas

TEEP (Total Effective Equipment Productivity)

=Equipment Utilization (EU) x
Overall Equipment Effectiveness (OEE)

OEE (Overall Equipment Effectiveness)
=Equipment Availability (EA) x
Performance Efficiency (PE) x
Rate of Quality (RQ)

NEE (Net Equipment Effectiveness)

=Uptime (UT) x
Performance Efficiency (PE) x
Rate of Quality (RQ)

Figure 7

Therefore a third formula, that clearly reflects the true quality and effectiveness of
equipment while it is running, seems to be in order. Net Equipment Effectiveness (NEE)
is this formula.

It excludes not only planned downtime (as does OEE), but also downtime required for
set-ups and adjustments. It is a reflection of the true mechanical condition of your
machine.


Equipment Losses

In order to calculate these three indices--TEEP, OEE, and NEE--you need to know what
your equipment losses are. TPM focuses on equipment losses that cut into your
equipment effectiveness. There are at least five categories:

• Set-up and adjustments
• Equipment failures
• Idling and minor stoppages
• Reduced speed
• Process defects (see Figure 8)

In many companies, there are more, such as warm-up losses, test runs, etc. Those losses
must be identified beforehand and included in the appropriate formula. It has been found
that the "reduced yield" or "start-up" loss (the difference from equipment start-up to
stable production) as described in other publications can not be measured as such, since it
normally consists of a combination of above five losses during the equipment de-bugging
or start-up phase. It is recommended to calculate the OEE at equipment

installation and then again at stable production to determine the "yield loss".

In the semiconductor industry, the term "yield" is used for the percentage of usable chips
obtained from a wafer and could be used as part of the OEE formula under Quality Level.
However, caution is advised, since this has no connection with the actual effectiveness of
the machine in question.

The first equipment loss is set-up and adjustment. When you do a set-up, the machine is
down, although it is not broken down. Of course, it's a necessary part of production, but
since it is a variable and can be reduced, it does qualify as an equipment loss. Frequently,
set-ups and changeovers are among the largest equipment losses, indicating the need to
carefully measure this loss and to develop improvements.

Unplanned downtime (equipment failures) is next. There are two types of equipment
failures: sporadic and chronic. Sporadic failures happen suddenly. Something on the
machine breaks. Usually you can identify it easily and fix it. It normally doesn't re-occur
often. Chronic failure is more difficult to deal with. Every once in a while, the machine
stops and you may not even know why. You suspect the cause, but you can't pin it down.
Eventually the plant learns to live with the defect. This compromise is not the right
solution and is not allowed to happen under TPM.

Both of these first two losses figure in the measurement of equipment availability. In
each case, the machine is down and therefore not available for production.

The next two losses are also called "hidden losses. " They're usually not measured and
not recorded as downtime because maintenance is not called and the equipment is not
broken down. It just runs less efficiently.

Idling and minor stoppages falls into this category. The machine's motor is running, but
no product is being processed. Perhaps there is a jam and no product is coming into the
machine, or the machine next in line is down and you are "blocked," or the operator is not
available for a few moments. Maybe you are momentarily out of parts, or the machine is
out of adjustment and needs to be re-adjusted. There are so many reasons for idling and
minor stoppages.

These little problems can cause some of the biggest losses in a factory. In one electronics
plant in Asia, a female operator was testing electronic parts that came down into the
machine through a channel. Every so often, the machine stopped (jammed) and the
operator used a small tool like an oversized toothpick to get it running again. It only took
her about four seconds to fix the problem, which happened on the average of three times
a minute. That's 12 seconds, and if you stop to figure it out, it's 20% of each production
minute. Multiply those 12 seconds per minute by eight hours and you have a considerable
loss of production. Jams figure prominently on every chart of idling and minor stoppages
analysis and frequently account for a high percentage of loss. Yet the reasons for most
jams are relatively easily corrected.

Reduced speed is the fourth major equipment loss. It stems mainly from poorly-
maintained, worn out or dirty equipment. Some other causes of speed losses are
insufficient debugging during the start-up phase, defective mechanisms or systems,
design weaknesses and insufficient equipment precision.

These two losses figure in the calculation of performance efficiency. In each case, the
machine is not broken down, but performs at a lower level of efficiency.

The fifth equipment loss is process defects. If a part is rejected or must be reworked, the
equipment time producing it is lost. This loss is relatively small when compared to other
major equipment losses. However, in today's environment of Total Quality, no rejects,
especially those caused by a machine, are tolerable. Typically, as the equipment gets

improved and better maintained under TPM, quality losses are also reduced.
Nevertheless, the reason for each quality loss must be investigated and the equipment
problem causing it must be eliminated. This loss is used to calculate the rate of quality.

As discussed before, there may be other losses in your plant. You must identify these
during the feasibility study and include them in your calculations.

Measuring each of these losses will determine overall equipment effectiveness (OEE) and
net equipment effectiveness (NEE) of your machines. Without proper identification and
quantification of your equipment losses, it will be very difficult to establish an effective
and tailor-made TPM program.


Calculating Equipment Effectiveness

Once all these losses are known, you can calculate your equipment effectiveness on a
step-by-step basis. Figures 9 and 10 show the procedure and a typical example.

Equipment is sitting in your plant 24 hours a day. Therefore, start with the total available
minutes (1440) in a 24-hour day. The company used as a typical example here is running
two shifts, so subtract 480 minutes (8 hours) for one shift. Then subtract the planned
downtime, which includes breaks and meals for the other two shifts plus planned
maintenance and any other planned downtime, such as meetings and no scheduled
production. The calculation will establish your percentage equipment utilization (60.4%).

The remaining time left after deduction of unutilized time is called running time (870
minutes). At this point, the OEE calculation starts, since now actual equipment losses
come into play. First, deduct the time spent for set-ups, changeovers and adjustments (70
minutes). The resulting calculation will give the planned availability (92.0%), which is
one part of the equipment availability (EA).

The time left after deduction of the set-ups is the operating time. At this point, the
calculation of the Net Equipment Effectiveness (NEE) starts. The amount of time the
equipment was broken down due to failures (unplanned downtime) is now deducted and
the percent uptime (93.7%) can be calculated. Unfortunately, this is often the only
number reported to plant management, creating a totally wrong impression of the real
equipment situation, since it only covers one single loss. For that reason, plant and