detroit type of automation

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Detroit-Type AutomationIn this chapter we consider the automated equipment used in the processing of discrete parts in

large volumes. The equipment is often in the configuration of mechanically integrated tiow lines,

consisting of a number of workstations that perform the processing operations on the line. These

flow lines are called transfer machines or transfer lines. The methods used to transport partsbetween stations are examined in this chapter as well as the other features that characterize these

production systems. We emphasize machining operations as the typical process carried out on these

systems, although the automated flow line concept is used in a variety of industries and processes.

1 AUTOMATED FLOW LINESLeave a reply

An automated flow line consists of several machines or workstations which are linked together by

work handling devices that transfer parts between the stations. The transfer of work parts occurs

automatically and the workstations carry out their specialized functions automatically. The flow linecan be symbolized as shown in Figure 4.1 using the symbols presented in Table 4.1. A raw work part

enters one end of the line and the processing steps are performed sequentially as the part moves

from one station to the next. It is possible to incorporate buffer storage zones into the flow line, either

at a single location or between every workstation.

It is also possible to include inspection stations in

the line to automatically perform intermediate checks on the quality of the work parts.

Manual stations might also be located along the flow line to

perform certain operations which are difficult or uneconomical to automate. These various features of

mechanized flow lines will be discussed in subsequent sections. Automated flow lines are generally
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the most appropriate means of production in cases of relatively stable product life ; high product

demand, which requires high rates of production; and where the alternative method of manufacture

would involve a large labor content. The objectives of the use of flow line automation are, therefore :

To reduce labor costs

To increase production rates

To reduce work-in-process

To minimize distances moved between operations

To achieve specialization of operations

To achieve integration of operations

Although Figure 4.1 shows the flow pattern of operations in a straight line, there are actually two

general forms that the work flow can take. These two configurations are in line and rotary.

In-line type

The in-line configuration consists of a sequence of workstations in a more-or-less straight-line

arrangement. The flow of work can take a few 90 turns, either for work piece reorientation, factory

layout limitations, or other reasons, and still qualify as a straight-line configuration. A common

pattern of work flow, for example, is a rectangular shape, which would allow the same operator toload the starting workpieces and unload the finished workpieces. An example of an in-line transfer

machine used for metal-cutting operations is illustrated in Figure 4.2.

Rotary type

In the rotary configuration, the work parts are indexed around a circular table or dial. The work

stations are stationary and usually located around the outside periphery of the dial. The parts ride on

the rotating table and are registered or positioned, in turn, at each station for its processing or

assembly operation. This type of equipment is often referred to as an indexing machine or dial index

machine and the configuration is shown in Figure 4.3.

Selection

The choice between the two types depends on the application. The rotary type is commonly limitedto smaller workpieces and to fewer stations. There is generally not as much flexibility in the design of

the rotary configuration. For example, the dial-type design does not lend itself to providing for buffer

storage capacity. On the other hand, the rotary configuration usually involves a lower-cost piece of

equipment and typically requires less factory floor space.

The in-line design is preferable for larger

workpieces and can accommodate a larger number of workstations. The number of stations on the

dial index machine is more limited due to the size of the dial. In-line machines can be fabricated with
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a built-in storage capability to smooth out the effect of work stoppages at individual stations and

other irregularities.

2 METHODS OF WORKPART TRANSPORT

The transfer mechanism of the automated flow line must not only move the partially completed work

parts or assemblies between adjacent stations, it must also orient and locate the parts in the correct

position for processing at each station. The general methods of transporting work pieces on flow

lines can be classified into the following three categories :

1.Continuous transfer

2.Intermittent or synchronous transfer

3.Asynchronous or power-and-free transfer

These three categories are distinguished by the type of motion that is imparted to the work piece by

the transfer mechanism. The most appropriate type of transport system for a given application

depends on such factors as :

The types of operation to be performed

The number of stations on the line

The weight and size of the work parts

Whether manual stations are included on the line

Production rate requirements
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Balancing the various process times on the line

Before discussing the three types of work transport system, we should try to clarify a possible source

of confusion. These transfer systems are used for both processing and assembly operations. In the

case of automatic assembly machines, we are referring to the mechanisms that transport thepartially completed assemblies between stations, not the feed mechanisms that present new

components to the assemblies at a particular station. The devices that feed and orient the

components are normally an integral part of the workstation. We take a closer look at these devices

in Chapter 7 when we discuss automatic assembly in more detail.Continuous transferWith the

continuous method of transfer, the work parts are moved continuously at constant speed. This

requires the work heads to move during processing in order to maintain continuous registration with

the work part. For some types of operations, this movement of the work heads during processing is

not feasible. It would be difficult, for example, to use this type of system on a machining transfer line

because of inertia problems due to the size and weight of the work heads. In other cases, continuous

transfer would be very practical. Examples of its use are in beverage bottling operations, packaging,

manual assembly operations where the human operator can move with the moving flow line, and

relatively simple automatic assembly tasks. In some bottling operations, for instance, the bottles are

transported around a continuously rotating drum. Beverage is discharged into the moving bottles by

spouts located at the drums periphery. The advantage of this application is that the liquid beverage

is kept moving at a steady speed and hence there are no inertia problems.Continuous transfer

systems are relatively easy to design and fabricate and can achieve a high rate of

production.Intermittent transferAs the name suggests, in this method the workpieces aretransported with an intermittent or discontinuous motion. The workstations are fixed in position and

the parts are moved between stations and then registered at the proper locations for processing. All

work parts are transported at the same time and, for this reason, the term synchronous transfer

system is also used to describe this method of work part transport. Examples of applications of the

intermittent transfer of work parts can be found in machining operations, press working operations

or progressive dies, and mechanized assembly. Most of the transfer mechanisms reviewed in

Section 4.3 provide the intermittent or synchronous type of work part transport.Asynchronous

transferThis system of transfer, also referred to as a power-and-free system, allows each work

part to move to the next station when processing at the current station has been completed. Each

part moves independently of other parts. Hence, some parts are being processed on the line at the

same time that others are being transported between stations. Asynchronous transfer systems offer

the opportunity for greater flexibility than do the other two systems, and this flexibility can be a great

advantage in certain circumstances. In-process storage of work parts can be incorporated into the

asynchronous systems with relative ease. Power-and-free systems can also compensate for line


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balancing problems where there are significant differences in process times between stations.

Parallel stations or several series stations can be used for the longer operations, and single stations

can be used for the shorter operations. Therefore, the average production rates can be

approximately equalized. Asynchronous lines are often used where there are one or more manually

operated stations and cycle-time variations would be a problem on either the continuous or

synchronous transport systems. Larger work parts can be handled on the asynchronous systems. A

disadvantage of the power-and-free systems is that the cycle rates are generally slower than for the

other types.Pallet fixturesThe transfer system is sometimes designed to accommodate some sort of

pallet fixture. Work parts are attached to the pallet fixtures and the pallets are transferred between

stations, carrying the part through its sequence of operations. The pallet fixture is designed so that it

can be conveniently moved, located, and clamped in position at successive stations. Since the part

is accurately located in the fixture, it is therefore correctly positioned for each operation. In addition to

the obvious advantage of convenient transfer and location of work parts, another advantage of pallet

fixtures is that they can be designed to be used for a variety of similar parts.

The other method of work part location and fixturing does not use pallets. With this method, the

workparts themselves are indexed from station to station. When a part arrives at a station, it is

automatically clamped in position for the operation.

The obvious benefit of this transfer method is that it avoids the cost of pallet fixtures

3 TRANSFER MECHANISMS

There are various types of transfer mechanisms used to move parts between stations. These

mechanisms can be grouped into two types: those used to provide linear travel for in-line machines,

and those used to provide rotary motion for dial indexing machines.Linear transfer mechanisms

We will explain the operation of three of the typical mechanim! the wal"in# $eam tranfer $ar

ytem, the powered roller conveyor system, and the chain-drive conveyor system. This is not a

complete listing of all types, but it is a representative sample.WALKING BEAM SYSTEMS. With the

walking beam transfer mechanism, the work parts are lifted up from their workstation locations by a

transfer bar and moved one position ahead. to the next station. The transfer bar then lowers the

parts into nests which position them more accurately for processing. This type of transfer device is


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illustrated in Figure 4.4. POWERED ROLLER CONVEYOR

SYSTEM. This type of system is used in general stock handling systems as well as in automated flow

lines. The conveyor can be used to move pans or pallets possessing flat riding surfaces The rollers

can be powered by either of two mechanisms. The first is a belt drive, in which a flat moving belt

beneath the rollers provides the rotation of the rollers by friction. A chain drive is the second common

mechanism used to power the rollers. Powered roller conveyors are versatile transfer systems

because they can be used to divert work pallets into workstations or alternate tracks. We discuss

roller conveyor systems in Chapter 14.

CHAIN-DRIVE CONVEYOR SYSTEM. Figure 4.5 illustrates this type of transfer system. Either a

chain or a flexible steel belt is used to transport the work carriers. The chain is driven by pulleys ineither an over-and-under configuration, in which the pulleys turn about a horizontal axis, or an

around-the-corner configuration, in which the pulleys rotate about a vertical axis.

This general type of transfer system can be used for continuous, intermittent, or non synchronous

movement of work parts. In the non synchronous motion, the work parts are pulled by friction or ride

on an oil film along a track with the chain or belt providing the movement. It is necessary to provide

some sort of final location for the work parts when they arrive at their respective stations.

Rotary transfer mechanisms

There are several methods used to index a circular table or dial at various equal angular positions

corresponding to workstation locations. Those described below are meant to be a representative

rather than a complete listing.
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RACK AND PINION. This mechanism is simple but

is not considered especially suited to the high-speed operation often associated with indexing

machines. The device is pictured in Figure 4.6 and uses a piston to drive the rack, which causes the

pinion gear and attached indexing table to rotate. A clutch or other device is used to provide rotation

in the desired direction.

RATCHET AND PAWL. This drive mechanism is shown in Figure 4.7. Its operation is simple but

somewhat unreliable, owing to wear and sticking of several of the components.

GENEVA MECHANISM. The two previous

mechanisms convert a linear motion into a rotational motion. The Geneva mechanism uses a

continuously rotating driver to index the table, as pictured in Figure 4.8. If the driven member has six

slots for a six-station dial indexing machine, each turn of the driver will cause the table to advance

one-sixth of a turn. The driver only causes movement of the table through a portion of its rotation.

For a six-slotted driven member, 120 of a complete rotation of the driver is used to index the table.

The other 240 is dwell. For a four-slotted driven member, the ratio would be 90 for index and 270

for dwell. The usual number of indexings per revolution of the table is four, five, six, and eight.

EXAMPLE 4.1

Let us examine the operation of a six-slotted Geneva mechanism. Suppose that the driver rotates at

6 rpm. Determine the cycle time of the indexing machine, the process time, and the time spent each

cycle in indexing the table to the next work position.

Solution:

As indicated above, for a six-slotted Geneva mechanism, the driver spends 120 of its rotation to

index the table, and the remaining 240 of rotation correspond to dwell of the table. At 6 rev/min, the

cycle time of the indexing machine is 10 s. The portion of this cycle time devoted to processing (dwell

of the indexing table) is 240/360 = 0.667. This corresponds to 6.67 s. The indexing time is 120/360

= 0.333 10 s = 3.33 s.
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CAM MECHANlSMS. Various forms of cam

mechanism, an example of which is illustrated in Figure 4.9, provide probably the most accurate and

reliable method of indexing the dial. They are in widespread use in industry despite the fact that the

cost is relatively high compared to alternative mechanisms. The cam can be designed to give a

variety of velocity and dwell characteristics.

% &UFFE' STO'A(E

Automated flow lines are often equipped with additional features beyond the basic transfer

mechanisms and workstations. For example, the idea of using a buffer storage capacity between

stations was introduced in Section 4.1. It is not uncommon for production flow lines to include

storage zones for collecting banks of work parts along the line. One example of the use of storage

zones would be two intermittent transfer systems, each without any storage capacity, linked together

with a work part inventory area. It is possible to connect three, four, or even more lines in this

manner. Another example of work part storage on flow lines is the asynchronous transfer line. With

this system, it is possible to provide a bank of work parts for every station on the line.

There are two principal reasons for the use of buffer storage zones. The first is to reduce the effect of

individual station breakdowns on the line operation. The continuous or intermittent transfer system

acts as a single integrated machine. When breakdowns occur at the individual stations or when

preventive maintenance is applied to the machine, production must be halted. In many cases, the

proportion of time the line spends out of operation can be significant, perhaps reaching 50% or

more. Some of the common reasons for line stoppages are :
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Tool failures or tool adjustments at individual processing stations

Scheduled tool changes

Defective work parts or components at assembly stations, which require that the feed

mechanism be cleared

Feed hopper needs to be replenished at an assembly station

Limit switch or other electrical malfunction

Mechanical failure of transfer system or workstation

When a breakdown occurs on an automated flow line, the purpose of the buffer storage zone is to

allow a portion of the line to continue operating while the remaining portion is stopped and under

repair. For example, assume that a 20-station line is divided into two sections and connected by a

parts storage zone which automatically collects parts from the first section and feeds them to the

second section. If a station jam were to cause the first section of the line to stop, the second section

could continue to operate as long as the supply of parts in the buffer zone lasts. Similarly, if the

second section were to shut down, the first section could continue to operate as long as there is

room in the buffer zone to store parts. Hopefully, the average production rate on the first section

would be about equal to that of the second section. By dividing the line and using the storage area,

the average production rate would be improved over the original 20-station flow line. A quantitative

analysis of the effect of adding buffer inventory zones will be presented in Chapter 5. Figure 4.10

illustrates the case of two processing lines separated by a storage buffer.

The second reason for using storage on How lines

is to smooth out the effects of variations in cycle times. These variations occur cither between

stations or, in the case of flow lines with one or more manual stations, they can occur from cycle to

cycle at the same station. To illustrate the second case, suppose that we are considering an
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assembly line on which all the stations are mechanized except one. The manual station requires the

operator to perform an alignment of two components and the time required tends to vary from cycle

to cycle. For the transfer system in this line, we must choose between a synchronous system with no

parts storage capacity and an asynchronous system which allows a float of parts ahead of each

station. To illustrate this difference in operation, let us consider the following example.

EXAMPLE 4.2

Assume that we have collected data on the operation and found the following distribution of

operation times for a total of 100 cycles: 7 s; two occurrences, or 29; 8 s; 10%: 9 s; 18%; 10 s; 38%;

11s; 20% and 12 s; 12%. This gives an average of 10 s. If this manual operation were used on the

synchronous machine. the line would have to be set up with cycle time of 12 s to allow the operator

time to finish all assemblies. This would give a production rate of 300 units/h from the line. If the

cycle time were adjusted to 11 s, the cycle rate would increase to 327 per hour. but the operator

would be unable to complete 12% of the assemblies. Thus, the actual production rate of completed

assemblies would be only 288 units/h. If the cycl time were decreased to 10 s, the cycle rate would

increase to 360 per hour. However, the operator would be unable to complete the assemblies

requiring 11 s and 12 s. The actual production rate would suffer a decrease to 245 units/h.

With the asynchronous transfer system. the line could be arranged to collect a bank of work parts

immediately before and after the manual station. Thus, the operator would be allowed a range of

times to complete the alignment process. As long as the operators average time were compatible

with the cycle time of the transfer system, the flow line would run smoothly. The line cycle time could

be set at 10 s and the production rate would be 360 good assemblies per hour.

The disadvantages of buffer storage on flow lines are increased factory floor space. higher in-

process inventory, more material handling equipment, and greater complexity of the overall flow line

system. The benefits of buffer storage are often great enough to more than compensate for these

disadvantages.

) %*) +ONT'OL FUN+TIONS

Controlling an automated flow line is a complex problem, owing to the sheer number of sequential

steps that must be carried out. There are three main functions that are utilized to cont,l the operation


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of an automatic transfer system. The first of these is an operational requirement, the second is a

safety requirement, and the third is dedicated to improving quality.

1.Sequence control. The purpose of this function is to coordinate the sequence of actions of

the transfer system and its workstations. The various activities of the automated flow line

must be carried out with split-second timing and accuracy. On a metal machining transferline, for example, the work parts must be transported, located, and clamped in place before

the work heads can begin to feed. Sequence control is basic to the operation of the flow line.

2.Safety monitoring. This function ensures that the transfer system does not operate in an

unsafe or hazardous condition. Sensing devices may be added to make certain that the

cutting tool status is satisfactory to continue to process the work part in the case of a

machining-type transfer line. Other checks might include monitoring certain critical steps in

the sequence control function to make sure that these steps have all been performed and in

the correct order. Hydraulic or air pressures might also be checked if these are crucial to the

operation of automated flow lines.

3.Quality monitoring. The third control function is to monitor certain quality attributes of the

work part. Its purpose is to identify and possibly reject defective work parts and assemblies.

The inspection devices required to perform quality monitoring are sometimes incorporated

into existing processing stations. In other cases, separate stations are included in the line for

the sole purpose of inspecting the work part.

It is possible to extend the notion of quality monitoring and to incorporate a control loop into the flow

line as illustrated in Figure 4.11. An inspection station would be used to monitor certain quality

characteristics of the part and to feed back information to the preceding workstations so that

adjustments in the process could be made.

The traditional means of controlling the sequence of steps on the transfer system has been to use

electromechanical relays. Relays are employed to maintain the proper order of activating the work

heads, transfer mechanism, and other peripheral devices on the line. However, owing to their

comparatively large size and relative unreliability, relays have lost ground to other control devices,

such as programmable controllers and computers. These more modern components offer

opportunities for a higher level of control over the flow line, particularly in the areas of safety

monitoring and quality monitoring.

Conventional thinking on the control of the line has

been to stop operation when a malfunction occurred. While there are certain malfunctions

representing unsafe conditions that demand shutdown of the line, there are other situations where

stoppage of the line is not required and perhaps not even desirable. For example, take the case of a

feed mechanism on an automatic assembly machine that fails to feed its component. Assuming that
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the failures are random and infrequent, it may be better to continue to operate the machine and lock

out the affected assembly from further operations. If the assembly machine were stopped,

production would be lost at all other stations while the machine is down. Deciding whether it is better

to stay in operation or stop the line must be based on the probabilities and economics of the

particular case. The point is that there are alternative control strategies to choose between,

instantaneous control and memory control.

1.Instantaneous control. This mode of control stops the operation of the flow line immediately

when a malfunction is detected. It is relatively simple, inexpensive, and trouble-free.

Diagnostic features are often added to the system to aid in identifying the location and cause

of the trouble to the operator so that repairs can be quickly made. However, stopping the

machine results in loss of production from the entire line, and this is the systems biggest

drawback.

2.Memory control. In contrast to instantaneous control, the memory system is designed to

keep the machine operating. It works to control quality and/or protect the machine by

preventing subsequent stations from processing the particular work part and by segregating

the part as defective at the end of the line. The premise upon which memory-type control isbased is that the failures which occur at the stations will be random and infrequent. If,

however, the station failures result from cause (a work head that has gone out of alignment,

for example) and tend to repeat, the memory system will not improve production but, rather,

degrade it. The flow line will continue to operate, with the consequence that bad parts will

continue to be produced. For this reason, a counter is sometimes used so that if a failure

occurs at the same station for two or three consecutive cycles, the memory logic will cause

the machine to stop for repairs.

DESIGN AND FABRICATION CONSIDERATIONS

When a manufacturing firm decides that some form of automated flow line represents the best

method of producing a particular work part or assembly, there are then a series of specifications that

must be decided. In designing and building an automated flow line, some of the details to consider

are the following :

Whether the flow line is to be engineered in-house or by a machine tool builder

Size, weight, geometry, and material if a processed work part

Size, weights, and number of components if an assembly

Tolerance requirements

Type and sequence of operations

Production-rate requirements


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Type of transfer system

Methods of fixturing and locating work parts

Methods of orienting and feeding components in the case of assemblies

Reliability of individual stations and transfer mechanisms, as well as overall reliability of the

line

Buffer storage capability

Ease of maintenance

Control features desired

Floor space available

Flexibility of line in terms of possible future changes in product design

Flexibility of line to accommodate more than a single work part

Initial cost of the line

Operational and tooling cost for the line

In developing the concept for a mechanized flow line, there are two general approaches that can be

considered. The first is to use standard machine tools and other pieces of processing equipment at

the workstations and to connect them with standard or special material handling equipment. The

material handling hardware serves as the transfer system and moves, feeds, and ejects the work

between the standard machines. The line of machines is sometimes referred to as a link line. The

individual machines must either be capable of operating on an automatic cycle or they must be

manually operated. There may also be fixturing and location problems at the stations which are

difficult to solve without some form of human assistance during the cycle. A firm will often prefer the

link line because it can be made up from machine tools that are already in the plant, these machine

tools can be reused when the production run is finished, and there is less debugging and

maintenance. These flow lines can also be engineered by personnel within the firm, perhaps with the

aid of material handling experts. The limitation of these flow lines is that they favor simpler workpiece

shapes and smaller sizes since the work handling equipment is less sophisticated and more general-

purpose. Greater future use of industrial robots as material handling devices will increase the

attractiveness of the link line. This type of flow line is used in such processes as plating and finishing

operations, press-working, rolling mill operations, gear manufacturing, and a variety of machining


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operations.

The alternative approach to developing an automated line is to turn the problem over to a machine

tool builder specializing in the fabrication of transfer lines, assembly machines, or other flow line

equipment. Using the customers blueprints and specifications, the builder will submit a proposal for

the line. Typically, several machine tool builders will be asked to make proposals. Each proposed

design will be based on the machine components comprising the builders product line as well as the

ingenuity and experience of the engineer preparing the proposal.

Once a particular proposal is accepted, the machine

tool builder will proceed with the final detailed design. The resulting machine will utilize a building-

block principle. That is, the specialized flow line, designed to produce the customers particularproduct, will be constructed out of standard components. These standard components consist of the

base or transfer system, and work heads for performing the -various processing or assembly

operations. These standard components will be fabricated into the special configuration required for

the customers product. Several examples of these standard transferline components are pictured in

Figures 4.16 and 4.17. For metal-cutting transfer lines, the work heads consist of the feed

mechanism, spindle, and power source. The work heads must then be fitted with special tools to

carry out the particular process. These work heads do not have a frame or worktable. Instead, they

are attached to the transfer system frame, which has been specially adapted for the work part. When

a flow line has been fabricated using this building-block principle, it is sometimes referred to as a

unitized flow line. The standard machine tool components all go together and act as a single

mechanical unit.
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Higher production rates are generally possible with

unitized construction compared to link lines. Also, less floor space is required because the unitized

lines are typically much more compact. The higher cost of unitized equipment makes it suitable only

for long production runs and on products not subject to frequent design changes. Equipment

obsolescence becomes a danger if these two requirements are not met. Applications of this type of

flow line construction are found in transfer lines for machining automotive engine parts and in

assembly machines for pens, small hardware items, electrical assemblies, and so on. Figures 4.2,

4.12, 4.13, 4.14, and 4.15 illustrate this type of flow line construction.

AUTOMATION FOR MACHINING OPERATIONS

Transfer systems have been designed to perform a great variety of different metal-cutting processes.

In fact, it is difficult to think of machining operations that must be excluded from the list. Typical

applications include operations such as milling, boring, drilling, reaming, and tapping. However, it is

also feasible to carry out operations such as turning and grinding on transfer-type systems.

There are various types of mechanized and automated machines that perform a sequence of

operations simultaneously on different workparts. These include dial indexing machines, trunnion

machines, and transfer lines. To consider these machines in approximately the order of increasing

complexity, we begin with one that really does not belong in the list at all, the single-station machine.
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Single-station machine

These mechanized production machines perform several operations on a single work part which is

fixtured in one position throughout the cycle. The operations are performed on several different

surfaces by work heads located around the piece. The available space surrounding a stationary

workpiece limits the number of machining heads that can be used. This limit on the number of

operations is the principal disadvantage of the single-station machine. Production rates are usually

low to medium.

Rotary indexing machine

To achieve higher rates of production, the rotary indexing machine performs a sequence

of machining operations on several workparts simultaneously. Parts are fixtured on a horizontal

circular table or dial, and indexed between successive stations. An example of a dial indexing

machine is shown in Figure 4.12.

Trunnion machine

This machine, shown in Figure 4.13. uses a vertical drum mounted on a horizontal axis, so it is a
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variation of the dial indexing machine. The vertical drum is called a trunnion. Mounted on it are

several fixtures which hold the workparts during processing.

Trunnion machines are most suitable for small workpieces. The configuration of the machine, with a

vertical rather than a horizontal indexing dial, provides the opportunity to perform operations on

opposiJe sides of the workpart. Additional stations can be located on the outside periphery of the

tnmnion if this is required. The trunnion-type machine is appropriate for workparts in the medium

production range.

Center column machine

Another version of the dial indexing arrangement is the center column type, pictured in Figure 4.14.

In addition to the radial machining heads located around the periphery of the horizontal table, vertical

units are mounted on the center column of the machine. This increases the number of machining

operations that can be performed as compared to the regular dial indexing type. The center column

machine is considered to be a high-production machine which makes efficient use of floor space.
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Transfer machine

The most highly automated and versatile of the machines is the transfer line, illustrated in Figures

4.2 and 4.15. The workstations are arranged in a straight-line flow pattern and parts are transferred

automatically from station to station. The transfer system can be synchronous or asynchronous,

workparts can be transported with or without pallet fixtures, buffer storage can be incorporated into

the line operation if desired, and a variety of different monitoring and control features can be used to

manage the line. Hence, the transfer machine offers the greatest flexibility of any of the machines

discussed. The transfer line can accommodate larger workpieces than the rotary-type indexing

systems. Also, the number of stations, and therefore the number of operations, which can be

included on the line is greater than for the circular arrangement. The transfer line has traditionally

been used for machining a single product in high quantities over long production runs. More recently,

transfer machines have been designed for ease of changeover to allow several different but similar

workparts to be produced on the same line. These attempts to introduce flexibility into transfer line
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design add to the appeal of these high-production systems.

PROBLEMS

4.1. For the eight-slotted driven member of the Geneva mechanism in Figure 4.8, determine the

proportion of each complete revolution of the driver that motion is being imparted to the driven

member. Express this in degrees. How many degrees of rotation represent dwell ?

4.2. A Geneva mechanism with a six-slotted driven member is used in a dial-type assembly machine.

The longest assembly operation takes exactly 1s to complete, so the driven member must be in a

stopped (dwell) position for this length of time.

(a) At what rotational speed must the driver be turned to accomplish this 1-s dwell ?

(b) How much time will be required to index the dial to the next position ?

(c) Determine the ideal production rate of the assembly machine if each index of the dial produces a

completed work part.
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4.3. A certain transfer line performs a sequence of machining and assembly operations. One of the

assembly operations near the end of the line is performed manually. The distribution of operation

times for the manual station is given in the following table.

Operation time (s) Frequency of occurrence (%)

15 2.7

16 6.1

17 12.1

18 25.9

19 32.1

20 10.9

21 6.9

_ 100.0

The slowest automatic station has a cycle time of 18 s.

(a) If the line uses a synchronous (intermittent) transfer system, determine the cycle rate and

actual production rate per hour if the transfer system were set to index parts at each of the following

intervals : once every 22 s. 21 s, 20 s, 19 s, 18 s. Time taken to move parts is negligible.

(b) If an asynchronous parts transfer system were used on this line to accumulate a float of parts

before the manual station, at what cycle rate should the system be operated ?

Determine the corresponding production rate.


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ANALYSIS OF AUTOMATED FLOW LINES

In analyzing the performance of automated flow lines, two general problem areas must be

distinguished. The first is related to the production processes used on the line. For example,

consider a transfer line that performs a series of machining operations. There is an extensive body of

knowledge related to the theory and practice of metal machining. This technology includes the

proper specification and use of cutting tools, the metallurgy and machinability of the work material,

chip control, machining economics, machine tool vibrations, and a host of other topics. Many of the

problems encountered in the operation of a metal-cutting transfer line are directly related to and can

be solved by the application of good machining principles. The same is true for other production

processes. In each area of production, a technology has developed after many years of research

and practical experience in the area. By making the best use of the given process technology, each

individual workstation on the line can be made to operate at or near its maximum productive

capability. However, even if it were possible to operate each station in an optimal way, this does not

guarantee that the overall flow line will be optimized.

It is with this viewpoint of the overall flow line operation that we identify the second general problem

area of flow line performance. This second area is concerned with the systems aspects of designing

and running the line. Normally associated with the operation of an automated flow line is the problem

of reliability. Since the line often operates as a single mechanism. failure of one component of the

mechanism will often result in stoppage of the entire line. There are approaches to this problem that


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transcend the manufacturing processes at individual stations. What is the effect of the number of

workstations on the performance of the line How much improvement can he obtained by using one

or more buffer storage zones? What is the effect of component quality on the operation of an

automated assembly machine? How will the use of manual workstations affect the line? These are

questions that can he analyzed using a systems approach.

In addition to the reliability problem, another systems design problem is one of properly balancing the

line. ln line balancing. the objective is to spread the total work load as evenly as possible among the

stations in the line. The problem is normally associated with the design of manual assembly lines. lt

is also a consideration in automated flow lines, but the reliability problem usually predominates We

will consider line balancing in Chapter 6. The reliability of the line will be the principal concern of the

present chapter.

GENERAL TERMINOLOGY AND ANALYSIS

Flow line performance can be analyzed by means of three basic measures : average production rate,

proportion of time the line is operating (line efficiency). and cost per item produced on the line. We

will concern ourselves initially with flow lines that possess no internal buffer storage capacity.

To begin the analysis. we must assume certain basic characteristics about the operation of the line.

A synchronous transfer system is assumed. Pans are introduced into the first workstation and are

processed and transported at regular intervals to succeeding stations. This interval defines the ideal

or theoretical cycle time Tc of the flow line. 7] is equal to the time required for parts to transfer plus

the processing time at the longest workstation. The processing times at different stations will not bethe same. Long holes take more time to drill than short holes. A milling operation may take longer

than a tapping operation. The stations which require less time than the longest station will have a

certain amount of idle time. The components of Tc are illustrated graphically in Figure 5.1.


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Because of breakdowns of the line, the actualaverage production time Tp will be longer than the ideal cycle time. When a breakdown occurs at

any one station, we assume that the entire line is shut down. Let Ts represent the average downtime

to diagnose the problem and make repairs when a breakdown occurs. Since there may be more than

one reason why a line is down, it is sometimes convenient to distinguish between the different

reasons by subscripting the term as Tdj, The subscript jis used to identify the reason for the

breakdown (e.g., tool failure, part jam, feed mechanism, etc.). The frequency with which line stops

per cycle occur for reason j is denoted by Fj. Multiplying the frequency Fj by the average downtime

per stop Tdj, gives the mean time per cycle the machine will be down for reason j. If there is only one

reason why the transfer machine may be down, the average production time Tp is given by

The effect of this apportioned downtime on the cycle

time is illustrated in Figure 5.2.

If there are several reasons why the line is down and we wish to distinguish among them, the

average production time becomes

It is possible that there are interactions between

some of the causes of line stops. For example, if we were to introduce a scheduled maintenance or

tool change program, this would presume to favorably influence the frequency of unplanned station

breakdowns.

One of the important measures of performance for a transfer line is the average production rate Rp.

We must differentiate between this production rate, which represents the actual output of the

machine, and the theoretical production rate. The actual average production rate is based on the

average production time Tp :

II the flow line produces less than a 100% yield, this

production rate must be adjusted for the yield. For example, if 2% of the work parts are scrapped

during processing, the production rate would be 98% of that calculated by Eq. (5.3). The theoretical

production rate of the flow line, rarely achieved in practice, is computed as

The machine tool builder will use this value in

describing the flow line, and will speak of it as the production rate at 100% efficiency. Unfortunately,

the machine will not operation at 100% efficiency.
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The line efficiency E is simply the proportion of time

the line is up and operating We can compute it as follows :

An alternative measure of performance is the

proportion of downtime D on the line :

Certainly, the downtime proportion and the uptime

proportion must add to 1 :

The third measure of flow line performance is the

cost per item produced. Let Cm equal the cost of raw materials per product, where the product refers

to the unit of output from the line. Let Ce represent the cost per minute to operate the line. This

would include labor, overhead, maintenance, and the allocation of the capital cost of the equipment

over its expected service life. The last item, capital cost, will likely be the largest portion of CL. The

cost of any disposable tooling should be computed on a per workpiece basis and is symbolized by

C1. Using these components, the general formula for calculating the cost per workpiece C, is

This equation does not account for such things as

yield or scrap rates, inspection cost associated with identifying defective items produced, or repair

cost associated with fixing the defective items. However, these factors can be incorporated into Eq.

(5.8) in a fairly straight forward manner.We will let n represent the number of workstations on the flow line and Q will designate the quantity

of work part produced off the line. Q may represent a batch size or it may mean the number of parts

produced over a certain time period. We will use it in whatever way we find convenient. However, one

note of caution is this: Q may include a certain number of defects if the flow line has a habit of not

producing 100% good product.

Lot us consider an example to illustrate the terminology of flow line performance.

EXAMPLE 5.1

Suppose that a 10-station transfer machine is under consideration to produce a component used in a

pump. The item is currently produced by more conventional means, but demand for the item cannot

be met. The manufacturing engineering department has estimated that the ideal cycle time will be Tc

= 1.0 min. From similar transfer lines, it is estimated that breakdowns of all types will occur with a

frequency, F = 0.10 breakdown/cycle, and that the average downtime per line stop will be 6.0 min.

The scrap rate for the current conventional processing method is 5% and this is considered a good

estimate for the transfer line. The starting casting for the component costs $1.50 each and it will cost
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The lower cost per unit is associated with the policy

of scrapping the 5% defects rather than repairing them. Unless the extra units are needed to meet

demand, the scrap policy seems preferable.

ANALYSIS OF TRANSFER LINES WITHOUT STORAGE

In this section we consider the analysis of continuous and intermittent transfer machine

without internal storage capacity. e will supplement the results of Section !." #

considering what happens at a wor$station when it #rea$s down. %here are two

possi#ilities and we refer to their analyses as the upperound approach and the lower&

#ound approach In practical terms' the di(erence #etween the two approaches is simply

this) ith the upperound approach' we assume that the wor$part is not removed from

the station when a #rea$down occurs at that station. ith the loweround approach'

the wor$ part is ta$en out of the station when the station #rea$s down. %he

circumstances under which each approach is appropriate are discussed in the following

su#sections.Upper-bound approach

%he upperound approach provides an estimate of the upper limit on the fre*uency on

line stops per cycle. e assume here that a #rea$down at a station does not cause the

part to #e removed from that station. In this case it is possi#le' perhaps li$ely' that there

will #e more than one line stop associated with a particularwork part. An example of this

situation is that of a hydraulic failure at a workstation which prevents the feed mechanism from

working. Another possibility is that the cutting tool has nearly worn out and needs to be changed. Or,

the workpart is close to being out of tolerance and a tool adjustment is required to correct the

condition. With each of these examples, there is no reason for the part to be removed from the

transfer machine.

Let pi represent the probability that a part will jam at a particular station i, where i = 1, 2, . . . , n.

Since the parts are not removed when a station jam occurs, it is possible (although not probable) that

the part will jam at every station. The expected number of line stops per part passing through the line

can be obtained merely by summing up the probabilities p, over the n stations. Since each of the n

stations is processing a part each cycle, this expected number of line stops per part passing through

the line is equal to the frequency of line stops per cycle. Thus,

If the probabilities p are all equal, p1 = p2 = . . . =

pn = p, then

EXAMPLE 5.2
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In a 10-station transfer line, the probability that a station breakdown will occur for a given work part is

equal to 0.01. This probability is the same for all 10 stations. Determine the frequency of line stops

per cycle on this flow line using the upper-bound approach.

Solution :

The value of F can be calculated from Eq. (5.10).

This is the value of F used in Example 5.1.

Lower-bound approach

The lower-bound approach gives an estimate of the lower limit on the expected number of line stops

per cycle. In this approach we assume that the station breakdown results in the destruction or

damage of the workpiece. For example, a drill or tap breaks off in the part during processing. The

broken tool must be replaced at the workstation and the work part must be removed from the line for

subsequent rework or scrap. Accordingly, the part cannot proceed to the next stations for further

processing.

Again, let p1 be the probability that a part will jam at a particular station i. Then, considering a given

work part as it proceeds through the line, p1 is the probability that the part will jam at station 1, and

(1- p1) is the probability that the part will not jam at station i and thus be available for subsequent

processing. The quantity p2(1- p1) is the probability that this given part will jam at station 2.

Generalizing, the quantity

is the probability that the given part will jam at any

station i. Summing all these probabilities from i = 1through n would give the probability or what has

previously been called the frequency of line stops per cycle. There is an easier way to determine this

frequency.

The probability that a given part will pass through all n stations without a line stop associated with its

processing is given by

Therefore, the frequency of line stops per cycle is provided by

if the probabilities pi that a part will jam at a

particular station are all equal, p1 = p2 . . . = pn = p. then

We might be tempted to view this frequency, F, asthe probability of a line stop per cycle, except that it is possible, in the upper-bound approach, for the

frequency of line stops per cycle to exceed unity. A probability greater than l cannot be interpreted.

With the lower-bound approach, the number of work parts coming off the line will be less than the

number starting. If the pans are removed from the line when a breakdown occurs, they are not

available to be counted as part of the output of the line. Therefore, the production-rate formula given
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by Eq. (5.3) must be amended to reflect this reduction in output. Using the lower-bound approach,

the production-rate formula becomes

where F not only stands for the frequency of line stops but also the frequency of

pars removal. If no rework is performed, F is the scrap rate. Therefore, the term (1 F) represents

the yield of the transfer machine. Tp is interpreted to mean the average cycle time of the machine.

EXAMPLE 5.3

Compute the value of F using the lower-bound approach for the data of Example 5.2. Also compute

the production rate.

Solution:

From Eq. (5. l2) the value of F is

Although the value of F is smaller as calculated by the lower-

bound approach, the difference is small. This difference grows as the value of p and the number of

workstations increase.

To compute the production rate, we must adjust Eq. (5.3), as indicated in the previous discussion, by

the value of F, Production time Tp was calculated in Example 5.1 as 1.6 min. Therefore, the

production rate

This is somewhat below the 35.6 pieces/h obtained

in Example 5.1, where the scrap rate was 0.05 rather than the 0.0956 computed here. We can

reason that the 5% scrap rate probably represents a mixture of the two cases assumed by the upper-

and lower-bound approaches. When breakdowns occur, the work parts are sometimes removed from

the line and other times they are not, The upper- and lower-bound approaches, as their names imply,

provide upper and lower limits on the frequency of downtime occurrences. However, these two

approaches also generate upper and lower limits on the production rate, assuming that station

breakdowns are the sole cause of scrap. Of course, the 5% scrap figure may also include other

conditions, such as poor quality of the work parts.

Some comments and observations

Determining whether the upper- or the lower-bound approach is more appropriate for a particular

transfer line requires knowledge about the operation of the line. The operator may be required to usejudgment in each breakdown to determine whether the work part should be removed. lf pans are

sometimes removed and sometimes not, the actual frequency of breakdowns will fall somewhere

between the upper and lower bounds. Of the two approaches, the upper-bound approach is

preferred, certainly for convenience of calculation and probably for accuracy also.

There are other reasons why line stops occur which are not directly related to workstations (e.g.,
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transfer mechanism failure, schedule tool changes for all stations, preventive maintenance, product

changeover, etc.). These other factors must be taken into consideration when determining line

performance.

The biggest difficulty in using Eqs. (5.9) through (5.12) lies in determining the values of p1 for the

various stations. Perhaps the best approach is to base the values of p1 on previous historical data

and experience with similar workstations.

There are a number of general truths about the operation of transfer lines which are revealed by the

equations of Sections 5.1 and 5.2. First, the line efficiency decreases

substantially as the number of stations increases. It is not uncommon for large transfer lines

consisting of up to 100 stations to be down more than 50% of the time. lt is doubtful that such lines

achieve the return on investment which their owners anticipated from them. The influence of the

number of stations on line efficiency is dramatically displayed in Figure 5.3 for several assumed

values of station breakdown probabilities.

This figure was calculated using the upper-bound

approach. In comparing the upper- and lower-bound approaches, we find that the upper-bound

calculations lead to a lower value of line efficiency but a higher value for the production rate. The

reason for this apparent anomaly is this : Using the lower-bound approach with its assumption of

parts removal. the removed parts are not available at subsequent workstations to cause line

stoppages. Hence, the proportion of uptime on the line is greater. However, if the parts are removed

from the line, the production rate of the line is reduced.

PARTIAL AUTOMATION
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There are many examples of How lines in industry that contain both automated and manually

operated workstations. These cases of partially automated production lines occur for two main

reasons. First, mechanization of a manually operated How line is often introduced gradually.

Automation of the simpler manual operations is carried out initially, and the transition toward a fully

automated line is accomplished progressively. A long

period of time may elapse before the transformation has been completed. Meanwhile, the line

operates as a partially automated system. The second reason for partial automation is based strictly

on economics. Some manual operations are difficult to automate and it may be uneconomical to do

so. Therefore, when the sequence of workstations is planned for the flow line, certain stations are

designed to be manually operated while the rest are designed to be automatic. Examples of

operations that are often difficult to automate are processes requiring alignment or special human

skills to carry out. Many assembly operations fall into this category. Also, inspection operations often

create problems when automation is being considered to substitute for a human operator. Defects in

a work part that can be easily perceived by a human inspector are sometimes extremely difficult to

identify by an automatic inspection device. Another problem is that the automatic device can only

check the defects for which it was designed, whereas a human operator is capable of sensing a

variety of unforeseen imperfections in the part.

To analyze the performance of a partially automated How line, we will build on the developments of

the previous sections. Our analysis will be confined to the operation of a system without buffer

storage. Later in this section we will speculate on the improvements that would result if in-process

inventory banks were used for the manual workstations. The ideal cycle time Tc will be detenninedby the slowest station in the line, which generally will be one of the manual stations. We assume that

breakdowns occur only at the automatic stations and that the reasons for breakdowns are as varied

as they are for a completely automated system (e.g., tool failures, defective components, electrical

and mechanical malfunctions, etc.). Breakdowns do not occur at the manual stations because the

human operators are flexible enough, we assume, to adapt to the kinds of variations and disruptions

that cause an automatic workhead to stop. This, of course, is not always true, but our analysis will be

based on this assumption. Let p equal the probability of a station breakdown for an automatic work

head. The values of p may

be different for different stations, but we will leave to the reader the task of generalizing on our

particular case of equal p for all stations.

Since n is the total number of workstations on the line, let na equal the number of automatic stations

and no equal the number of manual operator stations. The sum of na and no must be n. ln our

previous discussion of costs in Section 5.1, CL was used to represent the cost to operate the line,


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including labor, capital, and overhead. For a partially automated system, we must separate this term

into several components. Let Co equal the operator cost per manual station. This is a labor cost in

$/minute which applies to each manually operated workstation. The cost per automatic workstation in

$/minute will be denoted by Cas This cost will often vary for different stations, depending on the level

of sophistication of the mechanism. We can allow for these differences in Cas, with little difficulty, as

will be shown in a subsequent example. The last cost component of CL is the cost per minute of the

automatic transfer mechanism which will be used for all stations, manual and automatic, to transport

the work parts. Let this cost be symbolized by Cat. It is not a cost per station, but rather a cost that

includes all n stations. Combining these costs. the total line cost CL is given by

To figure the average production time, the ideal

cycle time is added to the average downtime per cycle as follows :

By using no (the number of automatic workstations),we are consistent with our assumption that breakdowns do not occur at manual stations. Equation

(5.14) is based on the upper- bound approach of Section 5.2.

By substituting Eqs. (5.13) and (5, 14) into Eq. (5.8), we obtain the cost per unit produced :

EXAMPLE 5.4

A proposal has been made to replace one of the current manual stations with an automatic work

head on a 10-station transfer line, The current system has six automatic work heads and four manual

stations. The current cycle time is 30 s. The bottleneck station is the manual station that is the

candidate for replacement. The proposed automatic station would allow the cycle time to be reduced

to 24 s. The new station is costed at $0.25/min. Other cost data for the existing line :

Breakdowns occur at each of the six automatic workstations with a probability

p = 0.01. The average downtime per breakdown is 3 min. lt is estimated that the value of p for the

new automatic station would be p = 0.02. The average downtime for the line would be unaffected.

Material for the product costs $0.50/unit. Tooling costs can be neglected (C1 = 0). It is desired to

compare the challenger (the new automated station) with the defender (the current manual station)on the basis of cost per unit.

Solution:

For the current line, the production time is
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For the transfer linc with the new workhead

replacing thc current manual station, similar computations are as follows :

The conclusion is that the improved performance of

the automatic work head does not justify its greater cost. It should be noted that the reduced

reliability of the automatic workhcads figures

prominently in the cost calculations. If the value of p for the new automatic work head had been

equal to 0.0l instead of 0.02, the conclusion would have been reversed.

Buffer storage

The preceding analysis assumes no buffer storage between stations on the line. Therefore, when the

automated portion of the line breaks down, the manual stations must also stop working for lack of

work parts, It would be beneficial to the line operation to build up an inventory of parts before and

after each manual station. ln this manner, these stations, which usually operate at a slower pace

than the automatics, could continue to produce while the automated portion of the line is down. This

would tend to help solve the line balancing problem that often occurs on partially automated

machines. To illustrate, consider the previous example of the current transfer system whose ideal

cycle time is 0.50 min. Under the current method of operation, both manual and automatic stations

are out of operation when a breakdown occurs. The 0.50-min cycle time is caused by one of the

manual stations. Suppose that the ideal cycle time on the automatic portion of the line could be set

at 0.32 min. The resulting average production time on the automatic stations would then be

lf a bank of work parts could be provided for the

human operators to work on during the downtime occurrences, the average production time of the

entire assembly system would be 0.50 min rather than 0.68 min as computed in Example 5.4. The

resulting cost per assembly, ignoring any added cost as a result of the buffer storage capacity, would

be

which is a substantial reduction from the $1.384/unit

previously calculated for the current line.

We treat the topic of buffer storage in more detail in the next section.
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AUTOMATED FLOW LINES WITH STORAGE BUFFERS

One of the methods by which flow lines can be made to operate more efficiently is to add one or

more parts storage buffers between workstations along the line. The buffer zones divide the line into

stages. If one storage buffer is used, the line is divided into two stages. If two storage buffers are

used at two different locations along the line, a three-stage line results. The upper limit on the

number of storage zones is to have in-process inventory capacity between every workstation. Die

number of stages will then be equal to the number of stations. For an n-stage line, there will be n l

storage buffers. This, of course, does not include the inventory of raw work parts which are available

for feeding onto the front of the flow line. Nor does it include the inventory of finished pans that

accumulate at the end of the line.

In a flow line without internal work part storage, the workstations are interdependent. When one

stations breaks down, all the other stations will be affected, either immediately or by the end of a few

cycles of operation. The other workstations will be forced to stop production for one of two reasons :

1.Starving of stations. If a workstation cannot continue to operate because it has no pans to

work on, the station is said to be starved of parts. When a breakdown occurs at a given

station in the line, the stations following the broken-down station will become starved.

2.Blocking of stations. This occurs when a station is prevented from passing its work part along

to the following station. When a station breakdown occurs, the preceding or upstream

stations are said to be blocked, because they are unable to transfer work parts to the station

that is down.

The terms starving and blocking are often used in reference to manual how lines. However, they

are useful for explaining how buffer storage zones can be used to improve the efficiency of

automated flow lines. When an automated flow line is divided into stages by storage buffers, the

overall efficiency and production rate of the line are improved. When one of the stages in the line

breaks down, the storage buffers prevent the other stages from being immediately affected. Consider

a two-stage transfer line with one storage buffer between the stages. If the first stage breaks down,

the inventory of work parts that has accumulated in the storage buffer will allow the second stage to

continue operating until the inventory has been exhausted. Similarly, if the second stage breaks

down, the storage zone will accept the output of the first stage. The first stage can continue to

operate until the capacity of the storage buffer is reached.

The purpose served by the storage buffer can be extended to flow lines with more than two stages.

The presence of these in-process inventories allows each stage to operate somewhat independently.

It is clear that the extent of this independence between one stage and the next depends on the


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storage capacity of the buffer zone separating the stages.

The two extreme cases of storage buffer effectiveness can be identified as

1.No buffer storage capacity at all

2.Storage buffers with infinite capacity

NO BUFFER STORAGE CAPACITY. In this case, the flow line acts as one machine. When a station

breakdown occurs, the entire line is forced down. The efficiency of the line was previously given by

Eq. (5.5). We will rewrite this equation as

The subscript 0 identifies this as the efficiency of the

line with no buffer storage capacity. It represents a starting point from which the line efficiency can be

improved by adding provision for in-process storage.

INFINITE-CAPACITY STORAGE BUFFERS. The opposite extreme is the case where buffer zones

of infinite capacity are installed between each stage. If we assume that each of these buffer zones is

half full (in other words, each buffer zone has an infinite supply of parts as well as the capacity to

accept an infinite number of additional parts), then each stage is independent of the rest. Infinite

storage buffers means that no stage will ever be blocked or starved because of the breakdown of

some other stage.

Of course, a flow line with infinite-capacity buffers cannot be realized in practice. However. if such a

line could be approximated in real life, the overall line efficiency would be determined by the

bottleneck stage. We would have to limit the production on the other stages to be consistent with that

of the bottleneck stage. Otherwise, the in-process inventory upstream from the bottleneck would

grow indefinitely, eventually reaching some practical maximum, and the in-process inventory in the

downstream buffers would decline to zero. As a practical matter, therefore, the upper limit on the

efficiency of any such a line is defined by the efficiency of the bottleneck stage. If we assume the

cycle time Tc to be the same for all stages, the efficiency of early stage is given by

where the subscript k is used to identify the stage.

The overall line efficiency would be given by

where the subscript oo is used to indicate the infinite

storage buffers between stages.

BUFFER STOCK EFFECTIVENESS IN PRACTICE. By providing one or more buffer zones on a

flow line, we expect to improve the line efficiency above Eo but we cannot expect to achieve Eoo,

simply because buffer zones of infinite capacity are not possible. Hence, the actual value of E will lie
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We assume that the downtime distributions of all stations within a stage are the same, and that the

average downtimes of stages I and 2 are Td1 and Td2.

Over the long run, both stages must have equal efficiencies. For example, if the efficiency of stage I

would tend to be greater than the stage 2 efficiency, the inventory in the storage buffer would tend to

build up until its capacity b is reached. Thereafter, stage l would eventually be blocked when it tried

to outproduce stage 2. Similarly, if the efficiency of stage 2 is greater, the buffer inventory would

become depleted, thus starving stage 2. Accordingly, the efficiencies of the two stages would tend to

equalize over time.

The overall line efficiency for the two stages can be expressed as

where Eo is the line efficiency without buffer

storage. The value of Eo was given by Eq. (5.l6), but we will write it below to explicitly define the two-

stage system efficiency when b = 0 :

The term D1h(b) appearing in Eq. (5.25 represents

the improvement in efficiency that results from adding buffer capacity (b > 0).

D1 is the proportion of total time that stage 1 is down. However, Buzacott defines D1 as

Note that the form of this equation is different from

the case given by Eq. (5.6) when we were considering the How line to be an integral mechanism,

uncomplicated by the presence of buffer stocks.

The term h(b) is the ideal proportion of the downtime D1 (when stage 1 is down) that stage 2 could

be up and operating within the limits of buffer capacity b. Buzacott [2] presents equations for

evaluating h(b) using a Markov chain analysis. The equations cover several different downtime

distributions and are based on the assumption that the probability of both stages being down at the

same time is negligible. Four of these equations are presented in Table 5.l.

In sample calculations, the author has found that Eq. (5.21) tends to overestimate line efficiency. This

is because of the assumption implicit in the computation of h(b) that both stages will not be broken

down at the same time. Another way of stating this assumption is this: During the downtime of stage

l, stage 2 is assumed to always be operating. However, it is more realistic to believe that during the

downtime of stage 1, stage 2 will be down a certain portion of the time, this downtime determined by

the efficiency of stage 2. Hence, it seems more accurate to express the overall line efficiency of atwo~stage line as follows, rather than by Eq. (5.21) :

where E2 represents the efficiency of stage 2 by Eq.

(5.l7). In work subsequent to the research documented in reference [2], Buzacott develops a more

exact solution to the two-stage problem than that provided by Eq. (5.2l). This is reported in reference

5 and seems to agree closely with the results given by Eq. (5.29).
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EXAMPLE 5.5

This example will illustrate the use of Eq. (5.29) and Table 5.l to calculate line efficiency for a transfer

line with one storage buffer. The line has I0 workstations, each with a probability of breakdown of

0.02. The cycle time of the line is I min, and each time a breakdown occurs, it takes exactly 5 min to

make repairs. The line is to be divided into two stages by a storage bank so that each stage will

consist of five stations. We want to compute the efficiency of the two-stage line for various buffer

capacities.

Solution :

First, let us compute the efficiency of the line with no buffer capacity.

By Eq. (5.16),

Next, dividing the line into two equal stages by a buffer

zone of infinite capacity, each stage would have an efficiency given by Eq. (5.l7).

By Eq. (5.18), Eoo = 0.6667 represents the

maximum possible : efficiency that could be achieved by using a storage buffer of infinite capacity.

TABLE 5.1 Formulas for Computing h(b) for a Two-Stage Flow line under Several Downtime

Situations

The following definitions and assumptions apply to Eqs. (5.25) through (5.28) used lo compute h(b) :

Assume that the two stages have equal repair limes and equal cycle times. That is,

and

let

when B is the largest integer satisfying the relation

and L represents me leftover units, the number by which b exceeds BTd/Tc. Enally, let r

= F1/F1, as given by Eq_ (5.20).

With these definitions and assumptions, we can express the relationships for two theoretical

downtime distributions as developed by Buzacott [2]:

1. Constant repair distribution. Each downtime occurrence is assumed to require a constant require

time Td.
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2.

Geometric repair distribution. This downtime distribution assumes that the probability that repairs are

completed during any cycle is independent of the time since repairs began. Define the parameter K :

Then for the two cases

No

w, we will see how Eq. (5.29) and the formulas in Table 5.1 are used. Let us investigate the following

buffer capacities: b = 1, 10, 100, and .

When b = 1, according to Eq. (5.24) in Table 5.1, the buffer capacity is convened to B = 0, and L = 1.

We are dealing with a constant repair-time distribution and the breakdown rates are the same for

both Stages, so r = 1. We will therefore use Eq. (5.26) to compute the

value of h(1) :

Using Eq. (5.23) to get the proportion of total time

that stage 1 is down

Now the line efficiency can be computed from Eq. (5.29)

Only a very modest improvement results from the

use of a storage buffer with capacity of one work part.

When b = 10, B = 2 and L = 0. The value of h(10) is again computed from Eq. (5.26) :

The resulting line efficiency is

We see a 12% increase in line efficiency over Eo

from using a buffer capacity of 10 work units

When b = 100, B = 20, L = 0, and

Therefore,

E = 0.50 + 0.25 ( 0.952 ) ( 0.6667 ) = 0.6587

A 32% increase in line efficiency results when the buffer capacity equals 100. Comparing this to the

case when b = 10, we can see the law of diminishing returns operating.

When the storage capacity is infinite (b = ),
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E = 0.50 + 0.25 ( 1.0 ) ( 0.6667 ) = 0.6667

according to Eq. (5.29), which equals the result given by Eq. (5. 18).

It is of instructional value to compare this result for b = 0 given by Eq. (5.29), with the result given

by Eq. (5.21), which neglects the possibility of overlapping stage downtime occurrences. According

to Eq. (5.2l), the line efficiency would be calculated as

E = 0.50 + 0.25 ( 1.0 ) = 0.75

which, of course, exceeds the maximum possible value given by Eq. (5. l8). The corrected formula,

Eq. (5.29), provides better calculated results, since it accounts for the occurrence of overlapping

breakdowns for the two stages.

Flow lines with more than two stages

We will not consider exact formulas for computing efficiencies for How lines consisting of more than

two stages. lt is left to the interested reader to consult some of the references, in particular [1], [2],and [5]. However, the previous discussion in this section should provide a general guide for deciding

on the configuration of multistage lines. Let us consider some of these decisions in the context of an

example.

EXAMPLE 5.6

Suppose that the flow line under consideration here has I6 stations with cycle time of 15 s (assume

that all stations have roughly equal process times). When station breakdowns occur, the average

downtime is 2 min. The breakdown frequencies for each station are presented in the following table.

Station pt Station pt

1 0.01 9 0.03

2 0.02 10 0.01

3 0.01 11 0.02

4 0.03 12 0.02

5 0.02 13 0.02

6 0.04 14 0.31

7 0.01 15 0.33

8 0.01 16 0.31

We want to consider the relative performances when the line is separated into two, three, or fourstages.

Solution:

First, the performance of the line can be determined for the single-stage case (no storage buffer)

From the table, the frequency of downtime occurrences is given by Eq. (5.9) :
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The line efficiency is, by Eq. (5.16)

To divide the line into stages, we must first decide the

optimum locations for the storage buffers. The stations should be grouped into stages so as to make

the efficiencies of the stages as close as possible. Then, to assess relative performances for the

two-, three-, and four-stage lines, we will base the comparison on the use of storage buffers with

detroit type of automation

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