Ch04-Assembly Lines(1) (1)

Work Systems and the Methods, Measurement, and Management of Workby Mikell P. Groover, ISBN 0-13-140650-7.

©2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.

Manual Assembly Lines

Sections:

1. Fundamentals of Manual Assembly Lines

2. Analysis of Single Model Assembly Lines

3. Line Balancing Algorithms

4. Other Considerations in Assembly Line Design

5. Alternative Assembly Systems

Chapter 4



Manual Assembly Lines

Work systems consisting of multiple workers organized to produce a single product or a limited range of products

Assembly workers perform tasks at workstations located along the line-of-flow of the product Usually a powered conveyor is used Some of the workstations may be equipped with portable

powered tools.

Factors favoring the use of assembly lines: High or medium demand for product Products are similar or identical Total work content can be divided into work elements To automate assembly tasks is impossible



Why Assembly Lines are Productive

Specialization of labor

When a large job is divided into small tasks and each task is assigned to one worker, the worker becomes highly proficient at performing the single task (Learning curve)

Interchangeable parts

Each component is manufactured to sufficiently close tolerances that any part of a certain type can be selected at random for assembly with its mating component.

Thanks to interchangeable parts, assemblies do not need fitting of mating components



Some Definitions

Work flow

Each work unit should move steadily along the line

Line pacing

Workers must complete their tasks within a certain cycle time, which will be the pace of the whole line



Manual Assembly Line

A production line that consists of a sequence of workstations where assembly tasks are performed by human workers

Products are assembled as they move along the line At each station a portion of the total work content is

performed on each unit

Base parts are launched onto the beginning of the line at regular intervals (cycle time) Workers add components to progressively build the

product



Manual Assembly Line

•Configuration of an n-workstation manual assembly line

•The production rate of an assembly line is determined by its slowest station.

Assembly workstation: A designated location along the work flow path at which one or more work elements are performed by one or more workers



Two assembly operators working on an engine assembly line (photo courtesy of Ford Motor Company)



Manning level

There may be more than one worker per station.

Utility workers: are not assigned to specific workstations.

They are responsible for

(1) helping workers who fall behind,

(2) relieving for workers for personal breaks,

(3) maintenance and repair



Manning level

Average manning level:

where

M=average manning level of the line,

wu=number of utility workers assigned to the system, n=number of workstations,

wi=number of workers assigned specifically to station i for i=1,…,n

n

wwM

n

iiu

1



Work Transport Systems-Manual Methods

Manual methods Work units are moved between stations by the

workers (by hand) without powered conveyor

Problems: Starving of stations

The assembly operator has completed the assigned task on the current work unit, but the next unit has not yet arrived at the station

Blocking of stations The operator has completed the assigned task on the

current work unit but cannot pass the unit to the downstream station because that worker is not yet ready to receive it.



Work Transport Systems-Manual Methods

To reduce starving, use buffers

To prevent blocking, provide space between upstream and downstream

stations.

But both solutions can result in higher WIP, which is economically undesirable.



Work Transport Systems-Mechanized Methods

Continuously moving conveyor: operates at constant velocity1. Work units are fixed to the conveyor

The product is large and heavy Worker moves along with the product

2. Work units are removable from the conveyor Work units are small and light Workers are more flexible compared to synchronous lines, less flexible than

asynchronous lines

Synchronous transport (intermittent transport – stop-and-go line): all work units are moved simultaneously between stations. Problem:

Task must be completed within a certain time limit. Otherwise the line produces incomplete units;

Excessive stress on the assembly worker. Not common for manual lines (variability), but often ideal for automated production

lines

Asynchronous transport : a work unit leaves a given station when the assigned task is completed. Work units move independently, rather than synchronously (most flexible one). Variations in worker task times Small queues in front of each station.



Coping with Product Variety

Single model assembly line (SMAL) Every work unit is the same

Batch model assembly line (BMAL ) – multiple model line Two or more different products Products are so different that they must be made in

batches with setup between batches

Mixed model assembly line (MMAL) Two or more different models Differences are slight so models can be made

simultaneously with no setup time (no need for batch production)



Coping with Product Variety

Advantages of mixed models over batch order models

No production time is lost during changeovers

High inventories due to batch ordering are avoided

Production rates of different models can be adjusted as product demand changes.

Disadvantages of mixed models over batch order models

Each station is equipped to perform variety of tasks (costly)

Scheduling and logistic activities are more difficult in this type of lines.



Analysis of Single Model Lines

The formulas and the algorithms in this section are developed for single model lines, but they can be extended to batch and mixed models.

The assembly line must be designed to achieve a production rate sufficient to satisfy the demand.Demand rate → production rate→ cycle timeAnnual demand Da must be reduced to an hourly production rate Rp

whereDa = annual demandRp = hourly production rateSw = number of shifts/weekHsh = number of hours/shift

shw

ap HS

DR

52



Determining Cycle Time

Now our aim is to convert production rate, Rp, to cycle time, Tc.

One should take into account that some production time will be lost due to

equipment failures power outages, material unavailability, quality problems, labor problems.

Line efficiency (uptime proportion): only a certain proportion of the shift time will be available.

where production rate, Rp, is converted to a cycle time, Tc, accounting for line efficiency, E. p

c R

ET

60



Number of Stations Required

Work content time (Twc): The total time of all work elements that must be performed to produce one unit of the work unit.

The theoretical minimum number of stations that will be required to on the line to produce one unit of the work unit, w*:

w* = Minimum Integer

where Twc = work content time, min; Tc = cycle time, min/station

If we assume one worker per station then this gives the minimum number of workers

c

wc

T

T



Theoretical Minimum Not Possible

Repositioning losses: Some time will be lost at each station every cycle for repositioning the worker or the work unit; thus, the workers will not have the entire Tc each cycle

Line balancing problem (imperfect balancing): It is not possible to divide the work content time evenly among workers, and some workers will have an amount of work that is less than Tc



Repositioning Losses

Repositioning losses occur on a production line because some time is required each cycle to reposition the worker, the work unit, or both

On a continous transport line, time is required for the worker to walk from the unit just completed to the the upstream unit entering the station

In conveyor systems, time is required to remove work units from the conveyor and position it at the station for worker to perform his task.



Repositioning Losses

Repositioning time = time available each cycle for the worker to position = Tr

Service time = time available each cycle for the worker to work on the product = Ts

Service time Ts = Max{Tsi} ≤Tc – Tr

where Tsi= service time for station i, i=1,2,..,n

Repositioning efficiency Er = c

rc

c

s

T

TT

T

T



Cycle Time on an Assembly Line

•Components of cycle time at several stations on a manual assembly line

Tsi=service time, Tr=repositioning time



Line Balancing Problem

Given: The total work content consists of many distinct

work elements The sequence in which the elements can be

performed is restricted The line must operate at a specified cycle time

(=service time + repositioning time)

The Problem: To assign the individual work elements to

workstations so that all workers have an equal amount of work to perform



Assumptions About Work Element Times

1. Element times are constant values

But in fact they are variable

2. Work element times are additive

The time to perform two/more work elements in sequence is the sum of the individual element times

Additivity assumption can be violated (due to motion economies)



Work Element Times

Total work content time Twc

Twc =

where Tek = work element time for element k

Work elements are assigned to station i that add up to the service time for that station

Tsi =

The station service times must add up to the total work content time

Twc =

en

kekT

1

ik

ekT

n

isiT

1



Constraints of Line Balancing Problem

Different work elements require different times.

When elements are grouped into logical tasks and assigned to workers, the station service times, Tsi, are likely not to be equal.

Simply because of the variation among work element times, some workers will be assigned more work.

Thus, variations among work elements make it difficult to obtain equal service times for all stations.



Precedence Constraints

Some elements must be done before the others.

Restrictions on the order in which work elements can be performed

Can be represented graphically (precedence diagram)



Example:

Grommet : sealant like ring



Example:




Example: A problem for line balancing

Given: The previous precedence diagram and the standard times. Annual demand=100,000 units/year. The line will operate 50 wk/yr, 5 shifts/wk, 7.5 hr/shift. Uptime efficiency=96%. Repositioning time lost=0.08 min.

Determine

(a) total work content time,

(b) required hourly production rate to achieve the annual demand,

(c) cycle time,

(d) theoretical minimum number of workers required on the line,

(e) service time to which the line must be balanced.



Example: Solution

(a) The total work content time is the sum of the work element times given in the table

Twc=4.0 min(b) The hourly production rate

(c) The corresponding cycle time with an uptime efficiency of 96%

(d) The minimum number of workers:w* = (Minimum Integer 4.0 /1.08=3.7)=4 workers(e) The available service time

Ts=1.08-0.08=1.00 min

units/hr 33.53)5.7)(5(50

000,100pR

min08.133.53

)96.0(60cT

en

kekwc TT

1

shw

ap HS

DR

50

pc R

ET

60

c

wc

T

Tw *

rcs TTT



Measures of Balance Efficiency

It is almost imposible to obtain a perfect line balance

Line balance efficiency, Eb:

Eb = Perfect line: Eb = 1

Balance delay, d:

d = Perfect line: d = 0

Note that Eb + d = 1 (they are complement of each other)

s

wc

wT

T

s

wcs

wT

TwT



Overall Efficiency

Factors that reduce the productivity of a manual line

Line efficiency (Availability), E,

Repositioning efficiency (repositioning), Er,

Balance efficiency (balancing), Eb,

Overall Labor efficiency on the assembly line = br EEE

c

rc

c

sr T

TT

T

TE

s

wcb wT

TE

pc R

ET

60



Skip - Worker Requirements

Skip this partThe actual number of workers on the assembly line is given by:

w = Min Int

where

w=number of workers required

Rp=hourly production rate, units/hr

Twc=work content time per product, min/unit

c

rc

c

sr T

TT

T

TE

s

wcb wT

TE

sb

wc

cbr

wc

br

wcp

TE

T

TEE

T

EEE

TR

60

pc R

ET

60



Continously moving conveyors - Workstation considerations

Total length of the assembly line

where L=length of the assembly line, m: Lsi=length of station i, m

Constant speed conveyor: (if the base parts remain fixed during their assembly)Feed rate

fp=1/Tc

where fp=feed rate on the line, products/min

Center-to-center spacing between base parts

sp=vc/fp=vcTc

where sp= center-to-center spacing between base parts, m/part and vc=velocity of the conveyor, m/min

n

iisLL

1



Continously moving conveyors - Tolerance Time

Defined as the time a work unit spends inside the boundaries of the workstation

Provides a way to allow for product-to-product variations in task times at a station

Tt =

where

Tt = tolerance time, min;

Ls = station length, m (ft);

vc = conveyor speed, m/min (ft/min)

c

s

v

L



Continously moving conveyors -Total Elapsed Time

The time a work unit spends on the assembly line.

ET =

where

ET = total elapsed time, min;

Tt = tolerance time, min;

L = length of the assembly line, m (ft);

vc = conveyor speed, m/min (ft/min)

tc

nTv

L



Line Balancing Objective

To distribute the total work content on the assembly line as evenly as possible among the workers

Minimize (wTs – Twc)

or Minimize

Subject to:

(1)

(2) all precedence requirements are obeyed

sikek TT

w

isis TT

1



Line Balancing Algorithms – Heuristics

1. Largest candidate rule

2. Kilbridge and Wester method

3. Ranked positional weights method, also known as the Helgeson and Birne method

In the following descriptions, assume one worker per workstation



Largest Candidate Rule List all work elements in descending order based on their

Tek values; then,

1. Start at the top of the list and selecting the first element that satisfies precedence requirements and does not cause the total sum of Tek to exceed the allowable Ts value

When an element is assigned, start back at the top of the list and repeat selection process

2. When no more elements can be assigned to the current station, proceed to next station

3. Repeat steps 1 and 2 until all elements have been assigned to as many stations as needed



Solution for Largest Candidate Rule



Example:










•Physical layout of workstations and assignment of elements to stations using the largest candidate rule



Ranked Positional Weights Method

A ranked position weight (RPW) is calculated for each work element

RPW for element k is calculated by summing the Te values for all of the elements that follow element k in the diagram plus Tek itself

Work elements are then organized into a list according to their RPW values, starting with the element that has the highest RPW value

Proceed with same steps 1, 2, and 3 as in the largest candidate rule



Solution for Ranked Positional Weights Method



Example:






Other Considerations in Line Design

Methods analysis To analyze methods at bottleneck or other troublesome

workstations improved motions, better workplace layout, special tools to facilitate manual work elements product design

Utility workers To relieve congestion at stations that are temporarily

overloaded

Preassembly of components Prepare certain subassemblies off-line to reduce work

content time on the final assembly line



Other Considerations - continued

Storage buffers between stations To permit continued operation of certain sections of

the line when other sections break down To smooth production between stations with large

task time variations

Parallel stations To reduce time at bottleneck stations that have

unusually long task times

Worker (Labor) Shifting with crosstraining Temporary (or periodic) relocation to expedite or to

reduce subassembly stocks



Most Follower Rule

1

6

5

4

3

7

8

2

9 10

8

5

3

4

9

5

4

6 10

6

19191919

Item iMost Follower

1 9

2 5

3 4

4 4

5 4

6 4

7 3

8 3

9 2

10 1

19/19 16/19 15/19 10/19



We omit

Worker Requirements in 4.2.2

4.3.2 Kilbridge and Western Method

4.5 Alternative Assembly Systems

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