04 Sisprod - Line Balancing

8/10/2019 04 Sisprod - Line Balancing

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KESEIMBANGANESEIMBANGAN LINTASANINTASAN

Presentasi KuliahTKI-313 Sistem Produksi

Jurusan Teknik Universitas Muhammadiyah Surakarta

Dosen : Much Djunaidi


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Manual Assembly Lines

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

Assembly workers perform tasks at workstations locatedalong the line-of-flow of the product Usually a powered conveyor is used

Some of the workstations may be equipped with portable

Work Systems and the Methods, Measurement, and Management of Work

by Mikell P. Groover, ISBN 0-13-140650-7.2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.

.

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


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Why Assembly Lines are Productive

Specialization of labor

When a large job is divided into small tasks and eachtask is assigned to one worker, the worker becomeshighly 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 beselected at random for assembly with its matingcomponent.

Thanks to interchangeable parts, assemblies do notneed fitting of mating components


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Some Definitions

Work flow

Each work unit should move steadily along the line

Line pacing



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


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Manual Assembly Line

A production line that consists of a sequence ofworkstations where assembly tasks areperformed by human workers

Products are assembled as they move along



At each station a portion of the total work content isperformed on each unit

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

product


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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


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Two assembly

operators working

on an engine



assembly line

(photo courtesy of

Ford Motor

Company)


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Manning level

There may be more than one worker per

station.

Utility workers: are not assigned to

s ecific workstations.



They are responsible for

(1) helping workers who fall behind,

(2) relieving for workers for personal breaks,

(3) maintenance and repair


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Manning level

Average manning level:

where

n

ww

M

n

i

iu

1



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 stationifori=1,,n


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Work Transport Systems-Manual Methods

Manual methods

Work units are moved between stations by theworkers (by hand) without powered conveyor

Problems:

Starving of stations


by Mikell P. Groover, ISBN 0-13-140650-7.

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

The assembly operator has completed the assignedtask on the current work unit, but the next unit has notyet arrived at the station

Blocking of stations

The operator has completed the assigned task on thecurrent work unit but cannot pass the unit to thedownstream station because that worker is not yetready to receive it.


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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.


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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-goline): all work units are moved simultaneously between stations. Problem:

Task must be completed within a certain time limit. Otherwise the lineproduces 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 whenthe 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.


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Coping with Product Variety

Single model assembly line (SMAL)

Every work unit is the same

Batch model assembly line (BMAL ) multiple modelline

Two or more different products




Products are so different that they must be made inbatches with setup between batches

Mixed model assembly line (MMAL)

Two or more different models

Differences are slight so models can be madesimultaneously with no setup time (no need for batchproduction)


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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




pro uc eman c anges.

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 thistype of lines.


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Analysis of Single Model Lines

The formulas and the algorithms in this section aredeveloped for single model lines, but they can beextended to batch and mixed models.

The assembly line must be designed to achieve aproduction rate sufficient to satisfy the demand.

Demand rate production rate cycle time




Annual demand Da must be reduced to an hourlyproduction rate Rp

where

Da = annual demand

Rp = hourly production rate

Sw= number of shifts/week

Hsh = number of hours/shift

shw

ap

HS

DR

52


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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 willbe lost due to

equipment failures




,

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.

pc

R

ET

60


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Number of Stations Required

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

The theoretical minimum number ofstations that willbe 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 theminimum number ofworkers

c

wc

T


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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 Tceach 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 isless than Tc


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Repositioning Losses

Repositioning losses occur on a productionline because some time is required eachcycle to reposition the worker, the work unit,or both




,for the worker to walk from the unit justcompleted to the the upstream unit entering thestation

In conveyor systems, time is required toremove work units from the conveyor andposition it at the station for worker to performhis task.


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Repositioning Losses

Repositioning time = time available eachcycle 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


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Cycle Time on an Assembly Line

Components of cycle time at several stations ona manual assembly line




Tsi=service time, Tr=repositioning time


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Line Balancing Problem

Given:

The total work content consists of many distinct

work elements

The sequence in which the elements can be

erformed 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


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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 (dueto motion economies)


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Work Element Times

Total work content time Twc

Twc=

where Tek= work element time for element k

en

k

ekT1




the service time for that station

Tsi=

The station service times must add up to the total workcontent time

Twc=

ik

ekT

n

i

siT1


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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 toobtain equal service times for all stations.


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Precedence Constraints

Some elements must be done before the

others.

Restrictions on the order in which work

elements can be erformed




Can be represented graphically (precedence

diagram)


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Example:




Grommet : sealant like ring


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Example:






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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.


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Example: Solution

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

Twc=4.0 min

(b) The hourly production rate

units/hr33.53)5.7)(5(50

000,100

pR

en

k

ekwc TT1

shw

a

p HS

DR

50




(c) The corresponding cycle time with an uptimeefficiency 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

min08.133.53

)96.0(60cT

p

cR

ET

60

c

wc

T

Tw *

rcs TTT


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Measures of Balance Efficiency

It is almost imposible to obtain a perfect line balance

Line balance efficiency, Eb:

Eb = Perfect line: Eb = 1s

wc

wT

T




Balance delay, d:

d= Perfect line: d= 0

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

other)

s

wcs

wT

TwT


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Overall Efficiency

Factors that reduce the productivity of amanual line

Line efficiency (Availability), E,




, r,

Balance efficiency (balancing), Eb,

Overall Labor efficiency on the assembly line = br EEE

c

rc

c

s

r T

TT

T

T

E

s

wc

b wT

T

E pc R

E

T

60


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Skip - Worker Requirements

Skip this part

The actual number of workers on the

assembly line is given by:

w= Min Int wcwcwcp TTTR




where

w=number of workers required

Rp=hourly production rate, units/hrTwc=work content time per product, min/unit

c

rc

c

sr

T

TT

T

TE

s

wcb

wT

TE

sbcbrbr TETEEEEE60

p

cR

ET

60


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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

n

iis

LL1




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


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- Tolerance Time

Defined as the time a work unit spends inside theboundaries 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


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-Total Elapsed Time

The time a work unit spends on the assembly

line.

ET= tc

nT

v

L




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)


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Line Balancing Objective

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

Minimize (wTs Twc)

or

w




Subject to:

(1)

(2) all precedence requirements are obeyed

s

ik

ek TT

i

sis

1


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Line Balancing Algorithms Heuristics

1. Largest candidate rule

2. Kilbridge and Wester method




. ,

known as the Helgeson and Birne method

In the following descriptions, assume one

worker per workstation


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Largest Candidate Rule

List all work elements in descending order based on theirTekvalues; then,

1. Start at the top of the list and selecting the first elementthat satisfies precedence requirements and does notcause the total sum of Tek to exceed the allowable Tsvalue




When an element is assigned, start back at the top of thelist 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 beenassigned to as many stations as needed


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Solution for Largest Candidate Rule





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Example:






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Physical layout of workstations and assignmentof elements to stations using the largest

candidate rule





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Ranked Positional Weights Method

A ranked position weight (RPW) is calculated for eachwork element

RPW for element k is calculated by summing the Tevalues for all of the elements that follow element kin the

diagram plus T itself




Work elements are then organized into a list according to

theirRPWvalues, starting with the element that has the

highest RPWvalue

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

candidate rule


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Solution for Ranked Positional Weights

Method





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Example:






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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 temporarilyoverloaded

Preassembly of components

Prepare certain subassemblies off-line to reduce workcontent time on the final assembly line


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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


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Most Follower Rule

1

4

38

9 10

8

5

3

5

Item iMost Follower

1 9

2 5

3 4

4 4




6

5

72 4

9

4

6 10

6

19191919

5 4

6 4

7 3

8 3

9 2

10 1

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


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We omit

Worker Requirements in 4.2.2

4.3.2 Kilbridge and Western Method




.

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04 Sisprod - Line Balancing