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Manual Assembly Lines

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Manual Assembly Lines. Chapter 4. Sections: Fundamentals of Manual Assembly Lines Analysis of Single Model Assembly Lines Line Balancing Algorithms Other Considerations in Assembly Line Design Alternative Assembly Systems. Manual Assembly Lines. - PowerPoint PPT Presentation
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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
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Page 1: Manual Assembly Lines

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

Page 2: Manual Assembly Lines

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

Page 3: Manual Assembly Lines

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

Page 4: Manual Assembly Lines

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

Examples Automobile, camera, furniture, lamp, luggage, personal computer, mobile phone, video game console …

Page 5: Manual Assembly Lines

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

Page 6: Manual Assembly Lines

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

Page 7: Manual Assembly Lines

Two assembly operators working on an engine

assembly line

Final inspection of a car

Page 8: Manual Assembly Lines

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

Page 9: Manual Assembly Lines

Manning level

Practically, 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

n

wM Average manning level:

Page 10: Manual Assembly Lines

Work Transport System

Manual method Mechanized Methods

Page 11: Manual Assembly Lines

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.

Page 12: Manual Assembly Lines

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.

Page 13: Manual Assembly Lines

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.

Page 14: Manual Assembly Lines

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)

Page 15: Manual Assembly Lines

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.

Page 16: Manual Assembly Lines

Analysis of Single Model LinesThe 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/shiftOw = number of operation weeks

shww

ap HSO

DR

Page 17: Manual Assembly Lines

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.Rc = Ideal cycle rate for the line (cycle/hr)

pc R

ET

60

Cycle time Ideal cycle time

cc TR

60

Analysis of Single Model Lines

Page 18: Manual Assembly Lines

p

c

c

pc T

T

R

RE

Rc < Rp [Ideal cycle rate must be less than required production rate]

Line efficiency, Tp = average production cycle time =Tp = 60/ Rp

AT

WLw

wcpTRWL

EAT 60 *60 minute

WL = workload in a given time periodAT = available time in the period

Rp = production rateTwc = work content time

No of worker,

Workload to be accomplished

Available time

Analysis of Single Model Lines

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

Page 19: Manual Assembly Lines

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

Analysis of Single Model Lines

Page 20: Manual Assembly Lines

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

Page 21: Manual Assembly Lines

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.

Page 22: Manual Assembly Lines

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

Page 23: Manual Assembly Lines

Cycle Time on an Assembly Line

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

Tsi=service time, Tr=repositioning time

Page 24: Manual Assembly Lines

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

Page 25: Manual Assembly Lines

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)

Page 26: Manual Assembly Lines

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

Page 27: Manual Assembly Lines

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.

Page 28: Manual Assembly Lines

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)

Page 29: Manual Assembly Lines

Example:

Grommet : sealant like ring

Page 30: Manual Assembly Lines

Example:

Page 31: Manual Assembly Lines

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.

Page 32: Manual Assembly Lines

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

Page 33: Manual Assembly Lines

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

Page 34: Manual Assembly Lines

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

Page 35: Manual Assembly Lines

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

Page 36: Manual Assembly Lines

Largest Candidate Rule

1. List all work elements in descending order based on their Tek values; then,

2. 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

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

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

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

Page 37: Manual Assembly Lines

Solution for Largest Candidate Rule

Page 38: Manual Assembly Lines

Example:

80.0)0.1(5

0.4

s

wcb wT

TE

Balance efficicency

Page 39: Manual Assembly Lines
Page 40: Manual Assembly Lines

Solution for Largest Candidate Rule

Page 41: Manual Assembly Lines

Solution for Largest Candidate Rule

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

Page 42: Manual Assembly Lines

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

Page 43: Manual Assembly Lines

Solution for Ranked Positional Weights Method

Page 44: Manual Assembly Lines

Example:

Page 45: Manual Assembly Lines
Page 46: Manual Assembly Lines

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

Page 47: Manual Assembly Lines

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

Page 48: Manual Assembly Lines

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

Page 49: Manual Assembly Lines

End of Lecture


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