Kilbridge and wester method example

Manual Assembly Lines

Sections:

1. Fundamentals of Manual Assembly Lines

2. Analysis of Single Model Assembly Lines

3. Line Balancing Algorithms

4. Mixed Model Assembly Lines

5. Workstation Considerations

6. Other Considerations in Assembly Line Design

7. Alternative Assembly Systems

• Example: Chrysler Transmission Assembly Line 2014

https://www.youtube.com/watch?v=447sSED91v0

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Manual Assembly Line Defined

A production line consisting of a sequence of workstations where assembly tasks are performed by human workers as the product moves along the line • At each station, a worker performs a portion of the total work on the unit

• The common practice is to “launch” base parts on the beginning of the line at regular intervals ((cycle time).

• A mechanized material transport system is typically used to move the base parts along the line as they are gradually transformed into final products.

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

2***Total work content - the sum of all work elements required to assemble one product unit on the line***

Manual Assembly Lines

• Most consumer products are assembled on manual assembly lines • Automobiles, personal computers, cooking ranges, power tools, dishwashers,

refrigerators

• Factors favoring the use of assembly lines: • High or medium demand for product

• Identical or similar products

• Total work content can be divided into work elements

• It is technologically impossible or economically infeasible to automate the system

• Why assembly lines are so productive? • Specialization of labor: each task is assigned to one worker, the worker becomes

highly proficient at performing the single task.

• Interchangeable parts: Components made to close tolerances that any part of a certain type can be selected for assembly with its mating components

• Work flow principle: Products are brought to the workers rather than vice versa

• Line pacing: Workers must complete their tasks within the cycle time of the line

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Assembly Workstation and Work Transport Systems

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

• Typical operations performed at manual assembly stations • Adhesive application, Sealant application, Arc welding, Spot welding

• Electrical connections, Component insertion

• Work Transport Systems • Manual

• Mechanized

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Manual Work Transport Systems

• Work units are moved between stations by the workers without the aid of a powered conveyor • Types:

• Work units moved in batches

• Work units moved one at a time

• Problems: • Starving of stations: is the situation in which 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: 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.

• No pacing: workers are unpaced in lines that rely on manual transport methods, and production rates tend to be lower.

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Mechanized Work Transport Systems

• Work units are moved by powered conveyor or other mechanized apparatus • Categories:

• Work units attached to conveyor

• Work units are removable from conveyor

• Problems • Starving of stations

• Incomplete units

• Material Handling Equipment for Mechanized Work Transport • Continuous transport: Overhead trolley conveyor, Belt conveyor, Drag chain

conveyor

• Synchronous transport: Walking beam, Rotary indexing machine

• Asynchronous transport: Power-and-free conveyor, Cart-on-track conveyor, AGV

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https://www.youtube.com/watch?v=447sSED91v0
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Types of Mechanized Work Transport

Conveyor moves at constant velocity vc

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All work units are moved

simultaneously to their respective next workstations with quick,

discontinuous motion

Continuous Transport Synchronous Transport

Work units move independently, not

simultaneously. A work unit departs a given station when the worker releases it.

Small queues of parts can form at each

station.

Asynchronous Transport

Line Pacing

• A manual assembly line operates at a certain cycle time – that is established to achieve the required production rate of the line

• On average, each worker must complete his/her assigned task within this cycle time

• Pacing provides a discipline for the assembly line workers that more or less guarantees a certain production rate for the line

• Manual assembly lines can be designed with three alternative levels of pacing: 1. Rigid pacing

2. Pacing with margin

3. No pacing

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Rigid Pacing

• Each worker is allowed only a certain fixed time each cycle to complete the assigned task • Allowed time is set equal to the cycle time less repositioning time

• Synchronous work transport system provides rigid pacing

• Undesirable aspects of rigid pacing: • Emotionally and physically stressful to worker

• Incomplete work units if task not completed

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Pacing with Margin

• Worker is allowed to complete the task within a specified time range, the upper limit of which is greater than the cycle time

• On average, the worker’s average task time must balance with the cycle time of the line

• How to achieve pacing with margin: • Allow queues of work units between stations

• Provide for tolerance time to be longer than cycle time

• Allow worker to move beyond station boundaries

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No Pacing

• No time limit within which task must be completed

• Each assembly worker works at his/her own pace

• No pacing can occur when: • Manual transport of work units is used

• Work units can be removed from the conveyor to perform the task

• An asynchronous conveyor is used

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Coping with Product Variety

• Manual assembly lines can be designed to deal with differences in assembled products. In general, the product variety must be relatively soft. Three types of assembly line can be distinguished: • Single model assembly line (SMAL)

• Produces only one product in large quantities

• Every work unit is identical, so the task performed at each station is the same for all product

• Batch model assembly line (BMAL) • Designed to produce two or more products or models, but different approaches are

used to cope with the model variations.

• Hard product variety

• Products must be made in batches

• Mixed model assembly line (MMAL) • Soft product variety

• Models can be assembled simultaneously without batching

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MMAL vs. BMAL

• Advantages of mixed model lines over batch models lines: • No lost production time between models

• High inventories typical of batch production are avoided

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

• Difficulties with mixed model line compared to batch model line • Line balancing problem more complex due to differences in work elements

among models

• Scheduling the sequence of the different models is a problem

• Logistics is a problem - getting the right parts to each workstation for the model currently there

• Cannot accommodate as wide model variations as BMAL

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Analysis of Single Model Assembly Lines

• Cycle Time and Workload Analysis

• Repositioning Losses

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Exercise 1

• A product whose work content time = 47.5 min is to be assembled on a manual production line. The required production rate is 30 units per hour. From previous experience, it is estimated that the manning level will be 1.25, proportion uptime = 0.95, and repositioning time = 6 sec. Determine (a) cycle time, and (b) ideal minimum number of workers required on the line. (c) If the ideal number in part (b) could be achieved, how many workstations would be needed?

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Exercise 2

• A manual assembly line has 17 workstations with one operator per station. Work content time to assemble the product = 28.0 min. Production rate of the line = 30 units per hour. The proportion uptime = 0.94, and repositioning time = 6 sec. Determine the balance delay.

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Exercise 3

• A single model assembly line is being planned to produce a consumer appliance at the rate of 200,000 units per year. The line will be operated 8 hours per shift, two shifts per day, five days per week, 50 weeks per year. Work content time = 35.0 min. For planning purposes, it is anticipated that the proportion uptime on the line will be 95%. Determine (a) average hourly production rate, (b) cycle time, and (c) theoretical minimum number of workers required on the line. (d) If the balance efficiency is 0.93 and the repositioning time = 6 sec, how many workers will actually be required?

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Exercise 4

• The required production rate = 50 units per hour for a certain product whose assembly work content time = 1.2 hours. It is to be produced on a production line that includes four workstations that are automated. Because the automated stations are not completely reliable, the line will have an expected uptime efficiency = 90%. The remaining manual stations will each have one worker. It is anticipated that 8% of the cycle time will be lost due to repositioning at the bottleneck station. If the balance delay is expected to be 0.07, determine (a) the cycle time, (b) number of workers, (c) number of workstations needed for the line, (d) average manning level on the line, including the automated stations, and (e) labor efficiency on the line.

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Exercise 5

• A final assembly plant for a certain automobile model is to have a capacity of 225,000 units annually. The plant will operate 50 weeks/yr, two shifts/day, 5 days/week, and 7.5 hours/shift. It will be divided into three departments: (1) body shop, (2) paint shop, (3) general assembly department. The body shop welds the car bodies using robots, and the paint shop coats the bodies. Both of these departments are highly automated. General assembly has no automation. There are 15.0 hours of work content time on each car in this third department, where cars are moved by a continuous conveyor. Determine (a) hourly production rate of the plant, (b) number of workers and workstations required in trim chassis final if no automated stations are used, the average manning level is 2.5, balancing efficiency = 90%, proportion uptime = 95%, and a repositioning time of 0.15 min is allowed for each worker.

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Line Balancing Problem

• Given: • 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

• 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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Components of Cycle Time Tc

Components of cycle time at several workstations on a manual assembly line. At the bottleneck station, there is no idle time.

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Precedence Constraints

• Restrictions on the order in which work elements can be performed

Precedence

diagram

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Example 15.1

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• Measures of line balance efficiency

• Worker requirements

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Exercise 6

• Production rate for a certain assembled product is 47.5 units per hour. The assembly work content time = 32 min of direct manual labor. The line operates at 95% uptime. Ten workstations have two workers on opposite sides of the line so that both sides of the product can be worked on simultaneously. The remaining stations have one worker. Repositioning time lost by each worker is 0.2 min/cycle. It is known that the number of workers on the line is two more than the number required for perfect balance. Determine (a) number of workers, (b) number of workstations, (c) balance efficiency, and (d) average manning level.

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Exercise 7

• The work content for a product assembled on a manual production line is 48 min. The work is transported using a continuous overhead conveyor that operates at a speed of 5 ft/min. There are 24 workstations on the line, one-third of which have two workers; the remaining stations each have one worker. Repositioning time per worker is 9 sec, and uptime efficiency of the line is 95%. (a) What is the maximum possible hourly production rate if the line is assumed to be perfectly balanced? (b) If the actual production rate is only 92% of the maximum possible rate determined in part (a), what is the balance delay on the line?

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Line Balancing Algorithms

• Largest Candidate Rule • Assignment of work elements to stations based on amount of time each work

element requires

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Example 15.2

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Exercise 8

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Line Balancing Algorithms

• Kilbridge and Wester Method • Assignment of work elements to stations based on position in the precedence

diagram

• Elements at front of diagram are assigned first

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Example 15.3

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Exercise 9

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Exercise 10

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Line Balancing Algorithms

• Ranked Positional Weights • Combines the two preceding approaches by calculating an RPW for each element

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Example 15.4

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Exercise 11

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Exercise 12

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Mixed Model Assembly Lines

A manual production line capable of producing a variety of different product models simultaneously and continuously (not in batches)

• Problems in designing and operating a MMAL: • Determining number of workers on the line

• Line balancing - same basic problem as in SMAL except differences in work elements among models must be considered

• Model launching - determining the sequence in which different models will be launched onto the line

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Exercise 13

Two product models, A and B, are to be produced on a mixed model assembly line. Hourly production rate and work content time for model A are 12 units/hr and 32.0 min, respectively; and for model B are 20 units/hr and 21.0 min. Line efficiency = 0.95, balance efficiency = 0.93, repositioning time = 0.10 min, and manning level = 1. Determine how many workers and workstations must be on the production line in order to produce this workload.

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Other Considerations in Line Design

• Line efficiency • Management is responsible to maintain line operation at efficiencies (proportion

uptime) close to 100% • Implement preventive maintenance

• Well-trained emergency repair crews to quickly fix breakdowns when they occur

• Avoid shortages of incoming parts to avoid forced downtime

• Insist on highest quality components from suppliers to avoid downtime due to poor quality parts

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Other Considerations - continued

• Methods analysis • To analyze methods at bottleneck or other troublesome workstations

• Subdividing work elements • It may be technically possible to subdivide some work elements to achieve a better

line balance

• Sharing work elements between two adjacent stations • Alternative cycles between two workers

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Other Considerations - continued

• Utility workers • To relieve congestion at stations that are temporarily overloaded

• Changing workhead speeds at mechanized stations • Increase power feed or speed to achieve a better line balance

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

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Other Considerations - continued

• Zoning constraints - limitations on the grouping of work elements and/or their allocation to workstations • Positive zoning constraints

• Certain work elements should be grouped at same station

• Example: spray painting elements

• Negative zoning constraints • Elements that might interfere with each other

• Separate delicate adjustments from loud noises

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Other Considerations - continued

• Position constraints • Encountered in assembly of large products such as trucks and cars, making it

difficult for one worker to perform tasks on both sides of the product

• To address, assembly workers are positioned on both sides of the line

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Parallel work stations for better line balance

• Example 15.5

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Alternative Assembly Systems

• https://www.youtube.com/watch?v=DfGs2Y5WJ14

• Single-station manual assembly cell

• Worker teams

• Automated assembly systems

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Single-Station Manual Cell

• A single workstation in which all of the assembly work is accomplished on the product or on some major subassembly • Common for complex products produced in small quantities, sometimes one-of-a-

kind • Custom-engineered products

• Prototypes

• Industrial equipment (e.g., machine tools)

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https://www.youtube.com/watch?v=DfGs2Y5WJ14
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Assembly by Worker Teams

• Multiple workers assigned to a common assembly task • Workers set their own pace

• Examples • Single-station cell with multiple workers

• Swedish car assembly (job enlargement) - product is moved through multiple workstations by AGVS, but same worker team follows it from station to station

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Reported Benefits of Team Assembly

• Greater worker satisfaction

• Better product quality

• Increased capability to cope with model variations

• Greater ability to cope with problems that require more time rather than stopping the entire production line

• Disadvantage: • Team assembly is not capable of the high production rates of a conventional

assembly line

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