Determining numbers of workstations and operators for a linear walking-worker assembly line

TLDR: The research concluded that this multiskilled linear WW assembly line is able to sustain a higher flexibility and efficiency as compared with a conventional linear fixed-worker (FW) assembly line under similar conditions.

Abstract: This paper presents an investigation into a system of linear walking-worker (WW) assembly lines that have been implemented in a local small and medium enterprise (SME). The work aims to observe the relevant impact on logical interactions and interrelationships between the number of workstations and the number of walking workers against the system performance and to minimize these numbers providing a quick response in re-configuration of the system to meet a given output and cycle time. This has been achieved by developing a simulation model for a theoretical study into the linear WW system using a manufacturing-focused simulation tool. By examination of the results of a series of simulation experiments, a minimization of the numbers of workstations and walking workers for a linear WW line configuration can be quantified. The research concluded that this multiskilled linear WW assembly line is able to sustain a higher flexibility and efficiency as compared with a conventional linear fixed-worker (FW) assembly line under similar conditions.

Figures

Figure 1. Layout of the linear WW assembly system (built by Witness)

Table 4. Comparison of highest blocking rates (%) between linear FW and WW lines with varying levels in work time variations

Full text

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Determining numbers of workstations and operators for
a linear walking-worker assembly line
Qian Wang, G W Owen, Antony Roy Mileham
To cite this version:
Qian Wang, G W Owen, Antony Roy Mileham. Determining numbers of workstations and operators
for a linear walking-worker assembly line. International Journal of Computer Integrated Manufactur-
ing, Taylor & Francis, 2006, 20 (01), pp.1-10. �10.1080/09511920600667358�. �hal-00513375�

For Peer Review Only
Determining numbers of workstations and operators for a
linear walking-worker assembly line
Journal:
International Journal of Computer Integrated Manufacturing
Manuscript ID:
TCIM-2005-IJCIM-0045.R1
Manuscript Type:
Original Manuscript
Date Submitted by the
Author:
22-Feb-2006
Complete List of Authors:
Wang, Qian; University of Bath, Department of Mechanical
Engineering
OWEN, G W; University of Bath, Department of Mechanical
Engineering
Mileham, Antony; University of Bath, Department of Mechanical
Engineering
Keywords:
MODELLING, PERFORMANCE ANALYSIS, ASSEMBLY LINES,
SIMULATION APPLICATIONS, MANUFACTURING SYSTEMS,
FLEXIBLE ASSEMBLY
Keywords (user):
WALKING WORKERS
URL: http://mc.manuscriptcentral.com/tandf/tcim Email:ijcim@bath.ac.uk
International Journal of Computer Integrated Manufacturing

For Peer Review Only
1
Determining numbers of workstations and operators for a linear
walking-worker assembly line
QIAN WANG*, GERAINT WYN OWEN and ANTONY ROY MILEHAM
Department of Mechanical Engineering, University of Bath, Bath, BA2 7AY, UK
E-mail: A.R.Mileham@bath.ac.uk, G.W.Owen@bath.ac.uk, Q.Wang@bath.ac.uk
*Corresponding author. Email: Q.Wang@bath.ac.uk
Abstract
This paper presents an investigation into a system of linear walking-worker (WW)
assembly lines that have been implemented in a local SME (small and medium
enterprise). The work aims to observe the relevant impact on logical interactions and
interrelationships between the number of workstations and the number of walking
workers against the system performance and to minimise these numbers providing a
quick response in re-configuration of the system to meet a given output and cycle time.
This has been achieved by developing a simulation model for a theoretical study into
the linear WW system using a manufacturing-focused simulation tool. By examination
of the results of a series of simulation experiments, a minimisation of the numbers of
workstations and walking workers for a linear WW line configuration can be quantified.
The research concluded that this multi-skilled linear WW assembly line is able to
sustain a higher flexibility and efficiency as compared to a conventional linear fixed-
worker (FW) assembly line under similar conditions.
Keywords
Page 1 of 31
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International Journal of Computer Integrated Manufacturing

For Peer Review Only
2
Walking workers, assembly lines, simulation, flexible assembly, modelling
1. Introduction
A typical manual assembly line has separate workers performing assembly tasks at each
workstation whilst assembled items are moved on a conveyor system from workstation
to workstation. The operating times at each workstation are expected to be almost equal
in order to achieve a balanced assembly system. These times are highly contingent on
the variation of speed or skill of the workers and the processes used at each workstation.
The slowest worker often dictates the line output and the production rate, which are also
affected and controlled by the workstation with the longest processing time;
nevertheless, this longest processing time also determines the level of line balance. In
most conventional manual assembly lines, there is one (or more) seated or standing
assembly worker who repeatedly performs a single assembly task at each workstation
(or machine) and when this task has been accomplished, the assembled part will be
passed on to the next assembly worker at a downstream workstation for the next
assembly task to be done. This procedure will carry on until all the assembly work has
been completed. In this article, we name this type of serial production line with one
stationary worker per workstation as a linear fixed-worker (FW) assembly line, i.e., an
assembly worker always stays at a fixed-position workstation to do a single and often
repetitive assembly work along a sequential production line. One of the main drawbacks
for this type of production line is the difficulty of ‘in-process’ line balancing. Any
variance at any link in a theoretically balanced production chain will result in
disruptions in production and could even stop the entire production line. Because each
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International Journal of Computer Integrated Manufacturing

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3
workstation needs an assembly worker, the number of operators is normally equal to (or
greater than to cover breaks etc.,) the number of workstations regardless of the volume
of production. This is wasteful when the production volume is relatively low as the
production line still has to be fully staffed at each workstation leading to a poor
efficiency, responsiveness and the system’s re-configuration flexibility to a varying
demand of outputs. Moreover, the variation in individual working speed for each
worker increases the complexity of balancing the workload of individuals in terms of
percentage utilisation. To deal with this line-balancing problem, the most commonly
used approach is to add appropriate buffers into the production line. This simply
increases unnecessary costs and work-in-process (WIP). Previous work (Mileham et al.
2000) outlines that fixed-fitter lines tend to have low flexibility (in terms of fitters and
products), need constant demand and can be difficult to balance.
The fundamental distinction for the definition of linear WW assembly lines compared to
the definition of conventional linear FW assembly lines is that each worker is cross-
trained so that they are capable of assembling each product completely from beginning
to end and do this by walking from workstation to workstation along the production
line. Unlike a linear FW line where each worker is permanently engaged at one
workstation, in a linear WW line, each worker travels with his / her own assembled item
downstream and stops at each workstation to carry out the essential assembly work as
scheduled. When a walking worker finishes the assembly of a product, the worker walks
back to the front end of the production line to start the assembly of another and the
above-described procedure is repeated. This method attempts to combine some
advantages that originate from a workbench system or a cellular system with its high
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International Journal of Computer Integrated Manufacturing

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Determining numbers of workstations and operators for a linear walking-worker assembly line" ?

This paper presents an investigation into a system of linear walking-worker ( WW ) assembly lines that have been implemented in a local SME ( small and medium enterprise ). The work aims to observe the relevant impact on logical interactions and interrelationships between the number of workstations and the number of walking workers against the system performance and to minimise these numbers providing a quick response in re-configuration of the system to meet a given output and cycle time. This has been achieved by developing a simulation model for a theoretical study into the linear WW system using a manufacturing-focused simulation tool. The research concluded that this multi-skilled linear WW assembly line is able to sustain a higher flexibility and efficiency as compared to a conventional linear fixedworker ( FW ) assembly line under similar conditions. 

Q2. What is the common approach to deal with this problem?

To deal with this line-balancing problem, the most commonly used approach is to add appropriate buffers into the production line. 

Q3. What are the animation facilities used in the Witness simulation system?

The animation facilities within the Witness simulation system wereused to provide visual insight about material flows, WIP levels, bottlenecks and observing WW movement. 

Q4. What is the effect of adding more workstations on the output of a linear WW line?

Because adding more workstations will actually increase the length of the production line thereby increasing the total amount of walking time, this may affect the maximum output as each walking worker spends too much time on walking rather working along the line. 

Q5. What is the effect of increasing the number of walking workers on the line?

The research concludes that for a known overall cycle time in a linear WW line, increasing the numbers of walking workers and workstations together will significantly increase the line output; increasing the number of walking workers without increasing the number of workstations will not increase the line output but will decrease the utilisation of each worker. 

Q6. What is the definition of a linear walking-worker assembly line?

Walking workers, assembly lines, simulation, flexible assembly, modellingA typical manual assembly line has separate workers performing assembly tasks at each workstation whilst assembled items are moved on a conveyor system from workstation to workstation. 

Q7. What is the effect of walking time on the output of a linear WW line?

Since the total amount of walking time in this case study is very small compared to the overall cycle time of the production line, this variation caused by walking time appears not to be a major factor causing this fluctuation. 

Q8. What are the main drawbacks of a fixed-fitter line?

Previous work (Mileham et al. 2000) outlines that fixed-fitter lines tend to have low flexibility (in terms of fitters and products), need constant demand and can be difficult to balance. 

Q9. What is the effect of adding more workstations to the line?

It is also interesting to observe from figure 3 that when a linear WW line has the same number of walking workers and workstations, i.e., k = n where the line produces a maximum output, adding one more workstation (i.e., n = k + 1) to the line will increase the value of this maximum output and then this value becomes stable with a fluctuation as stated above. 

Q10. What is the effect of increasing the number of walking workers on a linear WW line?

It can generally be seen in figures 4 and 5 that increasing the number of walking workers k on an n-workstation (i.e., indicated as n WKs, where n varies from 1 to 10) line will decrease the percentage utilisation for each walking worker.