Joy M. Ebben, PhD, CPE, CHFEP, DABPS
I am often given the opportunity to tour manual assembly environments, either as a guest or as part of an ergonomic workplace evaluation. One of the common problem areas I have observed is how workstations are used in conjunction with conveyor systems. Often conveyor/workstation systems require the user to work in awkward, non-neutral postures and the conveyors create contact force on arms, wrists, and legs
Advancements in understanding ergonomic requirements in workstation design have provided significantly improved conveyor/workstation systems for seated operations. However, providing proper working height for standing operations is still a cost challenging proposition.
Why Use Conveyors?
The first question to ask is do you want a conveyor as part of your workstation configuration. In addition to creating difficult ergonomic problems for the workstation configuration, conveyors are often costly but yet inflexible. For example, you can not usually change a straight-line conveyor into a U shaped conveyor. So, you want to be sure that a workstation/conveyor system is really the right approach for your manufacturing environment.
Conveyors are used to move product between people or between people and automated equipment. The primary application for using a conveyor system to move product between people is for performing in-line assembly when each person completes a small number of tasks of short duration and a number of people are required to complete the assembly. Alternatively, the employee completes a batch of products and then uses the conveyor to transport the batch to the next station. Used in this fashion, material handling time and exerted force is reduced for employees.
Sometimes it is better for one person to complete all the assembly tasks. This provides "job enlargement" for the employee which could result in increased job satisfaction and possibly a reduction in the risk of musculoskeletal disorders. Having one person complete all steps is a useful concept to consider. But, it is most useful when all the required tools and piece-part components can be properly incorporated into a single workstation, when another person does not need to check the quality of each manual operation, and when the training approach supports everyone learning how to complete all tasks.
If someone is completing all tasks, then the conveyor can be used to move the item from a single workstation to an automated operation. However, if the next station is a manual station it may be better to move the assembled unit using some other type of material handling equipment, like adjustable customized carts. This is particularly true if the preferred design for the test station allows the employee to sit or stand. If it is determined that part of the task should be performed standing, the conveyor system becomes much more complicated in order to accommodate different working heights for standing individuals.
So, conveyors are not always the best solution for material handling when all factors are taken into consideration. But, if you have determined that the best approach to performing a task is seated, in-line progressive assembly, a conveyor may be a very appropriate part of your workstation configuration.
Two different workstation/conveyor layouts are typical -- the workstation positioned perpendicular or parallel with the conveyor.
The perpendicular orientation usually requires the employee to use awkward postures in order to move items on and off the conveyor. The extent of this problem depends upon the size of the workstation, where the person is seated at the workstation, and the size, weight, and shape of the item to be moved. Another potential shortcoming of a perpendicular layout is the limited viewability of other workers on the line; the person can easily see operations only in the direction they are facing.
One advantage of the perpendicular arrangement is that the workstation can have a much larger surface size without increasing reach distances to the conveyor. This is useful if there are large pieces of equipment or components that require worksurface space. The surface can be larger without negatively impacting accessing items on the conveyor system.
Parallel layouts, if designed properly, can reduce awkward postures and facilitate cooperative work because of the improved line-of-sight. A good design for seated operations in a parallel configuration addresses the following:
Providing parts cups can be a problem with parallel configurations because of limited space and required access to the conveyor and required clearance above the conveyor. Possible approaches include:
Workstations used sitting/standing or simply standing that are associated with a conveyor systems should provide some method of adjusting the workstation's height. This adjustment allows employees to adjust the height of the object they are working on to be appropriate for their body dimensions and the type of task to be performed. Systems are available (albeit expensive) that provide height adjustable workstations used in conjunction with a conveyor. Using control systems and conveyor "bridges" connected to the workstation surfaces, an individual employee can properly adjust the height of their workstations and still be connected to the adjustable conveyor ramp that would take the product to the fixed height main conveyor line. This sophisticated approach may be the design of choice when using sitting/standing or standing workstations and when a conveyor transportation system is the preferred material handling approach.
Perpendicular Arrangement with Conveyor "Bridge"
Another concern with conveyor systems is the potential for creating unacceptable work loads. Workload is a recognized risk factor for occupational stress and injuries. Two main methods for controlling the pace of work along conveyors are:
Conveyor systems can be designed to be machine paced, self-paced, or some combination of the two.
Self-paced operations are superior because they allow individuals to work at a rate that is safe and effective for themselves, not at a pace that is externally determined. The issue of pace works in both directions -- underworked and overworked. An underworked person on a machine-paced conveyor becomes bored, may use risky posture like extended reach to acquire an item before the conveyor presents it, or becomes distracted when waiting for the next task reducing productivity or quality.
Probably more of concern is the person that is paced by the machine to work faster than they are comfortable. Perceptions of heavy workload and associated stress and strain occur in response to fast-paced, short-cycle, and highly demanding tasks that do not provide opportunities for variety, interruption of the work cycle, or rests breaks - a close description of many workstation/conveyor operations. As workload increases there is more work pressure and heightened performance demands, forcing employees to remain longer at the activities. Increased psychological stress and repetition can encourage risky shortcuts which can increase the risk of WMSDs.
A conveyor system should be designed to allow for self-paced operations. This can be accomplished in a number of ways including providing individual holding areas, multiple person performing the same task, or returning items that are "passed-through" incomplete.
In conclusion, if you are considering using workstation with conveyor systems you have a number of ergonomic questions to address to help minimize exposing employees to musculoskeletal risk factors of awkward postures and contact stress. In addition, but not addressed here, are the issues of conveyor noise levels, conveyor control method and devices, and layout flexibility. Find a workstation manufacturer that has the expertise to help guide you through these ergonomic design issues to help ensure a successful implementation.
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Last Updated: 08/14/99