Students will be able to recognize the best work surface height for a given task and identify the basic principles behind seated, standing, and leaning workstations. They will be able to identify common workstation solutions and how to apply work space envelopes.
Most all workers perform their tasks at some sort of workstation. Whether the worker is a truck driver in a vehicle, an office worker at a desk, or a healthcare worker in a hospital, they work at some sort of workstation. This chapter discusses specific industrial types of workstations that involve assembly repair and fabrication types of tasks (Ostrom, 1994).
A workstation is a location where a person performs one or more tasks that are required as part of his/her job. The design of the workstation can have a profound impact on the person’s ability to safely and effectively perform the required tasks. Reach capability, body size, muscle strength, and visual capabilities are just a few of the factors that should be considered in workstation design. The design guidelines to be discussed in this section include the following:
1. Accommodate people with a range of body sizes or anthropometric dimensions.
2. Permit several working positions/postures to promote better blood flow and muscle movement.
3. Design workstations from the working point of the hands. People work with their hands so we want their working height to be relative to their hand height.
4. Place tools, controls, and materials between the shoulder and waist height, where they have the greatest mechanical advantage.
5. Provide higher work surfaces should be provided for precision work, and lower work surfaces for heavy work.
6. Round or pad work surface edges should be rounded or padded to reduce compressive forces.
7. Provide well-designed chairs in order to support the worker.
As each of the design guidelines is discussed, keep in mind that some of these principles may not be applicable to designs for individuals with special needs. Different tasks may also require different design guidelines, such as a shorter bus driver who may need a brake pedal extension.
GENERAL WORKSTATION GUIDELINES
Workstations should be designed to accommodate the anthropometric characteristics of a range of workers. In general, a well-designed workstation should be able to accommodate 90–95% of a worker population. This helps to ensure that workers can perform their job tasks comfortably and in a safe manner. There are several different sources of data that can assist with ergonomic design of the workplace.
Anthropometric tables contain data on human body dimensions and can be extremely useful in designing a work environment. When designing a new workstation, you should first identify the target audience, determine the appropriate anthropometric measurements, and ideally, create a mock-up for trial.
Figure 11.1 is an example of a height-adjustable table with a hand crank. This is an example of how you can accommodate workers with a range of body dimensions.
Figure 11.1 Hand crank workstation (Photo with permission from Pro-Line)
For operations where highly repetitive actions are required, where existing tools are not optimal, and where the workplace cannot be adjusted, a specially designed tool should be considered. Examples of special purpose tools include the following:
· Spring-loaded scissors that prevent irritation of the backs and sides of the fingers caused by opening conventional shears
· Bent pliers to maintain the wrist in a neutral posture
· Adjustable position (straight to pistol) screwdriver for multiple uses that encourages the use of neutral postures (Refer to the Hand Tool Section for more information on tools and neutral postures).
Situations when special tool designs are not recommended include the following:
· If the tool is used for a short part of a multitask cycle and thus becomes an extra tool to pick up and set down frequently.
· If the operation is not a continuous one and occurs only occasionally in the work shift
· If the workspace around the operator is limited, and there is no place to set tools between uses (Eastman-Kodak, 2003).
MOCK-UP OR FITTED TRIALS
The following illustrates the advantages of using a mock-up before actually purchasing equipment. In this example, aircraft access doors were being repaired. The workers were bending over into awkward, stressful postures when working on the access doors. One of the workers created a fixture for holding the access doors, which would allow them to rotate 180°. Figure 11.2 is the original design developed by a worker. As you can see, the heavy-duty locking pin and open support structure allowed almost 180° of rotation. The large diameter casters created low push forces, even when the door was attached to the fixture.
Figure 11.2 Original fixture
Navy Hazard Abatement funding was used to improve and mass produce these fixtures. Prior to mass production of the fixture, a prototype (mock-up) was fabricated and put into use by the workers. The first prototype ( Figure 11.3 ) was inferior to the one made by the workers, and needed improvements were identified during this trial use. The wheels were too small, which required high forces to move the fixture and access door. The locking mechanism was unstable, and the support structure actually reduced the amount of rotation. A great amount of information was gained from creating and testing the mock-up that helped in designing the final product ( Figure 11.4 ).
Figure 11.3 Prototype fixture
Figure 11.4 Final fixture
The final fixture design, shown on the left, permits 180° of rotation, and is height adjustable and has a heavy-duty locking pin and large diameter casters to reduce push/pull forces.
The original fixture design was built in-house; we borrowed the design and expanded on it. A great amount of worker pride was shown by using the original design. The new fixture can be shared with other activities, and they save 3–5 days of labor per door now.
GENERAL WORKSTATION GUIDELINES PERMIT SEVERAL WORKING POSITIONS/POSTURES
One of the tenets of biomechanics is that static loading of muscles and joints is something that should be avoided. Static loading is considered a potential risk factor for WMSD development. To avoid static loading, it is recommended that workers be allowed to periodically change their working posture. When changing posture, the individual utilizes different muscle groups, increases blood flow, and allows the muscles to rest.
There are many possible postures for working. Each has benefits and disadvantages to be considered. We will discuss a few of them in detail:
Each of the postures a worker uses should be equally safe and free from ergonomic stresses. One approach for permitting different working postures is to allow the worker to both sit and stand while performing job tasks.
Seated workstations are very common, but there are guidelines to follow when determining if a workstation should be seated. A seated workstation is not appropriate when working with heavy items, repetitive extended reaching, or heavy forces.
Sitting is preferred in the following situations:
· All repeatedly used items are easily located within the seated workspace (i.e., “reach envelope” shown in Figure 11.5)
· The items being handled do not require the hands to work more than 6 in. above the work surface on average.
· No large forces are required (heavy objects greater than approximately 10 lb). Using mechanical assists, such as a counterbalance, may eliminate or reduce large forces.
· Fine assembly or writing tasks are done for a majority of the shift (e.g., precision work or visual inspection).
Figure 11.5 Horizontal reach envelope
Note: Workers should not handle more than 10 lb from a seated position.
If the workstation is a seated workstation, they need to provide the following:
· All tools, parts, and controls to be within easy reach.
· Appropriate height work surface for the individual/task, height adjustable preferred. Figures 11.6 and 11.7 illustrate adjustable work benches.
· Chair providing good support
· Foot support if necessary (solutions for this will be shown later in this module)
· Part/task should be oriented toward the employee.
· Padded work surface edge, to reduce compression
· Allow clearance for head, thighs, and knees.
· If feasible, alternate between sitting and standing, or leaning.
Figure 11.6 Adjustable workbench (Photo with permission from Pro-Line)
Figure 11.7 Manual adjustable workbench (Photo with permission from Pro-Line)
Providing a well-designed, ergonomic chair can be extremely important in designing or choosing a safe and comfortable workstation. Many manufacturers market their chairs as being “ergonomic”. What makes a chair ergonomic? With regard to vendors and chair manufacturers, the term “ergonomic” is often overused. In order to select a good ergonomic chair, a few issues need to be considered:
· How many hours does a typical user spend in a chair? (24-h chairs are more durable than 8-h chairs)
· Is the user population extremely tall, short, or heavy?
· Is the work surface adjustable?
· What is the height of the work surface?