Eng. Suleiman Daifi

KEY POINTS Fit the workplace to the operator. Provide adjustability. Maintain neutral postures (joints in midrange). Minimize repetitions. Use power grips when force is required. Use pinch grips for precision and not force.

Ergonomics: Designing the workplace, tools, equipment, and work environment to fit the human operator This chapter presents the principles of work design and appropriate checklists to facilitate the use of these design principles. This approach will better assist the methods analyst in designing the (1) workplace, (2) equipment, and (3) tools to meet the simultaneous goals of: (1) increased production and efficiency of the operation and (2) decreased injury rates for the human operator.

ANTHROPOMETRY AND DESIGN The primary guideline is to design the workplace to accommodate most individuals with regard to structural size of the human body. Anthropometry : The science of measuring the human body and typically utilizes a variety of caliper like devices to measure structural dimensions, for example, stature and forearm length. The CAESAR (Civilian American and European Surface Anthropometry Resource) project collected over 100 dimensions on 5,000 civilians using three-dimensional body scans. A summary of useful dimensions that apply to the particular postures needed for workplace design for U.S. males and females is given in Table 5.1.

DESIGN FOR EXTREMES Design for extremes implies that a specific design feature is a limiting factor in determining either the maximum or minimum value of a population variable that will be accommodated. For example (Max): clearances, such as a doorway or an entry opening into a storage tank, should be designed for the maximum individual, that is, a 95th percentile male stature or shoulder width. Then 95 percent of all males and almost all females will be able to enter the opening. Obviously, for doorways, space is usually not at a premium, and the opening can be designed to accommodate even larger individuals. Example (Min): added space in military aircraft or submarines is expensive, and these areas are therefore designed to accommodate only a certain (smaller) range of individuals. Reaches, for such things as a brake pedal or control knob, are designed for the minimum individual, that is, a 5th percentile female leg or arm length. Then 95 percent of all females and practically all males will have a longer reach and will be able to activate the pedal or control.

DESIGN FOR ADJUSTABILITY Design for adjustability: it is typically used for equipment or facilities that can be adjusted to fit a wider range of individuals. Example: Chairs, tables, desks, vehicle seats, steering columns, and tool supports are devices that are typically adjusted to accommodate the worker population ranging from 5th percentile females to 95th percentile males. Obviously, designing for adjustability is the preferred method of design, but there is a trade-off with the cost of implementation. (Specific adjustment ranges for seat design are given later in Table 5.2)

DESIGN FOR THE AVERAGE Design for the average: It is the cheapest but least preferred approach. Even though there is no individual with all average dimensions, there are certain situations where it would be impractical or too costly to include adjustability for all features. For example, most industrial machine tools are too large and too heavy to include height adjustability for the operator. Designing the operating height at the 50th percentile of the elbow height for the combined female and male populations (roughly the average of the male and female 50th percentile values) means that most individuals will not be unduly inconvenienced. However, the exceptionally tall male or very short female may experience some postural discomfort.

PRACTICAL CONSIDERATIONS Reasonable effort must be made to accommodate individuals with all abilities. Special accessibility guidelines (U.S. Department of Justice, 1991) have been issued regarding parking lots, entryways into buildings, assembly areas, hallways, ramps, elevators, doors, water fountains, lavatories, restaurant or cafeteria facilities, alarms, and telephones. It is also very useful, if practical and cost-effective, to build a full-scale mock-up of the equipment or facility being designed and then have the users evaluate the mock-up.

PRINCIPLES OF WORK DESIGN: THE WORKPLACE

1. DETERMINE WORK SURFACE HEIGHT BY ELBOW HEIGHT 2. ADJUST THE WORK SURFACE HEIGHT BASED ON THE TASK BEING PERFORMED 3. PROVIDE A COMFORTABLE CHAIR FOR THE SEATED OPERATOR 4. PROVIDE ADJUSTABILITY IN THE SEAT 5. ENCOURAGE POSTURAL FLEXIBILITY 6. PROVIDE ANTIFATIGUE MATS FOR A STANDING OPERATOR 7. LOCATE ALL TOOLS AND MATERIALS WITHIN THE NORMAL WORKING AREA 8. FIX LOCATIONS FOR ALL TOOLS AND MATERIALS TO PERMIT THE BEST SEQUENCE 9. USE GRAVITY BINS AND DROP DELIVERY TO REDUCE REACH AND MOVE TIMES 10. ARRANGE TOOLS, CONTROLS, AND OTHER COMPONENTS OPTIMALLY TO MINIMIZE MOTIONS

(1) DETERMINE WORK SURFACE HEIGHT BY ELBOW HEIGHT The work surface height (whether the worker is seated or standing) should be determined by a comfortable working posture( وضع ) for the operator. Typically, this means that the upper arms are hanging ( معلق ) down naturally and the elbows are flexed at 90°so that the forearms are parallel to the ground The elbow height becomes the proper operation or work surface height. If the work surface is too high, the upper arms are abducted, leading to shoulder fatigue. If the work surface is too low, the neck or back is flexed forward, leading to back fatigue

(2) ADJUST THE WORK SURFACE HEIGHT BASED ON THE TASK BEING PERFORMED For rough assembly involving the lifting of heavy parts, it is more advantageous to lower the work surface For fine assembly involving minute visual details, it is more advantageous to raise the work surface by up to 8 in (20 cm) to bring the details closer to the optimum line of sight of 15 These principles also apply to a seated workstation. A majority of tasks, such as writing or light assembly, are best performed at the resting-elbow height. If the job requires the perception of fine detail, it may be necessary to raise the work to bring it closer to the eyes. Seated workstations should be provided with adjustable chairs and adjustable footrests (see Figure 5.6). Ideally, after the operator is comfortably seated with both feet on the floor, the work surface is positioned at the appropriate elbow height to accommodate the operation. Thus, the workstation also needs to be adjustable. Short operators whose feet do not reach the floor, even after adjusting the chair, should utilize a footrest to provide sup- port for the feet.

(3) PROVIDE A COMFORTABLE CHAIR FOR THE SEATED OPERATOR The seated posture is important from the standpoint of reducing both the stress on the feet and the overall energy expenditure. Because comfort is a very individual response, strict principles for good seating are somewhat difficult to define.

(4) PROVIDE ADJUSTABILITY IN THE SEAT Seat height is most critical, with ideal height being determined by the person’s popliteal height, A seat that is too high will uncomfortably compress the underside of the thighs. A seat that is too low will raise the knees uncomfortably high and decrease trunk ( جذع ) angle, again increasing disk pressure. Specific recommendations for seat height and other seat parameters (shown in Figure 5.6) are given in Table 5.2.

(5) ENCOURAGE POSTURAL FLEXIBILITY The workstation height should be adjustable so that the work can be performed efficiently either standing or sitting. The human body is not designed for long periods of sitting. An alternate compromise is to provide a sit/stand stool so that the operator can change postures easily.

(6) PROVIDE ANTIFATIGUE MATS FOR A STANDING OPERATOR Standing for extended periods on a cement floor is fatiguing. The operators should be provided with resilient ( مرن ) anti-fatigue mats. The mats allow small muscle contractions in the legs, forcing the blood to move and keeping it from tending to pool in the lower extremities ( أطراف الجسم ).

(7) LOCATE ALL TOOLS AND MATERIALS WITHIN THE NORMAL WORKING AREA LOCATE ALL TOOLS AND MATERIALS WITHIN THE NORMAL WORKING AREA In every motion, a distance is involved. The greater the distance, the larger the muscular effort, control, and time. It is therefore important to minimize distances. The normal working area in the horizontal plane of the right hand includes the area circumscribed by the arm below the elbow when it is moved in an arc pivoted at the elbow (see Figure 5.12). This area represents the most convenient zone within which motions may be made by that hand with a normal expenditure of energy. The normal area of the left hand may be similarly established. Since movements are made in the third dimension, as well as in the horizontal plane, the normal working area also applies to the vertical plane.

(8) FIX LOCATIONS FOR ALL TOOLS AND MATERIALS TO PERMIT THE BEST SEQUENCE Example: In driving an automobile, we are all familiar with the short time required to apply the foot brake. The reason is obvious: since the brake pedal is in a fixed cation, no time is required to decide where the brake is located. The body responds instinctively ( بصورة غريزية )and applies pressure to the area where the driver knows the foot pedal is located. If the location of the brake foot pedal varied, the driver would need considerably more time to brake the car. Conclusion: providing fixed locations for all tools and materials at the workstation eliminates, or at least minimizes, the short hesitations required to search for and select the objects needed to do the work.

(9) USE GRAVITY BINS AND DROP DELIVERY TO REDUCE REACH AND MOVE TIMES The time required to perform both of the transport actions :Reach and Move is directly proportional to the distance that the hands must move in performing these Actions Utilizing gravity bins, components can be continuously brought to the normal work area, thus eliminating long reaches to get these supplies (see Figure 5.15). Likewise, gravity chutes ( مزالق ) allow the disposal of completed parts within the normal area, eliminating the necessity for long moves to do so.

(10) ARRANGE TOOLS, CONTROLS, AND OTHER COMPONENTS OPTIMALLY TO MINIMIZE MOTIONS The optimum arrangement depends on many characteristics: 1. human (strength, reach, sensory) and 2. task (loads, repetition, orientation). The most important, or most frequently used components, should be placed in the most convenient locations. For example, an emergency stop button should be placed in a readily visible, reachable, or convenient position. Similarly, a regularly used activation button, or the most often used fasteners, should be within easy reach of the operator. Once the general location has been determined for a group of components, that is, the most frequently used parts for assembly, the principles of functionality and sequence of use must be considered. Functionality refers to the grouping of components by similar function, for example, all fasteners in one area, all gaskets and rubber components in another area. Since many products are assembled in a strict sequence, cycle after cycle, it is very important to place the components or subassemblies in the order that they are assembled, since this will have a very large effect on reducing wasteful motions. These principles of work design for workstations are summarized in the Workstation Evaluation Checklist (see Figure 5.16). The analyst may find this useful in evaluating existing workstations or implementing new workstations.