11

DESIGN OF WORK SURFACESDESIGN OF WORK SURFACES

Work-space envelope is the three-dimensional Work-space envelope is the three-dimensional space within which an individual works.space within which an individual works.

Within this envelope, specific design decisions Within this envelope, specific design decisions need to be made about various features of the need to be made about various features of the workspace.workspace.

Decisions include the location and design of Decisions include the location and design of benches, desks, tables, and control panels.benches, desks, tables, and control panels.

22

The following is a discussion of:The following is a discussion of:

1)1)The horizontal work-surface area, andThe horizontal work-surface area, and

2)2)Work-surface height (seated and Work-surface height (seated and standing). standing).

33

1.1. The horizontal Work-Surface AreaThe horizontal Work-Surface Area

Workspace design should be Workspace design should be performed with the shortest arm in performed with the shortest arm in mind and according to postural mind and according to postural considerations and task requirements considerations and task requirements (Corlett and Clark, 1995).(Corlett and Clark, 1995).

Ayoub, (1973), suggested an optimal Ayoub, (1973), suggested an optimal work area approximately 25 X 25 cm work area approximately 25 X 25 cm located at 10 cm from the table edge located at 10 cm from the table edge with the body mid-line crossing at the with the body mid-line crossing at the middle of the area (see figure1).middle of the area (see figure1).

44

FIGURE 1

55

The previous suggestion falls into the The previous suggestion falls into the optimal area suggested by Squires, optimal area suggested by Squires, (1956), which takes into account the (1956), which takes into account the dynamic interaction of the movement of dynamic interaction of the movement of the forearm as the elbow is also moving the forearm as the elbow is also moving (see figure 13-11 in your text book).(see figure 13-11 in your text book).

66

77

2. 2. Work-Surface Height (seated and Work-Surface Height (seated and standing)standing)

Backaches?, neck aches?, shoulder pains?Backaches?, neck aches?, shoulder pains?

Are they a result of the height of the work-Are they a result of the height of the work-surface?surface?

There might be a difference between the There might be a difference between the work-surface height and the working height.work-surface height and the working height.

88

Work-surface height is simply the height of Work-surface height is simply the height of the upper surface of the table, bench, desk, the upper surface of the table, bench, desk, counter, etc. measured from the floor.counter, etc. measured from the floor.

Working height depends on what the Working height depends on what the operator is working on.operator is working on.

Arm posture and thigh clearance (if seated) Arm posture and thigh clearance (if seated) should be considered in choosing work-should be considered in choosing work-surface height.surface height.

99

The arms should be relaxed at about 90The arms should be relaxed at about 90° to ° to provide comfort and help maintaining provide comfort and help maintaining straight wrists.straight wrists.

Three general principles are offered in your Three general principles are offered in your text book regarding seated work surfaces.text book regarding seated work surfaces.

Corlett and Clark, (1995), suggested Corlett and Clark, (1995), suggested providing adjustable chairs to achieve providing adjustable chairs to achieve appropriate elbow height in relation to work-appropriate elbow height in relation to work-surface height if the work-surface height is surface height if the work-surface height is fixed.fixed.

1010

Sanders and McCormick, (1987), suggested Sanders and McCormick, (1987), suggested that work-surface height should be at a level that work-surface height should be at a level that permits the arms to hang in a that permits the arms to hang in a reasonably relaxed position from the reasonably relaxed position from the shoulder, with the forearm usually being shoulder, with the forearm usually being near horizontal or sloping down slightly.near horizontal or sloping down slightly.

The work-surface height should be at a level The work-surface height should be at a level

that does not require excessive flexing of the that does not require excessive flexing of the spine. spine.

1111

Pheasant, (2001), stated that the major Pheasant, (2001), stated that the major determinant of a standing worker’s posture is determinant of a standing worker’s posture is the height at which manual activities are the height at which manual activities are performed.performed.

Too high working levels will cause the Too high working levels will cause the shoulders and upper limbs to be raised. This shoulders and upper limbs to be raised. This will probably lead to fatigue and strain in the will probably lead to fatigue and strain in the muscles of the shoulder region. If the task muscles of the shoulder region. If the task requires any downward application of force requires any downward application of force the upper limbs will be in a position of poor the upper limbs will be in a position of poor mechanical advantage for providing that mechanical advantage for providing that force. A lower working level could overcome force. A lower working level could overcome this problem. this problem.

1212

If, however, the working level is too low the If, however, the working level is too low the trunk, neck, and head will be inclined trunk, neck, and head will be inclined forwards with consequent postural stress for forwards with consequent postural stress for the spine and its muscles.the spine and its muscles.

Pheasant, (2001), added that it may be Pheasant, (2001), added that it may be presumed that somewhere between a presumed that somewhere between a working level that is too high and one that is working level that is too high and one that is too low there may be found a suitable too low there may be found a suitable compromise at which neither the shoulders compromise at which neither the shoulders nor the back are subjected to excessive nor the back are subjected to excessive postural stress. postural stress.

1313

This limits the design in two directions to This limits the design in two directions to give a two-tailed constraint. Pheasant, give a two-tailed constraint. Pheasant, (1990), suggested two options for such a (1990), suggested two options for such a situation. situation.

The first option is to provide adjustable The first option is to provide adjustable workstation which could be set by the user workstation which could be set by the user according to his/her optimum dimensions. according to his/her optimum dimensions.

1414

The second option is to settle on a single The second option is to settle on a single overall compromise value which will overall compromise value which will maximise the number of users who are maximise the number of users who are accommodated and minimise the accommodated and minimise the inconvenience suffered by the reminder.inconvenience suffered by the reminder.

Most authors consider elbow height and the Most authors consider elbow height and the type of work performed as the most type of work performed as the most important two critical features of working important two critical features of working level.level.

1515

• Pheasant, (2001), recommended: Pheasant, (2001), recommended:

1)1) A work level of 50-100 mm above elbow A work level of 50-100 mm above elbow height for delicate manipulative tasks,height for delicate manipulative tasks,

1)1) A work level of 50-100 mm below elbow A work level of 50-100 mm below elbow height for manipulative tasks involving a height for manipulative tasks involving a moderate degree of both force and moderate degree of both force and precision, andprecision, and

1)1) A work level 100-250 mm below elbow A work level 100-250 mm below elbow height for heavy manipulative tasks. height for heavy manipulative tasks.

1616

Grandjean, (1988), recommended Grandjean, (1988), recommended three different working levels for three different working levels for three types of work:three types of work:

1)1)100-200 mm below elbow height for 100-200 mm below elbow height for heavy work, heavy work,

2)2)0-100 mm below elbow height for 0-100 mm below elbow height for light work, andlight work, and

3)3)0-100 mm above elbow height for 0-100 mm above elbow height for precision work.precision work.

1717