The five general classes of hand motions emphasize that material and tools should be located as close as possible to the point of use. The motions of the hands should be as short as the work permits. In the listing of classifications shown below, the one requiring the least amount of time and effort is shown first:
1. Finger motions
2. Motions involving fingers and wrist
3. Motions involving fingers, wrist, and forearm
4. Motions involving fingers, wrist, forearm, and upper arm
5. Motions involving fingers, wrist, forearm, upper arm, and shoulder (causes posture change)
It should be pointed out that finger motions have been found to be less accurate, slower, and more fatiguing than motions of the forearm. Evidence seems to indicate that the forearm is the most desirable member for performing light work. In highly repetitive work, motions about the wrist and elbow are superior to those of the fingers or shoulders.
The fifth principle of motion economy states that momentum should be employed to assist the worker wherever possible, and it should be reduced to a minimum if it must be overcome by muscular effort. The momentum of an object is defined as its mass multiplied by its velocity. In the factory environment, the total weight moved by the operator may consist of the weight of the material moved, the weight of the tools moved, and the weight of the part of the body moved. It should be a real possibility to employ momentum to advantage when a forcible blow or stroke is required. The motions of the worker should be so arranged that the blow is delivered when it reaches its greatest momentum.
The sixth principle of motion economy states that smooth, continuous, curved motions of the hands are preferable to straight-line motions involving sudden and sharp changes in direction. Abrupt changes in direction are not only time-consuming but also fatiguing to the operator.
The seventh principle of motion economy states that ballistic motions are faster, easier, and more accurate than restricted or controlled movements. Ballistic move- ments are fast, easy motions caused by a single contraction of a positive muscle group, with no antagonistic muscle group contracting to oppose it. A ballistic stroke may be terminated by the contraction of opposing muscles, by an obstacle, or by dissipation of the momentum of the movement, as in swinging a sledge hammer.
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Ballistic movements are preferable to restricted or controlled movements and should be used whenever possible.
The eighth principle of motion economy states that work should be arranged to permit an easy and natural rhythm wherever possible. Rhythm is essential to the smooth and automatic performance of any operation. Rhythm, as in a regular sequence of uni- form motions, aids the operator in performing work. A uniform, easy, and even rate of work is aided by proper arrangement of the workplace, tools, and materials. Proper motion sequences help the operator to establish a rhythm that helps make the work a series of automatic motions where the work is performed without mental effort.
The ninth principle of motion economy states that eye fixations should be as few and as close together as possible. Where visual perception is required, it is desirable to arrange the task so that the eyes can direct the work effectively. The workplace should be laid out so that the eye fixations are as few and as close together as possible.
The Workplace
The first principle of motion economy related to the workplace states that there should be a definite and fixed place for all tools and materials. The operator should always have tools and materials in the same location, and finished parts and assembled units should be placed in fixed positions or locations. For example, in the assembly of mechanical hardware, the hand should move without mental direction to the bin containing flat washers, then to the bin containing lock washers, then to the bin containing bolts, and finally to the bin containing hex nuts. There should be no thinking required on the part of the operator to do any of this.
The second principle of motion economy related to the workplace states that tools, materials, and controls should be located close to the point of use. In the hori- zontal plane, there is a definite and somewhat limited area that the worker can use with a normal expenditure of effort. This includes a normal working area for the right hand and one for the left hand for each working separately, and another for both hands working together. Figure 1.16 shows this and the dimensions of normal and maximum working areas in the horizontal and vertical planes. Both the standing and sitting positions are included. It also shows normal bench work surface heights, which can have a significant adverse effect if they are not correct.
Figure 1.17 shows in greater detail the areas of easiest reach for the left and right hands, for both hands working together, and the area in which small objects can be most easily picked up.
The third principle of motion economy related to the workplace states that gravity-feed bins and containers should be used to deliver the material close to the point of use. This can sometimes be accomplished by using parts bins with slop- ing bottoms that feed parts by gravity to the front of the bin, eliminating the need for the assembly operator to reach down into the bin to grasp parts.
The fourth principle of motion economy related to the workplace states that drop deliveries should be used wherever possible. This requires configuring the work- place, for example, so that finished units may be disposed of by releasing them in the position in which they are completed, delivering them to their next destination by
24 Assembly Process: Finishing, Packaging, and Automation
gravity. Besides the savings in time, this frees the two hands so that they may begin the next cycle immediately without breaking the rhythm.
Other principles of motion economy related to the workplace include the following:
Materials and tools should be located to permit the best sequence of motions. Provision should be made for adequate lighting.
FIGURE 1.16 Normal and maximum working areas and heights.
Total height Eye level Shoulder Elbow Seat Knee Normal working height 28" 14" 15" 10" 2" 9½" 20 " 29 " 39" 52 " 58 " 63 " 23.5 " 14" 14" 40" 58" 2" Work position Maximum work area Edge of work height Normal work area 37 " 9½ "
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The height of the workplace and chair should be arranged so that alternate sitting and standing at work are easily possible.
A chair of the type and height to permit good posture should be provided for the operator.