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Metallurgical Engineering

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Chrono Cycle Graph

These are the techniques of analyzing the paths of motion made by an operator and were originally developed by the Gilbreths. To make a cycle graph , a small electric bulb is attached to the finger, hand, or any other part of the body whose motion is to be recorded. By using still photography, the path of light of bulb (in other words, that of the body member) as it moves through space for one complete cycle is photographed. The working area is kept relatively less illuminated while photograph is being taken. More than one camera may be used in different planes to get more details. After the film is developed, the resulting picture (cycle graph) shows a permanent record of the motion pattern employed in the form of a closed loop of white continuous line with the working area in the background. A cycle graph does not indicate the direction or speed of motion.

It can be used for

• Improving the motion pattern, and

• Training purposes in that two cycle graphs may be shown with one indicating a better motion pattern than the other.

The chrono cycle graph is similar to the cycle graph, but the power supply to the bulb is interrupted regularly by using an electric circuit. The bulb is thus made to flash. The procedure for taking photograph remains the same. The resulting picture (chrono cycle graph), instead of showing continuous line of motion pattern, shows short dashes of line spaced in proportion to the speed of the body member photographed. Wide spacing would represent fast moves while close spacing would represent slow moves. The jumbling of dots at one point would indicate fumbling or hesitation of the body member. A chrono cycle graph can thus be used to study the motion pattern as well as to compute velocity, acceleration and retardation experienced by the body member at different locations. Figures show a cycle graph and a chrono cycle graph.

The world of sports has extensively used this analysis tool, updated to video, for the purpose of training in the development of form and skill.

Principles of Motion Economy:

These principles can be considered under three different groups.

• Those related to the use of the human body.

• Those related to the workplace arrangement, and

• Those related to the design of tools and equipment.

1. Principles related to the use of human body:

(i) Both hands should begin and end their basic divisions of activity simultaneously and should not be idle at the same instant, except during the rest periods.

(ii) The hand motions should be made symmetrically and simultaneously away from and toward the centre of the body.

• 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.

• Continuous curved motions should be preferred to straight line motions involving sudden and sharp changes in the direction.

• The least number of basic divisions should be employed and these should be confined to the lowest practicable classifications. These classifications, summarized in ascending order of time and fatigue expended in their performance, are:

• Finger motions

• Finger and wrist motions.

• Finger, wrist, and lower arm motions.

• Finger, wrist, lower arm, and upper arm motions.

• Finger, wrist, lower arm, upper arm motions and body motions.

• Work that can be done by the feet should be arranged so that it is done together with work being done by the hands. It should be recognized, however, that it is difficult to move the hand and foot simultaneously.

• The middle finger and the thumb should be used for handling heavy loads over extended periods as these are the strongest working fingers. The index finger, fourth finger, and little finger are capable of handling only light loads for short durations.

• The feet should not be employed for operating pedals when the operator is in standing position.

• Twisting motions should be performed with the elbows bent.

• To grip tools, the segment of the fingers closed to the palm of the hand should be used.

2. Principles related to the arrangement and conditions of workplace:

• Fixed locations should be provided for all tools and materials so as to permit the best sequence and eliminate search and select.

• Gravity bins and drop delivery should be used to reduce reach and move times. Use may be made of ejectors for removing finished parts.

• All materials and tools should be located within the normal working area in both the vertical and horizontal plane, and as close to the point of use as possible.

• Work table height should permit work by the operator in alternately sitting and standing posture.

• Glare-free adequate illumination, proper ventilation and proper temperature should be provided.

• Dials and other indicators should be patterned such that maximum information can be obtained in minimum of time and error.

3. Principles related to the design of tools and equipment:

• Use colour, shape or size coding to maximize speed and minimize error in finding controls.

• Use simple on/off, either/or indicators whenever possible. If simple on/off indicator is not sufficient, use qualitative type indicator, and use quantitative type indicator only when absolutely essential.

• All levers, handles, wheels and other control devices should be readily accessible to the operator and should be designed so as to give the best possible mechanical advantage and utilize the strongest available muscle group. Their direction of motion should conform to stereo-typed reactions.

• Use quick acting fixture to hold the part or material upon which the work is being performed.

• Use stop guides to reduce the control necessary in positioning motions.

• Operating, set-up and emergency controls should be grouped according to the function.

Design of Workplace Layout

The design of workplace layout involves the following

• Determination of work surface height

• Design of operator chair (if work is to be done in sitting posture), or allowing the use of antifatigue mats for standing operator

• Determination of location of tools, materials, controls, displays and other devices.

We shall consider these briefly.

Work Place Height

This should be decided from the standpoint of comfortable working posture for the operator. Generally, it is equal to the elbow height of operator whether work is done in standing or sitting posture. However, for work involving lifting of heavy parts, it is useful to lower the work surface height by as much as 20 cm. This would reduce the fatigue to the trunk of operator. Similarly, it may be useful to raise the work surface height when work involves visual examination of minute details of fine parts. This would reduce the eye fatigue to the operator. Alternatively, the work surface may be inclined by 15 degrees or so. Work surface height may also be made adjustable in situations where operator is permitted to do work in alternatively sitting and standing postures.

Design of Operator Chair

A seated posture is better than standing posture from the standpoint of stress reduction on the feet and the overall energy expenditure. A well-designed seat should

• Provide trunk stabilization so that a good posture is maintained,

• Permit change of posture, and

• Not unduly press the thigh tissues.

This requires the use of ergonomic considerations and anthropometric dimensions of operator so that appropriate dimensions are chosen for the following features of chair

(i) Seat Height

(ii) Seat Depth

(iii) Seat Width

(iv) Seat Inclination

(v) Arm Rests

(vi) Back Rest

(vii) Foot Rest

It is necessary to provide adjustability, particularly with respect to seat height, in order that the same seat (or chair) is useable by many operators doing same job.

Standing for long periods of time on a cemented floor is fatiguing. If operator has to work only in standing posture, it is essential to provide resilient anti-fatigue floor mats. Such mats allow small muscle contractions in the legs and force the blood to keep circulating.

Determination of location of tools, materials, controls, displays and other devices.

We all know that greater the distance through which operator moves his body member while doing work, larger is the requirement of muscular effort, control and time. This means that all tools, materials, controls, etc need to be located within close reach of the operator. In this context, two areas can be identified: normal working area and maximum working area.

Within these areas, all tools, materials, controls, displays and other devices must be located on the basis of any of the following principles.

(i) Importance Principle: According to this principle, the most important item or group of items is first located within the normal area in the best position. The next important component item or group of items is then selected and located in the best location within the remaining area. In this way, all the items are located.

(ii) Frequency of Use Principle: According to this principle, the item with the greatest frequency of use has the highest priority for location at the optimum position. From within the remaining items to be located in the remaining area, the same principle can then be applied repetitively.

(iii) Functional Principle: This principle provides for grouping of items according to their function. For instance, all controls that are functionally related may be grouped together and located at one place.

(iv) Sequence of Use Principle: According to this principle, items are located according to sequence of their use. For illustration, let us consider the case of assembly. As we know, an assembly is made by assembling the sub-assemblies in a specific order. From motion economy or production efficiency point of view, it would be better if sub-assemblies and other items are located in the sequence in which they are to be used in assembly.

Further, for better productivity, it is important that location of all tools, materials and controls be fixed so that their "search" and “select" is minimized.