These common possibilities are indicated by principles of motion economy.
A good analysis of efficiency improvement opportunities has to include examining the 10 efficiency aids.
Gravity Delivery Chutes.
Gravity delivery chutes are useful for bringing material close to the point of use, thereby shortening the motions required to obtain the material. The usual arrangement consists of a hopper that will hold a reasonable supply of material with an opening at the bottom through which a few pieces may pass. Material may be removed directly from the opening at the bottom of the hopper. If the workplace is crowded, the hopper may be set out of the way and a chute provided between the bottom of the hopper and the point of use along which the parts may slide by gravity.
If parts are of a suitable shape, special delivery devices may be built that are more effective than the common chute. Small, uniform parts with no projections may be handled in an arrange-ment that delivers the parts at the bottom in predetermined quantities. The coin holders used by street-railway conductors, newsboys, and others who must make change frequently are a well-known example of this sort of delivery device.
Many parts are by no means free from projections or even symmetrical in shape. The design of chutes and hoppers that will handle irregular parts is more difficult, and considerable cutting and trying may be necessary before an arrangement can be devised that will deliver parts uniformly at a given point and will neither jam nor overflow. If the chute is used in conjunction with moving machinery, the delivery problem is much easier. Even the smoothest running machine has a certain amount of vibration, and if the chute Is rigidly attached to some part of the machine, the vibration mill cause the parts to move slowly and uniformly down the chute and even around bends.
Illustration: Chute used in conjunction with a trimming machine for a leather of machine.
As originally designed, the parts tended to jam in the hopper. Removing the key of the jam brought a rush of parts which sometimes overflowed the sides of the chute. The parts did not slide easily, and, therefore, the chute had to be steep. An angle sufficient to overcome starting friction was too steep when the parts were in motion, and the parts shot down so quickly that they were continually falling to the floor. These difficulties were overcome by slight design changes, but principally by attaching the chute to the machine so that the vibration from the machine kept the parts in motion. After this, the parts fed uniformly down the chute and arrived without interruption at a point where they could conveniently be grasped by the operator.
Drop delivery, as the name implies, consists of getting rid of a part by dropping it. It is used when placing finished parts aside. Sometimes, it is possible to arrange a setup In such a way that the finished part falls off into a container or chute as it Is completed, and the operator does not have to handle it after completing work upon it. For example, after completing the trimming operation on the machine, the operator merely opens his fingers, and the finished part falls into a box placed directly beneath the cutter. On operations where the finished part must be carried aside by the operator, drop delivery is still obtained if the part is carried over a container or a chute and is released by opening the fingers as the hand continues on its way to the next point, which is usually the raw-material supply. Not all parts can be dropped, of course. Fragile, brittle, or soft parts would be damaged if dropped with any appreciable jar. Even with parts of this kind, however, drop delivery can sometimes be used if the parts are dropped onto some sort of soft, yielding surface. A canvas chute may be provided, for example, which first breaks the fall of the part and then permits It to slide gently into a container.
When drop delivery is employed, the relative position of the raw- and the finished-material containers is Important. Many times workplace layouts are encountered In which the raw material Is close to the operator and the finished material farther away. This is Incorrect. The finished material should be closer to the operator and the raw material farther away and in the same line. When the operator finishes work on a part, he grasps the part. He moves toward the raw-material container and drops the part in the finished-material container on the way. With a little practice, he can do this without hesitation. Finished material is laid aside and raw material is obtained with two motions, one over to the raw-material container and one back to the work point. If the position of the material containers is reversed, three motions will be required, one to the finished-material container where the part is dropped, one to the raw-material container, and one from the raw-material container to the work point.
In order that parts may be dropped during a motion without hesitation, the object into which they are dropped must be large enough so that there is no danger of missing it. If the container itself is small or if the part must pass through a small hole in the bench, a funnel should be provided to make it easy to drop the part in the desired location.
Drop delivery suggests that the part falls away owing to the force of gravity. The same effect may be obtained by the use of springs that carry the released part aside, usually in an upward direction. The most common application of this arrangement is in the suspension of tools above the workplace. The tools are hung on a "spring. After the tool has been used, it is released by opening the fingers. The spring carries it away without further attention on the part of the operator.
A similar application of this principle may be made to the levers of small hand-operated arbor presses. When the handle of the press is released after the operation has been performed, a spring carries it out of the way and raises the arbor. The hand of the operator at the point of release is thus near the point where it must next go. Instead of some distance away as it would be if the hand had to return the press lever to the aside position.
Methods Used by Two or More Operators.
If no detailed instruction has been given, in at least 95 per cent of all cases observed by the authors, different operators on the same job will use different methods, even if the operation is fairly simple. The methods will all resemble each other, to be sure, but the trained observer will be able to detect many minor differences, and it is these differences that account for variations in production, fatigue, and quality of work.
As a matter of fact, where no specific instruction regarding proper methods has been given, it is not uncommon to see the same operator using two or three different methods on the same operation. Questioning fails to reveal the reason for this. Most operators do not seem to realize that they are using different methods. They have not been taught to regard their job as a series of elemental motions, and therefore an extra motion or two may be made without conscious recognition.
On repetitive work, considerable difference is found in the output of different operators doing the same operation. The usual tendency is to attribute this to differences in skill and perhaps effort. In reality, however, the difference is usually primarily due to a difference in method. The high producers have the best methods. These may have been developed as the result of long experience, or they may have been hit upon the first day on the job. The low producers have poor methods. These operators may be new to the work, or they may be old operators following a poor method from habit.
With proper operator instruction, this condition will not exist. If the best existing method is first recognized and then taught, all operators but the obvious misfits may be raised to the levels of the highest producers.. This can be done by any supervisor who is able to recognize different methods when he sees them and who realizes the difference that minor variations make. If he is sufficiently interested to decide which of several methods is best and to teach that method in detail to each operator in the department, he can raise the performance level of his department within a short time without any outside assistance.
It must be recognized, of course, that it is not always easy to teach operators new methods. Old methods, because of constant repetition, become habitual, and habits are hard to change. Very often, the easiest and best method will seem harder and slower to an operator than his own method. His production will fall off at first, and he will want to return to his own way of doing things. Patience and persistence on the part of both the operator and his instructor will overcome these difficulties, however, and a better performance and higher earnings will eventually result.
Chairs for Industrial Workers.
The subject of chairs for industrial workers has received a good deal of attention, and most progressive concerns have tried to do something along these lines. Interest in the subject is usually not sustained, however, and therefore the analyst often finds room for improvement. Many chairs designed for industrial use have been placed upon the market, some of which are good.
To minimize fatigue, work should be done alternately seated and standing. Although it is less fatiguing to work seated than standing, even the seated position becomes tiring after long periods of time. Therefore, a workplace arrangement that permits the operator to vary his position from time to time is the best from the standpoint of fatigue.
In order to permit the use of the same motions seated or standing, the height of the chair must be such that the elbows of the operator are the same distance from the floor when he is seated as when he stands. The proper height of the workplace should be determined while the operator is standing.
This is the ideal condition, and like many ideals, is difficult to attain under everyday conditions. Operators vary in size which makes adjustable chairs and even adjustable work-station heights necessary- Where two or more shifts use the same equipment, the problem is further complicated. A tall operator may work a given operation on one shift and a short operator on the next. For example, a certain plant operated a large sewing department on a two-shift basis. When the first shift finished work, all the operators were required to leave the department. Then ; after a signal was given, the second-shift operators entered. The first few minutes were occupied by a confused search for suitable chairs. The sewing machines were all the same height from the floor, and so each operator had to search for a chair that was adjusted so that it would enable her to assume a fairly comfortable working position. Considerable time was lost in starting work, and it was not always possible for an operator to find a suitable chair.
Variable conditions of this sort may best be met by providing equipment that is suitable for a certain size range. Chairs may be adjusted for several classes of operators as very short, short, medium, tall, and very taU. If the chairs are marked as to class and the operator is informed of the class to which she belongs, she will have no difficulty in locating a proper chair at the beginning of the shift, provided that a sufficient number of all classes is available.
The height of the workplace is a point that has received too little attention throughout industry. Benches are made to a standard height. Thus, when an operator stands at the bench, if he is short, he stands on a box or a platform if he can get one. If he is tall, he stoops and as a result has an aching back at the end of the day. Conditions of this sort should be corrected wherever found. A slight change in the height of a workplace will often result in more production of a better quality and a more satisfied and less fatigued operator.
An industrial chair, besides being adjustable for height, should have a wide seat from side to side and an adjustable back rest. If, however, the seat is wide from front to back, many operators will sit on the front edge of the chair and will not use the back rest. This apparently is. because, when one is sitting far back on a wide seat, the front edge of the chair presses the underside of the thighs, cutting off circulation from, the feet and legs and causing general discomfort. A tired back seems preferable, and so operators sit on the front edge of their chairs. This condition may be avoided by providing narrow seats not greater than 13 inches from front to back.
Ejectors and Quick-acting Clamps.
The possibility of improving jigs and fixtures and of providing ejectors, quick-acting clamps, and other time-saving devices should have been considered when the tool equipment was analyzed. The point is so important, however, that it is brought up for consideration again under item 7 of the analysis sheet so that it will not be overlooked. Quick-acting clamps, for example, materially reduce the time required to fix a part in a holding device. Ejectors kick the part out of the holding device and make the removal of the part easier.
Any time that an operation can be performed by parts of the body other than the hands, it should be so done, if there is other work that the hands can perform at the same time. In this way, the hands are relieved of performing certain motions, and time is saved. If, however, there is no other work for the hands to do, there is usually no point in transferring operations to the feet.
The foot-operated drill press is a common example of a foot-operated mechanism. The operator works the drill spindle by a foot pedal, leaving both hands free to place drilled parts aside and to get other parts to be drilled. Foot-operated ejectors are sometimes advantageous, as they leave both hands free to grasp the part as it is ejected. Vises may be opened and closed by foot with a considerable saving of time. When chips or cuttings must be removed from a fixture at the end of an operation, an air jet built into the fixture and controlled by a foot-operated valve may be provided. The possibilities for employing foot-operated mechanisms are many, and the analyst should constantly be on the watch for them.
Two-handed setups which permit the use of motions made simultaneously by both arms moving in
opposite directions over symmetrical paths are highly desirable, because they yield far greater output with the same or less expenditure of energy than do setups on which one hand only is able to work effectively.
Although when two-handed setups are once devised they are fairly simple to operate, it requires considerable ingenuity and a thorough understanding of the principles of motion economy to make them correctly. Two-handed-operation setups are usually made only after detailed motion study. The possibility of making such a setup should be considered during the analysis of all operations, however, for throughout the analysis process, the desirability of a subsequent more detailed study must be kept in mind.
Normal Working Area.
The concept of normal and maximum working areas (a principle of motion economy) has been discussed in under the head of "The Workplace Layout." If the arrangement of tools and material was not considered during the analysis of item 6, it should be studied during the analysis of item 7, for the proper arrangement of the workplace is highly important to effective performance.
Layout Changes and Machine Coupling .
(One operator manning multiple machines)
As the result of detailed analysis, the possibility of coupling machines may have occurred. Machine coupling or multiple machine operation is possible when the operator is idle during part of the operation cycle, usually because a machine is doing the work without attention on his part. The idle time can often be utilized in running another machine if the second machine is located near the first.
If no machine is available near by, it may be desirable to change the layout and move one or more machines about. It is usually best to avoid making many minor layout changes separately, for if all factors affecting the department as a whole are not considered, the layout is likely to become inefficient. Unless a change is obviously desirable and easy to make, it is better to accumulate suggestions for change until sufficient are at hand to make a detailed layout study advisable.
The possibilities for machine coupling are brought out by man and machine process charts. (discussed in another chapter). Plant layout is a study in itself. Some of the issues in related to making layout studies are described in another chapter.
Utilization of Improvements Developed for Other Jobs.
Each operation analysis should not be regarded as an entirely new investigation. Many different operations present points of similarity; if a good method has been worked out for one operation, parts of it may often be applied to another.
Full Knol Book - Method Study: Methods Efficiency Engineering - Knol Book
Updated 4 July 2015
First published 24 Nov 2013