Lecture 11

Predetermined Motion Time System

A predetermined motion time system (PMTS) may be defined as a procedure that analyzes any manual activity in terms of basic or fundamental motions required to perform it. Each of these motions is assigned a previously established standard time value and then the timings for the individual motions are synthesized to obtain the total time needed for performing the activity.

The main use of PMTS lies in the estimation of time for the performance of a task before it is performed. The procedure is particularly useful to those organizations which do not want troublesome performance rating to be used with each study.

Applications of PMTS are for

(i) Determination of job time standards.

(ii) Comparing the times for alternative proposed methods so as to find the economics of the proposals prior to production run.

(iii) Estimation of manpower, equipment and space requirements prior to setting up the facilities and start of production.

(iv) Developing tentative work layouts for assembly lines prior to their working in order to minimize the amount of subsequent re-arrangement and re-balancing.

(v) Checking direct time study results.

A number of PMTS are in use, some of which have been developed by individual organizations for their own use, while other organizations have developed and publicized for universal applications.

Some commonly used PMT systems are:

  • Work factor (1938)
  • Method Time Measurement (1948)
  • Basic Motion Time (1951)
  • Dimension Motion Time (1954)

Important considerations which may be made while selecting a PMT system for application to particular industry are:

  1. Cost of Installation. This consists mainly of the cost of getting expert for applying the system under consideration.

  2. Application Cost. This is determined by the length of time needed to set a time standard by the system under consideration.

  3. Performance Level of the System. The level of performance embodied in the system under consideration may be different from the normal performance established in the industry where the system is to be used. However, this problem can be overcome by 'calibration' which is nothing but multiplying the times given in the PMT Tables by some constant or by the application of an adjustment allowance.

  4. Consistency of Standards. Consistency of standards set by a system on various jobs is a vital factor to consider. For this, the system can be applied on a trial basis on a set of operations in the plant and examined for consistency in the so obtained operation times.

  5. Nature of Operation. Best results are likely to be achieved if the type and nature of operations in the plant are similar to the nature and type of operations studied during the development of the system under consideration.

Advantages and limitations of using PMT systems

Advantages

Compared to other work measurement techniques, all PMT systems claim the following advantages:

  1. There is no need to actually observe the operation running. This means the estimation of time to perform a job can be made from the drawings even before the job is actually done. This feature is very useful in production planning, forecasting, equipment selection, etc.
  2. The use of PMT eliminates the need of troublesome and controversial performance rating. For the sole reason of avoiding performance rating, some companies have been using this technique.
  3. The use of PMT forces the analyst to study the method in detail. This sometimes helps to further improve the method.
  4. A bye-product of the use of PM times is a detailed record of the method of operation. This is advantageous for installation of method, for instructional purposes, and for detection and verification of any change that might occur in the method in future.
  5. The PM times can be usefully employed to establish elemental standard data for setting time standards on jobs done on various types of machines and equipment.
  6. The basic times determined with the use of PMT system are relatively more consistent.

Limitations

There are two main limitations to the use of PMT system for establishing time standards. These are: (i) its application to only manual contents of job and (ii) the need of trained personnel. Although PMT system eliminates the use of rating, quite a bit of judgment is still necessarily exercised at different stages.

Physiological Methods for Work Measurement

The physiological cost to an operator of performing any given physical work results from the activities of the muscles of arms, legs, back and other parts of the body and is, therefore, affected by the number and type of muscles involved in either moving the body member(s) or controlling antagonist contraction.

The activities of body muscles cause changes in oxygen consumption, heart rate, body temperature, lactic acid concentration in blood, 17-ketosteroid excretion in urine, pulmonary ventilation, and other factors. Studies have shown that some of these factors are only slightly affected by muscular activity. The important factors which have linear correlation with the physiological cost of work performed by an individual are oxygen consumption, heart rate, and pulmonary ventilation.

Increase in Heart Rate

When a person is at rest, his heart rate is at a fairly steady level (generally at about 70 beats/minute). Then when he starts doing some muscular work his pulse rate increases rapidly to about 110 beats/minute and remains near to this level during the working period. When work ends, the recovery begins and his heart rate drops off and finally returns to the original resting level ( Figure ).

The increase in heart rate during work has been used as an index of the physiological cost of the job. Some physiologists have also proposed the use of 'the rate of recovery immediately after work stops' for the evaluation of physiological cost of certain types of work. It is to be noted that the total physiological cost of a task consists of the energy expenditure during work and the energy expenditure above the resting rate during the recovery period. It is generally agreed that the optimum limit of industrial performance is reached when the average pulse rate during the work lies 30 beats/minute above the resting pulse rate.

Measurement. With every heart beat, a small electric potential is generated. This signal can be picked up by placing silver electrodes on either side of the chest, and transmitted to a receiver, where these can be counted directly or recorded continuously on a ruled graph paper or integrated over time to measure in units of beats per minute with the help of a cardiotachometer.

Another method of getting the heart beat signals is through the use of an ear lobe unit, which is a photo duodiode with a light source. This unit is mounted on an ear of the subject in such a way that the duodiode is on one side and the light source is on the other side of the ear. As the capacity of the ear lobe changes due to the blood surges through the ear with beats of the heart, impulses are created which are transmitted and recorded.

A simple method to get the heart beat rate is through the use of a stethoscope and stop watch. Studies have shown that the data obtained in this manner are fairly reliable and also easy to obtain.

Oxygen Consumption. It may be defined as the volume of oxygen which a person extracts from the air he inhales. Increase in the rate of oxygen consumption from the resting level to the working level is also taken as a measure of the physiological cost of the work done. The oxygen consumption per unit time is usually measured indirectly. To do this the volume of air exhaled by a person in a certain time is collected and the oxygen content of this air is determined. For this, use is made of a portable respirometer. It is a lightweight gas meter which is worn on the back of the subject. A mask is fitted on the face of the subject, and the exhaled air is collected in the respirometer through a rubber tube. The respirometer directly shows the volume of exhaled air in litres.

A sample of the exhaled air is taken out at random intervals into a rubber bladder and an analysis is carried out of its content. Comparison is then made between the oxygen content of the two samples-drawn from the exhaled air and another from the room air. For each litre of oxygen consumed by the human body, there is an average energy turnover of 4.8 Kcal.

Table gives the general values of oxygen consumption, lung ventilation, rectal temperature and heart beats at the different work loads.

Physiological measurements can be used to compare the energy cost to the operator on a job for which no time standard exists, with the energy cost to the same operator on a similar operation for which a satisfactory time standard already exists. By this comparison it is possible to establish the time standard on the job for which it does not exist already. For the sake of illustration, consider a job of lifting boxes weighing 2-3 kgs. from the floor level and placing it on a conveyor belt. For this job a time standard of 6 seconds (10 boxes/min.) is being used. When energy measurements were taken, it was found that to Mr. Singh, the operator on the job, the energy cost of this job was 300 W. Let us suppose now that there is another jab, similar to the first one, with the difference that here, the weight of the boxes is 5-6 kgs. If it is required to establish the t ime standard for this job, we need Mr. Singh to do this job of handling 5-6 kg. boxes at various speeds. From the energy cost data collected on him, we can select the speed of working that gives an energy cost of 300 W. So, by keeping the energy cost of the two jobs same, the time standard (the number of 5-6 kg. boxes/min.) can be determined.

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