PMT Systems are methods of setting basic times for doing basic human activities necessary for carrying out a job or task.
The definition in BS 3138, Glossary of Terms Used in Work Study is: 'Tables of time data at defined rates of working for classified human movements and mental activities. Times for an operation or task are derived using precise conventions. Predetermined motion time data have also been developed for common combinations of basic human movements and mental activities'.
The principle of analyzing work into into basic actions was first published by F. Gilbreth in 1920, as his Therbligs. The first commercial and internationally recognized system was devised in the 1930's to circumvent the banning by the government of the United States time study and the stop-watch as the means of measuring work performed on US government contracts. It was devised by Quick, Malcolm and Duncan under the title Work-Factor and appeared in 1938. Other methods followed, the main one, some ten years later, being Methods-Time Measurement (MTM). Both systems share basic similarities but are based on different standards of time.
The concept of PMTS is to analyse a job into its fundamental human activities, apply basic times for these from tables and synthesize them into a basic time for the complete job. The basic elements include the following:
other elements for assembling to, or inserting an object into, its intended location.
For each of these actions basic times are tabled. For example, in Work-Factor the time unit is one thousandth of a minute (the Work-Factor Time Unit) whereas in MTM the unit is one hundred-thousandth of an hour (time measurement unit, tmu).
The times for basic actions are adjusted for other factors which take into account such variables as:
The above basic motions cover most of the actions performed by humans when carrying out work. Other basic activities include:
Mental activities include times for: See, Inspect, Identify, Nerve Conduct, React, Eye focus, Eye travel times, Memorize, Recall, Compute (calculate) and others, mostly from Work-Factor.
In order to speed up measurement time the major systems all include different levels of detail, such as:
One example of simplifying in the second level system MTM-2 is the combining of MTM elements reach, grasp and release to produce a new MTM-2 element of "Get".
PMTS is often used to generate synthetic data or (standard data banks) which are overall basic times for more complex tasks such as maintenance or overhauling of equipment. This is achieved by synthesizing the hundreds of small jobs measured using PMTS into a time for the complete project.
Basic times produced by PMTS need to have relaxation allowances and other necessary allowances added to produce standard times.
An example of part of a typical analysis in MTM-2 is given in the Appendix.
An extract from an MTM analysis showing the first seven elements.
|Job description:||Analyst:||E J H|
|Assemble r.f. transformer to base-plate||Date:||3 May|
|1||Move hand to washer||R14C||15.6||R14B||Move hand to transformer|
|2||Grasp first washer||G4B||9.1||G1A||Grasp transformer|
|3||Move hand clear of container||M2B||---||---||Hold in box|
|5||To second washer||R2C||5.9||---||ditto|
|7||Move washers to area||M10B||16.9||M14C||Transformer to plate|
The codes in the LH and RH columns refer to those in the MTM time tables. For example: R14C is translated as "Reach 14 in. to an object jumbled with other objects in a group, so that search and select occur" (Class C reach). R14B is translated as "Reach 14 in. to a single object in location which may vary slightly from cycle to cycle." G2 is a grasp Case 2 which is a Regrasp to move the washer into the palm G4B is a Grasp Case 4B which is for grasping *object jumbled with other objects so search and select occur. Objects within the range 0.25 x 0.25 x 0.125 in. to 1 x 1 x 1 inch."
One tmu is one hundred-thousandth of an hour