Predetermined time systems: logic and limits of use
In one sentence
Predetermined time systems combine defined motions with times tied to their conditions; the original training table in this lesson does not produce a real work standard.
By the end of this lesson
- Combine the training times of eight motions with running subtotals to reach 5.6 s.
- Calculate the five-motion option as 4.2 s, taking the new transport condition into account.
- Separate the reduction in motions from the reduction in time and find the 25% difference in training time.
- Reject the decision that derives a real standard time or a worker target from a synthetic training table.
Why it matters
The method study in M3 taught how to question why and in what sequence the work is done. Motion economy also made unnecessary reaching, transport and regrasping visible. The general logic of predetermined time systems is to analyze a division of work by using time values that fit the conditions of defined motions. In this approach the name of the motion alone is not enough. The distance reached, how the object is presented, how hard the grasp is, the need for positioning and the relationship among motions all affect the result. Defining the work wrongly ties a correctly added total to the wrong work. This is why method study comes first; a time table is not a tool that proves a method is necessary.
In this lesson the values or classification of no outside table are used. All the times are original values produced only for training. The aim is to see the relationship between the definition of a motion and the logic of adding. These numbers are not the normal time, standard time or worker target of a real job. That the table is numerically consistent does not mean it is valid under real conditions. The seconds found at the end of the calculation show the time in the training model. Writing the same motion name onto a real job and applying this value goes beyond the scope. Keeping within the limit of the training model is as important a gain as adding correctly.
Measuring an unnecessary motion does not make it necessary. Regrasping a part set down on an intermediate surface can enter a tidy time calculation; but method study asks why that intermediate set-down is needed. Merely coding every motion in finer detail does not improve the method. On the other hand, removing one motion does not guarantee that the rest will take the same time under the same conditions. When the intermediate surface is gone, the direct transport may become longer or the orientation of the part may change. The safety, quality condition and feasibility of the new arrangement must be preserved. Deleting the old rows and simply adding up the remaining times can overlook the changed condition of the motion. On both sides of the comparison the same start and completion state are required.
The times belonging to motions do not show that elapsed time and a person's total activity time are the same in every case. Consecutive motions are added; if there are safe, truly simultaneous motions, the time relationship is resolved separately. The automatic running of a machine does not replace the table of human motions either. In this lesson's example there are only the consecutive motions of one hand; there is no automatic time, allowance or performance adjustment. Keeping the scope narrow makes the logic of the calculation clear. The measurement and verification needed in the move to a real job cannot be obtained by adding arbitrary coefficients on top of this training total. In the result report, the model assumption and the observed evidence must be clearly separated.
Formula
Consecutive training time (s) = Sum, over each motion, of Number of repetitions × Training time
Training time difference (s) = Old model total − New model total
Training time reduction (%) = Difference ÷ Old model total × 100
New model total (s) = Old total − Removed motions − Old time of the changed motion + New time of the changed motion
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- Step-by-step instructions
- Worked example with real numbers
- Where the method misleads
- Common mistakes