Direct Time Study Sections: 1. Direct Time Study Procedure 2. Number of Work Cycles to be Timed 3. Performance Rating 4. Time Study Equipment Direct Time Study - Defined Direct and continuous observation of a task using a stopwatch or other timekeeping device to record the time taken to accomplish the task
While observing and recording the time, an appraisal of the workers performance level is made to obtain the normal time for the task The data are then used to compute a standard time for the task Direct Time Study Procedure 1. Define and document the standard method 2. Divide the task into work elements 3. Time the work elements to obtain the observed time Tobs
4. Evaluate workers pace relative to standard performance to obtain normal time Tn Called performance rating (PR) Tn = Tobs(PR) 5. Apply allowance factor to compute standard time Tstd = Tn(1 + Apfd) Document the Standard Method Determine the one best method Seek workers advice if possible
Documentation should include: All of the steps in the method Special tools, gauges, equipment and equipment settings (e.g., feeds and speeds) if applicable Irregular elements and their frequency Once the standard method is defined, it should not be possible for the operator to make further improvements 13.1
Form to document the standard method Why Documentation is Important Batch production repeat orders after a significant time lapse Methods improvements by operator to restudy task, must be able to prove a change has occurred
Disputes about method operator complains that standard is too tight Is operator using the standard method? Data for standard data system good documentation is essential for developing a standard data system Divide Task into Work Elements Guidelines: Each work element should consist of a logical group of motion elements
Beginning point of one element should be the end point of the preceding element Each element should have a readily identifiable end point Work elements should not be too long nor too short Separate irregular elements, machine elements, internal elements Time the Work Elements
Each element should be timed over several work cycles to obtain a reliable average Stopwatch timing methods: 1. Snapback timing method stopwatch is reset to zero at the start of each work element 2. Continuous timing method stopwatch is allowed to run continuously throughout the duration of the work cycle
Advantages of Each Timing Method Advantages of snapback method: Analyst can readily see how element times vary from cycle to cycle No subtraction necessary to obtain individual element times Advantages of continuous method: Elements cannot be omitted by mistake Regular and irregular elements can be more readily distinguished
Manipulation and resetting of the stopwatch is reduced Performance Rating Analyst judges the performance or pace of the worker relative to the definition of standard performance used by the organization Standard performance PR = 100% Slower pace than standard PR < 100% Faster pace than standard PR > 100% Normal time Tn = Tobs(PR)
13.2 Direct time study form Apply Allowances A PFD allowance is added to the normal time to compute the standard time Tstd = Tn(1 + Apfd) where Apfd = allowance factor for personal time,
fatigue, and delays The function of the allowance factor is to inflate the value of standard time in order to account for the various reasons why the worker loses time during the shift Example A direct time study was taken on a manual work element using the snapback method. The regular cycle consisted of three elements, a, b, and c. Element d is an irregular element
performed every five cycles. Work element Observed time (min) Performance rating a 0.56 100% b 0.25
80% c d 0.50 1.10 110% 100% Determine (a) normal time and (b) standard time for the cycle
Solution (a) Normal time: Tn = 0.56(1.00) + 0.25(0.80) + 0.50(1.0) + 1.10(1.0)/5 = 1.53 min (b) Standard time: Tstd = 1.53(1 + 0.15) = 1.76 min Machine Cycle in the Task If the work cycle includes machine-paced elements, then standard time may include a machine allowance applied to the machine time
Tstd = Tnw(1 + Apfd) + Tm(1 + Am) where Tnw = normal time for worker (external) elements, Tm = machine cycle time (assumed constant), and Am = machine allowance Cycle-to-Cycle Time Variations Result from the following: Variations in hand and body motions Variation in the placement of parts and tools Variations in the quality of the starting work units
Operator mistakes Variations in worker pace Timing errors by analyst Number of Cycles to be Timed After several cycles, calculate sample standard deviation s s= x x
2 n 1 Using the Student t distribution, t / 2 s n= kx 2
Performance Rating Analyst observes and rates the workers performance relative to the definition of standard performance The most difficult and controversial step in direct time study Potential conflict of interest between the worker and the analyst Most common performance rating method is based on speed or pace
Traits of a Good Rating System Consistency among tasks A worker who can perform at 125% on one task should be able to do the same on other tasks Consistency among analysts Easy to explain and easy to understand Based on a well-defined concept of standard performance Rate performance during the observation
Worker notification Time Study Equipment 1. Stopwatch Mechanical stopwatches Electronic stopwatches 2. Video cameras Provides visual and audio record of method used by worker 3. Computerized techniques in direct time study Use of PCs and PDAs
Mechanical Stopwatch Calibrated in decimal minutes Electronic Stopwatch LED read-out
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