Page 284 - DMGT206_PRODUCTION_AND_OPERATIONS_MANAGEMENT

Unit 14: Operations Scheduling

‘thrash’. ‘Thrashing’ is the problem of the system keeping itself busy re-planning rather than Notes
producing, which effectively reduces capacity. This results in problems similar to mainframe
computers where this problem was first documented, lots of expediting, customer panics and
increased changeovers. The result is an effective reduction in batch sizes, as you spend more
time changing over than producing due to batch splitting.
Figure 14.1 shows a gradual increase in lead time as load increases up to the point where
‘thrashing’ occurs and where lead times go through the roof.

14.1.7 Input-output Control

Output plans and schedules call for certain levels of capacity at a work center and those jobs are
completed at a specific time on every facility. In a real world situation, the utilization of the
capacity of a facility may be different from the plans – under-utilization of capacity means
wasted resources and over-utilization may cause disruption, failure, and delays. These differences
are monitored through input-output reports.
Figure 14.2: Thrashing due to ‘Failure to Adjust’

These reports form the basis for adjustments to the schedule. If arrears are not rescheduled in
conjunction with the customer, they create a short-term overload of equal due dates, which
induce ‘thrashing’. The customers are still expecting their order, unless the adjustments are
communicated to customers.

Figure 14.2 shows the impact of arrears on scheduling causing ‘thrashing’. The solution is to
reschedule arrears, in conjunction with customers and manage lead times.

14.1.8 Non-cyclic Personnel Schedules

Demand variations are often caused by trend and seasonal factors, holidays, etc. Depending on
the nature of the particular operations, suppose that we are faced with labor requirements that
vary from hour to hour, day to day, week to week, and so on. Staffing this operation would
require continuous adjustment to the changing requirements. These types of personnel scheduling
use the ‘first-hour’ principle. The principle can be stated as follows:
“Assign the work in the first period to a number of workers equal to the number required for that
period. For each subsequent period, assign the exact number of additional workers needed to meet
requirements. When workers come to the end of their shifts, do not replace them if they are not
needed”.
This procedure is best exampled with the aid of an example. The sequence of worker requirements
for the first 12 hours of a continuous operation (one assigned, workers continue working for an
8-hour shift) are given in Rows ‘1’ and ‘2’ of Table 14.1.

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