Optimisation of the maintenance force in a company producing drop forgings

Abstract

Working conditions in a forge are such that machinery deteriorates rapidly, that maintenance is essential, and that breakdowns are inevitable. It is costly to have forging units idle maintenance because all the maintenance men are already at work elsewhere, but it is also costly to have maintenance men standing idle because all the units are working or already under repair. These two costs must be balanced, to give a minimum total cost. The work reported here studies the pattern of breakdown and repair times in a forge in order to identify the optimum balance.Breakdowns have been considered in two groups: those for which the repair required is less than one working day, and referred to as minor breakdowns; others, referred to as major breakdowns.Based on about 100 observations, the intervals between major breakdowns were found to follow a negative exponential distribution. The repair times also followed a negative exponential distribution. Queuing theory can therefore be used to predict the average number of hammers working/under repair/awaiting repair, for various numbers of repair crews. With the addition of basic cost information, the number of repair crews for minimum cost has been calculated.Most minor breakdowns occur near the beginning of a shift, so queuing theory cannot be applied. Instead, using detailed analyses of the hour-by-hour breakdown pattern, the situation has been simulated on a computer. When the simulation was run with the number of repair crews set equal to the number actually employed, it produced a breakdown pattern very similar to that observed. The simulation was therefore re-run with other numbers of repair crews, and the optimum number of repair crews calculated, as for major breakdowns.The investigation has shown that the patterns of breakdown and repair in a forge correspond to well-established mathematical models and that, provided suitable records are available, it is possible to calculate the optimum number of maintenance workers. When cost figures are introduced, it is apparent that an idle forge is much more expensive than an idle man, and that there should be so many maintenance staff available that only rarely will a hammer be broken down and waiting for repair to start. The shape of the cost/maintenance staff curve is such that the extra cost of having a few men more than the optimum is lower than the cost of having the same number of men fewer than the optimum.