Rui Assis
February 6th, 2007, 06:27 PM
Another problem…
Suppose an item that presents two different failure modes: one can be described by a Weibull distribution (lambda = 250 hours; beta = 4; eta = 800 hours) and the other by an exponential distribution (MTTF = 1,000 hours). The item exists in 6 pieces of equipment and each one of these has 4 units of that item, totalling 24 units. Each piece of equipment operates 15 hours a day in average. Preventive maintenance consists of replacing each item by a new one and its periodicity was set at 700 hours. Whenever an item fails, it takes in average 15 hours to replace it. When preventive maintenance is accomplished, it takes an average of 10 hours instead. The item is purchased from a distant supplier who takes an average of 1.5 month to deliver any quantity what so ever.
Parts needed for preventive actions are ordered just in time, (not kept permanently in stock). Only parts that are needed as a result of random failures or wear out failures (that small number that doesn’t live long enough to be replaced preventively at 700 hours), are kept in stock (just in case).
If a service level of 0.99 is aimed, which must the reorder point be?
I solved this problem in Excel and I obtained 21 units as the reorder point. I tried to enter these data in BS and became lost. I got the impression that two or more failure modes cannot be modelled using blocks in BS. Am I right? If not, how should I do?
Thanks in advance,
Rui
Suppose an item that presents two different failure modes: one can be described by a Weibull distribution (lambda = 250 hours; beta = 4; eta = 800 hours) and the other by an exponential distribution (MTTF = 1,000 hours). The item exists in 6 pieces of equipment and each one of these has 4 units of that item, totalling 24 units. Each piece of equipment operates 15 hours a day in average. Preventive maintenance consists of replacing each item by a new one and its periodicity was set at 700 hours. Whenever an item fails, it takes in average 15 hours to replace it. When preventive maintenance is accomplished, it takes an average of 10 hours instead. The item is purchased from a distant supplier who takes an average of 1.5 month to deliver any quantity what so ever.
Parts needed for preventive actions are ordered just in time, (not kept permanently in stock). Only parts that are needed as a result of random failures or wear out failures (that small number that doesn’t live long enough to be replaced preventively at 700 hours), are kept in stock (just in case).
If a service level of 0.99 is aimed, which must the reorder point be?
I solved this problem in Excel and I obtained 21 units as the reorder point. I tried to enter these data in BS and became lost. I got the impression that two or more failure modes cannot be modelled using blocks in BS. Am I right? If not, how should I do?
Thanks in advance,
Rui