14.1

Spares need to be provided for a critical unit in a system for 500 cumulative hours of operation at a confidence level of 95%. If the unit's failure rate is 0.009 fr/hr, how many spares should be provided for this period of operation?

14.2

Work out Problem 14-1 again but at confidence levels of 80%, 90% and 99%, and discuss your results comparatively.

14.3

Spares need to be provided for a critical unit in a system for 1,000 cumulative hours of operation at a confidence level of 95%. If the storage space for the spares to be provided is limited to 150 units what should be the MTTF of the units be so that the desired confidence level is met?

14.4

Work out Problem 14-3 at the confidence levels of 80%, 90% and 99%, and discuss your results comparatively.

14.5

Identical units have a Weibull times-to-failure distribution with the following parameters: ? = 1.75, ? = 3,000 hr and ? = 0 hr. The replacement policy is as follows: Those units that are found to have failed at 1,500 hr are replaced with units which have a Weibull times-to-failure distribution with the following parameters: ? = 2.5, ? = 1,500 and ? = 0 hr. Do the following:

1. If 1,000 such units are operating in identical equipment at the same application and operation stress level, how many will fail if each one operates T ₁ = 1,500...