The Weibull distribution gives the fraction failed by time t as 1 minus e to the power of minus (t over eta) to the beta. The shape parameter beta tells which failure mode is at work, and the scale parameter eta, the characteristic life, is the time by which 63.2 percent of units have failed whatever the value of beta. Plotted on Weibull probability paper, data that fit fall on a straight line whose slope is beta.
Weibull fit on the pump bearings gives beta 2.8 and eta 14,000 hours, so it's wear-out. Move the PM interval up.
With fewer than about 5 to 10 failures the slope estimate is shaky; Weibayes, which borrows a beta from past experience, is the usual fallback.
Also heard as
- Weibull
Related terms
Shape parameter
Weibull beta: the slope of a Weibull plot, which shows whether failures are early-life, random or wear-out.
Censoring
Incomplete life data where a unit's exact failure time is unknown because the test ended or the unit was removed first.
Bathtub curve
Plot of failure rate over a product's life: falling early failures, a flat random period, then rising wear-out.
MTTF
Mean time to failure: expected life of a non-repairable item, the average time until its first and only failure.
Dogleg
Bend in a Weibull plot where the data change slope, a sign of two failure modes or two populations mixed together.