1. RELIABILITY OF MECHANICAL COMPONENTS
The following provides methodology for designing-in reliability into
mechanical components
Cycle Life = Life before wearout
Reliability Of Wear Related Parts
The wear related parts should be tested for 2 x Cycle Life ( or to any equivalent Accelerated
Life) to assure that no wearout failures could occur during the Operational Use (Note 1) of
TBD hours of the unit. At the end of the test, the components should be examined for wear.
Should use industry published failure rate data or Empirical Reliability Models like Naval
Surface Warfare Center (NSWC-06/LE10) to calculate the Reliability. The expected reliability
is TBD
Reliability Of None -Wear Related Parts
For mechanical components subjected to reversing mechanical loads that exhibit a single
failure mode (fatigue, creep, degradation and the like), the reliability should be designed-in by
selecting the probability number representing the Safety Margin (SM) (Note 2)
SM =
( µS − µs )
(σS
2
+ σs
2
Where µs = Mean Stress of the failure governing stress function
)
σs = Standard Deviation of Stress function
µS = Mean Strength of the material
σS = Standard Deviation of the material Strength
This is called the Stress/Strength Interference and the Probabilistic Design method. This
method assumes that the material properties are time independent because of their slow
change. All these components should be designed to have a Reliability better than TBD for
the Operational Use (Note 1) of the unit
---------------------------------------------------------------------------------------------------------Note 1: Operational Use Time = (In House Testing Time) + (Test Time during Storage)
+ (Mission Flight Time)
Note 2: When SM = 3.0, Reliability = 0.99865
When SM = 3.5, Reliability = 0.99977
When SM = 4.0, Reliability = 0.99996
Reliability (R)
=1-
http://www.linkedin.com/in/hilaireperera
Hilaire Ananda Perera
Long Term Quality Assurance
1
2π
∫
SM
−∞
−t2
e 2
dt