NSWC - Mechanical Reliability Prediction

The Handbook of Reliability Prediction Procedures for Mechanical Equipment (NSWC-06) was originally released in the 1980's by the Naval Surface Warfare Center. The development of this handbook was coordinated with the military, industry and academia. It was developed to present a proposed methodology for predicting the reliability of mechanical equipment.

Following are some excerpts from the standard:

Due to the wide dispersion of failure rates, which occur for apparently similar components, mechanical reliability predictions do not rely on failure rate data alone. It also takes into consideration the material properties, operating environment and critical failure mode at the component level. Failure modes such as corrosion, erosion, creep and fatigue can simultaneously impact a mechanical component, causing an adverse effect on the reliability. This handbook provides prediction models that take all of these factors into consideration.

Individual mechanical devices often perform more than one function, and failure data for a specific application may not be available. Valves for example, may contain a manual shut-off function, as well as an automated control ability.

Failure rates of mechanical devices do not normally follow a constant failure rate distribution, due to wear, fatigue, and other stresses which degrade the equipment. Failure date reliability is complicated when a constant failure rate distribution cannot be assumed. Individual failure times must be recorded in addition to total operating hours and failures.

Mechanical equipment reliability is more sensitive to loading, operating mode, and utilization rates than electronic equipment reliability. Failure data based upon operating time alone is usually inadequate.

The definition of a failure of mechanical equipment will depend upon its application. Failure due to excessive noise or leakage cannot be modeled generically. Lack of a clear definition limits the usefulness of the data.

The approach to predicting reliability of mechanical equipment used in the Handbook considers the intended operating environment and determines the effect of that environment at the lowest level where the material properties can be considered. Combining these factors permits the use of design parameters to determine the design life of the equipment in the operating environment and the rate and pattern of failures.

Models developed in the handbook are based upon identified failure modes and their causes. Equations were derived for each failure mode from design and published experimental data. From these equations the failure rate per hour or per cycle is determined. Limitations of the models are also presented in the Handbook.

For example, a Poppet Valve assembly will have an equation and parameters as follows:

Failure rate = bfr * Cp * Cq * Cf * Cv * Cn * Cs * Cdt * Csw * Cw failures/million operations

Where:

bfr = Base failure rate as determined by the handbook
Cp = Fluid pressure factor
Cq = Allowable leakage factor
Cf = Surface finish factor
Cv = Fluid viscosity factor
Cn = Contaminants factor
Cs = Seat stress factor
Cdt = Seat diameter factor
Csw = Seat land width factor
Cw = Fluid flow rate factor

Validation of the Handbook equations was performed by a variety of government and academic organizations to a limited extent. Not all potential operating conditions and applications were able to be tested.