The failure rates of certain mechanical components used in solenoid valves, actuators, and valves vary substantially depending on operation. Seals such as O-rings, for example have fundamentally different failure modes when used in applications with frequent movement (dynamic) versus applications with infrequent movement (static). 

Static is generally defined as “stationary or fixed.”  We at exida define a mechanically static application as a low demand mode operation. Dynamic is generally defined as “energetic, capable of action and/or change, or forceful.”  exida defines a mechanically dynamic application as a high or continuous mode operation. 

Mechanical failure data can be found by four main sources:

1. Failure Mode Effects and Diagnostic Analysis (FMEDA)
2. End-User Field Failure Studies
3. Manufacturer Field Return Data Studies
4. Cycle Test Data

Each source has its own positive aspects, as well as negative. 

FMEDAs

This will identify the failure modes within the device which could relate those failures to the operation of the safety instrumented function. 

   PLUS:

1. Can provide detailed / complete information
2. Predictive – does not require a multi-year lag time to gather statistics

   MINUS:

1. Depends on component failure mode/failure rate data

FMEDAs are safely used in both static and dynamic applications, or in high and low demand. 

End-User Field Failure Studies

  PLUS:

1. Real Data

  MINUS:

1. Variations of amount of data collected
2. Different definitions of “FAILURE”
3. Categorizing and Merging Technologies

Manufacturer Field Return Data Studies

  PLUS:

1. Real Data

  MINUS:

1. Calculation methods vary widely
2. Cannot know what percentage of actual failures are returned
3. Different definitions of “FAILURE” 

Cycle Test Data

Cycle testing the accelerated motion testing until either a failure or a given number of cycles used to simulate years. 

  PLUS:

1. Helps establish end of life value

  MINUS:

1. Assumes application has constant dynamic operation

Since cycle testing has the assumption of constant dynamic operation, it is for high demand applications only; not for static or low demand.  This assumption stems from premature wear-out being the dominant failure mechanism and that no other failure mechanisms are significant.

Low demand or static applications have hazards that high demand applications do not.  While cycle testing is an acceptable failure rate prediction technique for high or continuous demand applications, using it for low demand is DANGEROUS.  Low demand applications can generate failures that high demand applications cannot develop since they are in an inactive state for long periods of time.  Issues such as stiction can transpire which are not accounted for in a dynamic application.  So if your application is a static application, rule out the dangers and choose a mechanical failure mode data source that can support your operation.