FMEA, FTA and design for reliability

Hazard analysis finds what can go wrong, how likely it is and what it causes, so that design changes can remove or reduce it.

FMEA (bottom-up)

For each component list the failure mode, its effect on the system, its cause and current controls. Rate severity (S), occurrence (O) and detection (D) from 1 to 10 and calculate the risk priority number $RPN = S \times O \times D$. Highest RPN items are fixed first.

FTA (top-down)

Start with an undesired top event and trace the combinations of causes down to basic events using logic gates.

        [ Machine stops ]
              |
            (OR)
         /          \
  [Motor fails]   [Power lost]
                       |
                     (AND)
                   /      \
            [Mains off] [Backup fails]

AND gate: all inputs needed; OR gate: any input is enough. Probabilities of basic events give the top-event probability.

Design for reliability

• Use proven components, adequate factor of safety and derating (use below rated load).
• Redundancy (parallel units, standby) for critical functions.
• Simplify: fewer parts mean fewer failures in a series system.
• Fail-safe and fail-soft design, protective devices, good lubrication and sealing, controlled manufacturing and testing.

Maintenance and repair

Preventive maintenance (inspection, replacement) reduces failure rate during useful life. Availability $= \dfrac{MTBF}{MTBF + MTTR}$, so easy access, modular parts and a short repair time (MTTR) raise availability.