---
title: "FMEA, FTA and design for reliability"
subject: "Machine Design II"
chapter: 4
key: "semester_7/machine_design_ii:hazard-analysis-fmea-fta"
source: https://www.bctengineer.com/q/semester_7/machine_design_ii/hazard-analysis-fmea-fta
site: BCT Engineering
---

# FMEA, FTA and design for reliability

Machine Design II · Chapter 4: Risk and Reliability of Design

- Asked in: year not given, 6 marks
- Video: [Mastering FMEA | Failure Modes and Effects Analysis Explained | Lean Six Sigma| Anexas | FMEA Guide](https://www.youtube.com/watch?v=EaD1qgsRNr0) — Anexas
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/machine_design_ii/hazard-analysis-fmea-fta
- Whole chapter: https://www.bctengineer.com/md/semester_7/machine_design_ii/chapter-4

## Question

Explain failure mode and effects analysis (FMEA) and fault tree analysis (FTA) as hazard analysis tools. Also state the measures used in design for reliability and the importance of maintenance and repair.

## Short answer

FMEA (bottom‑up) rates severity, occurrence, detection to compute RPN and prioritises fixes. FTA (top‑down) uses AND/OR gates to trace top events to basic causes. Design for reliability uses derating, redundancy and simplicity. Maintenance raises availability = MTBF/(MTBF+MTTR) by shortening repair time.

## Answer

**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.

| Part | Failure mode | Effect | S | O | D | RPN |
|---|---|---|---|---|---|---|
| Brake hose | Leak | Loss of braking | 9 | 3 | 4 | 108 |

#### FTA (top-down)
Start with an undesired **top event** and trace the combinations of causes down to basic events using logic gates.
```text
        [ 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is the risk priority number in FMEA calculated?
   A. Sum of severity, occurrence and detection
   B. RPN = S x O x D, each rated 1 to 10
   C. Severity divided by occurrence
   D. Occurrence x detection only
   Answer: B. RPN = S x O x D, and the highest RPN items are fixed first. (Key point: RPN)

2. How does fault tree analysis proceed?
   A. Bottom-up from each component's failure mode
   B. From cost to design
   C. Top-down, starting with an undesired top event and tracing causes with logic gates
   D. By rating severity from 1 to 10
   Answer: C. FTA is top-down from the top event to basic events. FMEA is bottom-up. (Key point: FTA direction)

3. What does an AND gate in a fault tree mean?
   A. Any one input is enough
   B. The inputs are added in probability
   C. The event never occurs
   D. All inputs must occur for the output event
   Answer: D. An AND gate needs all inputs, while an OR gate needs only one. (Key point: Logic gates)

4. How is availability raised, using Availability = MTBF/(MTBF + MTTR)?
   A. Increase the mean time to repair
   B. Shorten the repair time through easy access and modular parts
   C. Reduce MTBF
   D. Stop preventive maintenance
   Answer: B. A lower MTTR increases the ratio, and preventive maintenance also reduces failure during useful life. (Key point: Availability)

## Related in Machine Design II

- [Exponential reliability and system reliability](https://www.bctengineer.com/question.md?key=semester_7%2Fmachine_design_ii%3Areliability-exponential-series-parallel) — Machine Design II, chapter 4
- [Reliability from stress-strength interference](https://www.bctengineer.com/question.md?key=semester_7%2Fmachine_design_ii%3Astress-strength-interference-reliability) — Machine Design II, chapter 4
- [Internal expanding shoe brake analysis](https://www.bctengineer.com/question.md?key=semester_7%2Fmachine_design_ii%3Ainternal-expanding-shoe-brake) — Machine Design II, chapter 7

## Related in other subjects

- [Importance of repair and maintenance](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Amaintenance-importance-prolong-life) — Construction Management, chapter 9, asked 2×
- [Fault tolerance and redundancy for reliability](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Afault-tolerance-schemes) — Instrumentation II, chapter 7, asked 4×
- [Remedial measures for slope failure](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aslope-remedial-measures) — Soil Mechanics, chapter 11
- [Maintenance vs rehabilitation; flexible failure causes](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Amaintenance-vs-rehabilitation-flexible-failure-causes) — Transportation Engineering II, chapter 4