RELIABILITY Reliability is - the ability to continue to be fit for purpose or function.

Assure reliability by: Use proven designs. Use the most simple design possible i.e. few components with simple designs. Use components with a known or high probability of survival.

Assure reliability by: Employ redundant parts where there is a likelihood of failure. Design to 'fail safe'. Specify proven and capable methods of manufacture.

Failure. Types of Failure: Total failure - results in the complete loss of ability to perform the required task. Partial failure - results in the task not performing to expectations.

Types of Failure cont. Gradual failure - progressive failure over a period of time, could and should be expected and discovered by examination / inspection. Sudden failure - unexpected failure, not easily detected by examination / inspection.

Causes of Failure: Weakness inherent in the product or component when subject to the normal stresses of use. May be due to poor design, choice of materials, poor manufacturing performance or incapable machines / processes.

Causes of Failure: Misuse the application of stresses that are outside the usual capability of the product or component.

FMECA Failure Mode, Effect and Criticality Analysis FMECA is used to determine the features of product or component design and the manufacturing processes or operations that are critical to the various modes of failure.

FMECA FMECA uses all available expertise and experience from: marketing, design, technology, purchasing, production, distribution, service, etc..... and customer feedback, to identify the importance levels or criticality of potential problems and to stimulate action which will reduce these levels.

The elements of the analysis Failure Mode - How will it fail ? Considers the anticipated operating conditions used to study the most probable failure mode.

The elements of the analysis Failure Effect - What will happen ? potential failures are studied to determine the probable effect on the performance of the whole product or service and the effects of the various components on each other.

The elements of the analysis Failure Criticality - How serious ? potential failures in each part of the product or service are studied to determine the severity of each failure effect; in terms of lowering performance, safety, total loss of function, etc...

The elements of the analysis FMECA uses a simple rating system (1 - 10) for each potential failure which could occur in a product, component or service.

The elements of the analysis Each potential failure is rated with regard to: P probability of each failure occurring (1=low, 10 =high) S seriousness or criticality of the failure (1=low, 10=high) D difficulty of detecting the failure (1=low, 10=high) C criticality, C = P x S x D

FMECA Method 1. Identify all components. 2. List all failure modes for each component. 3. List failure effects for each failure mode. 4. List causes of each failure. 5. Assess failures in terms of P, S, & D. 6. Calculate the criticality, C, for each failure. 7. Rank all failures in terms of C. 8. Indicate Corrective Action for each failure.

FMECA Analysis Analysis headings.

Example - Anti-Roll Bar

Part FMECA for Anti-Roll Bar

Measures of Reliability A number of measures of reliability exist. Four of the most common follow.

Measures of Reliability R(t) The reliability, in terms of the probability of a product to still function after time t. Number surviving at time t R(t) = ------------------------------------ Number that existed at time = 0

Measures of Reliability F(t) The cumulative distribution of failure. Cumulative number of failures by time t F(t) = -------------------------------------------- Number that existed at t=0

Measures of Reliability f(t) The probability density of Failure. Number failing in unit time at time t f(t) = -------------------------------------------- Number that existed at time t = 0

Measures of Reliability (t) The Failure or Hazard rate. Number failing in unit time at t (t) = ------------------------------------------ Number surviving at time t

Life table for 150 light bulbs Operating Period (Months) Number operating at start of period 0 - 1 150 1 - 2 123 2 – 3 108 3 - 4 99 4 - 5 91 5 - 6 84 6 - 7 78 7 - 8 74 8 - 9 69 9 -10 65 10 -11 60 11 -12 57 12 -13 54 13 -14 51 14 -15 49 15 -16 47 16 -17 44 17 -18 42 18 -19 40 Operating Period (Months) Number operating at start of period 19 -20 38 20 -21 35 21 -22 32 22 -23 29 23 -24 26 24 -25 24 25 -26 22 26 -27 20 27 -28 17 28 -29 15 29 -30 13 Calculate values for R(t), F(t), f(t) and (t) for each month between 0 & 30.

R(t) and F(t) Curves

f(t) and (t) Curves

(t) - Smoothed Bath Tub Curve

The bath-tub curve. The bath-tub curve can be considered to have three distinct parts: 1. The ‘infant’ or early failure phase, (failure rate decreases rapidly). 2. The ‘adult’ or ‘useful life’ phase, (failure rate is almost constant). 3. The ‘ageing’ or ‘wear out’ phase, (failure rate increases).

Manufacturers strategy. 1. The ‘infant’ phase should be as short as possible. Manufacturers should understand how long this period is. Manufacturers should ensure that the ‘test’, ‘burn-in’, ‘run-in’ or ‘load’ period for the product should encompass this phase if possible.

Manufacturers strategy. 2. The ‘adult’ phase should be long enough to meet the customer’s expectation of reliability. This period exhibits a constant value of failure rate for the useful life of the product (or component). Manufacturers will quote ‘failure rates’ for this period only. Warranty, or Guarantee, periods are normally determined with reference to this phase.

Manufacturers strategy. 3. The ‘ageing’ phase gives an indication to the manufacturer (or supplier) of the product as to when service, repair or replacement is likely to be required. This information can be used in forecasting future demand on spares production, service engineering resources and increased manufacturing requirements.

Mean Time Between Failures (MTBF). The MTBF figure quoted for a product is the reciprocal of the constant failure rate. Evaluation time period MTBF = --------------------------------------------- Number of failure in the time period

Mean Time Between Failures (MTBF). For example: If during a period of 10 months 25 of the 150 light bulbs fail the MTBF for the light bulbs will be; 10 months MTBF = ------------------------- 25 failures = 0.4 months (or 2.5 failures per month)