Mean Time Between Failures (MTBF) — definition
Average operating time expected between one failure of a repairable system and the next, used as a reliability indicator for equipment and components.
MTBF is calculated from historical failure data or supplier reliability testing and is most meaningful when paired with mean time to repair (MTTR) to estimate overall equipment availability. It applies to repairable assets; for non-repairable components, mean time to failure is the equivalent measure.
Why it matters to industrial buyers
MTBF figures inform spare parts strategy, maintenance budgeting and the reliability assumptions used in total cost of ownership comparisons between competing equipment offers.
Key reference points
Related metric
MTTR (mean time to repair) combines with MTBF to estimate availability.
Data source
Often derived from field failure logs, supplier reliability tests or industry reliability databases.
Commonly confused with
Mean time to failure (MTTF)
MTTF applies to non-repairable items that are replaced rather than repaired after failure.
How it is used in practice
The technical specification requested MTBF data for the pelleting die to compare reliability across two competing machine suppliers.
MTBF is one of the few reliability numbers a buyer can actually compare across competing offers — provided both suppliers state the measurement basis. This section covers how to calculate it, how to convert it into availability and lost output, what values are typical, and where it belongs in a capital equipment decision.
MTBF formula and a worked calculation
MTBF = total operating time ÷ number of failures. Operating time counts only the hours the asset was scheduled to run; planned maintenance, changeovers and idle shifts are excluded, which is why two suppliers can quote very different figures for the same machine.
- Feed pelleting line, one year: 6,000 scheduled operating hours, 8 unplanned stoppages → MTBF = 6,000 ÷ 8 = 750 hours.
- Repairs consumed 60 hours in total → MTTR = 60 ÷ 8 = 7.5 hours.
- Availability = MTBF ÷ (MTBF + MTTR) = 750 ÷ 757.5 = 99.0%.
- Annual unplanned downtime = 6,000 × (1 − 0.990) ≈ 60 hours. At 12 t/h and a €90/t margin, that is roughly €65,000 of lost contribution per year.
Always ask whether a quoted MTBF is measured field data, an accelerated test result, or a calculated prediction from component failure rates — the three are not comparable.
Turning MTBF into an availability target
Buyers rarely need a specific MTBF; they need an availability outcome. Fix the availability you require, estimate realistic MTTR from your own spare parts and technician coverage, then derive the minimum MTBF you can accept.
| Target availability | MTTR 4 h | MTTR 8 h | MTTR 24 h |
|---|---|---|---|
| 95% | 76 h | 152 h | 456 h |
| 98% | 196 h | 392 h | 1,176 h |
| 99% | 396 h | 792 h | 2,376 h |
| 99.5% | 796 h | 1,592 h | 4,776 h |
Minimum MTBF = MTTR × availability ÷ (1 − availability). The table shows why cutting repair time — local spares, trained operators, remote diagnostics — is often cheaper than buying a more reliable machine.
Indicative benchmarks by asset type
Published reliability data varies widely by duty cycle, ambient conditions and maintenance discipline, so treat the ranges below as orientation for specification discussions rather than guarantees. Ask each supplier for their own field data on comparable installations.
| Asset | Typical MTBF range | Usual failure driver |
|---|---|---|
| Industrial pumps (process duty) | 8,000–25,000 h | Seals and bearings |
| Screw compressors | 10,000–30,000 h | Air end wear, oil condition |
| Refrigeration compressor packs | 15,000–40,000 h | Controls, valves, refrigerant leaks |
| Conveyor drives and gearboxes | 20,000–50,000 h | Lubrication, misalignment |
| Pelleting dies and rollers | 300–1,500 h (consumable wear life) | Abrasion, formulation |
| Robotic palletisers | 30,000–80,000 h | Grippers, cabling, sensors |
| VFDs and control cabinets | 40,000–100,000 h | Heat, dust, power quality |
How to use MTBF when comparing supplier offers
- Request MTBF and MTTR together in the technical specification, with the measurement basis stated.
- Normalise the figures: same duty cycle, same shift pattern, same definition of a failure.
- Model availability and lost output in money, then add it to the purchase price — this is what separates two offers that look similar on capital cost.
- Check spare parts lead time and local service coverage; they drive MTTR far more than the machine itself.
- For critical lines, tie an availability figure to the performance test rather than relying on a headline MTBF number.
Where MTBF fits in the capital decision
Reliability assumptions belong in the business case, not only in the maintenance plan. Lost output from downtime is an operating cost that changes payback, and it is one of the numbers a capital committee will challenge.
- Feed the availability estimate into the total cost of ownership comparison and the payback calculation.
- State the reliability assumption explicitly in the capital request so reviewers can test it.
- Reflect critical spares holding as a working-capital line in the budget, not an afterthought.
Frequently asked questions
How is MTBF calculated?
Divide total operating time by the number of failures in that period. A line running 4,000 hours with 5 failures has an MTBF of 800 hours. Planned downtime is excluded from operating time.
What is a good MTBF value?
There is no universal target — it depends on the asset and duty cycle. Judge it against availability: MTBF ÷ (MTBF + MTTR). Most continuous industrial lines are specified for 95–99% availability, so the acceptable MTBF follows from your repair time.
Is a higher MTBF always better?
Generally yes, but it should be weighed against purchase cost and repair time within a total cost of ownership analysis. A machine with double the MTBF but a four-week spare part lead time can deliver worse availability.
What is the difference between MTBF and MTTF?
MTBF applies to repairable assets that are fixed and returned to service; MTTF applies to non-repairable items such as bearings, seals or lamps that are replaced on failure.
How do MTBF and MTTR combine?
Availability = MTBF ÷ (MTBF + MTTR). With an MTBF of 800 hours and an MTTR of 8 hours, availability is 800 ÷ 808 = 99.0%.
Can MTBF predict when a specific machine will fail?
No, it is a statistical average across a population of similar units, not a prediction for an individual machine.
Should MTBF be written into the purchase contract?
Buyers of critical lines commonly request supplier MTBF data in the technical specification and, for large packages, link a guaranteed availability figure to the performance test at commissioning.
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Related terms
Preventive Maintenance (PM)
Planned maintenance programme performed at predetermined intervals or usage thresholds to reduce the probability of unplanned equipment failure.
Spare Parts Strategy
Planned approach to identifying, stocking and sourcing replacement components based on criticality, lead time and failure risk to limit unplanned downtime.
Total Cost of Ownership (TCO)
Sum of acquisition, operating, maintenance and disposal costs of an asset over its useful life, used to compare purchase options beyond initial price.
Technical Specification
Structured document describing the functional, performance, materials, and interface requirements an item of equipment or system must meet.
Pelleting Line
Processing sequence within a feed mill that compresses conditioned mash into dense pellets using a pellet press, followed by cooling, screening and, where required, coating.
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Reference content only. Global B2B Group is independent of equipment manufacturers and financing institutions; definitions are provided for education and do not constitute engineering, financial or legal advice.