Lesson 7 of 10
Planned Maintenance, MTBF and MTTR
Lesson six handed the natural wear over to this pillar. Here are the two numbers that decide how much of it reaches your availability figure — and a piece of arithmetic that tells you, exactly, which of the two is worth your money.
Step one
Read the material
Three ideas. The two numbers and the availability they produce, why maintaining everything is the wrong goal, and the equivalence that settles where to spend.
Concept one
Two numbers, and the fraction they make
MTBF — mean time between failures — is total operating time divided by the number of failures. It answers: how long does this machine run before it stops?
MTTR — mean time to repair — is total repair time divided by the number of failures. It answers: when it stops, how long is it down?
Together they produce availability directly:
Availability = MTBF ÷ (MTBF + MTTR)
Which is the same fraction as lesson three, arrived at from the other direction. There, availability was run time over planned production time. Here it is built from the failure pattern itself, which is more useful because it tells you why the number is what it is rather than only what it is.
One honesty point that decides whether MTTR is a real measurement or a comfortable one. Strictly, MTTR is repair time. But the clock the machine experiences starts when it stops, not when a technician picks up a spanner — and that gap is the waiting from lesson six, plus diagnosis, plus fetching a part that is not on the shelf. If your MTTR counts only the spanner time, it will look excellent while the machine sits idle for two hours. Measure time to restore, not time to repair, and if you must report both, report both.
Concept two
Maintaining everything is not the goal
Planned Maintenance is not "do preventive work on all of it." There are four strategies, and choosing the wrong one is expensive in both directions.
Run to failure is a legitimate strategy, not a confession. For a cheap, non-critical item that is easy to replace and has a spare on the shelf, planning around it costs more than it saves. The failure has to be safe, cheap and quick — if it is any of those things and not all three, choose something else.
Time-based replaces or services on a fixed interval. Simple, schedulable, and wasteful when the interval is conservative — you are throwing away good life.
Condition-based acts on a measurement: vibration, temperature, oil analysis, current draw. You intervene when the equipment tells you to.
Predictive extends that with trend and models to forecast when the condition will cross the line, so the work can be scheduled rather than reacted to.
Now the part that surprises people. Every intervention reintroduces risk. A machine that has been opened, reassembled and restarted is at its most vulnerable — a seal seated wrong, a fastener missed, a setting not restored, contamination let in while it was open. In plants with heavy time-based programmes, a real share of all failures happen shortly after planned maintenance, and they were caused by it.
So more preventive work is not automatically better maintenance. Beyond a point it buys you failures. The measure of a good programme is not how much is done, it is how little needs doing.
Concept three
Doubling MTBF and halving MTTR are exactly the same thing
This is the most practically useful result in the lesson, and it is exact rather than approximate.
Double MTBF, and availability becomes 2M ÷ (2M + R). Halve MTTR instead, and it becomes M ÷ (M + R/2) — multiply top and bottom by two and you have 2M ÷ (2M + R). The identical number. For every machine, at every ratio.
So the question is never which one helps more. They help identically. The question is only which one is cheaper to achieve, and that is not a close contest.
Doubling MTBF means the machine must fail half as often. That is redesign, better components, condition monitoring, eliminating the forced deterioration from lesson six — months of work, usually capital, sometimes not achievable at all.
Halving MTTR means the same failure is fixed in half the time. That is a spare on the shelf instead of on order. A written procedure instead of a person remembering. The right tool in a shadow board by the machine instead of somewhere in the shop. A diagram of what to check first. Someone qualified on shift instead of on call.
None of that is glamorous and almost all of it is cheap. Yet nearly every improvement programme starts on MTBF, because failing less often sounds like the real fix and repairing faster sounds like an admission that things break.
Start with MTTR. It is the same arithmetic and a fraction of the money, and it pays out the first time the machine stops.
Step two
Work it with your hands on it
One machine over a year. Set the failure pattern, then split the repair into the part where somebody is working and the part where the machine is only waiting. The last two rows are concept three — drive them apart if you can.
Hands on it, off the screen. Pick the machine that stops most often. For its last ten failures, write down two times: when it stopped, and when a technician actually started work. You are measuring the gap, not the repair.
Then look at what filled those gaps. If most of them are a part that was not on the shelf, you have found a spares problem wearing a maintenance costume — and halving that gap does exactly as much for availability as making the machine twice as reliable, at a fraction of the cost.
Step three
Prove you understood it
One question per idea, new numbers every time.
All three solid.
You can build availability from the failure pattern, choose a maintenance strategy instead of defaulting to one, and prove where the cheap improvement is. Lesson eight steps back and lays out all eight pillars, so you can see where these two sit in the whole structure.
Modern Total Productive Maintenance (TPM)
Lesson seven of ten. This class is taught from the Total Productive Maintenance series — the eight pillars, the six big losses, and the roadmap into AI and predictive manufacturing. Written by our founder, Dr. Gene A Constant, and donated to the Foundation.
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