On which machines monitoring pays for itself

In brief

• Do the math first, then buy the sensor: ISO 17359 puts the cost-benefit analysis first.
• The benefit is not the whole downtime avoided, but the difference between an unplanned and a planned intervention.
• Time-based maintenance remains the right choice for many components.
• Start with two or three machines, chosen with a criticality grid, not twenty.

Four strategies, with the right names

On the shop floor people say “run to failure”, “by the calendar”, “on condition”. EN 13306:2018 uses the names you find in specifications and audits.

What you call it	Standard term	When the intervention happens
Run to failure	Corrective maintenance	When the machine has already stopped working
By the calendar, by hours	Predetermined preventive maintenance	When hours or months decided in advance run out
On condition, CBM	Condition-based maintenance	When a measurement says the condition has changed enough
Predictive, PdM	Predictive maintenance	As above, with a date estimated from the trend of the measurements

Two clarifications that avoid misunderstandings:

• Predictive maintenance is a subset of condition-based maintenance. If you measure and intervene when a threshold is exceeded, you are already doing the latter; to call it predictive you need a forecast built on the trend of the degradation parameters. No measurement gives you the date of the failure.
• “Preventive” is not a synonym of “by the calendar”: predetermined and condition-based are both branches of preventive maintenance. “Ordinary” and “extraordinary” are not strategies but accounting categories (UNI 11063:2017).

The calendar is not a mistake

Replacing bearings and belts at fixed intervals is often the economically correct choice. The literature confirms it: the advantage of condition-based over time-based maintenance depends on how the component degrades, how severe the failure is and how accurate the measurement is (de Jonge, Teunter and Tinga, 2017).

The opposite also holds: opening a machine that is running well is not free. The Nowlan and Heap report (1978), on civil aircraft components, observes that on complex assemblies scheduled overhaul can raise the failure rate, introducing reassembly defects into a stable system. It is not a study on industrial machines, but you will recognize the mechanism.

The calendar is the right choice when:

• the component has a recognizable, repeatable wear life: filters, oil, seals, belts;
• the intervention is required by law or by the manufacturer for the warranty;
• the failure mode has no measurable symptom in advance;
• the intervention is cheap inside an already scheduled stop: measuring would cost more than the spare part;
• the time between first symptom and failure is shorter than the time you need to react.

The math first, then the sensor

ISO 17359:2018 gives guidelines for setting up a monitoring program. It is not mandatory and not certifiable: nobody is “compliant with ISO 17359”. The useful part is the order of the steps, and the first one surprises almost everyone.

1. Cost-benefit analysis (clause 5): does monitoring pay for itself? Count what the machine costs over its life and what is lost when it stops.
2. Inventory of the machines, their function and their operating conditions (clause 6).
3. Criticality of all machines: this yields the priority order and the list of those to leave out (clause 7.2).
4. Failure modes: which parameter can signal them, and whether it is measurable (clauses 7.3 and 7.4).
5. Method, measurement points, interval, initial thresholds and baseline; then trending, action and periodic review (clauses 8-11).

The question “how much does it earn me” comes before “which sensor do I buy”. Thresholds are not fixed: the review cycle puts them back on the table.

The cost of downtime, item by item

No standard gives you a formula for the cost of downtime: you do the math. Copy the table and fill in the last column.

Item	How to estimate it	Example	Your value
Duration of the stop (reference, not added)	From the call to restart at full speed	9 h	—
Internal labor and overtime	Man-hours for recovery and catching up out of hours	€210	—
Emergency external intervention	Travel, out-of-contract rate, spare part	€800	—
Lost or shifted production	Parts not made or moved to another machine	€480	—
Delivery delay	Penalties, express shipping	€300	—
Consequential damage	Workpiece lost, downstream damage	€200	—

The example is ours, not market data: €1,990 for nine hours of downtime on a machining center. If you work to order you often do not lose the margin, you shift it to a Saturday or another machine. But overtime and delivery delay stay real: fill them in honestly, otherwise the math says zero and it is not zero.

The benefit is not the whole downtime

A failure seen coming still costs money: the bearing is replaced anyway, the machine stops, the technician is paid. What changes is when and how.

Benefit per event = cost of the unplanned intervention − cost of the same intervention, planned. Then multiply by the events per year you realistically expect to catch: on a single machine that is often a fraction of an event.

On the example in the table, the same failure seen coming costs €1,050: labor €120 with no overtime, external intervention €400 at the ordinary rate, delay zero, consequential damage €50, lost production €480 as before. The benefit is €940 out of €1,990. Whoever presents the whole downtime as savings is inflating the math.

When the answer is no, and where to start

On some machines the honest answer is no: a twin ready to go, a spare on the shelf and replacement within an hour, a stop that blocks nothing downstream, a failure mode with no measurable symptom. The list of machines to leave out is a result of the method, not a shortcoming.

Factor	Weight	Score from 1 to 5
Cost of downtime and lost production	×3	1 = negligible, 5 = stops the line or the customer
No redundancy and no spare ready	×3	1 = twin and part ready, 5 = unique machine, months of lead time
Failure frequency and repair time	×2	1 = never, fixed in an hour, 5 = recurring, days of work
Consequential damage, safety, environment	×2	1 = none, 5 = risk to people or a spill
Feasibility of the measurement	×1	1 = inaccessible point or variable regime, 5 = easy access, repeatable cycle

Weighted sum, maximum 55. Rank the machines from the highest score and start with the top two or three. Almost all factors come from ISO 17359 (clause 7.2); the weights and the 1-to-5 scale are ours, so calibrate them on your plant.

Before buying anything

1. I have written down the cost of downtime on this machine, not an industry average.
2. I have calculated the difference between unplanned and planned intervention, not the whole downtime.
3. I know which failure mode I want to catch and which parameter shows it.
4. I know, as an order of magnitude, how long passes between first symptom and failure.
5. I know who looks at the data and what they do when an alarm arrives.