Usage-based maintenance from run hours and part counts

You trigger maintenance from hours run or parts made by giving each task a usage meter instead of a date. The meter adds up the machine’s run hours or part count, warns the maintenance team as the total nears the task’s interval, and opens the work order when it gets there. When the job is done, a reset starts the next cycle, so every service lands after the same amount of work.

This guide is for maintenance leads moving tasks off the calendar. It covers which tasks belong on a meter, which unit to count, how to set the first interval and how early to warn.

What is usage-based maintenance?#

It isn’t a separate strategy. NIST describes preventive maintenance as work scheduled on predetermined units, and names machine run time and cycles as the examples.^[1] NASA’s reliability-centered maintenance guide defines preventive maintenance as work at predefined intervals of time, operating hours or cycles.^[2]

The record that does the counting is a meter: a running total on one machine that rises with use and returns to zero only when someone resets it. A trigger point is the reading at which the task is due. The glossary entry for a meter uses the same terms.

Why the calendar drifts when the load changes#

A calendar interval is a usage interval with a usage rate baked in. “Monthly” on a grease task means “about as many hours as the machine ran in a month when someone wrote the plan.” When the machine runs more or less than that, the work lands late or early.

For example, take a CNC lathe with a 250-hour grease interval, scheduled every 4 weeks on the assumption that it runs about 62.5 hours a week. Over 12 weeks, its load follows the order book.

The two early services spend labor and grease the bearing didn’t need yet. The late one runs it 70 hours past its interval in the busiest month, when a failure costs the most.

Altium Packaging met the same problem with a Slide Knife that needs inspecting every 100,000 bottles. A line that ran fast reached the count early, a slow one reached it late, and a date-based schedule was always early or overdue. How far the calendar drifts depends on how much a machine’s hours swing, which is what machine utilization measures.

Which tasks belong on a meter?#

Put a task on a meter when the wear it prevents builds up with use. NASA’s RCM guide notes that simple items subject to metal fatigue or mechanical wear, and items designed as consumables, often show a direct link between age and reliability. For those items, it says, an age limit based on operating time or stress cycles can work.^[2]

Most equipment doesn’t behave that way, as how failures behave explains. In the three studies NASA summarizes, drawn from airline, Swedish and US Navy data, 77% to 92% of failures were random rather than age-related.^[2]

The same guide warns that timed maintenance can be unnecessary and sometimes harmful. NIST relays an estimate from a 2016 paper that preventive maintenance is applied unnecessarily up to 50% of the time in manufacturing.^[1]

A meter fixes when a task happens. It can’t make the task worth doing.

You don’t need a meter on a part that fails at random. Watch its condition instead, or let it run to failure if a breakdown costs little.

Choose what the meter counts#

Pick the wrong unit and every interval you set on it is wrong too. Count what the wear item experiences, not whatever number happens to be on the screen.

Run hours#

Decide which hours count before you set an interval. Power-on hours include a machine sitting idle between jobs; running hours count only while it works.

For wear on a spindle, a gearbox or a pump, count running hours. A Haas VF-2 left powered on through a two-hour wait for material adds two hours to its power-on clock and nothing to the spindle’s wear.

Parts, cycles and strokes#

A part count and a cycle count differ whenever one cycle makes more than one part. A four-cavity mold makes four parts per shot, but its ejector pins move once per shot. A meter counting parts reaches a shot-based interval four times too early unless you multiply the interval by the cavities.

The multiplier is the quantity per cycle. Check the count against a hand count over ten parts before you trust a meter with it. A count that reads double or half points to that multiplier, not to the sensor, as the part count troubleshooting guide explains.

Set the first interval#

1. Start from the manual’s unit

   Use the interval the manufacturer gives in hours, cycles or strokes as it stands. If the manual gives both an hours limit and a time limit, keep both and do the work at whichever comes first.

2. Convert dated tasks from measured hours

   For a task you only have as a date, take the run hours the machine averaged per period while that task kept it healthy. A monthly task on a machine that averaged 230 run hours a month becomes a 230-hour trigger point.

3. Don’t use MTBF as the interval

   Mean time between failures (MTBF) is the average run time between breakdowns. It gives the average age at failure, not the most likely one, and says nothing about whether failures rise with age. NASA’s guide notes that MTBF is often the starting point for preventive intervals and names exactly that weakness.^[2]

4. Record the as-found condition

   At every service, have the technician note what they found: worn past the limit, borderline or like new. That record is your evidence for changing the interval.

5. Adjust the interval in steps

   If three services in a row find the part like new, lengthen the interval one step, for example from 230 to 260 hours. If one finds it worn past the limit, shorten it. NASA calls this age exploration: adjusting a task’s interval until you know the rate at which the part’s resistance to failure declines.^[2]

Warn early enough to stage the job#

A meter that acts only at 100% opens the work order the moment the task is due. That is too late when the job needs a part you don’t stock. Give the meter an early warning, and size it from how fast the machine uses up the interval.

The same arithmetic applies to part counts, and it shrinks fast on a quick line. On a 100,000-bottle meter, the last 15% is 15,000 bottles: about 107 minutes at 140 bottles a minute, and 12.5 hours at 20 a minute. On a line that fast, the warning is a cue to stage the kit and book the stop, not to order parts.

When the warning can’t cover a part’s lead time, keep the part on the shelf instead of ordering it per job. One spare with a reorder point covers every machine that uses it.

Reset on the work, not the work order#

A meter measures the work since the last service, so the reset has to mark the service. Reset when the technician finishes the job, and date the reset to the day the work was done.

Resetting when the work order opens starts the next cycle before the grease is in. Every day the order waits in the backlog then comes out of the next interval, and nobody sees it.

Three more rules keep the count honest:

• Reset early work too. A technician who greases the lathe at 210 hours during a changeover resets the meter then, so the next service lands 250 hours after the real one.
• Leave skipped work unreset. The meter keeps counting past the trigger point, and the overrun records how late the job is.
• Keep a dated task beside the meter when the manual also sets a time limit, on the same preventive maintenance schedule as your calendar work. A meter on an idle machine stops counting, and a time limit doesn’t.

Run usage-based maintenance in IoTFlows#

In Maintain, a meter counts a machine’s runtime hours or parts toward a trigger point from the machine’s own data, so nobody types a reading in. Each meter has three behaviors you switch on separately: an email at 85%, an email at the trigger point and an automatic work order at 100%.

The meter card forecasts the time left at the rate the machine has run since its last reset. A reset, which you can backdate to the day of the job, starts the next cycle. Altium Packaging inspects its Slide Knife this way, every 100,000 bottles, with the work order opened automatically.

See how to set up a meter, or how Maintain ties work orders to the machines that need them.

Before you move a task to a meter#

• The wear the task prevents builds up with use, not with time and not at random.
• The unit matches what the wear item sees: running hours, cycles, strokes or parts.
• The count matches a hand count over ten parts.
• The trigger point comes from the manual or from measured hours, never from MTBF.
• The warning leaves more time than the job’s lead time at your busiest usage rate.
• Resets are dated to the day the work was done, with the as-found condition recorded.
• Any time limit from the manual or a regulation stays on a dated task.