Hour-based PM beats calendar PM in heavy-equipment fleets
Calendar reminders are easy to set up and easy to ignore. Engine-hour thresholds are harder to game and surface real risk. Here's the math.
Calendar reminders are the default PM scheme in most fleet software. They're easy to configure, easy to ignore, and wrong about half the time on heavy equipment. Hour-based PM — “every 250 engine hours” instead of “every 90 days” — is harder to game and surfaces real risk. Here is the math.
Where calendar PM goes wrong
Suppose you have a 40-ton excavator scheduled for an oil change every 90 days. In a steady season, that excavator runs 6 hours a day for 22 working days a month — call it 396 hours per quarter. That's a reasonable interval; the OEM spec says 500 hours.
Now imagine a wet October. The site shuts down for two weeks of rain, then runs catch-up overtime in November. The excavator logs 180 hours in 90 days. You change the oil at quarter-end anyway because the calendar says so. You've wasted oil + tech time + downtime.
The opposite case is worse. A drought year + a tight bid schedule pushes the excavator to 720 hours in 90 days. The calendar PM says you're on time; the engine says you're 220 hours overdue. That's the failure mode that puts injectors on the floor.
The fix
Set the threshold in the units the OEM specifies — engine hours for excavators / dozers / loaders, miles for on-road trucks, date for trailers and gensets that don't have a meter. When the meter crosses the threshold, the system raises an AUTO_PM flag with the right severity and notifies the right people.
Two-line trick: log meter readings as drivers do their normal end-of-shift workflow (don't require a separate “PM check” entry), and let the system compute the threshold delta. In DirtFleet that's the maybeAutoFlagPm hook fired from createHoursLog; you don't see it, you just see the flag when it matters.
When calendar still wins
Trailers, gensets in standby, and any unit without a meter. We support both axes — set the interval in days for those, and the same flag system kicks in when the calendar threshold crosses. Mixed fleets pick per-asset.