How to Set Gearbox Maintenance Intervals by Duty

A maintenance interval cannot be set once for every unit — the per-series maintenance pages on this site already say so, and this article does not repeat it. What those pages leave open is how to set the interval at all. This page supplies that: classify duty on four dimensions, merge the grades into one level, keep the ruler in cumulative running hours instead of calendar time, and carry that ruler one step further with equivalent running hours. Intermittent units judged by the calendar drift at both ends.

Where this article stands

The premise is already written down on this site's per-series maintenance pages: the oil change interval depends on lubricant type, temperature, contamination, load and running hours, together with the manufacturer's or service provider's requirements, and different duties cannot share one fixed interval. Restating that here would add length and no information.

The division of labour is therefore explicit. The premise is not re-argued; only the method is supplied. The four sections below answer four questions, in order:

1. How duty is classified — four dimensions, one merged level.
2. What the interval is measured against — cumulative running hours, and equivalent running hours where a unit runs a mix of duties.
3. What gets inspected at each level.
4. When the interval is shortened, and what has to be true before it is lengthened.

A planner who has worked through all four has an interval that can be written into a plan and defended at review. Nothing about why intervals differ is repeated below.

Classifying duty on four dimensions

Duty classification is not a mood or an impression of the plant floor. It splits into four dimensions that can be observed, argued about and written down, and each one is graded on its own before the four grades are merged into a single duty level.

Two merge methods exist. Choose one and freeze it:

1. Highest-level rule. The heaviest of the four grades becomes the overall level. It is simple, and it errs on the safe side.
2. Scored merge. Each dimension scores differently at light, medium and heavy; the scores are summed and the sum falls into a band. It is finer, but it needs the dimension weights settled first.

Neither the weights nor the band boundaries are given here. Both depend on the model and on the site, so they are settled against the machine's own maintenance manual and confirmed with engineering, and then frozen. Change a weight after the fact and every level recorded before the change stops being comparable with every level recorded after it.

The result goes into the asset file, together with which four dimensions were used and which merge method produced the level. A grade that lives only in one engineer's head disappears with the next staff change, and the interval quietly reverts to one setting for the whole plant.

The ruler: cumulative running hours, not the calendar

This is the most consequential step in the method.

Where an interval is hung on the calendar — a date on the wall chart, a recurring entry in the maintenance system — a unit that runs continuously comes out roughly right. A unit that runs intermittently does not, and it fails at both ends: when the date arrives, the light-duty unit has not yet accumulated the running hours the interval was written for, and the heavy-duty unit has already passed them while the calendar still says it is not due.

The ruler that does not fail that way is cumulative running hours. Each unit counts its own hours under load, and the inspection falls due against that count rather than against a square on the wall chart.

Equivalent running hours

Cumulative hours carry one defect of their own: an hour under heavy load and an hour under light load both add one hour to the counter. On a unit with one steady duty that hardly matters. On a unit whose load swings — a press line, a crane, a conveyor that alternates between full and empty — the counter treats very different wear as identical.

Equivalent running hours fold those segments onto one scale:

H_eq = Σ (h_i · w_i)

h_i is the running hours in the i-th duty segment, w_i is the duty weight for that segment, and H_eq is the equivalent running hours.

Two points decide whether the figure is usable:

1. `w_i` follows the duty level of that segment. The actual value comes from the machine's maintenance manual and from engineering confirmation. It is not given in this article, and no figure should be borrowed from another source and treated as settled.
2. The segmentation has to be fixed. By day, by shift, or at each change of duty — whichever is chosen, keep it identical period after period. Change the segmentation later and two H_eq values measure different things, which is worse than having no figure at all.

With H_eq in place, the interval finally has a common denominator: whatever mix of duty a unit runs, the accumulated load lands on one scale, and units, periods and sites can be compared on it.

What gets inspected

At any duty level the inspection items are the same seven. What changes with the level is how hard you look at each one.

This table sets attention intensity, not frequency. How often a check falls due follows from the merged duty level, from the record history and from the machine's maintenance manual. It is not implied by the table above.

Two adjacent subjects belong to other articles and are not repeated here: how to build a vibration and temperature baseline is covered in the condition monitoring article on this site, and oil type and viscosity selection is covered in the lubrication guide.

When to shorten, when to lengthen

An interval is not a figure to fit once and forget. It should move with the evidence — but the two directions do not need the same evidence, and that asymmetry is the point of this section: shortening can rest on a judgement about site conditions; lengthening needs records, and consecutive ones at that.

The asymmetry is deliberate and worth stating plainly: a shortening that turns out to be wrong costs maintenance hours; a lengthening that turns out to be wrong costs the gearbox. The rule that follows from it: shortening may be carried out on the basis of an on-site risk judgement; lengthening must be supported by consecutive records. Where the records do not support a change, run the interval that is already in the manual.

Maintenance record template

A record is not an archive exercise. Its purpose is to make the next adjustment defensible, in either direction. At minimum it carries these fields:

Fields 3 and 5 carry the whole method. Without cumulative hours the interval can only be guessed from the calendar; without equivalent hours, two units under different duty cannot be compared at all. The other nine fields exist so that those two stay honest when someone else takes the file over.

Condition-by-condition judgment table

Field checklist

When the grading is first set up:

1. Record the actual observation against all four dimensions, unit by unit.
2. Settle the merge method — highest-level or scored — and write down which one.
3. Put the grade and the reason for it into the asset file.
4. Record the version of the machine's maintenance manual and the requirements it imposes.

On routine execution:

1. Record cumulative running hours at every check, not just the date.
2. Convert the segments through equivalent hours wherever the duty varies.
3. Fill in a result for every inspection item; a blank is read as "not checked".
4. Mark the turning point in the record whenever the environment or the duty changes.

When adjusting the interval:

1. A shortening may be executed on the strength of a site judgement.
2. A lengthening needs consecutive records behind it.
3. After an oil change or an overhaul, reset the cumulative origin and update the grading basis with it.

Common errors and where this article stops

Common errors:

1. Setting the interval by the calendar and never looking at cumulative running hours, which puts intermittent units systematically out of step in both directions.
2. One interval for every gearbox in the plant, with no duty classification at all.
3. Grading the duty but leaving the result out of the asset file, so the next person reverts to one setting for everything.
4. Changing the weights in a scored merge at will, so levels recorded before and after the change no longer compare.
5. Wanting a longer interval without consecutive records to support it.
6. Leaving the interval as it was after the environment changed — new dust, higher temperature.
7. Recording dates but not running hours, so nothing can be converted after the fact.

Where this article stops:

• The four-dimension grading, the cumulative and equivalent hour rulers, the inspection item table and the record fields here apply to building a maintenance plan for industrial gearboxes.
• This article gives no maintenance interval values of any kind — no hours, months, years or running mileage — and no duty weights, no scoring thresholds, no band boundaries.
• This article gives no oil condition limits, no temperature limits and no vibration limits.
• This article makes no claim about prevailing practice in the wider installed base; it offers a method and a set of record fields, and nothing that would need a statistical source behind it.
• The specific interval is set by the machine's own maintenance manual and by the manufacturer's requirements. What this article supplies is the method for landing those requirements on your site's duty; it does not replace the manual.

Maintenance record data sheet

About this article and the publisher

This article was written by the technical team of Chengfa Reducer (Changzhou) Co., Ltd. (CHINFAR) and published after the company's technical review. CHINFAR products are produced and delivered on the joint manufacturing system shared with HUIGUANG.

Related material:

• Resource centre
• Gearbox lubrication guide: oil types, fill quantities and change intervals
• Industrial gearbox installation, lubrication and first-run checklist
• Contact the CHINFAR technical team

Oil type, viscosity and fill quantity are handled in the lubrication guide. The commissioning checks are handled in the first-run checklist. Neither is repeated here.

Next step

Send the eleven fields above to sales@chinfar.com. If you are at the start of the exercise, two things come first: finish the four-dimension grading and put it into the asset file, and move the interval ruler from the calendar to cumulative running hours. Until those two exist, the inspection items and the adjustment rules have nothing to stand on.

To be explicit: the specific interval is set by the machine's own maintenance manual. This article supplies the method for landing the manual's requirements on your plant's duty, and it does not replace the manual.