Designing for 500,000-cycle gearmotor life
Half a million cycles is often under fifty hours of running. The wear driver is starts, reversals and grease age, and the test that proves it takes a month and a half of real time.
- Wedge
- Obsolescence, second source, and reshoring
- Catalog reviewed
- 2026-08-18
- Configurations screened
- 32 published
Any mechanism specified in cycles rather than hours: a cam that indexes, a door that opens and closes, a carousel that steps, a valve that strokes, a dispenser that meters. The distinguishing feature is that each cycle is a short move followed by a long dwell. This is the single most common life specification in OEM motion, and it is also the one most often carried across from an incumbent supplier without anyone having established what it means physically.
Convert the cycle specification into the quantities that actually wear things, and do it before selecting anything. Total output revolutions is cycles times revolutions per cycle. Total motor revolutions is that times the ratio. Total running time is cycles times the moving time per cycle. Number of starts is cycles times the starts per cycle, which is two for anything that reverses. Then the thermal quantity: mean-square current is the integral of i²dt over one period divided by the period, and the starting transient contributes to it out of all proportion to its duration because the starting current is an order of magnitude above the running current.
What normally controls the selection
Which of four independent mechanisms runs out first: brush and commutator wear, which tracks commutation events; gear surface fatigue, which tracks tooth contact cycles and is a hundred times worse at the first stage than at the last; grease condition, which tracks calendar time and temperature more than it tracks motion; and bearing life, which tracks revolutions. These do not scale together. A cycle count alone determines none of them, which is why the first task is to convert the specification into all four quantities and see which one is close to a limit.
Four conditions, and they are usually decided by four different numbers.
Sizing on one of them is how a mechanism gets a part that works on the bench and fails in the field.
Failure modes and misleading specifications
What follows is what actually fails, and which published number sends people the wrong way.
| Failure mode | Physical cause | What it looks like in the field |
|---|---|---|
| Commutator wear concentrated at a few segments | The mechanism always starts and stops at nearly the same rotor position, so the same segment pair carries the starting current on every cycle. | Localized commutator erosion or brush transfer at regular angular intervals, on a motor whose total running hours are far below any brush-life figure. |
| First-stage pinion wear with an undamaged output stage | Contact cycles scale with the ratio. At 100:1 the first-stage pinion sees a hundred times the tooth contacts of the output stage. | Pitting or polishing on the motor pinion and the first wheel, with the output stage still showing its original tooth flank finish. |
| Grease failure before any wear limit is reached | The mechanism runs for minutes a day over ten years. The grease ages by calendar and temperature, not by motion, and the first stage throws it out of the mesh. | Rising current and rising noise on units with very low accumulated running time. Dry or separated grease at teardown with intact tooth flanks. |
| Backlash growth at the reversal point | A million reversals, each traversing lost motion and impacting the trailing flank under acceleration. | Increasing reversal error and increasing reversal noise, with flank damage confined to one side of the teeth. |
| Specification | Why it misleads in this application |
|---|---|
| Rated life in hours | It describes continuous running and it is normally set by brush wear. A cycling mechanism accumulates hours very slowly and starts very quickly, and the hours figure says nothing about starts. |
| A cycle count with no cycle definition | Half a million cycles of a 5 degree nudge and half a million cycles of a full revolution differ by a factor of 72 in every quantity that matters. The definition is the specification. |
| Duty cycle expressed as a percentage | A 3 percent duty implies a thermal margin, and the thermal margin is real. It also implies that the remaining 97 percent is free, which it is not: grease ages and mechanisms sit still under load. |
| MTBF | It is a constant-hazard figure derived from a population running continuously. It does not describe wear-out, and every mechanism on this page fails by wear-out. |
The tradeoffs that matter here
Spur or planetary reduction
Brushed coreless or brushless
How life is demonstrated
Convert a 500 000-cycle specification into the quantities that wear things, for an indexing cam drive.
- Cycle definition
- 90° forward, 90° reverse, then dwell
- Cycle count
- 500 000
- Cycle period
- 8 s
- Output torque
- 6 mN·m running, 12 mN·m peak
- Configuration
- MM-C1226-S01002-120A, 100.22:1 spur, 12 V
| Step | Expression | Result |
|---|---|---|
| Move time per cycle | 0.25 rev ÷ (100 rpm ÷ 60) × 2 moves | 0.30 s |
| Total running time | 500 000 × 0.30 s | 150 000 s, or 41.7 hours |
| Total output revolutions | 500 000 × 0.5 rev | 250 000 output revolutions |
| Total motor revolutions | 250 000 × 100.22 | 25 055 000 motor revolutions |
| Contact cycles, first stage against last | 25 055 000 against 250 000 | The first-stage pinion sees 100 times the tooth contacts of the output stage |
| Total starts | 500 000 × 2 | 1 000 000 starts |
| Running current | 6 mN·m ÷ (100.22 × 0.59) ÷ 10.997 mN·m/A + 2.9 mA | 12.1 mA, 71 percent of the 17 mA rated current |
| Electrical loading from running | (0.0121 A)² × 150 000 s | 22.1 A²·s |
| Electrical loading from starting | (0.6 × 0.301 A)² × 0.020 s × 1 000 000 | 652 A²·s, 30 times the running contribution |
| RMS current over the whole program | √[(652 + 22.1) ÷ 4 000 000 s] | 13.0 mA, inside the 17 mA rated current |
| Real time to run the full count | 500 000 × 8 s | 4 000 000 s, or 46.3 days |
Result
Half a million cycles is 41.7 hours of running. Almost no brushed motor fails from 42 hours, which means brush wear is not the risk here and any life argument built on a rated-hours figure is answering the wrong question. What the conversion exposes is that the mechanism performs a million starts, and that those starts contribute 30 times more electrical loading than all the running combined, concentrated at a nearly stationary rotor. It also exposes that the first-stage pinion accumulates 25 million tooth contacts against the output stage's 250 000, so the wear inspection belongs at the motor end of the train and not at the output. Thermally there is nothing to worry about: the RMS current over the full program is 13.0 mA against a 17 mA rating. The four risks in rank order are commutation wear from the starts, first-stage contact fatigue, grease age over the calendar life, and reversal-driven backlash growth. Testing the specification honestly takes 46.3 days at real duty, and that is a program schedule input.
What would change it
The cycle definition dominates everything. Changing the move from 90 degrees to a full revolution multiplies every revolution and contact-cycle figure by four while leaving the start count alone. Shortening the cycle period from 8 s to 2 s quadruples the RMS current, which at 26 mA would exceed the 17 mA rating and turn this into a thermal problem. Removing the reversal halves the starts and removes the backlash-growth mechanism entirely. And the assumed starting transient is the weakest number in the calculation: a measured current trace through a real start would change the 30-to-1 ratio in either direction and should be the first measurement taken.
A short qualification plan
Each step names the measurement, not the intention.
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Convert the specification before selecting anything
Cycles converted to output revolutions, motor revolutions, running hours, starts and RMS current, with the cycle definition written down.
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Measure the starting transient
Current trace through a real start on the assembled mechanism, at both temperature extremes, integrated to give the actual per-start loading.
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Run at real duty, not compressed
Full-count cycling at the real period, so the grease ages and the mechanism thermal-cycles at the rate it will in service.
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Tear down at intervals, not only at the end
Units removed at 10, 25 and 50 percent of the count and inspected. First-stage pinion flanks, commutator surface, grease condition and reversal backlash, each measured against its own baseline.
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Include the jam
Stall events distributed through the life test at the rate the mechanism will really see them, with the torque limit active.
Tools, products and articles for this problem
Send the duty definition and a cycled unit.
Send the cycle definition, the period and a unit that has accumulated real cycles. You get back the converted quantities, a teardown against the four wear mechanisms, and a test plan sized to the schedule.