An electric motor gearbox is usually bought as the second half of a gearmotor, and it is usually specified with far less care than the motor. Buyers check the power, the ratio and the flange, then discover six months later that the unit runs hot, leaks oil through the breather, or has enough backlash to spoil an indexing move. This guide covers the six gearbox characteristics that decide whether the drive survives — service factor, thermal rating, backlash, lubrication, overhung load and mounting position — and how to read each one off a datasheet.
The motor supplies speed and power. The gearbox trades speed for torque through a fixed ratio, minus losses. Everything else the gearbox does — absorbing shock, holding a load, resisting radial load from a pulley, dissipating heat — is a side effect of its mechanical design, and each of those side effects has its own rating. A gearbox can be perfectly adequate in torque and still fail in service, because torque rating is only one of four limits.
| Limit | What it protects | When it governs |
|---|---|---|
| Nominal output torque | Gear tooth and shaft strength | Steady running load |
| Service factor | Tooth strength under shock and starts | Frequent starts, reversing, jam-prone loads |
| Thermal power rating | Lubricant and seal life | Low output speed, high ambient, long duty |
| Overhung load rating | Bearings and output shaft | Pulley, sprocket or belt loads on the output shaft |
Service factor is a multiplier applied to the calculated running torque to cover the load cases the calculation does not describe: starts under load, jams, reversing, vibration and the fact that real machines are less polite than their datasheets. Selection torque is therefore:
Tselect = Trunning × SF
The gearbox must be rated at or above Tselect at the actual output speed. Service factor is not a safety margin you add because you feel cautious; it maps to a duty class, and the duty class is decided by what the machine does all day:
| Duty class | Typical machines | SF, up to 10 h/day | SF, 24 h/day |
|---|---|---|---|
| Uniform | Centrifugal pumps, light fans, steady conveyors | 1.0 – 1.25 | 1.25 – 1.5 |
| Light shock | Loaded conveyors, machine tools, crane travel | 1.25 – 1.5 | 1.5 |
| Moderate shock | Mixers, packaging indexers, screw conveyors | 1.5 | 1.75 – 2.0 |
| Heavy shock | Crushers, kilns, reversing drives | 1.75 – 2.5 | 2.5 + |
Two corrections are worth making on top of the table: add roughly 0.25 when the motor starts direct-on-line with no soft starter and the machine is heavily loaded at rest, and add about 0.1 above 10 running hours per day in small steps rather than jumping at 24 hours. A conveyor that starts fully loaded five times an hour is not a uniform duty, whatever the catalogue classifies it as. Note also that vendor service factor tables are not interchangeable — one manufacturer’s 1.4 is not another’s 1.4 — so compare required torque against rated torque rather than comparing factors.
Every mesh loses energy as heat: a well-finished helical stage transmits roughly 97–98% per stage, while a worm stage can drop to 60–85% at high ratios because the contact is mostly sliding. That heat has to leave through the housing. The power the housing can continuously dissipate without pushing the oil past its working temperature is the thermal power rating, and it is usually lower than the mechanical rating.
This is why the same gearbox that is comfortable driving a machine at 200 r/min may be marginal at 20 r/min: at low output speed the oil is churned less, the housing sees less airflow, and the heat per unit of output power rises. High ambient temperature compounds it. Practical responses, in order of cost:
Backlash is the small rotation at the output shaft that occurs before the gear teeth re-engage after a reversal. It exists by design in every gearbox, and its size depends mostly on the gear type. General-purpose parallel-shaft gearmotors commonly land in the region of one to three degrees at the output, planetary units are much tighter and often quoted in arcminutes, and worm units vary with ratio and manufacturing grade.
Backlash only matters when the load reverses direction or when position must be held. An indexing conveyor, a machine-tool axis, a valve actuator or a labelling head all care; a fan, a pump or a unidirectional belt conveyor does not. If backlash does matter, three things follow:
Small and medium gearmotors are usually grease-filled for life: no oil changes, but also no tolerance for over-speeding or running above the rated ambient, because there is no way to top up degraded lubricant. Larger units use an oil bath with a fill level set for a specific mounting position. Mount the unit in a different orientation from the one it was filled for and the oil level lands in the wrong place — too low starves the upper bearings, too high pressurises the seals and pushes oil out through the breather.
So mounting position belongs on the order, not in the installation conversation. Standard designations run from M1 (horizontal, floor-mounted) through vertical shaft-up and shaft-down variants, and the correct breather, drain and oil-level plug positions change with each. Practical rules that prevent most lubrication failures:
If the output shaft carries a pulley, sprocket or timing belt, the radial force at the shaft centre is an overhung load, and it has its own limit. A belt tensioned tighter than necessary, or a pulley mounted further out on the shaft than the rated position, can exceed the bearing rating even when the torque is well within limits. Symptoms are bearing noise within months, shaft deflection, and seal leakage on the pulley side.
Keep the pulley as close to the housing as the layout allows, use the smallest reasonable pulley diameter, tension the belt to the manufacturer’s specification rather than by feel, and check the published overhung load figure at the actual distance from the shoulder. Where a large pulley is unavoidable, an outboard bearing support is the correct fix — not a larger gearbox.
If the gearbox type itself is still an open question, the differences between helical, worm, planetary and right-angle units are compared in what a gear reducer is and its types, and the mechanical accessories that complete the installation — torque arms, flanges and couplings — are covered in the gear motor accessories guide.
Treat the gearbox as four separate ratings rather than one, and most of the failures that look like bad luck disappear: service factor for shock, thermal rating for slow and hot duty, overhung load for belt drives, and backlash for anything that reverses or positions. Add the mounting code and the lubricant type to the purchase order, and the unit will be filled, vented and installed the way it was designed. Browse the AC gear motor and gearbox range to see how these ratings appear on real datasheets.
Around 1.25 to 1.5 for a belt conveyor running up to 10 hours a day, rising to 1.5 or more for continuous duty or frequent loaded starts. Screw conveyors handling dense material usually warrant 1.5 to 1.75.
Usually yes, and that is the point: the extra capacity absorbs shock rather than passing it to the teeth. On large drives it can add a case size, so apply service factors deliberately rather than by habit.
Because torque rating does not cover heat. Check output speed, ambient temperature, mounting position and lubricant condition; a unit running at low speed in a hot enclosure can exceed its thermal rating at half its mechanical rating.
Only if the breather, drain and oil level plugs are relocated and the oil quantity is reset for the new orientation. Rotating an oil-filled unit without doing this leads to leakage or starvation.
Backlash is the designed clearance between gear teeth. Noticeable movement can also come from the key, the coupling or a worn bearing, so measure before assuming the gearbox needs replacing.
