A gear reducer (gearbox) slows a motor down and multiplies its torque. To replace one, you match what it does mechanically — not just the model number, which is often worn off. Here's what the nameplate tells you.
The core specs
- Ratio: e.g. 30:1 — input turns 30 times for one output turn. Sets the output speed.
- Output torque: the usable torque at the output shaft (Nm or in-lb) — the headline rating.
- Input/output RPM and input power (HP/kW).
- Service Factor (SF): how much margin over the rating (1.0 = none, 1.4+ for shock loads).
Type of gearing
The gearing type affects size, efficiency and how the shafts are arranged:
- Helical / in-line: efficient, input and output in line.
- Worm: right-angle, compact, often self-locking, lower efficiency.
- Helical-bevel: right-angle, high efficiency.
- Planetary: very high torque in a small package.
Mounting and shafts — the fit-critical part
- Mounting: foot, flange (face), or shaft-mount (mounts on the driven shaft with a torque arm).
- Input: solid input shaft, or a motor-mount (NEMA C-face / IEC) that the motor bolts onto.
- Output: solid keyed shaft, or hollow bore (with shrink disk / keyway) — and its diameter.
Match the function, then the brand
Reducers from SEW-Eurodrive, NORD, Sumitomo, Bonfiglioli, Dodge and Boston Gear can substitute for one another when ratio, torque, mounting and shafts match. Send us the nameplate (or a photo) plus how it mounts, and we'll quote an exact or equivalent unit.