Guide
Mounting a servo or stepper motor to a gearbox: flange dimensions and coupling guide
The four dimensions that fit any motor to any gearbox, reference tables for NEMA 17, 23 and 34 steppers and common servo frames, and how clamping hubs work.
Key takeaways
- Four motor dimensions decide the fit: shaft, pilot, bolt circle and flange size.
- An adapter flange plus a hub bushing fits almost any motor to a standard gearbox.
- Always work from the motor maker's dimension drawing; tables are a starting point.
The four dimensions
- Shaft diameter and length, and whether it has a key or flat.
- Pilot (spigot) diameter, the machined boss that centres the motor.
- Bolt circle diameter (PCD) and bolt size.
- Flange square size, which sets the minimum adapter size.
Stepper motors: NEMA frame dimensions
| Frame | Flange | Pilot Ø | Hole spacing | Bolt circle | Typical shaft Ø |
|---|---|---|---|---|---|
| NEMA 17 | 42.3 sq | 22.0 | 31.0 sq, M3 | 43.8 | 5 |
| NEMA 23 | 56.4 sq | 38.1 | 47.14 sq, Ø5.1 | 66.7 | 6.35 or 8 |
| NEMA 34 | 86 sq | 73.0 | 69.6 sq, Ø5.5 | 98.4 | 9.53, 12.7 or 14 |
AC servo motors: common metric frames
| Frame | Typical power | Shaft Ø | Pilot Ø | Bolt circle |
|---|---|---|---|---|
| 40 mm | 50–100 W | 8 | 30 | 46 |
| 60 mm | 200–400 W | 14 | 50 | 70 |
| 80 mm | 750 W–1 kW | 19 | 70 | 90 |
| 130 mm | 1–2 kW | 22 | 110 | 145 |
Makers differ in shaft length, key size and bolt size, and some use inch or special frames. Send us the model number and we check the drawing for you.
How the clamping hub works
Precision gearboxes clamp the motor shaft in a split hub tightened by one screw through an access hole in the adapter. A slotted bushing adapts the hub to smaller shafts. Because the clamp transmits torque by friction, the shaft key is usually removed or cleared by the bushing slot. Tighten the screw to the specified torque with the motor fully seated, before tightening the flange bolts, so the shaft is not side-loaded.
Adapter flanges, couplings and housings
An adapter flange sits between the motor and the gearbox: its motor side matches your pilot and PCD, its gearbox side matches ours. The same gearbox can later move to another motor by changing only the flange and bushing. When motor and gearbox are not directly mounted — a belt drive, or a housing carrying both — a zero-backlash bellows or disc coupling joins the shafts. We also make housings that hold motor and gearbox as one module for robots and AGVs.
References
- NEMA ICS 16, Motion/Position Control Motors, Controls and Feedback Devices (stepper frame sizes).
- IEC 60072-1, Dimensions and output series for rotating electrical machines.
Frequently asked questions
What are the flange dimensions of a NEMA 23 motor?
A NEMA 23 motor has a 56.4 mm square flange, a Ø38.1 mm pilot, mounting holes on a 47.14 mm square (66.7 mm bolt circle) and usually a 6.35 mm or 8 mm shaft. Always confirm on the motor's drawing.
Do I need to remove the motor shaft key?
With a clamping hub the torque is transmitted by friction, so the key is usually removed, or a slotted bushing is used that clears it. Follow the gearbox maker's instruction.
Can the same gearbox be moved to a different motor?
Yes, if it uses a separate adapter flange and bushing. Only those two parts change; the gearbox itself stays the same.