Start with the machine axis, then choose the gearbox. Motor power and flange size alone cannot tell you whether a unit will handle the required output torque, speed, reversals and positioning accuracy. This guide covers precision gearboxes used with servo motors, where the motion cycle matters as much as the load.

1. Establish the operating point before choosing a ratio

The reduction ratio is i = input speed / output speed. A gearbox trades speed for torque; it does not create mechanical power. Allow for efficiency when estimating the torque available at the output.

Output speed = motor speed / i
Estimated output torque = motor torque × i × efficiency

Consider a motor delivering 2.4 Nm at 3,000 rpm. With an illustrative efficiency of 95%, the results are:

These are transmission calculations, not gearbox ratings. Each candidate must still pass its own torque and speed limits. If your axis needs 300 rpm, a 20:1 gearbox requires 6,000 rpm at the motor. Check the motor's torque at that speed and the gearbox's permissible continuous input speed. Use load and transmission calculations alongside motor and drive sizing.

2. Match the torque rating to the actual duty

A catalogue may list continuous torque, cyclic torque, maximum torque and a separate emergency-stop torque. These limits have different conditions and permitted durations. An emergency-stop rating is not a torque allowance for every production cycle. Read the manufacturer's definitions before comparing numbers.

Acceleration adds a demand of torque = rotational inertia × angular acceleration, assessed together with process load and friction. A rotary table that turns easily by hand may need substantial torque to reach speed in a fraction of a second. For positioning applications, specify acceleration, deceleration, dwell time and reversals. Load mass alone is insufficient for sizing.

3. Translate backlash into a process tolerance

One arcminute is 1/60 of a degree. At a 100 mm arm radius, 3 arcmin of output backlash corresponds geometrically to about 0.087 mm at the tip. At a 500 mm radius, the same angle corresponds to approximately 0.436 mm. The small-angle calculation is tip displacement ≈ radius × angle in radians. This describes the gearbox backlash contribution, not the machine's complete positioning error.

Also examine torsional stiffness: the transmission twists elastically under load. A low backlash figure cannot guarantee accuracy by itself. A motor-mounted absolute encoder measures motor position, not mechanical errors beyond the gearbox. Load-side feedback or a linear encoder may be appropriate for demanding axes. Increasing servo loop gain does not remove mechanical clearance.

4. Check reflected inertia and understand the gear terminology

The load inertia reflected to the motor is approximately J_reflected = J_load / i². A 10:1 reduction divides that contribution by 100. Add the gearbox's input inertia and other rotating components when assessing the full system. There is no universal inertia-ratio target for every servo axis. The motor, drive and motion profile determine what is workable.

Planetary describes the gear arrangement; helical describes the tooth form. A gearbox can be both planetary and helical. Compare quantified backlash, stiffness, torque capacity, efficiency and envelope dimensions rather than treating those names as competing categories.

5. Verify both interfaces and output bearing loads

Confirm the exact motor model, shaft diameter and length, pilot diameter, bolt pattern and input clamping arrangement. Equal power ratings do not imply interchangeable mounting dimensions. Follow the manufacturer's specified assembly procedure and tightening torque.

A pulley, pinion or cantilevered arm adds radial force, axial force or tilting moment. Include the distance from the load to the bearing support. For vertical axes, do not assume the gearbox will hold the load when power is removed. A suitable braked servo motor and load-holding arrangement require their own assessment.

6. Compare complete configurations when requesting a quote

Maximum mechanical input speed is distinct from the speed allowed for continuous duty. Cycle frequency, ambient temperature, mounting and heat dissipation affect suitability. Lifetime lubrication means lubrication intended for specified service conditions; it does not promise unlimited life. Temperature and seal condition still matter.

Compare servo gearbox prices using the same ratio, backlash grade, frame size, motor adapter and output interface. For a useful quotation, provide the motor code, required output speed, continuous and peak loads, timing diagram, accuracy target, mounting drawing and operating environment. Check Derell servo gearboxes against the selected model's technical data as well.

Technical references

Published values apply to the documented products; they are not ratings for a different gearbox.

Neugart — technical gearbox data · Neugart — gearbox operating manual · Oriental Motor — load inertia calculation · Neugart — drivetrain sizing