Can You Keep Your Planetary Reducer After Switching Servo Motor Brands?

A practical guide to motor power, mounting dimensions, speed, torque, and servo motor–planetary reducer compatibility.

When upgrading a machine, replacing a discontinued servo motor, or changing motor suppliers, engineers often ask: “Can I keep using the existing planetary reducer?”

Possibly—but motor power alone is not enough to determine compatibility. Two servo motors may both be rated at 400 W or 750 W, yet use different flange dimensions, centering pilots, shaft diameters, shaft extension lengths, and mounting-hole patterns. Selecting by power alone can result in misalignment, vibration, abnormal noise, excessive temperature rise, or a motor that cannot be installed at all.

The Same Rated Power Does Not Mean the Same Mounting Dimensions

Rated power describes a motor’s output capability under specified conditions. It does not define a universal mounting standard. Motors with the same rated power may differ in:

• Motor flange dimensions
• Centering pilot diameter and height
• Mounting-hole pattern
• Shaft diameter and extension length
• Plain, keyed, or special shaft configuration
• Rated and maximum speed
• Rated and peak torque
• Rotor moment of inertia

What Does a “2 HP” or “3 HP” Motor Mean in Watts?

Some customers specify motor size in horsepower instead of watts or kilowatts. For a quick industrial conversion:

1 HP ≈ 0.746 kW = 746 W. For quick reference, 1 HP is often treated as approximately 0.75 kW.

Check These Six Mechanical Dimensions First

1. Motor flange dimensions

Confirm the flange shape, side length or outside diameter, mounting-hole pattern, and centering pilot. A similar frame size does not guarantee identical dimensions.

2. Centering pilot diameter and height

The motor’s centering pilot fits the pilot bore in the reducer flange and helps maintain concentricity. An incorrect fit can cause misalignment, vibration, noise, abnormal bearing loads, and reduced service life.

3. Mounting-hole pattern

Check the number of bolts, hole diameter, thread specification, center distance, and orientation. Do not enlarge holes or force installation.

4. Motor shaft diameter

The shaft diameter must be within the reducer input coupling’s permitted clamping range.

5. Motor shaft extension length

A shaft that is too short may not provide enough clamping length; one that is too long may interfere with internal components.

6. Shaft configuration

Confirm that the reducer input can correctly clamp a plain, keyed, or special motor shaft and transmit the required torque.

A Motor That Fits Mechanically May Still Be Unsuitable

Rated and maximum motor speed

If the new motor’s maximum speed exceeds the reducer’s permissible input speed, temperature, noise, bearing load, and wear may increase.

Rated and peak motor torque

Check acceleration, deceleration, reversing, frequent start-stop operation, emergency stops, and shock loads caused by a collision or machine jam.

Reduction ratio and output speed

If the new motor runs at a different speed, retaining the same ratio will change the machine’s output speed.

Reduction ratio = Motor input speed ÷ Reducer output speed

Motor and load inertia

A significant change in rotor inertia can affect acceleration, settling time, vibration, and servo stability. The servo drive may require retuning.

Positioning accuracy and backlash

An existing reducer may still transmit power but no longer meet updated requirements for backlash, torsional rigidity, accuracy, or cycle time.

When Can the Motor Mounting Flange Be Changed?

If the existing reducer still meets the ratio, torque, speed, accuracy, and condition requirements—and the main difference is the mounting interface—a suitable motor flange may allow reuse. Final compatibility still depends on the reducer series, frame size, motor model, tolerances, and drawings.

When Should You Select a New Reducer?

• The new motor has substantially higher rated or peak torque.
• Maximum speed exceeds the reducer limit.
• The load, speed, or duty cycle has changed.
• The existing ratio no longer provides the required output speed.
• The machine requires lower backlash or higher accuracy.
• The reducer shows abnormal noise, vibration, leakage, or accuracy loss.
• The model is discontinued and replacement parts are unavailable.
• The new motor dimensions are outside the existing mounting range.

What Information Should You Provide?

1. Servo motor brand and complete model number
2. Motor catalog or dimensional drawing
3. Existing reducer brand and complete model number
4. Reducer dimensional drawing
5. Required output speed
6. Rated, peak, and shock-load torque
7. Acceleration, constant-speed, deceleration, and stop times
8. Radial load, axial load, and tilting moment
9. Backlash or positioning accuracy requirement
10. Available installation space

Do Not Select a Reducer Based Only on Watts or Horsepower

Motor–reducer matching requires more than power and physical fit. Speed, torque, ratio, dynamic loads, inertia, positioning accuracy, installation space, and the operating environment must all be considered.