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  • October 29, 2026

How to calculate torque and transmission ratio when purchasing a gear reducer for the first time?

Guidelines for torque calculation and transmission ratio selection of helical gear reduction motors:

Calculation of speed ratio for horizontal helical gear reducer：

1.Gear Ratio = Motor Speed (RPM) ÷ Output Speed (RPM)

  • Example: Your motor spins at 1400 RPM. Your machine only needs 70 RPM.

  • Calculation: 1400 ÷ 70 = 20.

  • Result: You need a 20:1 ratio gearbox.

2. Torque: How Strong Do You Need It?

Torque is the twisting force. When a gearbox reduces speed, it multiplies torque.

The Simple Formula:

Output Torque (N·m) = 9550 × Motor Power (kW) ÷ Output Speed (RPM)

  • Example: You have a 1.5 kW motor and your output speed is 70 RPM.

  • Calculation: 9550 × 1.5 ÷ 70 = 204.6 N·m.

  • Result: Your motor provides about 205 N·m of twisting force.

3. The Golden Rule: Add a Safety Margin (Service Factor)

Never push a gearbox to 100% of its limit. If your machine jams or vibrates, the gearbox will break. You must add a safety margin.

Required Gearbox Torque = Calculated Torque × Service Factor

  • Service Factor Guide:

    • Smooth machines (fans, light conveyors): Use 1.0

    • Normal machines (mixers, packaging): Use 1.5

    • Rough machines (crushers, hoists): Use 2.0

  • Example: 205 N·m × 1.5 = 307.5 N·m.

  • Result: When buying, choose a gearbox rated for at least 308 N·m.

Speed ratio lookup table for helical gear reducer:

What You Want to KnowEasy FormulaExample Result
Gear RatioMotor RPM ÷ Output RPM1400 ÷ 70 = 20
Output Torque9550 × Motor kW ÷ Output RPM9550 × 1.5 ÷ 70 = 205 N·m
Safe Torque to BuyOutput Torque × Service Factor205 × 1.5 = 307.5 N·m

high efficiency helical gear reducer

4.high efficiency helical gear reducer Easy FAQ

Q: What happens if I pick a gearbox with too little torque?
A: The gears will break or the motor will burn out. Always use the Service Factor to buy a slightly stronger gearbox.

Q: Can I just buy a very strong gearbox?
A: Yes, but it will be bigger, heavier, and more expensive. It's best to calculate your exact needs and add the safety margin.

Q: Does a higher gear ratio always give me more torque?
A: Yes, lower speed equals higher torque. But if the ratio is too high, the gearbox may overheat. Stick to the manufacturer's recommended range.

Q: Why do I need the "9550" number in the formula?
A: It is just a standard math number used to convert electrical power (kW) and speed (RPM) into twisting force (N·m). Just put your numbers into the formula and calculate!

Q: How do I know the Output RPM of my machine?
A: You can measure it with a tachometer (a cheap tool that measures spinning speed). Or, check your machine's manual. If you can't find it, contact the machine manufacturer.


Send us your requirement: Power, Ratio, Torque, Working Condition or Drawing We will provide professional quotation & selection plan.

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