How to Select the Right Worm Gearbox Ratio
Choosing the wrong gear ratio is one of the most common (and costly) mistakes when buying a worm gearbox. Get it right, and your machine runs at the correct speed with safe torque margins. Get it wrong, and you risk premature wear, stalling, or an output speed that simply doesn't match your application.
This guide walks through the calculation step by step — no engineering degree required.
Step 1: Find your motor's output RPM
Standard Indian motors typically run at one of these speeds depending on their pole configuration:
- 2-pole motor: ~2800-2880 RPM
- 4-pole motor: ~1400-1440 RPM
- 6-pole motor: ~900-960 RPM
This number is your input speed — it's printed on the motor nameplate.
Step 2: Determine your required output speed
This depends entirely on your machine. A conveyor roller, an agitator shaft, or a packaging indexer will each need a different rotational speed to function correctly. If you're not sure, measure the RPM your current shaft needs to turn at, or check your machine's design specification.
Step 3: Calculate the gear ratio
The formula is straightforward:
Gear Ratio = Input RPM ÷ Required Output RPM
For example, if your motor runs at 1440 RPM and you need an output speed of 24 RPM:
1440 ÷ 24 = 60 — so you'd select a 60:1 ratio
TRV Series gearboxes are available in standard ratios from 5:1 up to 100:1, so most common applications fall within range.
Step 4: Check the Service Factor (SF)
Once you know the ratio, check that the gearbox's rated torque at that ratio comfortably exceeds the torque your motor will actually deliver. This margin is called the Service Factor:
SF = Gearbox Rated Torque ÷ Actual Output Torque
- SF ≥ 1.5 — comfortable margin, ideal for continuous duty
- SF 1.0–1.5 — acceptable for light, uniform loads
- SF < 1.0 — gearbox is undersized for this duty — consider a larger frame
Worked example: Conveyor drive
| Parameter | Value |
|---|---|
| Motor speed | 1440 RPM (4-pole) |
| Required roller speed | 30 RPM |
| Required ratio | 1440 ÷ 30 = 48:1 (round to nearest standard ratio, e.g. 50:1) |
| Motor power | 0.75 kW |
| Recommended model | TRV 63 (at 50:1, check SF > 1.0) |
Don't want to calculate manually?
Enter your motor HP and required output RPM into our free Gearbox Selector — it does this calculation instantly and recommends the right TRV model and ratio.
Standard TRV ratios at a glance
All TRV models (30 through 150) are available in the following standard ratios: 5:1, 7.5:1, 10:1, 15:1, 20:1, 25:1, 30:1, 40:1, 50:1, 60:1, 80:1, 100:1.
Still unsure which ratio fits your machine? WhatsApp us with your motor HP and required output speed — we'll recommend the right model and ratio for free.
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