How to Size a Worm Gear Reducer: Ratio, Torque, Service Factor
A worm gear reducer fails two ways: you under-size the torque and strip it, or you ignore service factor and cook it. Sizing one correctly takes four numbers - input speed, required output speed, load torque, and duty cycle. Here is the sequence we use on sizing calls.
Step 1 - Ratio
Ratio = input RPM ÷ required output RPM. A 1750 RPM motor driving a mixer at 58 RPM needs 1750÷58 ≈ 30:1. Standard single-reduction worm ratios run 5:1 to 100:1 - our HDR cast iron series stocks 5:1 through 100:1, and WINL extends to 180:1 with double-reduction options. If the math lands between ratios, round to the nearest and trim with sheaves or the VFD.
Step 2 - Output Torque
Required output torque (lb-in) = HP × 63,025 × ratio × efficiency ÷ input RPM. Worm efficiency falls as ratio climbs - roughly 90% at 5:1 down to 65–70% at 60:1 and beyond. A 3 HP, 1750 RPM input through 30:1 at ~80% efficiency delivers about 3 × 63,025 × 30 × 0.80 ÷ 1750 ≈ 2,594 lb-in. The reducer's rated output torque at your input speed must exceed the load's demand - check the series rating table, not just the HP number.
Step 3 - Service Factor
| Load Type | Hours/Day | Minimum SF |
|---|---|---|
| Uniform (centrifugal pumps, fans) | ≤10 | 1.00 |
| Uniform | 24 | 1.25 |
| Moderate shock (conveyors, mixers, augers) | ≤10 | 1.25 |
| Moderate shock | 24 | 1.50 |
| Heavy shock (crushers, reciprocating loads) | any | 1.75–2.00 |
Multiply required torque by SF before you pick the unit. Frequent starts, reversing duty, and hydraulic braking all push SF up.
Step 4 - Mounting and Material
- HDR cast iron - 367 configurations, the workhorse for washdown-free industrial duty; C-face or separate input shaft, hollow or solid output.
- CALM aluminum - lighter, corrosion-resistant housing for weight-sensitive mounts; 7.5:1–100:1.
- SSHDR stainless - food-grade washdown; 5:1–100:1.
- KAN / KHN - compact right-angle units, 4.5:1–120:1.
- FHN - fractional-HP gearmotors and reducers to 110:1.
- SMR shaft-mount - belt-primary shaft-mount reducers for conveyor head shafts (15:1, 25:1), with screw conveyor and hydraulic-drive variants.
Don't forget the hardware: torque arms, tapered bushings, belt guards, and motor mounts are all stocked.
Frequently Asked Questions
What ratio do I need for a target output speed?
Divide motor speed by target speed: 1750 RPM in, 35 RPM out means 50:1. Round to the nearest stock ratio and fine-tune with sheave selection on belt-input units or frequency on a VFD.
Why is worm gear efficiency lower at high ratios?
High ratios use fewer worm starts and more sliding contact per revolution, so friction losses rise - from roughly 90% efficiency at 5:1 to 65-70% around 60:1 and up. Factor efficiency into torque and motor HP calculations, and expect the housing to run warm.
What happens if I skip the service factor?
The reducer may survive the average load but not the peaks - tooth wear accelerates, the worm gear's bronze wheel pits, and seals cook from sustained overload heat. A conveyor with moderate shock on a 24-hour schedule needs 1.5 times the calculated torque capacity.
Can a worm reducer hold a load without a brake?
Sometimes - high-ratio worm sets resist back-driving, but self-locking is never guaranteed across temperature, vibration, and wear. For hoisting or held loads, use a positive brake or a backstop assembly rather than relying on worm friction.
Solid shaft or hollow bore output?
Hollow bore mounts directly on the driven shaft and saves a coupling but needs a torque arm to react the housing. Solid output suits foot-mounted installs with a coupling or chain drive. Both are stocked across HDR, CALM, and SSHDR series.
Related Resources
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