How each one works
A worm gearbox turns the drive through ninety degrees with a screw-like worm meshing a bronze wheel. One stage gives a high ratio, the unit is small and quiet, and the geometry lets the output shaft be hollow so it slides onto the driven shaft. The price is sliding friction: the worm rubs across the wheel rather than rolling, which is where the efficiency goes.
A helical gearbox uses angled teeth that roll into mesh gradually. Each stage costs only two or three percent, so a two-stage unit runs around 96 percent efficient and a three-stage around 94, at any ratio. Inline units keep the output on the motor axis; bevel helical units add a spiral bevel stage to turn the corner and keep most of the efficiency.
The numbers that decide it
| Worm | Inline helical | Bevel helical | |
|---|---|---|---|
| Efficiency | about 90 percent at 5:1, falling to 60 percent or lower at 100:1 | 94 to 97 percent | 94 to 96 percent |
| Ratio per stage | 5:1 to 100:1 in one stage | 3:1 to 50:1 in two, 200:1 in three | similar to inline |
| Output layout | right angle, hollow or solid shaft | in line with the motor | right angle |
| Noise | quietest | quiet | quiet |
| Heat | runs warm at high ratio and long duty | cool | cool |
| Purchase price | lowest | higher | highest |
| Running cost | highest | lowest | low |
| Best at | under 4 kW, intermittent duty, tight spaces, gates, small conveyors | continuous straight-line drives, pumps, mixers, process lines | continuous right-angle drives, heavy conveyors, elevators |
A running-cost example
A 3 kW drive running 16 hours a day, 250 days a year, needs about 12,000 kWh of output work a year. Through a worm unit at 70 percent the motor draws about 17,100 kWh; through a helical unit at 95 percent about 12,600 kWh. The 4,500 kWh difference, at whatever your site pays per kilowatt hour, is usually more than the price gap between the two units in the first year, and it repeats every year.
Where the worm unit is still right
- Low power and high ratio in one stage, for example 0.37 kW at 60:1 on a small conveyor or a dosing screw.
- Space: the right-angle package is the smallest available for the torque.
- Intermittent duty: gates, barriers, lifts and adjusters that run seconds at a time, where efficiency is irrelevant.
- Self-locking tendency at high ratios, useful for holding a load against drift (never as a safety brake).
Where neither is right
Very high torque in a small envelope wants a planetary gearbox; a load that must be held safely when the power fails wants a brake motor regardless of gearing; and a speed that changes with the process wants an inverter drive on whichever gearing suits the torque, not a mechanical variator. If the application does not fit the table above, describe it and we will say which of these it is.