CASE STUDIES
Worm Gear Motors: How They Work and Where They Are Used
How does a worm gear work?
A worm gear uses a screw-shaped part, called the worm, to turn a gear wheel called the worm wheel. Each rotation of the worm pushes the teeth of the gear wheel only a little, so the output is much slower than the input. This is the main strength of a worm gear: a large speed reduction in a compact unit.
The two parts
- The worm. A threaded shaft, much like a screw. It is driven by the motor.
- The worm wheel. A wheel whose teeth are shaped to mesh with that threaded shaft. It carries the slow, high-force output.
The worm and wheel sit at right angles to each other, so the output shaft is perpendicular to the input shaft. That makes a worm gear motor useful where the motor and the drive shaft need to point in different directions.
Why use a worm gear motor
The worm arrangement offers a few things that are hard to get from other gear types in the same space:
- Very slow output speed. A worm gear can achieve a large reduction in a single stage, where other gear types would need several stages, which takes up more space and introduces more points of error.
- Compact right-angle drive. The speed reduction and the change of direction happen in one unit.
- Smooth, quiet running. The sliding contact between worm and wheel runs quietly.
How it compares with other gear types
| Feature | Worm gear | Helical gear | Bevel gear |
|---|---|---|---|
| Speed output | Very slow, large reduction | Moderate | Moderate |
| Shaft layout | Right angle | In line | Angled or right angle |
| Best known for | Big reduction in one stage | Smooth, long-lasting drives | Changing drive direction |
Each of the other types is covered in its own guide: helical gearing and bevel gearing.
Where worm gear motors are used
Worm gear motors suit machinery that needs slow, controlled movement and a strong reduction from a small unit. Typical settings include conveyors, lifting and hoisting equipment, and packaging machines. Where a machine needs speed reduction and a right-angle drive together, the worm gear is often the simplest way to get both.
Back-driving, and why a brake is still needed
In many worm designs the wheel cannot easily turn the worm in reverse. This resistance to back-driving is a characteristic of the gearing, not a safety device, and it should not be treated as a way of holding a load. Where a load has to be held in place or a motor stopped safely, the right answer is a brake. How that works is covered in the guide to electromagnetic fail-safe brakes.
The trade-off with worm gears
The large reduction comes from the sliding contact between the worm and the wheel. That sliding contact also loses more energy to friction as heat, so a worm gear is less efficient than a gear type that rolls. For slow-speed jobs this matters little, and the compact size and single-stage reduction are worth it.
The materials help manage it. The worm is the harder part, normally hardened steel, and the worm wheel is the softer one, normally bronze. The softer wheel beds in against the worm and takes the wear, so the pair slides smoothly and the wheel, rather than the worm, is the part that wears out.
To see the worm geared motor range, visit the worm geared motors category.
Frequently asked questions
How does a worm gear work?
A screw-shaped worm turns a gear wheel. Each rotation of the worm advances the wheel only a little, so the output is much slower than the input, giving a large speed reduction.
Why is a worm gear so slow?
Each turn of the worm moves the wheel by about one tooth. That built-in reduction is what makes the output speed low and the torque high.
Can a worm gear drive itself backwards?
In many designs, not easily. This is a characteristic of the gearing and not a substitute for a brake, which is the proper way to hold a load or stop a motor.
What layout does a worm gear motor have?
The worm and wheel sit at right angles, so the output shaft is perpendicular to the input shaft.
Which part is harder, the worm or the wheel?
The worm is the harder part, normally hardened steel, and the wheel is the softer one, normally bronze, so the wheel takes the wear.
Are worm gears efficient?
They trade some efficiency for a large reduction in a compact unit, because the sliding contact loses energy as heat. For many slow-speed jobs that trade-off is worth it.