A worm gear is a gear system where a screw-shaped gear, called a worm, drives a toothed wheel called a worm wheel. In robotics, this mechanism is useful because it can produce a very large speed reduction in a compact space. Large speed reduction also means large torque multiplication, which helps small motors lift, clamp, or hold loads.
Worm gears are common in robot arms, winches, steering systems, and adjustable platforms.
The worm screw rotates and its helical thread pushes against the teeth of the worm wheel, turning the wheel slowly but with greater output torque. Because many worm gears have high friction and a shallow lead angle, the wheel often cannot drive the worm backward, creating self-locking behavior. This can help a robot hold a lifted load without constantly powering the motor, but it also wastes energy as heat.
Good worm gear design balances reduction ratio, efficiency, lubrication, material choice, and load capacity.
Key Facts
- Gear ratio for a single-start worm: ratio = number of teeth on worm wheel.
- Gear ratio for a multi-start worm: ratio = teeth on worm wheel / number of worm starts.
- Output speed: omega_out = omega_in / gear ratio.
- Ideal output torque: tau_out = tau_in × gear ratio.
- Real output torque: tau_out = tau_in × gear ratio × efficiency.
- Power relation: P = tau omega, so reducing speed increases torque when losses are small.
Vocabulary
- Worm
- A screw-shaped gear whose rotating thread drives the teeth of a worm wheel.
- Worm wheel
- A toothed gear that meshes with the worm and turns at a reduced speed.
- Gear ratio
- The ratio that compares input rotation speed to output rotation speed in a gear system.
- Self-locking
- A condition where the output gear cannot easily turn the input worm backward because friction and geometry resist reverse motion.
- Efficiency
- The fraction of input power that becomes useful output power instead of being lost as heat, sound, or friction.
Common Mistakes to Avoid
- Using the number of worm wheel teeth as the ratio for every worm. This is only true for a single-start worm, while a two-start or four-start worm gives a smaller reduction.
- Assuming torque increases with no losses. Real worm gears have sliding contact and friction, so the actual output torque is reduced by efficiency.
- Thinking all worm gears are self-locking. Self-locking depends on lead angle, friction, lubrication, and load, so some worm gears can back-drive.
- Ignoring heat and lubrication. Worm gears slide more than ordinary spur gears, so poor lubrication can cause wear, low efficiency, and overheating.
Practice Questions
- 1 A single-start worm drives a worm wheel with 40 teeth. If the motor turns at 1200 rpm, what is the output speed of the wheel?
- 2 A worm gear has a ratio of 30:1 and an efficiency of 70 percent. If the motor supplies 0.50 N m of torque, what is the approximate output torque?
- 3 A robot lift must hold a heavy platform in place when power is turned off. Explain why a worm gear might be chosen, and name one drawback of using it.