Worm gear is a gear that allows motion transmission between two perpendicular rotation axes placed on two different planes. The worm gear is made of worm (cylinder shaped with machined spiral teeth) and a spiral gear (worm gear) adjusted to the same pitch of the worm.
Worm gear is characterized by a high speed and torque application and high transmission ratio.
The transmission ratio of a worm gear is determined by the number of teeth of the worm gear (assuming that the worm cylinder has 1 tooth in contact each time). For example, for a 1:60 transmission ratio of a worm gear, the worm gear will have 60 teeth.
A very important feature of worm gear is self-locking. When you turn the worm, the worm gear will start to rotate. But when you try to rotate the worm gear, the worm won’t move. This fact is related to the relatively flat angle of the worm teeth. Because of the flat angle, the force that the worm gear exerting on the worm can’t overcome the friction and therefore the worm won’t spin.
In mechanical mechanism, machines and robots, the self-locking feature can save production costs of breaking systems or other mechanical restrainers. By selecting the appropriate transmission ratio, you can create a braking mechanism based only on the worm gear without adding additional mechanical or electro-mechanical components such as brakes.
An example for the self-locking mechanism of the worm gear:
Suppose a robot was designed to lift a certain part using a motor and a transmission gearbox.
Let’s assume a first case that the transmission gearbox is based on spur gears only. When the motor starts to turn, the gearbox transmits the torque and lifts the part. When the motor is turned off, the weight of the part overcome the internal friction of the gear and it starting to drop down. In order to keep in place the part after it was being lifted, a brake should be added to the application or the motor has to be kept turned on in with a constant torque (unnecessary power consumption).
To overcome this, we replace the spur gearbox with a worm gearbox with high transmission ratio. The motor attached to the worm gearbox and starts to rotate thus lifting the part. When the motor is turned on, the part exerts its weight on the worm gearbox. Because of the structure and the flat angle of the worm teeth, the worm gearbox opposes the part gravitational force and prevents it from falling.