What Is a Worm Gear?
A worm gear is a type of screw gear that consists of a worm (worm shaft), which is a threaded toothed shaft, and a worm wheel, which is a helical gear with its axis of rotation at 90 degrees to the worm’s axis direction. Rotation of the worm sends the teeth of the worm wheel, which are meshed with the worm, to rotate.
The worm gear transmits rotation from the worm to the worm wheel and vice versa. This inability to rotate from the output shaft side is called “self-locking.
Therefore, the worm side is always the input shaft side, where the power unit is mounted, and the worm wheel side is the output shaft side, where the power transmitting unit is mounted. Because of this feature, worm gears are used in reduction gears.
Uses of Worm Gears
Worm gears can transmit power by changing the axial direction by 90 degrees, and since they are compact and can obtain a large reduction ratio, they are used in belt conveyors for factory equipment, screw jacks, steering systems for automobile steering wheels, wiper drive units, pivoting mechanisms for electric fans, material input parts of food manufacturing equipment and music boxes, pegs of stringed instruments, etc.
The self-locking function of worm gears may also be utilized in reduction devices for elevators and escalators to serve as a safety device to prevent reverse rotation.
Since a particularly large reduction ratio can be obtained, deceleration can be achieved with a small input torque. Therefore, reduction gears can be manufactured relatively compactly.
Principle of Worm Gears
The principle of worm gear is that when the worm (input side) makes one rotation, the worm wheel rotates by one tooth. In this case, the number of worm teeth is called one worm. In the case of a worm with two worm strips, one rotation of the worm rotates the worm wheel with two teeth.
The reduction ratio of worm gear can be calculated as follows
Reduction ratio = Number of worm wheel teeth ÷ Number of worm strips
The worm gear can rotate in both directions (e.g., clockwise and counterclockwise when viewed from the input shaft side). This can be achieved by selecting the worm twist direction as right-hand or left-hand twist.
The features of worm gears are as follows:
- The axial direction can be orthogonal on the input and output sides (conversely, it cannot be coaxial).
- Large reduction ratios are possible (e.g., 1/10 to 1/100, etc.)
- Backlash (gap between gears, play) is reduced. This results in better tooth meshing and less meshing noise.
- Large output torque can be obtained from small input torque.
- Reduced self-locking occurs.
In contrast to the above advantages, there are some disadvantages, such as heat generation due to friction, which can cause seizure and wear of the gears due to the large slippage between gears. Therefore, lubrication between the gears is important, and lubricants with high extreme pressure resistance may be used.
Typical worm gear materials are as follows:
- Worms: Carbon steel, chrome molybdenum hardened steel, or stainless steel, whose tooth surfaces are heat treated to increase hardness and are polished to improve wear resistance.
- Worm Wheels: Copper alloy steel, a material that is more susceptible to wear than worms, is used to prevent seizure.