A worm-gear winch uses a specialized type of helical gear set called a worm-gear to create torque. The worm-gear is connected to a drum that turns as the gear set rotates. These winches are available in hand cranked and powered varieties. Due to the design of the worm-gear winch, it has a higher gear ratio than other winches of comparable size.
Worm-gear sets use a worm and a disc-shaped flat gear. Worms are long, cylindrical gears that may have a single tooth or multiple teeth that spiral around the worm at an angle. This creates the appearance of a screw. The worm intersects the flat gear at an angle that allows the teeth of the worm to loosely mesh with the teeth of the gear. Depending on the angle of the intersecting teeth, the set may be self locking, meaning that the worm can turn the gear but friction will not allow the gear to turn the worm.
The high gear ratio of the worm-gear winch makes this design perform its role faster than other designs. While the traditional helical gear has a 10:1 or lower gear ratio, worm gears increase that ratio to 500:1. This equals faster movement because every rotation of the worm turns the gear 500 times as compared with 10 revolutions generated by traditional helical gears.
While the high ratio of the worm-gear winch makes it operate faster than other gear sets, the trade-off is efficiency. The design of the worm-gear set leaves a great amount of play, or sliding action, between the teeth of the two gears. In some cases, the worm-gear winch has less than 50% efficiency. Despite this drawback, the worm-gear set is preferable for applications in which self-locking, fast action winches are required.
A common application for a worm-gear winch is the boat trailer. The worm-gear winch is usually mounted to the front of the trailer and the drum is loaded with rope or a braided metal cable. The boat is connected to the winch cable, and then the operator cranks the handle or activates the electric motor connected to the worm. As the worm begins to turn, its teeth connect with the gear, turning the drum and pulling the boat onto the trailer. Similar arrangements are used for flat-bed car hauling equipment and tow trucks.