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A worm wheel is one of the two parts of a worm gear. These one-way gears are used to prevent energy from flowing backwards into a system and to turn the output power of a gearing system perpendicular to the input power. Worm gears are also used to reduce gear sizes, since the worm screw can transfer energy to a worm wheel of any size. Historically, worm wheels were open for inspection and part replacement, but modern worm wheels are generally part of a fully contained worm gear assembly.
The worm gear is a very simple gear that operates on the same principle as a screw. The gear is made of two parts, the worm wheel and the worm screw. The worm wheel looks much like a typical spur gear; it is round and covered in teeth. The worm screw is basically a screw without a head. Each of these parts is connected to a shaft, and then they are placed so the worm screw sits flat on top of the worm wheel.
Energy is sent into the gear through the shaft connected to the worm. This causes the screw to rotate, just like a common household screw. Rather than screw into a surface, the teeth cause the worm wheel to rotate. This will cause the shaft connected to the worm wheel to turn, transferring the output power of the system by 90 degrees.
Worm gears have two properties that make them special. First, these gears only operate in one direction. As the worm screw turns, it will easily turn the worm wheel, but the worm wheel cannot turn the worm screw. The design of the screw makes it impossible for the wheel to move it, and the gear locks rather than transfers power.
The second important factor relates to gear size. In a mechanical system, the size of a gear, the speed of its rotation and the power it generates are all connected. A worm gear is a form of reduction system. Since the size of the worm wheel is only important in relation to the size of the worm screw, its power output can match that of a much larger gear.
Modern worm gears are generally totally contained devices. The two gears are placed inside a protective case. A permanent lubricant is pumped into the case and the entire thing is permanently sealed. The outside of the case has two small shafts that are used to connect to a larger machine.