A type of mechanical gear. A screw is driven, and the continuous rising action of the thread turns a cog. The advantage is that you can easily shift your axis of rotation by 90 degrees, and you can also get some good leverage.

There are a few disadvantages, however. Since the screw is continually sliding against the cog, you get a lot of friction effects. Also, worm gears should never be reverse driven. My clinic project as a Senior Engineering Major was to find out why a lawn sprinkler driving apparatus was slowly breaking down over time. It turned out that hydraulic forces on one of the drive shafts were causing the shaft to be periodically pulled forward, in essence briefly driving a worm gear with a cog. The result was that the thread became deformed over time, and eventually would not run on normal residential water pressure. We solved the problem by slapping on some hydrophobic grease.

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