Backlash is a term used to describe the looseness in a mechanical system. Backlash can occur anywhere in a machine, but is generally emphasized at the nut and lead screw connection. If the nut threading doesn't perfectly match the lead screw thread, there will be a wiggle, or play between both the screw and nut. This is backlash. Backlash can also be located in the motor bearings, coupling (connecting a shaft or lead screw to the motor shaft), collars not being hard against the bearing, or temperature change that causes various materials to expand or contract, making the connections loosened.
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WHAT IS "BACKLASH"