working part

plural working parts

1. ropeThe section of rope between the working end and a knot being made.
2. A part of a machine that moves and helps to operate it.

   2. Simplicity, with the least number of working parts, all readily visible and accessible.

   1951 July, “British Standard Locomotives”, in Railway Magazine, pages 438-439

   1 more example

   Every village had a flour-mill, worked by wind or water. The buildings were usually of wood, as were all the working parts of the mill, except for the stones.

   1980, AA Book of British Villages, Drive Publications Ltd, page 116