dusty wrote:??An opposing set of threads?? Are the threads in the casting different than the threads in the nut?
Does "opposing" mean left hand versus right hand threads?
Opposing as in resisting advance of the nut farther towards the surface of the solid object which contains the opposing thread. Tightening the jam nut against that surface causes the screw thread to be pulled against the thread surface closest to the surface thus removing any play(clearance) between the mating threads. The same is occurring between the screw thread and the nut.
In the case of the speed control, a spacer is between the nut and the the casting surface. This spacer leaves enough clearance between the jam nut and the gear which rotates on the spacer. The nut prevents the gear from slipping off the spacer.