Left handed threads and grinder

[Mike Minto]

Just a quick note - i recently changed from a hard stone to a wire wheel on my grinder, and when i went to screw a nut back on the shaft, it wouldn't go on. i wondered how i had damaged the threads without noticing having done so. after going over the threads with a 'thread file' and asking guys at work if they knew what might be wrong, one guy noticed that one end of the shaft was right threaded, and the other left threaded. i simply had mixed the nuts up, and when i got the right one on the right shaft, it screwed right on! oh, well. these things happen . mike

[Bill Huber]

The reason is that as the shaft rotates it will tighten both nuts, if they were both the same then one nut would want to come loose as the motor turned.