On Thu, 09 Feb 2012 16:05:58 +0000, The Natural Philosopher
wrote:

I would guess it's there to remove any free play in the drive system.
That is, make sure that the there's no microscopic movement caused by
there being no loading on the gears at any point. (There's a proper
technical term for this but I'm darned if I can remember what it is at
the moment. Whiplash maybe)

Tim
backlash, or simply lash.

What's happened to rum and sodomy?