[TheForge] Re: A&O

[Andrew Vida]

Mike Spencer wrote:

> In this case, I don't thing shock is an issue.  It's a compressor,
> right?  It's turning 180 rpm (? I forget...) and mooshing air.

	The bearing undergoes large spikes in stress when
	the crank throw reaches top- and bottom dead center.
	As long as the clearances are within spec., the
	shock (impulse) is so small as to be negligible.
	This is due to the difference in velocity between
	the throw and the rod upon reversal.  It cannot be 
	large enough to deliver a significant impulse to the
	bearing because the relative motion between the two
	components is almost nil.  But as the gap between the 
	two increases, the differential in velocity as the throw 
	reverses suddenly and the rod keeps on its merry way 
	become enough to cause a good slap between bearing 
	surfaces, which pounds out the bearing material.

	Once a bearing pounds out beyond some threshold,
	the shock grows enormously, further pounding out the
	bearing, creating greater shock loads... leading
	to failure. The noises a pounded out bearing makes
	is pretty well unmistakable and hard not to notice.

	Imagine trying to forge a piece of hot iron without
	ever raising your hammer from its surface, i.e. to
	move metal by holding the hammer to the surface of 
	the work and tryng to violently apply enough sudden
	force to make the metal flow.  T'ain't gwyne happen.
	But if you allow even a modest interval between the
	two, wherein the hammer's momentum can be grown,
	relative to the work, then upon impact the metal flows
	quite readily.  Same principle applies with bearings.