[TheForge] Bronze Bushings

[Andy Vida]

Rick at Rafter Lazy C wrote:
> 
> Hello Guys and Gals -
> 
> I am finding myself in a (not unfamiliar) quandry.  I am trying to
> design something and am missing some very needed training.  Never
> had any engineering training, but I need to size a bronze bushing
> for a 2 1/4 dia. shaft.  Where on the web can I find info on how
> large to bore the pocket for the bushing, and etc.?  Can someone
> help me out here?

	Several questions.  First, it would be good to have more
	information on the precise configuration of the mechanism,
	what stresses (ballpark guesstimates) it must endure, and
	what is the shaft attached to?  

	What do you mean by "pocket"?  Are you referring to the
	housing that holds it in place?  Is your question intended
	to divine what thickness the bushing ought to be?

	Rather than a bushing, could you use a self aligning pillow
	block?  They are a cast iron housing with usually two or four
	mounting holes.  The bearings can swivel in the housing so that
	any misalignment in the shaft is compensated for by the degree
	of freedom of motion the bearing has in the housing.  Does that
	make sense?  That way all you have to do is bore and tap the
	mounting holes.

>  This shaft will only be turned by hand, so speed
> of rotation is not an issue here.  If it were, would the requirements
> change?

	Impossible to say for certain, but the higher the shaft speed,
	the more likely this would in face be the case.  If the shaft
	is slow turning and loaded in one direction only, you may be
	able to get away with sloppier tolerances.  But if the shaft
	will take radial loadings from widely differing directions,
	chances are you will want to bore the bearings to pretty tight
	tolerances... no more than a few thousandths.

	You will also want to cut oil grooves and bore a radial
	lubrication gallery in both the bushing and its housing
	to you can keep the bearing from running dry, which is
	the express method of achieving bearing death.

	Simpler solution is to make the bushing from oilite, a permanently
	self lubricating porous bronze material.  If you go that route,
	do NOT ream the bore to final dimension.  You must never ream
	oilite because the reamer closes and burnishes over the pores
	causing the bearing to lose all its wonderful self lubricating
	properties.  You must also never use heat to affect an 
	interference fit of the bearing into its housing because it will
	degrade or even destroy the lubricant, which as far as I know
	cannot be replaced.

>  Where can I get this kind of info when I need it?

	I would recommend Mark's Handbook For Mechanical Engineers.  I
	have my uncle Bengtsson's from 1958.  It's a great resource.
	Machinist's Handbook is another excellent source and in fact
	may be better for these sorts of questions.  There are definite 
	guidelines for all these bearing issues and you're probably best
	advised to adhere to them.

	Best wishes.

	-Andy