Press-Fits into Cast Al?

[Vrogy]

I need to press some oil-impregnated bronze bushings into cast aluminum parts with machined bores.. should I worry about the parts cracking if the tension is too much? Should I be looking at flange bearings or something to distribute the load?

Details: 3/4" ID bearings, static side load of ~75 pounds, momentary loads of up to 300-400 pounds.

[awemawson]

If you have enough meat arround the bearing you shouldn't have a problem with a press fit - aim for 1 thou per inch of interferance. Remember the porous bushes tend to squash inwards when pressed in, so will need reaming in situ.

[Mcgyver]

read machinery's handbook on this. this squashing in is engineered to a degree, the sintered bearings are over sized such that when installed in a hole of the nominal dimension, a certain ID (and clearance to nominal shaft) will result. Machinery's handbook describes how to control the resultant bearing ID by varying the housing's ID. The sintered bearings shouldn't be reamed, this smears over the pours somewhat defeating on of their advantages; being self lubricating as they're oil impregnated.

As Andrew said, how much the casting expands or if its damaged depends on the cross section near the bearing. Unless it was very thin i wouldn't be too concerned, pressing in these sintered bearings is a light load - unlike a typical press fit where the outside material expands, these depend on the sintered bearing being compressed

[NC Cams]

Whenever you press a busing into a housing, the housing tries to grow from the oversized bushing OD and the bushing tends to shrink.

The amount of shrinkage/growth is a funtion of the individual material modulus, the wall thickness of both and the amount of press fit. To an extent, the surface finish of the parts also affects the fit and shrink/growth factor.

There are websites that allow you to put in the factors and calculate the proper press fit and yes, too much press can crack/yield the aluminum as well as cause the busing to yield as well.

Once you yield the material, you can not/will not retain a press.

Recall the the press does several things. It surely holds the busing in place. IT also facilitates heat transfer -higher press in smoother housing transfers more heat than loose fit in rough housing.

You also however, need some "friction" to retain the bushing. Thus there is a fine line between high friction (rough bore) and poor heat transfer and low friction (glass smooth bore) and great heat transfer.

The optimum finish for a pressed in bushing or steel backed bearing is in the 40-60 micro-inch finish range. This was extensively verified in studies by Clevite as they made many cast bronze and steel backed bushings over the years.

If you properly ream/cut the oilite, you'll not smear it and it will work well. Hack it up and smoosh over the oilite or bronze material and you could have siezue problems. Guys in the auto industry regularly hone/pin fit piston pin and king pin bushings with Sunnen equipment. A guy who does that is good to know.

Without specific info about your bushing and housings, I'd be HIGHLY disinclined to use a SWAG'd "give it a thou" press fit - it might work, it might not, especially if you expect to see a wide temp range in operation. Aluminum hsgs' are notoriously more sensitive to over enthusiastic press fits than steel or iron.

Hope this helps

[Kevin Taylor]

Thermal srink fit work's well without damage to the part's consult the machinest hand book for the expansion coaficant's of the two materials some where around .002 per inch for that size should be enough 200F will be enough and not damage your part a hot plate or a home range oven work's fine chilling the bushing shoulden't be needed I'v found that the chance of a stuck half way deal is more common when the Ex cold meet's the hot If everthing is right it should drop in Good luck Kevin