Interference Fit: Stuck Bushing Inside Hub Inside Outer Ring

[racookpe1978]

My lead machinist disagreed with each Friday, and I figured I'd bring it up for discussion with the group here because "Anything one person can believe logically, he can also be living but be logically wrong."

We've got a bushing stuck with what should be a 0.004 interference fit inside a hub. it won't move despite forces sufficient to bend a 2 deep 4 x wide strongback 1/2" from horizontal. (That strongback has a 1-1/2 dia hole through it for a 1-1/2 dia all-thread rod trhough it, so the strongback is not as strong as it appears at first.)

The bushing is 2.5 inch ID, 3 inch long, and 3.000 inch OD. (Not brass/bronze by color and texture, I do not know the alloy. (The bushing is to align a valve stem in a high temperature steam valve, so it faces no rotational forces - only axial.) What the actual interference fit is we do not know, this valve was last assembled 45 years ago.

The bushing is restrained within a 3 inch long hub, 3 inch ID, 5 inch OD.

That hub is held in place by three straight spokes, 120 degrees apart. Each spoke is approximately 1.5 inches thick, 2 - 2.5 inches deep - depending on where in the casting you measure.

The spokes connect to an outer ring, 20 inch OD, 18.5 inch ID approximately 20 deep axially. The outer ring is not constrained, but is "loose" to expand and move within the valve in this area by a thin seal weld. Assume for working purposes, the valve seat is sitting loose on a plywood plate.

Since the bushing will not pull out after applying axial force with a port-a-power jack pulling on the 1-1/2 all-thread, my foreman wants to use heat Monday to loosen the hub.

Normally, I would agree with him. Heat should loosen the hub because heat applied against the hub will expand the hub OD, which should expand the hub ID, and - if the bushing can be kept cooler - reduce the force between the bushing and hub.

But ...

In this case, I think heating only the hub would expand the hub OD against the still-cooler spokes, which are restrained by the cool outer rim, which in turn means the "outwards force" created by heating will turn around and compress the hub ID into the bushing OD.

I believe that - if heat is to be applied at all - it should be uniformly applied at the intersection between the spokes and outer rim, so the rim heats up, the rim expands, the spokes (still cold) pull outwards, and the hub is pulled outwards away from the bushing OD. If we can, the bushing should be cooled below room temperature as well.

Any comments or critiques? And recommendations?

 

[desertfox]

Hi racookpe1978

I think I agree with the comments you've made as regards heating, if the hub expands against the cooler spokes it will try and push them outwards putting the outer rim in tension, two problems I see though, how are you going to uniformly heat the intersections of the rim and spokes? How much heat will you put into the joint because you don't know how much expansion you will need?

 

[chicopee]

If it could be done, I would try, instead of heating the hub, cool the bushing with dry ice. As the temperature gradient lessens starting from the inner surface of the bushing, eventually there may be enough contraction to press out or punch the bushing out before the hub starts cooling down; be careful since the bushing may get brittle.

 

[Dougt115]

All you need is .002 of radial expansion to free the parts. The spokes are loose, I would expect there to be at least .002 clearance if not more.

I would go with the heat. Easier to focus, can create a larger temperature gradient faster, and metals generally loose mechanical strength when heated.

Cooling is hard to do unless the entire part fits in the frig which will be pointless in this case. And technically the heat flows from hot to cold so you will have a hard time imparting a temperature difference to the two parts press fit together.

 

[dhengr]

Racookpe1978:
I hate it when “My lead machinist disagreed with each Friday....”, your OP.
I think you want to expand the outer structure (ring?) by heating it, to make it move outward in circumference. Remember, less heat at a greater circumference still means more movemwnt. I think you want to make the spokes contract by cooling them. Dry ice in an old sock, taped to the spoke. I think you want to heat the hub, the most heat, to cause it to expand away from the bushing, and maybe push the spokes outward. And, I think you want to cool the bushing to cause it to contract away from the hub. All of this at the same time, a fairly inexact science, I agree/admit. Then, pull with the porta-power, and at the same time wack on the hub or the bushing with some sort of an impacting tool, impact wrench, impacting screw driver motor, or some such, to break it loose; maybe as inelegant as a sledge hammer. You only need 37 hands and arms to do this. 45 years is a long time for the parts to kinda nit together, so you gotta break then apart somehow, at their interface. Certainly some penetrating oil wouldn’t hurt. The two 4x4's side by side with the threaded rod in the space btwn. them and resting on a thick washer plate, on them, would be stronger than drilling and stacking the 4x4's, where you drill out 30% of the beam. You would do more with a couple 8" channels, back to back, with the pulling rod btwn. them, and a thick washer pl. atop them. Good Luck.

 

[dhengr]

Actually, a cycle or two of heating and cooling, applied to the hub and the bushing, along with some penetrating oil, and a little impact action, might help break the 45 year bond btwn. them, before you try to pull the bushing.

 

[chicopee]

The use of penetrating oil that DHENGR mentions might just work. I have seen mechanics using penetrating oil on rusted lugs to break the bond, however, this method took about a week of oil penetration during which time there were several applications of penetrating oil in order to free the works.

 

[1gibson]

Maybe I am missing something, but you are going to re-use a 45 year old bushing? Rent a PMI (positive material identification) gun, determine the alloy, then machine it out.

If you can't do that, consider that it may have been loctited in, and you may be able to "cook" the Loctite. So heating may help, eventually. Impact is probably going to help more than slowly applied jacking force, set up some sort of slide hammer.

 

[racookpe1978]

To clarify - The old was bushings could be destructively removed, they will be replaced.

Update. Bushings are all out now.

What worked? To tell the truth, we don't know. All above were used to one degree or another, including steady pressure over Saturday-Sunday-Monday while soaking with Kroil penetrating oil.
We replaced the previous strongback by stiffening it with 2x 4 inch CH underneath, and applied impact force before jacking again on Tuesday. Didn't use heat Tuesday, but had heated it Sat afternoon, then chilled the center.

My thanks to all who replied.

 

[desertfox]

Hi racookpe

When you applied the heat where bouts did you apply it? Just curious after your original post.