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Press Fit of Bearing into Aluminum Pulley
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Posted by: a2t

04/12/2008, 23:17:41

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I have a 6061-t651 pulley with inner bore that Im pressing a 6203 bearing into (outer bearing race dia 1.5748"). The pulley is used in a car engine and reaches 250 F during service. There is no thrust load on bearing, only radial.

In the past, Ive used .0005" interference and pressed bearing in (NOT using heat).

Ive had seveal pulleys fail where the bore grows until the bearing is so loose it will fall right out.

So, apparently .0005 isnt enough given the temp range. Im trying to correct this issue, and was running thru calculations based on thermal expansion of aluminum pulley and steel bearing.

From what Ive found, the 6061-t6 has thermal expansion of 13.1 microinch/inch/degree F, while the steel bearing is about 5 microinch/inch/degree F. The bearing thermal expansion is estimate, Im trying to be conservative too. Dont know how to find out true thermal expansion of bearing, perhaps I can call SKF. Anyways, 5 should be about right.

That yields a pulley "relative" thermal expansion of 13.1 - 5 = 8.1 microinch/inch/degree F.

So, if the metal pulley bolted to side of car engine reaches 250 F (which is highly likely), I can expect the bore to grow about .0032 inch. Thats over a thousandth per side. I cant press bearing in with that much pinch...

I must be doing something wrong here, pls advise where Im going wrong. Also, I was thinking of using some sort of loc-tite to help out, but that doesnt do well at 250F either.

The OEM just overmolded the bearing into plastic pulley, so they didnt have to address this issue.

Any help is appreciated!








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Posted by: randykimball
Barney
04/13/2008, 00:28:27

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You can't??
You can't heat the pulley in and oven and freeze the shaft? Even the end of a shaft hanging out can be placed in contact with a block of dry ice to result in a fair temperature drop.


...we call this a shrink fit....(wink)





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Posted by: a2t

04/14/2008, 10:16:52

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.0032 interference seems like a hell of alot on a 1.5xx" dia bore, doesnt it??

I dont even know if the pulley would crack, would prob have to run some calcs on stress levels.

Ive worked in airline maintenance for quite some time, we've never exceeded .001 per inch shrink. On this bore, that would result in about .0015 pinch.

According to my calcs, that still wouldnt be enough. Am I missing something with the thermal expansion of the 2 metals?








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Posted by: randykimball
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04/14/2008, 11:13:20

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Generally, fits for press and for shrink are about the same. You may be damaging the aluminum when you press fit the hub. If you shrink fit the hub you will know you have a good bore. Also, any reasonable texture on the shaft will add grip and not destroy or damage the bore during press processes. You may, in the long run, not be able to use aluminum for your hub because of the temperature changes.

(If it were me, I would not be afraid to "test" a hub with .005 shrink fit using 6061, "IF" the hub were about .5" thick) [note: this is only an opinion and must be tested and proved by yourself]





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Posted by: Kelly Bramble

04/14/2008, 11:36:33

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ANSI B4.2 gives recommended tolerance for force/shrink fits. I don't know what the reference material is though..







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Posted by: a2t

04/14/2008, 21:49:01

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Im going to take a finished pulley and pop it in the oven at 250F and leave it for an hour or so. Then wear some thick gloves, see if the bearing becomes loose...then I'll know for sure if .0005 pinch is too little. I suspect it is...







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Posted by: randykimball
Barney
04/14/2008, 23:19:57

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ooops... I failed to notice that you mentioned a bearing before.

'When" you try a tighter shrink fit. Be sure to see that the bearing still has some (not much) slack and rotates freely. A really tight shrink fit will take away some of the necessary bearing tolerance. Some of it is there for the press fit, but you must be sure all the tolerance in the bearing is not used up. As the pulley gets hot and loosens up the bearing will gain tolerance, but it needs at least some for start up.

Bearing.... ?? .. is this an idle pulley?

Also, does this pulley by any chance set close to an exhaust stack as in a turbo charger header? I'm just wondering if the pulley is getting hotter than you think. If so you are getting mighty hot for a bearing. 300f is knocking on the door of failure. You will cook off the oil and not have any thermo bearier left for the friction heat in the bearing to conduct to without going over the limits. AND, if so this could explain your thermo expansion problems.





The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them.

Modified by randykimball at Mon, Apr 14, 2008, 23:25:28


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Posted by: RWOLFEJR

04/16/2008, 16:25:33

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Popped on and saw this post and wanted to add a quick thought. Remember that the thermal expansion changes in size are from room temp. so 250 is only about 180-ish change. Also bearings can be had with suffix C3 (I think) that will be supplied with looser than normal internal clearance to accomodate both internal and external shrink fits on the same bearing. Thinking C1, C2, C3 are the options. Don't recall if C2 and C1 are necessarily calling out levels of loose but all refer to internal clearances.
I'd go .003/.0035 shrink and standard bearing and would feel really good that it won't go anywhere and it'll spin fine. The idler should move more than the bearing at shrink so long as you are very round or circular. If out of round then you'd have a better chance of crushing the bearing. Also you could just heat your pully to about 400/450 in your oven so you don't have to mess with freezing the bearing.

Another option would be to cap the bearing and pinch it width-wise.
Hope all goes well.








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Posted by: CCR5600Design

04/18/2008, 11:37:50

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Ok, maybe I am thinking too far outside of the box, but what would be wrong with machining a recieving groove in the hub for an internal snap ring to locate the bearing and keep it retained?

Ron





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