Dissassembling a Press fit

[monkeywrencher]

I have seen other topics on disassembling a press fit assembly. But I was curious what others would do with the tools I have available.

Specifically, I have a 1.250 OD steel pin going into a 1.240 ID set of aluminum sideplates. We use a hydro-electric press to push the steel pin into the sideplates, using a alum block to keep the plates a static distance apart. One plate tweaked after the spacer block spit out during the pressing, now the pin and one sideplate is bent.

There is alot of tension in this part, as the hyd press will not push this pin out.

I have a LED heat gun, and might be able to get dry ice. Liquid N2 would be harder to come by. Would it work to use the heat gun to expand the plate ID, then after heating, then drop dry ice into the hollow steel pin? Will the heat gun be too sploppy? Should I try to apply heat via conduction rather then convection? Thanks in advance!

 

[MikeHalloran]

LN2 in the pin may be enough all by itself, now that the aluminum part has been permanently deformed.

.. but given that the parts are farkled anyway, why bother trying to reclaim anything?

That's a huge press fit; are you sure you need that much?



Mike Halloran
Pembroke Pines, FL, USA

 

[monkeywrencher]

Its for a landing gear assembly. The sideplates are probably screwed, but the rest of the leg subassy is still good. I should see if I can get the LN2...

 

[Compositepro]

Sometimes a hammer IS the most appropriate tool.

 

[berkshire]

Not on a landing gear.
B.E.

 

[RPstress]

You have a worst case interference strain of maybe 8 millistrain. If the pin and lug were equally stiff it'd be shared to give maybe 4 millstrain, best case. (Also depends on how plastic the Al went.)

Dry ice would thermally strain the pin maybe 1 millistrain (delta-T -100 K, CTE martentitic steel ~11 microstrain/K). Getting the Al to expand by up to, say, 5 millistrain would need an increase in temperature of 200°C/375°F if Al CTE is 24 microstrain/K. (And bear in mind, because the two are in intimate contact, the temperature difference will be less in the vicinity of the I/F than you might think/hope.) After that the Al will almost certainly need re-heat treatment; depends on the alloy.

LN2 might get the pin to shrink by 2 millistrain. You'd still need quite a bit of heat on the Al.

Can you not cut it out? I assume the steel pin is the most expensive bit, although you do say it's bent.

Concur with MikeHalloran; that's a lot of interference. In the past we'd have used both heat (~100°C on Al alloy) and LN2 on the pin and a lubricant like PR1302 (I think that was what we used) and a press...

Can't help feeling that just putting it in with lube (I assume) is potentially going to give an interesting finish in an Al bore. (Any bushings? Getting those off is another little problem.) NB: Personally not worked with LG before, so not familiar LG common practice.

 

[WKTaylor]

Drilling-out the steel pin [small drill thru the center, then incrementally increasing the ID] till it loosens or crumbles is another time-tested method.

Regards, Wil Taylor

 

[berkshire]

rawk619 (Aerospace)
Which parts do you wish to save?
Do you need the pin, or can that be sacrificed?
Was this a trunnion fitting or some other part of a landing gear?
Some answers here may get you some sensible suggestions you can use.
B.E.

 

[MikeHalloran]

I don't think I'd want to try landing on an aluminum LG part that had been plastically deformed like that. In fact, I'd cut 'em up so _nobody_ could reuse them.



Mike Halloran
Pembroke Pines, FL, USA

 

[monkeywrencher]

LN2 might get the pin to shrink by 2 millistrain. You'd still need quite a bit of heat on the Al.Can you not cut it out? I assume the steel pin is the most expensive bit, although you do say it's bent.Concur with MikeHalloran; that's a lot of interference. In the past we'd have used both heat (~100°C on Al alloy) and LN2 on the pin and a lubricant like PR1302 (I think that was what we used) and a press...Can't help feeling that just putting it in with lube (I assume) is potentially going to give an interesting finish in an Al bore. (Any bushings? Getting those off is another little problem.) NB: Personally not worked with LG before, so not familiar LG common practice.

We got the pin out today! When the steel pin initially seized, we immediately flipped over the leg and tried to push it back out the way it came in. I stopped when the press pump pressure was around 600 psi, equaling around 10K PSI of force on the ram. We had a couple issues lately with that press and the work being done, so we stopped at that pressure. Didn't want anything to explode while in the press I am not sure if it helped (I was skeptical when the engineer suggested it), we had used a penetrating oil and let it soak in for several hours. We then allowed the press to exert more force then we did the last time (16,000K, which over came the initial friction.)

There are no bushing on the assy. The eng that was looking at the issue wondered out loud about bushings. They would have to both be hatted in the direction that pin pushes in from, or else the pin would pull them allong with it. I was suprised that the Al bores were actually not that bad after pushing the pin out. However, upon intriri-mic'ing them, we noticed that even the new ones are out of spec, on one half. We are suspecting deformation from the previously completely load test(before the leg gets to us).


Thanks everyone for the tips!!!!

 

[monkeywrencher]

Mike,

The aluminum sideplates sprung back to their orgiginal positions. We are assuming that the elastic limit was not exceeded, because they returned to their starting position.

Do you think there may have been granular damage? We don't have NDI equipment readily available...


Rock

 

[MikeHalloran]

Bushes that were threaded in, and staked against rotation, wouldn't need 'hats' to react axial loads.



Mike Halloran
Pembroke Pines, FL, USA

 

[SWComposites]

Rock - are you an engineer? (you referred to "the engineer" a couple of times). If not, you need to get an M&P engineer and stress engineer involved to figure out whether the lugs are still good.

Are you following a process spec for pin insertion? Does it allow the pin to be pressed back out?

And no, you cannot assume there was no yielding or other damage just because the lugs appear to have sprung back. You said in the original post that one side plate was "bent". A detailed investigation of the parts is warranted.

Are these parts for a military aircraft, an FAA certified aircraft or an experimental one?

 

[SAITAETGrad]

I'd be surprised if another answer than the one provided by wktaylor would be seen as superior.

Good topic. I'm interested to hear what else people have to say.

 

[monkeywrencher]

Hi Composites. No, I am not an engineer. I have about 10 years aviation maintenance background, was active duty for 8 and reserve currently. I now work manufacturing military aircraft. I don't want to go into to much detail as to compromise any proprietary details. I am however, getting more interested in engineering due to my work. I already have a BS in Aerospace management, but am tentatively working on adding an eng degree, not sure which type.

In answer to majority of your questions, I would have to say that we are a growing company. Many of the things you mentioned (pin removal, plate stresses) are not spelled out in black and white. I do see your point about non-visible damage (stress) to some of the components.

The engineer dealing with issue gave us the green light to press a new pin and continue. I think I will ask him about the issues raised here.