Pin on Steel bearing - tough one. 3

[Biggadike]

I'm struggling with this one..
I've got a sprung loaded pull lever (for a switch) the motion of which is governed by a pin in a slot profile. (This provides detents and self centring etc.) The environment is difficult (unsealed unit on landing gear of aircraft) and I'm having problems with galling on the 4mm dia pin - I can't use lubicants (attract dirt- washes off etc.) I need the pin to slide accross the angled slot surface under the load of the spring.

I've tried:

Phosphor bronze pin onto chemically polished stainless.
(Failed quite quickly)
Stainless steel pin onto chemically polished stainless - both suface treated with oil impregnation.
(Much better but failed at about 15k ops)
I need at least 40k ops - low temp performance (-55dgC to 80degC).
I don't want a little bearing in there as dirt and dust will cause problems.
I would love to use a machined plastic for the slot carrier but delrin will creep (constant load from spring) and glass filled will abrade the pin.

Any suggestions?

 

[unclesyd]

This information will let you become somewhat of an expert in wear and galling resistance of various alloys. It looks like 440C SS, Waukesha 88, Stellite 6B, Nitronic 60 and possibility of a hard Chrome Plate in some combination will be the way to go.

http://www.hpalloy.com/FTP/Alloys/Nitronic60/Nitronic60brochure.pdf

 

[MikeHalloran]

Examine some of the pins you've worn out. They're not round anymore, right? No law says the pin has to be round when it's new, either.

Where I'm going with this is that even the best coatings fail when the substrate fatigues away from under them. You have to reduce the contact stress, by reducing the force or by reducing the curvature.

When the cost of manufacturing a super- hard, exotically curved pin is finally revealed, some money might, er, fall out of the sky so to speak, for redesigning the entire mechanism.

I'm reminded of some guys who think they mass- produce big yachts. Truth is, whichever installation crew reaches a hull first gets to pick where their stuff goes, so the last trade to reach a particular hull faces a really knotty problem of snaking their pipes or cables or hoses or structure around whatever's already been installed. A friend of mine runs a fleet of them, all the same model, and they're all different; it drives him nuts.

Analogously, you've been suckered into patching a design that really should be re-done from scratch, with all affected specialties participating in the balancing act at the same time.



Mike Halloran
Pembroke Pines, FL, USA

 

[Biggadike]

Hi Mike-

Lots of points there - taking them in order..

I know the pin doesn't have to be round but if you could see the detail of the design you'd see that either you let the pin wear to a flat without galling or you'll be running on an edge from flat misalignment. This is largely due to the scale of the mechanism and the simlplicity it needs due to the need for reliability under dust ingress.

You're right about substrates - that's why I need a hard substrate to support my finish, this will also help with wear. A larger pin is possible but this weakens the slot carrier.

I'm afraid the design is all mine and it was done from scratch. Its got to a point where its very simple now and, ironically, the issues I've now got are more to do with failure in qualification rather than failure in the field. (I prefer it that way around).

 

[KiwiMace]

Have you looked at UHMWPE? Creep may be a factor again, but don't readily see it mentioned above. It has similar self-lubricating properties to teflon, but better wear resistance.

 

[Biggadike]

CincMace.

I haven't looked at it although aren't those specialist PE variants all moulded and fairly pricey?

I must admit I'd worry about chemical attack in this application and abuse by users but its a good suggestion and I'll have a look. I know that stuff is tough.

 

[KiwiMace]

wikipedia has a good summary:

http://en.wikipedia.org/wiki/Ultra_high_molecular_weight_polyethylene