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?

 

[iainuts]

It's hard to imagine exactly your situation, but in general I wouldn't create an unlubricated metal to metal sliding fit that has any significant load on it. Sometimes there's little choice because of stress, cost or other factors, so I'll leave that for you to decide.

If you can create an joint using plastic on metal, that's your best bet. If stresses are high and creep is a problem, I might suggest some highly filled plastics. I've used Vespel CR6100 which has a very high strength and modulus for a plastic. It's a carbon fiber impregnated Teflon, but that's not a particularly good description.

If you have to have a metal on metal joint, I'd suggest one of the parts being Nitronic 60. I've used that for a stainless valve guide, unlubricated, and it works very well without lubrication. Nitronic 60 is a special anti-galling austenitic stainless with much higher strength than 300 series stainless.

 

[diskullman]

You mentioned using a small bearing , but dirt & dust will cause problems. I assume because bearings usually have grease that will attract the dirt & dust. You could try a bearing with Cobra Solid Lubricant. It is a dry graphite based lubricant that will not attract dirt or dust.

Russell Giuliano

 

[TVP]

I suggest coating the pin with a hard, low-friction coating such as MoST from IonBond or one of the offerigns from General Magnaplate. Both the pin and the slot surface should be as smooth as possible. I would need a lot more information on the application before suggesting base materials for the pin or slot. Use the following websites for more information on the above coatings:

http://www.ionbond.com/startseite.cfm?ID_n=27&haupt=27&language=1&CFID=23307691&CFTOKEN=33595503

http://www.magnaplate.com/coatings/index.html

 

[Biggadike]

I am coating the pin and slot in Micro-mu micro-molecular tribofluid which gives a molecular layer of oil bonded to the suface.
The original trials with stainless steel on both sides went better than you might exppect so I'm now trying hardened a silver steel pin (its an easy test I can do now).

I think a graphite based coating is a possiblity but I am worried about the surface pressures. I've got 30N on a 4mm Dia pin running on two slots cut in a cylinder.

I have electro-polished the slot to give a good surface.

I'll have a look at the coatings you've suggested - please keep them coming!

From a small bearing point of view - I've only got 4mm Dia and imagine the super-fine dust you get in the desert.. I've done my best to baffle the entry but over 20 years of use it will get in there along with ice, de-iceing fluid and all sorts.

 

[TVP]

What is silver steel?

 

[iainuts]

I take it the pin is round where it contacts the slot in the cylinder. That results in the load being distributed along a line at the cylinder/pin interface which results in relatively high contact stress.

Can you make that interface a flat surface to reduce contact stress? If you can make it even a mm or so wide, the contact stress drops by an order of magnitude or more and the resulting wear should easily drop by an equivalent amount. The pin could be square in cross section instead of round, or flats machined into the round where it contacts the slot.

 

[eromlignod]

How about a S/S pin and Garolite for the slot? That's what's used in a lot of electrical switches.

Don
Kansas City

 

[Biggadike]

Answers in order...

1/ Silver Steel is a high nickel 1% carbon steel which has a hardness of 65 Rockwell C when hardened. Its pretty old school.

2/ The slot isn't a straight line - more of a wierd zig-zag, I would need multiple flats on it which would be difficult - but not impossible. Its likely that they would be out of position slightly and it would run on an edge. I take the point though, flat would be better.

3/ I don't know anything about garolite. Most switches are made with a thermoset aren't they? I need to machine my parts from stock I'm afraid.

 

[MikeHalloran]

You need to sleeve the pin. Sort of like the roller on a roller chain, except ... the 'od' of the sleeve has to be square, so its contact with the slot will have a finite surface area. You'll have to open the slot up a little where it turns, so you can slide a square object inside it.





Mike Halloran
Pembroke Pines, FL, USA

 

[unclesyd]

One combination that I've had great success with is nitrated 17/4 vs Nitronic 60. The 17/4 has to liquid nitrated as gas nitriding will not work. I use the Kolene QPQ process.

http://www.kolene.com/applications/adc.asp

One other that you might try is your "silver steel" vs a hard chrome plated "silver steel" pin. Heat treat both then hard chrome plate the pin.

 

[Biggadike]

Again, responses in order..

Mike -

If all of this was bigger, I would do just as you suggest. The whole pin arrangement would need to be approx 4x4mm with a central spindal within that. This thing needs to be durable and reliable in a harsh environment and I just can't feel condfident about such small parts. I know you haven't got visibility of this unit but the whole thing can be held in one hand.

unclesyd -

Generally I've steered away from nitride solutions because of poor corrosion resistance (I need 500 hours salt mist). I need to find out more about Nitronic 60. What is 17/4 ?

Galling and wear are the biggest issues I'm facing. The pin seems to take most of the wear so I've hardened that and I'm just about to put it back into the test chamber but it won't run until next week now...

 

[CoryPad]

17/4 and Nitronic 60 are stainless steels, so you needn't worry about cosmetic corrosion problems due to orange iron oxide. You can look for basic information at MatWeb:

http://www.matweb.com

and at materials suppliers like Carpenter Technologies:

http://www.cartech.com

AK Steel:

http://www.aksteel.com

and Allegheny Technologies:

http://www.alleghenytechnologies.com

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Screwman]

We have had good luck with electroless nickel with PTFE impregnation. It stays on pretty well and resists galling under load. We are putting it onto 302SS.

 

[unclesyd]

CoryPad, has given the better sources on information about N60 and 17/4.

Another possibility is make either part out of hardened 440C SS. This material has given some excellent wear and galling resistance in some sliding applications.

 

[Biggadike]

Thanks guys,

I'm in the UK and, looking at the materials you've mentioned, they seem easier to get in the US. I'll talk to my supplier about UK friendly equivalents for anti-galling, hardenable stainless steels.

Screwman - I like the Nickel plus PTFE coating idea. Do you have any suppliers or a brand name for that?

Chris

 

[Tmoose]

I imagine you've already thought this through, but how is the pin mounted to the pull lever? I'd consider moving the bearing function to the lever, so the pin could roll along the slot

 

[Biggadike]

Tmoose -

This has been suggested a few times - the problem is that its a long, small diameter pin. Even with a good bearing in the centre, the pin wouldn't rotate as the small torque due to the small OD wouldn't overcome the bending forces trying to jam it. Added to that, this is an unsealed unit which will get jammed up with fine dust.

There are plastic bearing materials which I could use which wouldn't creep (Nylon 4.6, PEEK) but looking at the abuse the unit might get in the field a steel solution is far more confidence inspiring albeit harder to get right.

My current thinking, based largely on the feedback I've had from this forum, is to make both the pin and the slot from a gall-resistant hardened steel and then apply a surface treatment to both which is wear resistant and self-lubricating. If I can I'll make the pin a little larger and the slot a little wider to help things along.

 

[Screwman]

Here is a link to the site of the company that we use. I saw that there is a licensee in the UK for Armoloy (the parent company)

http://www.armoloy.com/nyflon25.html

Hope this helps.

 

[Biggadike]

Thanks Screwman,

I did speak to Armaloy and got a quote. I've looked into lots of different surface treatments and its suprising how similar a large range of chemically different processes can be for a given application.
My current favourite is the hard graphite coating. One thing I like about it is that (to my thinking) if it does begin to flake, its still a lubricant. Some of the other coatings are ceramic and I worry that if they start to go they'll put an abbrasive dust all over the surface which will cause a rapid decilne.
The Armoloy coating and some others are very diificult to get a feel of their failure modes as they are a proprietary blend of unknown origins.
Some other processes (like ion based ones which are line of sight) don't give a warm and fuzzy feeling with regards to evenly coating the insides of a slot.