UHMW bushing issues... 4

[RikuY]

I have a slow speed bushing application for a project. The specifics are:
25 rpm max.
2" galvanized steel shaft
4 bushings per shaft
1500 lbs. per bushing
bushing I.D. is 2" O.D. is 3" and length is 2"

We have a UHMW PE (virgin homopolymer) bushing that is wearing okay. However, the bushing is bored a little over to account for tolerance of the cheap galvanizing. The problem is that the bushings chirp. I believe the "slop" of the hole is allowing the shaft to climb the bushing face and make noise as a result. The rest of the structure is aluminum, and therefore, is a tuning fork for this chirp as well.

We found the option of an oil-filled UHMW PE product that may lubricate the bushing enough to rid the chirp, but I am wondering if there are any other alternatives. Obviously, the cost is going to be an issue of this specialty polymer.

We are trying to design this to be end-user maintenance-free. Thanks in advance...

 

[RikuY]

Also, I should add that this is in a salty marine environment. The product may run for 15 minutes a day max.

 

[kjoiner]

RikuY,

You might want to check out a company called Pobco at

http://www.pobcoplastics.com.

They make bearings out of various materials including hardwood. Their hardwood bearings have the best PV values compared to other materials. I think they also use those types of bearings in some nasty environments including waste treatment. They might be able to provide some information/advice.

Kyle

 

[patprimmer]

You could try Acetal as it is harder than PE. It is not as forgiving as PE to dirt but has a better PV and might be quiet.

Acetal will not take as much set from the uneven surface, so the start up torque and noise factor might both improve.

Nylon is somewhere between PE and Acetal in most regards re bearings.

Polyester is another possibility.

The zinc from the galvanising, together with chlorine from salt water might causing stress cracking in Acetal and nylon, but I don't think so as many yacht fittings are in Acetal and nylon.

Regards

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[Demon3]

I should make it clear that I know next to nothing about bearings. I do know that graphite filled grades of nylon and other polymers are available and used for bearings due to the extra lubrication from the graphite. Likewise oil or MoS2 filled grades are available as well and should stop your problem.

 

[RikuY]

Good point on the Acetal.

The other thing that is a problem is water absorption. Nylon has been attempted before by competitors. Needless to say, the Nylon bushings took on water and seized the shafts!!

I think the market has a dirty taste in their mouths from the Nylon experiences, but I should look into that as well.

I am really appreciating this thread right now. Thanks to all of you and please keep it coming

 

[Demon3]

Yes, nylon not only adsorbs water but the properties change drastically depending on the water uptake and humidity. The modulus is halved by water uptake and the impact resistance goes way up. No-one like designing in a material where the properties change from day to day. Also of course the water uptake is likely to be accompanied by swelling so the size of your part bushing is likely to change as well.

 

[unclesyd]

Galvanizing isn’t a normal finish for a shaft.
It will be hard to pick a bearing material that you can predict it’s performance. I think a composite bearing might be a better option than a glass filled nylon based on my experienced around water.
I have also seen and used wood bearings, as mentioned above, in similar cases as yours.

This company has about any bearing material you desire and did have a very good technical department. You will have to register to get to the good stuff.

http://www.tstar.com/default.asp

This is a composite bearing supplier.

http://www.polygoncompany.com/

 

[PerBear]

On

http://www.igus.co.uk

you will find over 7000 various polymer bearings under the Iglidur trade name. It is a German product and they have special material for subsea use (ROV's etc).

 

[engineeringplastics]

UHMW-PE is an extremely soft material and has a very low PV limit of 2300 whereas PA6G (cast nylon) with an oil filler has a PV of 16,000. This oil lubricant however is only effective if there is constant pressure and speed which brings the liquid lubricant to the surface. If it is not constant try MOS2 filled nylon.
The materials are available in stock rod from major suppliers such as Dotmar or E-Plas.

 

[RikuY]

PerBear,
Thanks for that. We are currently testing the Igus products. I have been a little too busy to post this in the thread.

engineeringplastics,
Thank you as well. You might be ahead of your time. The more and more we test, the more it seems you can't replace a cheap nylon bushing. Cheap being the operative word.


Thank you all for the information to date. Once the testing is done, I will report back on the results. Maybe this thread will help someone down the road with the same problems. You all sure have helped me.

 

[patprimmer]

Igus do not declare the polymer types, and I suspect they use a wide variety of polymers and fillers ranging from Nylon, to UHMWPE and XLDPE and Acetal and Polyesters and PTFE and wood flour and cotton fibre reinforced Phenolic.

As they specialise in bearing solutions, their advice and product should be good.

If the usage rate increases to the extent that semi finished is not economical vs injection moulded, the compound can always be analysed and a compound made by a speciality compounder.

Nylon is a very good all round bearing material. It's main weakness is dimensional stability as it swells a little when it absorbs water. This results in it initially tightening on the shaft then becoming loose if free to expand, or just getting tighter if effectively restrained by a saddle or bearing cap.

Regards

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