Plain Bearing PV values? 3

[EdDanzer]

We have been working on a piston type air intensifier. 100 psi air in, 400 psi air out operating dry. The heat of compression has limited the elastomer sealing material choices to fluorocarbon based materials, and none seem to work well. The barrel of the compression cylinder has been hard chrome plated to reduce wear and the piston travels about 10” per stroke at about 380 ft/min.
When looking at plain bearing design materials are listed with a PV value. P being projected area pressure, and V being the velocity, or speed of movement. The published values are for lubricated bearings.
When sealing a surface air tight, the piston ring and barrel that the ring runs in must be have a zone that is dry with no leakage.
Is there any reliable published data for dry PV values of materials?
What are some recommended materials for this application?
What would be the lest expensive, and fastest way to determine the dry PV of several different material combinations?

 

[unclesyd]

Are you looking for a seal on the piston rod, or a piston ring, or a bearing for the piston rod?

 

[unclesyd]

Try this website out.

http://www.drakeplastics.com/torlon_4435.php

 

[unclesyd]

Try this search on Google or Scirus

"carbon piston rings"

 

[EdDanzer]

The application requires a 2" piston ring.
Torlon is very expensive, and an unknown as a dry bearing/seal material.
Carbon may work but the dust from wear may cause down stream problems, and I've found little data on running it dry.

 

[unclesyd]

On the drake site there are several plastics that have PV data. Also NASA has done a lot of work on Carbon based products for similar applications. Try the NASA Tech Briefs Site. C-C, C-Graphite, C-Graphite+ lubricants.
There is a material sometimes called glass carbon that maybe a possibility.

I think anything that could possibly be suitable is going to be expensive.

 

[kenvlach]

Try this site; they have glass-, graphite-, bronze-, MoS₂-filled Teflon called "Resilon." Specifically mention both lubricated and non-lubricator compressor applications.

http://www.ceconet.com/ring_shop/resilont.htm

 

[EdDanzer]

We have tried bronze filled Teflon with poor success, less than a days life. I have an old Bearings Inc. plane bearing handbook, 1989 copyright. They list glass filled Teflon with a max P of 200 psi, and virgin Teflon with a max P of 500 psi. We have been using Viton U-seals but they do not last as long as we would like, ideally 4 million cycles would be good. The current best is 1.2 million cycles.

 

[unclesyd]

Lets start out on a new site:

http://www.busakshamban.us/prod_global.htm?pid=234

 

[rnd2]

Ed
we sometimes use a crude little test rig that rotates with a fixed shaft speed of about 1.46 ms-1. If my calculation is correct your piston travels at about 1.9ms-1, so its in range. The shaft rotation speed can be changed. The rig is useful to quickly compare materials by dry-testing them as plain bearings under speed and load against temperature rise. A 1" shaft supported by a bearing 1/2 shell is loaded with a given weight so that the shaft presses against the bearing. The weight can be varied. The shaft is rotated at 1100 rpm and the temperature rise of the bearing measured against time. Running within the materials capacity there is first rapid temp rise and then easing to a plateau temp that varies depending on the load the shaft supports. Running beyond the materials capacity there is rapid temp rise followed by equally rapid failure of the bearing material. Depending on the materials make up it melts, extrudes, disintegrates or carbonizes.

 

[EdDanzer]

rnd2
Thanks for the reply.
We have discussed a similar test device, but have some concerns about results. Our objective would be to have values that would be similar to those in design books.
How long do you run a test to establish a PV value?
How much wear is acceptable?
Are there any SAE, ASTM, or similar groups that have established test criteria?

We think surface finish and the method of obtaining the surface finish effects the PV value. Honed, ground and burnished parts can have the same RMS finish, but the shape of the finish should effect wear.

 

[unclesyd]

Here is paper by Peter Min that covers Wear in a very thorough and concise manner. Peter also gives his references.

http://www.solorb.com/dat-heads/digests/V4.100/D100

 

[rnd2]

Ed
The rig really is only an in-house means of culling contenders.
Usually less than 5 minutes.
Given the short duration and severity I doubt the rig is useful for measuring wear. If, after a test, regardless of whether the material failed at a given load, visible grooves appear on the shaft beyond polishing, candidate material is rejected.
Pin on disc

 

[EdDanzer]

unclesyd,
Thanks for the link, I printed it so I can check the references. This raises more questions about what we are doing than solves, but does give perspective of what to look for and how to do it.

rnd2,
If you have a chance to review the information in the link unclesyd provided, your method, and observations of wear are a good way to determine materials.

We should be constructing a test system in the near future to further investigate materials, surface finishes, lubricants, and other factors of wear in this product.

If anyone can provide more ideas of how to construct and monitor a test, I would like to thank you in advance.

Ed Danzer

 

[Compositepro]

There was a good artcle on polymer bearings in , I think, Machine Design magazine a while back. The metal surface texture is very important to polymer wear. Too smooth is as bad as too rough. 32 RMS seemed close to optimum, as I recall.

 

[Boomerang]

EdDanzer
"Carbon may work but the dust from wear may cause down stream problems, and I've found little data on running it dry." & "We have tried bronze filled Teflon with poor success, less than a days life."
Browse through
thread404-32366

 

[rnd2]

Ed
I have had a quick read of Peter Min's Paper. That's a pile of good information that he has sifted through.
You are correct with your observations and good luck with your project.

Unclesyd
Well done for finding the article. Worth a star!

 

[wtcape]

To EdDanzer

You may wish to consider slick, ultra-fine particles of rounded titanium carbide embedded in "any" thermoplastic. TiC as an additive, modified to bond to the plastic matrix, achieves lowest self-lubed friction and potentially highest P-V facors running dry. Access

http://www.ppm.bc.ca

and click on TiC-FineParticles for reference.
wtcape