Seal materials

[MrClean]

I’m working on designing a seal to keep contamination away from a potentiometer in a greasy environment. The 1st pass was bare aluminum lightly spring loaded against Delrin 500. The surface roughness on the aluminum was about 63 micro-inches. Contact pressure about 7 psi. Action is reciprocating at 2 fpm. There is no pressure differential across the seal. At 30,000 cycles, the Delrin was badly shredded, though not badly enough to disable the pot. The aluminum showed little wear. The seal needs to last at least 10 times as long without serious damage.

I am considering changing the plastic to Nylatron GS, oil filled UHMW, oil filled nylon 6, or Delrin AF, specifying a smoother finish on the aluminum, as well as circumferential surface grain direction, and possibly anodizing or hard anodizing the aluminum.

Lightly lubricating the seal at assembly is possible, though subsequent lubrication is not.

Due to space limitations, purchasing an off-the-shelf sealed potentiometer is not an option.

Does anyone have any experience with these plastics or others in a similar application?

Any other thoughts?

 

[Eltron]

I'm having difficulty picturing your seal. It sounds like a reciprocating cylinder seal. Am I way off base? Maybe if you describe the system a little more we could offer some (more educated) suggestions.

 

[Philrock]

It's a rotary face seal - a narrow annulus - .796" i.d. x .800" o.d., spring loaded in the axial direction.

 

[Eltron]

Take a look at Rulon or PEEK. Both are good bearing materials with good wear reasistance. However, they are more costly than your above listed materials.

 

[EdDanzer]

Nylon and UHMW will change size with humidity. Too rough of a surface, and stainless might work better.

 

[MikeHalloran]

You should investigate the "V-ring seal", originally introduced by Forsheda, now available from other sources too:

http://www2.chicago-rawhide.com/v_rings.htm

It assembles axially over a shaft, and retains itself fairly well even if not trapped by a coupling or knob. A thin stainless washer makes a nice mating surface, and a dab of light silicone grease will help it survive a while.

;---

Anodizing aluminum produces a micro- cracked aluminum oxide ceramic surface that will abrade nearly anything. You can use it to hone a knife blade. Not good with dynamic seals.





Mike Halloran
Pembroke Pines, FL, USA

 

[Cockroach]

Rotary seal, you may have a gland problem.

Standard percentage compression for rotary applications are 0-11% for 004-045 elastomers, 1-8.5% for 102-163 and 0-7% for 201-258. In other words, your gland depth is 0.065/0.067, 0.097/0.099 and 0.133/0.135 inches respectively and gland width is 0.075/0.079, 0.108/0.112, 0.144/0.148 inches.

Have you checked the gland geometry? Sounds like you have excessive compression on the elastomer, possibly gland fill.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[MintJulep]

I have successfully used delrin (I don't recall what grade) against Teflon-hardcote anodized aluminum with a 4 micro-inch finish.

Worked fine and lasted longer than the rest of the machine.

 

[unclesyd]

Here are some possibilities if the action is reciprocating and you have the room. I've used all three in the design of machines. We used the small metal bellows on numerous machines.
Metal Bellows

http://www.servometer.com/products.asp?l=2

Elastomeric Bellows

http://www.edtcorp.com/a.products/i-miscproducts/i.c-bearingglove1.html

Convoluted Tubing

http://www.zeusinc.com/convoluted_tubing.asp

If the motion is rotary, even recip, you contact these people as they make some very good small seals.

http://www.balseal.com/techLib/catalogs.jsp