Spool valve seal design 3

[addax]

I am a mechanical engineer by training and my knowledge in the world of hydraulic valve design is quite limited so if my post needs clarification please advise.

I am designing a hydraulically activated 4/2 spool vlave that will work at relatively low pressures (10 bar max). The through fuid is water and the pilot fluid is also water (filtered at 20 microns) The body will be injected plastic rather than traditional machined or cast metal. The spool would be stainless steel.
I have cut open a Festo pneumatic valve and discoverd the sealing elements for the spool are metal rings with the sealing element embedded in the ring. In my searches I have stumbled upon something called bonded seals which look to be the exact same design but are applied as sealing washers for bolts in high pressure hydraulics. Although the materials available are correct, stainless and NBR I immagine tolerances are not acceptable for my application. The goal is to create a relatively cheap spool valve with similarly low performance requiremente. The tight tollerances required will be achived by the rings (not the plastic where the tollerances can't be maintaied). Can these sealing rings be bought OEM? Is there another type of sealing ring that would be appropriate.

Many thanks in advance

 

[hydromech]

If you have to seal the spool, then your best bet will be an 'O' ring of some sort and material.

It is unusual to seal the spool in the bore though...what are you trying to seal...again external leaks...water leaking out or just to stop water leaking from one flow path to another.

Adding grooves to fit the seals will add cost and will also make the valve more diffcult to move...how difficult depends on the materials that you use.


I think the first thing to do is to look and see if you really need to seal the spool at all.

Hydromech

 

[addax]

The advantage of sealing in the bore is manufacturability.
this valve will be incorporated in a larger hydraulic logic circuit.

small leakage, external, beteen water paths or from pilot to water path is acceptable.

My idea with the o-rings eencased in a secondary metal ring is that there you can maintain the tight tollerances.
I put a couple immages on web that I think will do a better job of explaining what I am trying to do than I can with words

http://www.addax.es/Spool1.html

here also is a link to bonded seals

http://supaseal.co.uk/bondedseals.htm

 

[MikeHalloran]

With a liquid medium, the valve spool would stick in either of the extreme end positions, because the end of the spool and the end face of the pilot cavity would form an effective seal against each other, so pilot pressure would not reach the end face of the spool at all, or would reach it after some indeterminate time delay. The spool needs to have a 'bump' on each end so the pilot pressure can act on the full end area of the spool. Machining roughness accomplishes the same thing in a pneumatic valve.

The horizontal split of the sleeve/ housing is very unusual in both air and liquid valves.

I have also seen the rubber/metal rings. Chances are you'll end up with a custom part, even if they happen to be available in the geometry you need.

Using a seal as a sliding bearing is likely to produce surprises.



Mike Halloran
Pembroke Pines, FL, USA

 

[addax]

You are correct in pointing out the omission of the bump. The drawing was a rough render, I should have included more detail.

The horizontal split of the housing is unusual - but given that the housing is not performing any critical funcion I'm hopefull it will work - insights welcome.

regarding using a seal as a sliding bearing - ¿by that you mean that the only contact with the spool is the seal and therefor is the guide/slide bearing? ¿Would it then be interesting to use a wear ring/bearing in addition to the seal? It sound like what I need for this design to work is a seal and wear ring attached to a rigid backing.

this is a very uselful exchage for me all posts welcome and appreciated

 

[budt]

Take a look at these sites for some available designs.

Hydraulic:

http://www.elwoodcorp.com/82_PDF.pdf

Check out many of the pneumatic valve companies. Many have spool valves with sealed spools. Their pressure is also high enough for your application. Probably have to do something with the solenoid operators exhaust though.




Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING, INC.

http://www.fluidpower1.us

 

[MikeHalloran]

I used to have a Marketing guy who claimed to be a ChemE, and used the word "hopefully" a lot, usually in reference to an expected and necessary suspension of the laws of physics. I still cringe when I hear it.

Yes, I think you'll need wear rings/ bearings in addition to the dynamic seals.

I'm a little fuzzy on how you effect static sealing and retention of the metal/rubber rings, and how you'll effect a seal against external leakage at the housing split.

A general caution about re-use/ mis-use/ derivative applications of existing products, e.g. the rubber/metal seal rings: Try not to focus on them as 'a' or 'the' solution to a problem. Back up a bit, define the problem you're trying to solve, synthesize a couple of different and/or traditional solutions to it, and evaluate the trick parts against that set of solutions.



Mike Halloran
Pembroke Pines, FL, USA

 

[budt]

Here is another hydraulic valve that uses seals on the spool. They don't have any bypass until the seals wear out.

http://www.huntvalve.com/

Herion is one pneumatic valve that has an excellent seal design for the spool and could run liquids.

http://216.87.71.202/norgrenValves/default.asp?reid=A3U297487DE#herion

Bud Trinkel, Fluid Power Consultant
HYDRA-PNEU CONSULTING, INC.

http://www.fluidpower1.us

 

[EdDanzer]

I see a couple of problems with your design:

1. How will you structurally segregate the pressures in the different chambers?
2. When oil moves under pressure and speed it will cut most materials like a water-jet cutting machine.
3. How do you plan on sealing the halves? Flat surfaces can be difficult to seal at high pressures.

You should post the working and peek pressures, shift time and flow rate across the spool and seal area.

Have you looked at flow induced spool shifting problems?