O-ring material selection 1

[Amberten]

Hi,

I'm new to this forum stuff and I'm not sure if this is the right place to ask, so if you could kindly read on and if this is not the right place, could you possibly point me in the right direction?

I'm in the process of designing a water diverter valve for use in a domestic product that will divert water flow (max pressure 5 bar) from one port to another, and want to use o-rings for sealing purposes.

Basically my design comprises of 2 parts (intended to be injection molded plastic parts) in a piston/internal bore configuration with the o-rings seated radially in grooves externally on the piston part. It is intended that the valve be hand operated over a quarter turn, and this is where I need some help ... My concern is, if I do not specify the right o-ring material and groove dimensions, the piston part will 'stick' in the internal bore and will not be able to be operated by hand, which will defeat the purpose of the design! (and it's got to be watertight!)

I have done some extensive research, using various o-ring design guides available on the internet, but my application seems to fall between the accepted applications of 'static' and 'dynamic' sealing methods ... So, any suggestions would be greatly appreciated.

Thanks for your time..!

 

[DesignerGuy16]

http://www.parker.com/literature/ORD 5700 Parker_O-Ring_Handbook.pdf

That's what I use, pretty extensive. Don't know if you've been through that or not yet.

James Spisich
Design Engineer, CSWP

 

[Eltron]

Yep, I use Parker unfailingly.

Dan

http://www.eltronresearch.com

Dan's Blog

 

[Pud]

Apart from the O ring grooves etc, you will have to consider the effect of the plastic body/O ring combination. Fortunately, acetal is a popular choice for potable water under continuous pressure due to it's good long-term creep behaviour. It is also a good material for other polymers to slide against.

http://www.tynevalleyplastics.co.uk

 

[Amberten]

Thanks for the info.

I have read the Parker guide, but couldn't identify a suggested method of sealing for my application. I think 'dynamic' sealing is the closest, but in my application the part is not reciprocating.

Pud ... Are you suggesting that the plastic molded parts be made of acetal, or is that the o-ring material?

 

[DesignerGuy16]

Static would be if the seal is never going to move after install.

Dynamic would be moving during operation.

James Spisich
Design Engineer, CSWP

 

[DesignerGuy16]

Plastic out of acetal would be good, as it holds its tolerances very well, doesn't swell, etc like UHMW/HDPE do.

O-ring material would probably be a viton or a buna-n material (most prevalent).

James Spisich
Design Engineer, CSWP

 

[Amberten]

I think I'm getting somewhere...

I've done a bit more research on the internet, and the o-ring seal I'm trying to accomplish is referred to as an 'oscillating' seal (eg. faucet valve). The general consensus being use the same design guidelines as for a 'reciprocating' seal.

I intend to use an o-ring with a cross section size of 3mm and according to the design guidelines the groove needs to be 2.6mm deep and 3.9mm wide. Although I have read somewhere that it might be advisable to reduce the 'squeeze' on the o-ring to reduce friction.

Which is why I'm concerned about material choice for the o-ring, as I don't want the two parts being sealed together to 'stick'. Would lubrication of the 0-ring at installation alleviate this?

Any suggestions welcomed...

 

[MikeHalloran]

Lubrication of a dynamic seal o-ring is mandatory.

In this case, a fingerprint's worth of silicone grease would probably be sufficient.



Mike Halloran
Pembroke Pines, FL, USA

 

[Pud]

Amberten:

The body - not the O rings.

Ticona (used to be Hoescht (sp?)) have had water valves on continuous pressure test for over 20 years in Germany (just to make sure, they say). They will tell you all you need to know about water valves and their like.

Not sure where in the World you are, but Ticona have Technical Support in most places on the planet.

H

http://www.tynevalleyplastics.co.uk

 

[EdDanzer]

Be sure to consider the effects of chlorine on acetal or delrin.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[HDS]

Again I vote for Parker. The facet handle is in the examples in the handbook. They also have an application on their webpage to find the correct material and groove dimensions.

Even with the guidance you will need to plan for testing and iterations to refine the design. Make you mold dimensions "steel safe" so it if things change you can do it by removing steel instead of adding it.

P.S. if you use someone's design guides specify them as the brand of the seal or material.

 

[synapseengineer]

Whether or not it is a dynamic or static seal, I believe that lubrication is always a necessity. O-rings work by moving dynamically in the gland to the point of greatest pressure differential across the gland. That is also why you will not want to crush the seal too much. If the o-ring doesn't move, then its ability to be a long term seal deteriorates over time. Using lubricant also create a temperature gradient to the o-ring. In my application, we use o-rings in temperatures up to 500c+, and lubricant takes the heat cycling and not so much the o-ring.

 

[hydtools]

Use PTFE coated or PTFE impregnated o-rings if you can not use a lubricant.

Ted

 

[Amberten]

Thanks everyone, for all your help.

After considering all the design requirements for o-rings and the close tolerances that are needed (that I'm sure will be difficult to attain with plastic molded parts), I have decided to use a 'T' shaped rubber extrusion to create the seal between the parts used in my design.

Again, thanks for your time!