[modeleng] Re: O rings

There are 2 types of sealing methods for reciprocating seals, when using 
"O"rings.
The first, and cheapest comercially, is the so called rolling ring (it 
doesn't actually roll at all). These are used
generally for compressed air equipment, where waistage and leaks are not an
issue. The second, and most expensive commercially, is the fixed ring. This
is the method used in hydraulic seals etc, where waistage and leakage is an
issue.

The difference is in the clearances within the ring groove. With all types 
of seals, the pressure on one side of the "O"ring pushes the "O"ring against 
the other side of the goove, and aginst the shaft. Once the "O"ring is 
pressed up against the side of the groove it deforms, and the force from the 
pressure holds it there. It does not move. The greater the pressure the 
greater the deformation, the better the seal.
With the hydraulic seals, the "O"ring is already partially compressed by 
both the groove and the shaft before the pressure acts against it. Generally 
the ring should be a neat fit in the groove before the shaft is in place. 
The clearance is determined by both the ring thickness and the shaft size. 
The greater the pre-compression, the better the seal, up to a point. This 
also requires lubrication of the "O"ring/shaft contact area to facilitate 
shaft movement and a long life of the "O"ring. This is not going to happen 
in our models to a large enough degree, so hence the force required to move 
the shaft through the "O"ring increases dramatically. This reduces the 
available force to do other things, like drive wheels etc. These seals work 
well in axle pumps and crosshead pumps, as the water acts as a lubricant, 
and they do not allow the ingress of air to the system.
With the (so called) rolling ring, the groove is wider and deeper which 
reduces the compression of the "O"ring to almost zero. The only forces on 
the "O"ring before the pressure enters the seal region, is a tensile force 
from the stretch of the ring over the shaft. Once the pressure is applied to 
the seal, the "O"ring is pushed against the side of the groove and shaft by 
the forces, thus creating the seal.

The problem with using hydraulic types of seals on steam, is the expansion 
of the "O"ring material due to both the temperature rise and the absorbsion 
of water. All of the "O"ring materials are hygroscopic. Which means that 
they absorb water and swell with it.this therefore requires larger 
clearances again above those used for compressed air service.

I hope this gives a better understanding of the different types of 
reciprocating seal designs. For more information, you can D/L the Parker 
"O"ring catalog, which has more than enough of the basics to let you design 
your own seals.


Cheers,

Phill.



----- Original Message ----- 
From: <deanrise@xxxxxxxxxxxxx>
To: <modeleng@xxxxxxxxxxxxx>
Sent: Friday, November 21, 2008 8:21 PM
Subject: [modeleng] Re: O rings


> The experts in our Hereford club advocate that an O ring should roll
> when in contact with a sliding surface. You have the make the groove
> large enough, so that the ring is not gripped. The groove should have a
> slightly bevelled edge as the ring wil try to leave the groove and you
> don't want to damage it. The ring should be slightly compressed, but
> not enough to trap it in the groove. The theory seems to be sound for
> water pumps and the like.
>
> Andy
>
>>----Original Message----
>>From: clif.gwr@xxxxxxxxxxxx
>>Date: 21/11/2008 10:58
>>To: <modeleng@xxxxxxxxxxxxx>
>>Subj: [modeleng] Re: O rings
>>
>>Hi All,
>>
>>I tend to use Viton O rings for steam and sliding joints, the only
> snag
>>being that untill I read Craig's e-mail I did not realise that there
> were
>>different types so perhaps I have been lucky.
>> Long before Martin Evans started specifing O rings for the steam
> pipe
>>joints at the front tube plate I had worked out that it was far
> easier to
>>make a compression joint using O rings than all the fun and games of
> trying
>>to screw a bush onto the steam pipe from the regulator while at the
> same
>>time screwing the bush into the front tube plate and get it all steam
> tight.
>>I also make compression joints for the steam and exhaust connections
> to the
>>steam chests.
>>I have never used Silicone O rings for sliding joints,despite the
> fact that
>>ME suppliers recommend them owing to the fact that the O ring
> manufacturers
>>say that Silicone cannot mechanical wear. Any comment please Craig?
>>
>>Regards
>>
>>Clif
>>
>>
>>
>>----- Original Message ----- 
>>From: "Craig Gluyas" <craig@xxxxxxxxxxxxxxx>
>>To: <modeleng@xxxxxxxxxxxxx>
>>Sent: Friday, November 21, 2008 10:08 AM
>>Subject: [modeleng] Re: O rings
>>
>>
>>> As a former seal specialist for a valve manufacturer, i guess this
> is my
>>> area!
>>>
>>> O-rings are very suited to our uses, we just have to be careful
> what
>>> material to use. Silicone is about the best & most widely suitable
> for -15
>>> to 200'C. Other materials vary, even to within blends. For example
> Viton A
>>> is not recommended for use with steam where as Viton B or Viton GF
> is.
>>> Viton is good for about the same temperatures as Silicone.
>>>
>>> Other seals such as EPDM and Nitrile are readily available, but are
> less
>>> tolerant to chemicals (Natural Rubber does not like Oils for
> example)
>>>
>>> An excellent compatibility website used in the industry is
> available here
>>> http://www.coleparmer.com/techinfo/ChemComp.asp
>>>
>>>
>>> Hope that helps!
>>>
>>> Craig
>>>
>>>
>>>
>>>> There is a discussion on a message board about the use of O rings
> in
>>>> model
>>>> steam engines.
>>>> Both the types and where they are, or are not, suitable, together
> with
>>>> the
>>>> advantages /disadvantages of them compared to other forms of seal.
>>>>
>>>> What opinions do members here have?
>>>>
>>>> Alan
>>>>
>>>>
>>>>
>>>> MODEL ENGINEERING DISCUSSION LIST.
>>>>
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>>>>
>>>
>>>
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