[modeleng] Re: O rings
- From: "Jesse Livingston" <fernj1@xxxxxxxxxxx>
- To: <modeleng@xxxxxxxxxxxxx>
- Date: Wed, 26 Nov 2008 12:20:03 -0600
Phill, et al,
When building my small stationary models and to a certain extent
locomotives, I hand fit the pistons into the cylinders to assure that there
is minimal drag. But first, I carefully measure the "O" ring thickness and
cut the groove only slightly less than this thickness. "O" ring tolerances
vary considerably between different makers so this can be a problem with
small stationary models. With the piston still in the lathe, I oil up the
cylinder and try the fit. If too snug, I will deepen the groove by maybe a
thousandth and try again. BTW: If you do happen to cut a groove too deep,
a narrow band of paper under the "O" ring will cure the problem and last
forever it seems.
Jesse in 45 degree F Sunny Troy, TN
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
270.9.10/1812 - Release Date: 11/25/2008 7:53 PM
This email was cleaned by emailStripper, available for free from
http://www.papercut.biz/emailStripper.htm
MODEL ENGINEERING DISCUSSION LIST.
To UNSUBSCRIBE from this list, send a blank email to,
modeleng-request@xxxxxxxxxxxxx with the word "unsubscribe" in the subject line.
Other related posts:
