'O' Ring Guide

Materials & properties
FEP encapsulated ‘O’ rings
These have a core of elastomer that is
completely covered with a seamless
sheath of fluorinated ethylene propylene
(FEP). The core is normally fluorocarbon
(FKM) or silicone (VMQ).
Encapsulated ‘O’ rings are generally used
when:
l A standard elastomeric ‘O’ ring has
inadequate chemical resistance for a
specific application, and
l A solid PTFE ‘O’ ring does not offer
sufficient elasticity for reliable, long-term
fluid sealing.
They are used where high levels of
chemical resistance or hygiene are
needed — typically in petrochemical,
chemical, food or pharmaceutical plant.
Although FEP encapsulated ‘O’ rings are
most suited to static duties, they may be
used with slow short movements on rotary
applications such as valve stem sealing.
Their advantages are manifold, including:
l Excellent chemical resistance to a wide
range of media. Please contact our
Technical Support Team for details.
l Operational temperature ranges of:
-60°C to +200°C with silicone core.
-20°C to +200°C with fluorocarbon core.
l Low friction and low ‘stick-slip’ effect.
l Far greater elasticity than solid PTFE.
Our FEP encapsulated ‘O’ rings are fully
interchangeable with standard ‘O’ rings.
However, due to the FEP sheath, they are
less flexible than elastomeric rings and
have limited stretch with higher permanent
deformation. Auxiliary tools may be
needed to facilitate efficient fitting.
Back-up rings
Back-up rings are installed to prevent
the extrusion of the ‘O’ ring. They are
manufactured from Fluolion ® PTFE (virgin
or filled), and PEEK TM .
They are recommended for applications
where:
Groove
length
1 back-up
Groove
length
1 back-up
Groove
length
1 back-up
Groove
length
2 back-ups
Groove Pressure
length from
2 back-ups either
direction
Groove
length
Groove
Groove
Pressure
1 back-up
length
length
from
Direction of pressure Direction of pressure
either
2 back-ups 2 back-ups direction
Pressure
from
either
direction
Pressure
from
either
direction
l System pressure exceeds 10MPa
(1450psi), or ‘O’ rings of low strength
elastomer are used, or
l Adverse mechanical conditions exist.
Direction of pressure Direction of pressure
Please consult our Technical Support
Team if system pressure exceeds 42MPa
(6092psi).
Two back-up rings — one either side of
the ‘O’ ring in its housing — are needed
when the application is double-acting.
Our back-up rings are usually supplied
as a spiral of two turns. This enables the
back-up ring to be opened with ease for
fitting over a shaft and ensures the ‘O’ ring
is supported around its entire diameter.
Single-turn back-up rings can also be
supplied, and these are usually endless to
ensure good support. However, they can
be scarf split if required, although we do
not recommend this because extrusion
can occur at the split.
Back-up rings less than 3mm ID are
available only as a single turn — 3mm ID
is the smallest size suitable for machining
as a two-turn spiral.
12
To order or get further details, call your local contact shown on rear cover or listed at www.jameswalker.biz