Basic O-Ring Seal Design Criteria

Written by Dale T. McGrosky

There are so many different types of seals and sealing principles that to try to cover them all would
require writing a book. So, to cover the basics we will talk about design criteria for O-ring seals. This
should give you a basic start into sealing principles and what to consider when designing a seal for your
application.

Types of O-Ring Seals

1st thing to consider is how is this seal going to seal? Is it static, nothing moves once installed, or
dynamic, the seal or sealing surface rotates or reciprocates in the application. Does the O-ring seal
axially or radially. Axially is when the O-ring is squeezed from the sides perpendicular to the parting
line or, if you think of the O-ring as a wheel, in line with the axle. Radially would be squeezed
perpendicular to the axle or in line with the O-ring parting line. So, we have static axial, static radial,
dynamic axial and dynamic radial O-ring seals. Each type of seal is going to have it own design criteria
to consider. There are other O-ring seal types like thread seals, tapered seats, or boss fittings which we
will not consider in this article. SAE.org is a great place to start you search for design criteria. They
have many military and aerospace design documents available for sale. A great start is ARP1231,
ìGland Design, Elastomeric O-Ring Seals, General Considerations.î This specification covers many
aspects to consider in O-ring seal design. Aerospace recommended Practice ARP1232, ARP1233 and
ARP1234 cover O-ring gland seal design for the AS568 series O-rings. These ARP documents contain
groove dimensions and stretch and squeeze specifications. These specifications are a great starting
point for a custom O-ring seal.

A couple of O-ring design flaws we encounter the most is excessive stretch and not enough groove
width. The O-ring should not be stretch more than 5% max. Also, rubber O-rings are subject to the
Poisson’s Effect (Poisson’s ratio). When solid rubber is compressed in one direction it expand in the
other direction. For practical purposes, rubber is non compressible and you must account for the
Poisson’s Effect this in the design of your groove width.

Tolerances

Many of the design specification take into consideration the tolerances of the O-rings in their given sealing gland dimensions. However, when you are straying from a standard design specification you must take into consideration the applicable tolerances for the type of seal you are designing. Will the seal work on the low end of the tolerances and also on the high end of the tolerances? Simple calculations can be done to check the seals stretch and squeeze at each end of the applicable tolerance
range. Don’t forget to consider the tolerances of all parts associated with the sealing gland.

Compound Selection

There are 36 types of elastomer compound and the proper selection is an important part of your design.
Selecting the wrong compound can cause premature failure in your application. Operating temperature,
fluid resistance and whether the seal is dynamic or static are the 3 main questions I ask when selecting
a compound. Weathering or ozone exposure , friction characteristics, abrasion resistance, compression
set, elongation, tensile strength are other physical properties to consider in your selection.
Below are the more popular material that are readily available. There are also many chemical
compatibility charts available on the internet to assist you in selecting a compound suitable for the fluid
in your application.

Common Name

ASTM D1418

Chemical Name

Nitrile, Buna

NBR

Acrylonitrile-Butadiene

* sometimes referred to as Buna-N

Ethylene propylene, EP

EPDM

Viton®

FKM

Silicone

PVMQ

Polysiloxane

* Not to be confused with the chemical element Silicon

Fluorosilicone

FVMQ

poly (trifluoropropyl) methylsiloxane

Neoprene®

CR

Chloroprene

Hydrogenated Nitrile, HSN, HNBR

HNBR

Hydrogenated Acrylonitrile-Butadiene

Styrene Butadiene

SBR

Styrene Butadiene

* Initially marketed as as Buna-S

Natural Rubber

NR

Cis-polyisoprene

Isoprene

IR

cis-polyisoprene, synthetic

Butyl

IIR

Isobutene-Isoprene

Aflas®

FEPM

Tetrafluoroethylene-Propylene (TFE/P)

Polyurethane

AU, EU

Polyester-urethane, polyether- Urethane

VitonÆ and NeopreneÆ are registered trademarks of DuPont Dow Elastomers.
AFLASÆ is a registered trademark of the Asahi Glass Co., Ltd.

Hardness

Elastomer compounds range from very soft, 20 Shore A, to very hard, 90 Shore A. Rubber comes in
various hardness’ for several reasons. The sealing surface should range from 8 to 32 micron finish. That
is pretty smooth. There are cases were the sealing surface may be porous or wavy such as is case metals
and plastic moldings. Softer rubbers will fill in the small voids, pits and scratches that are pathways for
fluid to escape. Softer rubbers are also easier to squeeze which can be useful during installation on
some applications.

Harder rubber is most commonly used in high pressure applications, up to 1500 psi, to prevent the seal
from extruding into the space between the two sealing surfaces. This will cause bits of rubber to be
nibbled away eventually leading to the seal failing.

Coefficient of friction is effected by the hardness of the rubber. Softer rubbers will cause higher
breakout and kinetic friction on dynamic seals compared to harder rubbers.

Prototyping

Before going to production on your new seal, getting prototypes made is a great way to check your
design work. There are several ways to prototype your O-ring design, cut and splice, prototype tooling, and cast mold via stereo lithography.

An O-ring can be made from cord stock or other o-rings that are cut to length and the ends glues
together. This can be done fast and cheap but slight leaking can occur around the splices. A more
accurate samples can be made from a 1 cavity prototype tool. This is more expensive, but less than a
production tool, and does take time to have the tool made and samples run in production.
Another option is to have the seal made from stereo lithography (SLA). A sample part is generated
with SLA. This sample is used to make a cast mold. Cast parts can be made from this. Turn around on
this can be quicker than a prototype tool but this is more costly and the prototype part may be silicone
and not the compound you need for production.