How to Properly Choose Commerically Available O-Ring Cross Sections

How to Properly Choose Commerically Available O-Ring Cross Sections

There are 400+ standard O-ring sizes, so which is the right one for an application?  Or maybe you are wondering if one O-ring thickness is better than another.  This question is posed to us several times a week and I suspect you have the same question if you have searched out this blog.  This short article will walk through some of the design considerations for selecting a standard, commercially available O-ring for an application.
 

How to Properly Choose Commerically Available O-Ring Cross Sections_fig 1_Parker O-Ring & Engineered Seals DivisionDesign Considerations

Hardware geometry and limitations are the first consideration. A traditional O-ring groove shape is rectangular and more wide than deep. This allows space for the seal to be compressed, about 25% (for static sealing), and still have some excess room for the seal to expand slightly from thermal expansion or swell from the fluid.  Reference Figure 1 as an example. Once the available real estate on the hardware is established, then we look at options for the O-ring inner diameter and cross section.

 
 

AS568 Sizes

From a sourcing perspective, selecting a commercially available O-ring size is the easiest option.  AS568 sizes are the most common options available both through Parker and from catalog websites.  A list of those sizes are found in a couple of Parker resources including the O-Ring Handbook and the O-Ring Material Offering Guide. They are also listed here.  The sizes are sorted into five groups of differing cross sectional thicknesses, as thin as 0.070” and as thick as 0.275”, shown in Table 1 below.
 
 

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Table 1 AS568 standard sizes

Cross Section; inches (mm)
Reference Width; inches
Inner Diameter Range; inches (mm)

0.070" (1,78)
1/16"
0.070 to 5.239" (1,78 to 133,07)

0.103" (2,62)
3/32"
0.049 to 9.737" (1,24 to 247,32)

0.139" (3,53)
1/8"
0.171 to 17.955" (4,34 to 456,06)

0.210" (5,33)
3/16"
0.412 to 25.940" (10,46 to 658,88)

0.275" (6,99)
1/4"
4.475 to 25.940" (113,67 to 658,88)

 
 
Most engineers understand that selecting an inner diameter is a good starting point. However, what if all five of these thickness options are available with an inner diameter that the hardware can be designed around? Why choose one thickness seal over another? There are several reasons to go with a thicker cross section, and a few different reasons why you might want to select a thin cross section.
 

Compression Set

Compression Set is a failure mode where the seal fails to return to its original size after time in service. A seal with good compression set resistance means simply that the seal resists “setting” in the shape of the group, so it can function longer as a seal.  There are many inputs to compression set, including material, time at temperature, amount of compression, fluid, to name a few.  If all these variables are held the same, a thicker seal tends to be better at resisting compression set.  
 

Tolerances

A second benefit to a thicker seal is that they are more able to compensate for tolerances in the hardware.  Particularly with the .103” (2,62) size and smaller, which have a tolerance of ±0.003” (0,08mm).  For hardware scenarios with an identical tolerance of ±0.002, since the tolerance on the seal does not decrease as the O-ring thickness decreases, the impact is that the range on the seal compression (Squeeze) increases.  See Table 2 for the squeeze details on three sizes placed in groove depths having a nominal compression of 25%.
 
 

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Table 2

Range of Compression for O-Rings with Same Tolerances in Grooves with Same Tolerances

O-Ring Thickness
Gland Depth Tolerance
Squeeze Range

.040 ± .003"*
±.002"
13.5 to 34.9% (21.4 range)

.070 ± .003"
± .002"
19.4 to 31.5% (12 range)

.103 ± .003"
±.002
21.0 to 29.2% (8.2 range)

*non-standard size but commonly used for applications with lacking space for larger seals
 
 

Contact Width

A third benefit to the thicker cross sections is the contact width between the seal and the hardware. If we again assume we are designing for 25% compression, the nominal contact width between a seal and the hardware increases.  A greater contact width can be helpful to overcome imperfections in hardware such as a scratch, debris or dirt in the groove, or an imperfection on the seal surface.  How much is the does the contact width increase?  Looking at the .070” and .103” sizes, each at a nominal compression of 25%, the distance increases from .058” to .078”.
 
How to Properly Choose Commerically Available O-Ring Cross Sections_FEA_Parker O-Ring & Engineered Seals Division
 
 

Dynamic Seals

A final benefit has to do with dynamic seals, which are subject to wear or abrasion damage.  A thicker seal has more cross section to withstand the movement and will theoretically handle more cycles than a thinner cross section.  A thicker seal will also have more stability in the groove and be less prone to rolling. However, thick seals will also have more friction in dynamic applications.  For this reason, it is recommended that the designed squeeze be reduced on dynamic seals which will improve the friction.
 

Installation

One advantage to thin cross section seals is that they are easier to install.  Thicker seals require more force to push the radial seal and piston into a housing, or to compress an axial seal with the mating flange.  Yet thin seals are easier since they require less installation force (radial seal) or compressive load force (axial seal).
The final, and in some cases, only a potential advantage to smaller cross section seals is the amount of material used to make the seal. A smaller cross section is manufactured with less raw material, which may be of some impact on the overall seal price.
 
If you have other questions about seal selection or any Parker product, please reach out to one of our Application Engineers and we will be happy to talk with you.
 
 
How to Choose an O-Ring Cross Section_Dorothy Kern_Parker O-Ring & Engineered Seals Division
 
 
 
 
 
 
 
 
Dorothy Kern, applications engineering lead, Parker O-Ring & Engineered Seals Division