How To Properly Choose Commercially 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.
Hardware geometry and limitations are the first consideration. A traditional O-ring groove shape is rectangular and wider 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.
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 is 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.
Table 1: AS568 standard sizes
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 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 the 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 the compression set.
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%.
Table 2: Range of Compression for O-Rings with Same Tolerances in Grooves with Same Tolerances
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 sealing 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”.
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.
One advantage of thin cross-section seals is that they are easier to install. Thicker seals require more force to push the radial seal and piston into 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.
This article was produced by Parker Chomerics. JHC Specialised Solutions are the exclusive authorised distributor for Parker Chomerics in Australia and New Zealand. Should you require any further information or wish to discuss a customised solution for your application, please contact JHC on +61 2 9531 7905 or firstname.lastname@example.org