Article re-posted with permission from Parker Hannifin Sealing & Shielding Team.
Original content can be found on Parker’s Blog.
This post is the first installment in a three-part series that describes the three main criteria for O-ring gland design: stretch, squeeze, and volume fill. These three related components must be balanced to create the ideal conditions for O-ring sealing.
Understanding the basics of stretch
A basic design principle often used as the starting point for O-ring grooves is stretch. One of the reasons elastomers are ideal materials for O-rings is because their elastic properties allow the O-ring to stretch around hardware and return to the original shape while within the material’s elastic deformation range.
The percent stretch, usually simply called the stretch, is defined by the diameter of the hardware minus the O-ring inner diameter (ID), all divided by the O-ring ID. The diameter of the hardware value represents the diameter the O-ring is being stretched about. For male designs, this diameter is the groove. For female designs, it is the rod. For face seals, this can be the ID of the groove. This formula can also be used for installation stretch where the hardware diameter is the outer diameter of the installation tool or the maximum diameter that the O-ring must pass over for installation.
The purpose of stretch in gland design is to appropriately size and sometimes secure the seal to the hardware for installation and functional purposes. For clarity, illustrations from the O-Ring Selector will be used for specific design configurations. Keep in mind, to increase the stretch, increase the related hardware diameter or decrease the ID of the O-ring. To decrease the stretch, decrease the related hardware diameter or increase the ID of the O-ring.
Stretch and groove designs
For male groove designs, up to 5% stretch is recommended. This means the ID of the O-ring, shown as d1 in the figure below, will be smaller than the groove diameter d3. This stretch “holds” the O-ring tight to the groove diameter which aids in a successful installation. When calculating stretch for a loose or no stretch condition can result in a negative value, which is sometimes called negative stretch. With no stretch or negative stretch, the O-ring may lay loose and sag out of the male groove. This creates a risk of pinching and damaging the O-ring during installation. This failure mode is often seen in “blind installations” which means the installation happens in a location where the seal cannot be watched and manually guided into the bore.
In female groove designs, low to no stretch around d5 shown in the below figure of the female gland design is recommended for easy installation. No stretch or a bit of interference between the outer diameter of the O-ring and the groove is usually preferred because if there is an excessive stretch, the O-ring may be pushed out of the groove by the rod interacting with the O-ring ID during installation causing damage to the seal. Female groove designs are typically more difficult as the installation is often blind.
For face seals, low to no stretch is recommended similarly to female designs. Face seal grooves designed with excessive stretch can cause the O-ring to “walk out” of the groove. General guidelines for face seal groove diameters are different for internal and external pressure. For internal pressure, design the OD of the O-ring to be the same as the OD of the groove. This results in a “no stretch” design which may appear as a negative or rounded to O% stretch value in some gland calculators. For external pressure, match the ID of the O-ring to the ID of the groove. This designs the O-ring to press up against the groove wall opposite of the pressure for best results.
Stretch affects an O-ring cross-sectional area
As an O-ring is stretched and elongated, its cross-sectional area decreases as a result. Too much stretch can impact compression or squeeze. When the centerline diameter is stretched more than two or three percent, the gland depth should ideally be reduced to retain the necessary squeeze on the reduced cross-section. This reduced cross-sectional area due to stretch is included in O-ring calculators like the O-Ring Selector and InPhorm.
When an O-ring is stretched, despite the cross-sectional area decreasing as the ID increases, the overall volume of the O-ring does not decrease. For this reason, the calculation for gland fills using O-ring volume compared to gland volume rather than comparing cross-sectional areas.
Excessive stretch
A seal with assembled stretch greater than five percent may decrease seal life. The stretching creates internal stress in the O-ring material which causes more rapid aging. Over five percent stretch may sometimes be used, however, if a shorter useful life is acceptable or if application experience and testing have proven the excess stretch is negligible to seal performance.
Common failure mode: installation stretch
O-rings are typically required to stretch over a rod or other features before being seated in the groove. This installation stretch is another important design consideration because a common failure mode is the O-ring snapping during installation. Installation stretch is defined by calculating the stretch over the widest feature during installation, which may be an installation aid or mandrel. An idea of how much an O-ring can be stretched is characterized by the ultimate elongation material property. Ultimate elongation is the percentage change in length from the original to the material’s breaking. For most materials, there is an inverse relationship between durometer and ultimate elongation.
The recommended limit for installation stretch is 25-50% and should not exceed 50% of the ultimate elongation of the chosen compound. Back-up rings may stretch up to 50% on installation.
Tolerancing issues and small O-rings
In the case of very small O-rings, it is important to realize the guidelines outlined may be impossible to achieve. This is due to the combination of O-ring tolerance and groove tolerance. For very small radial male designs, the combined values for the maximum material condition and least material condition cannot be narrowed down to 0-5% stretch. In this case, design the minimum stretch to be close to zero rather than designing for the maximum stretch to be 5%, This is consistent with having the O-ring sit snug on the groove. Since the stretch exceeds 5%, and coincidentally the installation stretch can run very high, select a material with as high ultimate elongation as possible. It is also wise to incorporate safeguards into the installation process so that the installed seal is checked to be sure it did not break while being seated in the groove.
For more information about glands or if you need assistance with a custom sealing application, contact Gallagher Fluid Seals today.
