Seal is a device, or combination of devices, intended to prevent the passage of liquid, vapor, or gas between two or more connected surfaces. Nowadays, countless varieties of seals are in use worldwide, differing in both their construction materials and geometry. One of the most widely used devices for sealing today is the O-ring.
The O-ring is a torus (type of surface), or doughnut-shaped object, generally made from an elastomer that seals when compressed or deformed in a groove. Even though O-rings have long been a widely used sealing solution, care must be taken to avoid some common flaws when selecting them.
The purpose of this article is to offer a basic method for O-ring selection. It will help prevent the time-consuming, costly, and potentially dangerous consequences of seal leakage due to improper choice of O-ring.
This type of seal is considered for a simple, reliable, and cost-effective sealing solution for several reasons:
- O-rings are capable of bidirectional sealing
- O-rings are able to compensate for tolerance runout
- This type of seal is reusable, within limits
- O-rings are resilient, thus eliminating re-tightening of bolts
- O-rings do not require any special installation tools or primers
- the symmetry of the O-ring’s circular cross-section allows it sealing in any direction (radially, axially, or angularly)
- O-rings consist of a varied number of compounds that can resist virtually all aggressive chemicals and environmental application.
Even though O-rings have long been a widely used sealing solution, care must be taken to avoid some common flaws in their selection. The following six-step guidelines should be followed systematically for every O-ring selection to minimize sealing problems downstream:
Determine the Type of O-ring Application You are Working with
Sealing applications can be divided into two general categories: static and dynamic. In reality, the vast majority of sealing applications are primarily pseudo-static or pseudo-dynamic in nature. True static applications, in which there is no vibrational movement, are quite rare.
Determine the Media to be Sealed
After establishing the O-ring application type, the next step is to determine the appropriate elastomer for the fluid to be sealed. There are various elastomers in terms of their compatibility with different media. Often, the design engineer is required to balance the compatibility of a variety of media with a single seal. For elastomers to be deemed compatible with the media in question, the material must exhibit only minimal changes in properties (i.e., tensile strength, volume, and elongation) when exposed. Choosing an elastomer that is compatible with all media it comes in contact with is imperative for a successful O-ring selection.
Define the Application Time, Temperature, and Pressure Envelope of Seal if Applicable
When selecting an elastomer for use in any sealing application, one of the most important factors to consider is the temperature range of the system. Choosing the correct elastomer, based on the temperature range required of the design, an iterative process between steps 2 and 3 may be needed. If the pressure differential across the seal is very large, then it is required increased hardness of O-ring. At this point in the selection, an iterative process between steps 1 and 3 may be required, depending on the specifics of the application.
Determine the Appropriate O-ring Size Based on the Application Type (i.e. Facial, Radial, Static, Dynamic, etc.)
Up to this point in the selection, the issues have involved the O-ring’s operating environment. Now is the time to decide what specific O-ring size is best suited for application. Many handbooks and a few software packages provide detailed tables when looking up the proper O-ring size.
Check Engineering Calculations either Manually or Using Available Software Programs
For every O-ring application, design engineers must concern themselves with three important design calculations: percent squeeze, compression percent stretch (if applicable), and volume/void ratio. Again, for most standard application dimensions, these calculations are available in many handbooks and software packages in the market, so there is no need for further calculations. However, if the O-ring application is unique, then these calculations will be required. Improper calculation of the seal size will most certainly result in leakage.
Test the O-ring Design, if Possible
In today’s harried engineering environment, the exercise of a test run or design-configuration run is somewhat of a luxury. Time constraints often do not allow verification of whether or not the selection of O-ring is proper, and that no unanticipated problems will arise. This lends even more credence to having an organized approach to O-ring selection.
You may not be a certified O-ring expert, but following these steps you will have a framework which may use as а guideline for O-ring selections. Each step may have details beyond the scope of this article, but it should provide sufficient food for thought when approaching the simple, yet elegant, O-ring seal.