What is an O-Ring?
An O-ring is a torus, or doughnut-shaped ring, generally molded from an elastomer, although O-rings are also made from PTFE and other thermoplastic materials, as well as metals, both hollow and solid. This handbook, however, deals entirely with elastomeric O-rings.
O-rings are used primarily for sealing. The various types of O-ring seals are described in the section called “Scope of O-ring Use.” O-rings are also used as light-duty, mechanical drive belts. More information, including design criteria on O-ring drive belts and their applications can be found in the O-Ring Applications Section of the ORD 5700 O-Ring Handbook.
What is an O-Ring Seal?
An O-ring seal is used to prevent the loss of a fluid or gas. The seal assembly consists of an elastomeric O-ring and a gland. An O-ring is a circular cross-section ring molded from rubber. The gland, usually cut into metal or another rigid material, contains and supports the O-ring. The combination of these two elements, O-ring and gland, constitute the classic O-ring seal assembly.
How an O-Ring Works
The rubber seal should be considered as essentially an incompressible, viscous fluid having a very high surface tension. Whether by mechanical pressure from the surrounding structure or by pressure transmitted through hydraulic fluid, this extremely viscous fluid is forced to flow within the gland to produce “zero clearance” or block to the flow of the less viscous fluid being sealed. The rubber absorbs the stack-up of tolerances of the unit and its internal memory maintains the sealed condition.
- Figure 1-4 illustrates the O-ring as installed, before the application of pressure. Note that the O-ring is mechanically squeezed out of round between the outer and inner members to close the fluid passage. The seal material under mechanical pressure extrudes into the microfine grooves of the gland.
- Figure 1-5 illustrates the application of fluid pressure on the O-ring. Note that the O-ring has been forced to flow up to, but not into, the narrow gap between the mating surfaces and in so doing, has gained greater area and force of sealing contact.
- Figure 1-6 shows the O-ring at its pressure limit with a small portion of the seal material entering the narrow gap between inner and outer members of the gland.
- Figure 1-7 illustrates the result of further increasing pressure and the resulting extrusion failure. The surface tension of the elastomer is no longer sufficient to resist flow and the material extrudes (flows) into the open passage or clearance gap.
Advantages of an O-Ring
- They seal over a wide range of pressure, temperature and tolerance.
- Ease of service, no smearing or retightening.
- No critical torque on tightening, therefore unlikely to cause structural damage.
- O-rings normally require very little room and are light in weight.
- In many cases an O-ring can be reused, an advantage over non-elastic flat seals and crush-type gaskets.
- The duration of life in the correct application corresponds to the normal aging period of the O-ring material.
- O-ring failure is normally gradual and easily identified.
- Where differing amounts of compression effect the seal function (as with flat gaskets), an O-ring is not effected because metal to metal contact is generally allowed for.
- They are cost-effective.
