An O-ring is a simple and versatile ring-shaped packing and sealing device with a circular cross section.
O-ring functions as compact and reliable sealing devices by absorbing the tolerance stack-up between closely mated surfaces in both dynamic and static applications. Although O-rings can be made from a variety of materials, they are most commonly molded in one piece from an elastomeric material.
O-ring seal is used to prevent the loss of a fluid and gas between two closely spaced surfaces. The O-ring is generally installed in a machined groove in one of the surfaces to be sealed. As the two surfaces are brought together, forming a gland, they squeeze the cross section of the O-ring. This squeezing action results in a deformation of the O-ring cross section. With O-rings, the greater the squeeze, the larger the deformation.
Listed below are some of the outstanding characteristics that make O-rings one of the most versatile, dependable, yet inexpensive seals available, with sealing capability from hard vacuum to high pressure.
- 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, and 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
- They are cost-effective
- Where differing amounts of compression effects the seals function, an O-ring is not effected because metal to metal contact is generally allowed for.
- O-ring compounds can be selected to resist most environmental extremes
It is a unique characteristics of the elastomer material used in O-rings that makes O-ring such a good seal. The elastomer, a highly viscous, incompressible fluid with high surface tension, has a capacity for remembering its original shape for a long time.
In low-pressure applications (that is, where the confined fluid exerts little or no pressure on the O-ring), the tendency of the elastomer to maintain its original shape creates the seal. As the O-ring is deformed when the mating surfaces are brought together, it exerts a force against the mating surfaces equal to the force it takes to squeeze it. The areas of contacts between the O-ring and the mating surfaces (contact bands) act as a barrier to block the passage of the fluid.
In applications where higher pressure is exerted by the confined fluid, the sealing action of the O-ring caused by the squeeze of its cross-section is augmented by fluid pressure, transmitted through the elastomer. The O-ring is forced to the side of the gland, away from the pressure. As it is pressed against the side, the O-ring, cross section is deformed. The elastomer exerts equal force in all directions and is forced up to the gap between the mating surfaces.
 |
 |
 |
 |
 |
A. The seals can be made perfectly leak-proof for cases of static pistons and cylinders for fluid pressures up to 5000psi (limited of test pressure). The pressure may be constant or variable
B. The seals can be made to seal satisfactorily between reciprocating pistons and cylinders at any fluid pressure up to 5000psi. There may be slight running leakage (a few drops per hundred strokes) depending on the film-forming between rotating members with similar results but in all cases the surface rubbing speed must be kept low.
C. A single O-ring will seal with pressure applied alternately on one side and then on the other, but in cases of severe loading or usage under necessarily unfavorable conditions, seal life can be extended by designing the mechanism so that each seal is subjected to pressure in one direction only. Seals may be arranged in series as a safety measure but the first seal exposed to pressure will take the full load.
D. O-ring seals must be radially compressed between the bottom of the seal groove and the cylinder wall for proper sealing action. This compression may cause the seal to roll slightly in its groove under certain conditions of piston motion, but the rolling action is not necessary for normal operation of the seals.
E. In either static or dynamic O-ring seals under high pressure the primary cause of seal failure is extrusion of the seal material into the piston-cylinder clearance. The major factors affecting extrusion are fluid pressure, seal hardness and strength, and piston-cylinder clearance.
F. Dynamic seals may fail by abrasion against the cylinder or piston walls. Therefore, the contacting surfaces should be polished for long seal life. Moving seals that pass over ports or surface irregularities while under hydraulic pressure are very quickly cut or worn to failure.
G. The shape of the seal groove is unimportant as long as it result in proper compression of the seal between the bottom of the groove and the cylinder wall, and provides room for the compressed material to flow so that the seal is not solidly confined between metal surfaces.
H. The seal may be housed in a groove cut in the cylinder wall instead of on the piston surface without any change in design limitations or seal performance.
I. Friction of moving O-ring seals depends primarily on seal compression, fluid pressure and projected seal area exposed to pressure. The effects of materials, surfaces fluid, and speeds of motion are normally of secondary importance, although these variables have not been completely investigated. Friction of O-ring seals under low pressures may exceed the friction of properly designed lip type seals, but at higher pressures, developed friction compares favorably with, and is often less than, the friction of equivalent lip type seals.
J. The effects of temperatures changes from +18oC to +121oC on the performaces of O-ring seals depends upon the seal material used. Synthetic rubber can be made for continual use at high or low temperatures, of for occasional short exposure to wide variations in temperature. At extremely low temperature, the seals may become brittle but will resume their normal flexibility without harm when warmed. Prolonged exposure to excessive heat causes permanent hardening and usually destroys the usefulness of the seal. The coefficient of thermal expansion of synthetic rubber is usually low enough so that temperature changes present no design difficulties.
K. Chemical interaction between the seal and the hydraulic medium may influence seal life favorably or unfavorably, depending upon the combination of seal material and fluid. Excessive hardening, softening, swelling, and shrinkage must be avoided.
L. O-ring seal are extremely dependable because of their simplicity and ruggedness. Static seals will seal at high pressure in spite of slightly irregular sealing surfaces and slight cuts or chips in the seals. Even when broken or worn excessively, seals may offer some measure of flow restriction for emergency operation and approaching failure becomes evident through gradual leakage.
M. The cost of O-ring seal and the machining expense necessary to incorporate them onto hydraulics mechanism designs are at least as low as for any other reliable type of seal. O-rings seals may be stretched over large diameters for installation and no special assembly tools are necessary.
N. Irregular chambers can be sealed, both as fixed or moving ¡V parts installations.
| ROTARY APPLICATIONS |
In rotary O-ring applications, the O-ring continuously moves against tire same portions to the shaft. Heat due to friction is continuously generated ion the same place, and elastomers are poor thermal conductors. If heat is generated more quickly than it can be dissipated, temperature rise is rapid and seal failure quickly follows. Where surface speeds do not exceed 180 feet/minute, or where rotation is brief and intermittent, this is rarely a problem and gland design criteria for reciprocating service are applicable. For continuous rotation at surface speeds over 180 feet/minute some developmental adjustments are often required to achieve acceptable performance.
In applications where rotating motions occur, the designer should consider the following:
i) Measures should be taken to reduce
heat buildup:
- Provide absolute minimal squeeze, as little as 0.002¡¨ to minimize friction.
This may permit some leakage
- Provide ample diametric clearance to increase fluid flow and facilitate
better dissipation of heat
- Select O-ring with smallest cross section.
- Maintain low system pressure (not over 250psi)
ii) Use a shaft of diameter no greater than that of the relaxed O-ring I.D. This is important because when an O-ring is heated under stress, it will tend to contract. Contraction of the O-ring could cause it to seize the shaft and increase friction and heat resulting in rapid failure.
iii) The gland should be located as close as possible to the lubricating fluid and as far as possible from the shaft support bearings. This allows the O-ring to receive the maximum amount of cooling lubricant and minimizes the effects of bearing-generated treat.
iv) Relative motion must occur
exclusively between the O-ring ID and the rotating shaft. Rotation of
the O-ring within the gland will lead to rapid
wear and leakage.
- Minimize out-of-round shafts and eccentric rotation. Maximum
eccentricity should not exceed 0.001¡¨
- Finish of the moving surface contacting the O-ring should not
exceed 16RMS. A rougher surface is desired within the groove to discourage
rotation; 32 RMS is recommended.
- Utilize an O-ring of a hard, self-lubricating compound specifically
developed for rotary service.
PERMEABILITY
Gases diffuse into and through elastomaric compounds at various rates depending on the elastomer type and nature of the individual compound. Generally, harder compounds which have more carbon black added have lower diffusion rates. Of the popular elastomer, epichlorohydrin and butyl have the lowest permeability, followed by fluorocarbons, polyurethanes, nitrites, heoprenes, polyacrylates, and SBR. The fluorosilicons and silicones have higher rates.
For any given compound, the permeability through the O-ring depends on the amount of its compression or squeeze, the area of the seal, and the pressure, temperature and type of gas begin sealed. For the majority of applications, the rate of gas permeation through the O-ring is inconsequential and standard groove dimensions are applicable.
Where gas pressure exceeds 500psi, and pressure is released alter a soak period, gas within the O-ring may exert considerable force under the lower external pressure and may cause damage. The O-ring may blister or chunks of rubber may even be blown out.
PNEUMATIC SEALS
No special consideration is usually warranted for pneumatic applications if they are static. With dynamic applications, the problem is lack of a system liquid to provide lubrication and cooling. If reasonable life is to be achieved sortie lubricant must be provided. Particularly where operating temperature approaches the capabilities of the O-ring, an elastomer resistant to oxygen should be chosen ¡V Gas temperature increase due to compression requires consideration in determining system temperature.
Conventional gland designs are applicable for pneumatic service. However, since slight leakage is usually not important, and friction is, reduced squeeze is desirable in reciprocating pneumatic applications.
VACUUM SEALS
Vacuum seals also warrant separate mention. Unlike pneumatic seals, even slight leakage is often unacceptable in vacuum applications. They have only one atmosphere differential pressure, so essentially all the sealing force must be provided by compression of the O-ring. The following factors should be considered:
Dynamic vacuum seals require proper lubrication
due to the absence of system liquids. Use of vacuum grease is also desirable
with static seals
An especially smooth finish in the gland is important to insure contact
between the elastomer and the metal parts.
In applications where absolute minimum leakage is a necessity, gland depth
should be reduced to increase the amount of squeeze
To minimize the possibility of gases begin trapped under the O-ring and
escaping into the vacuum, reduced groove width and the use of suitable
vacuum grease to fill the excess void are recommended.
O-rings may be used in series in vacuum applications, preferably with
a separate vacuum between them.
DRIVE BELTS
O-rings provide excellent service in low power drive belt applications. Tire primary concern for O-rings used as drive belts is the compound from which they are made.
Several elastomers have been used successfully in drive belt applications. Ethylenepropylene has provided superior performance due to its low stress relaxation, high temperature resistance, and overall reliability silicon has also been used in high temperature applications and lacking good wear and abrasion resistance, it provides reliable but somewhat limited service life. Polyurethane has been used successfully but it should not be used at temperatures above 158oF (70oC).
When utilizing O-rings as drive belts, the following factors should be considered:
I.D, stretch should be app. 10+/-2%,
Pulley grooves should be round, of depth and radius equal to the radius
of the O-ring cross sections.
Pulley diameter (at the bottom of groove) should be no less than four
times the O-ring cross section diameter
Comparison of Common Seal Types
A number of common seals types, T-seals, U-cups, V-packing and other devices, have been, and are still used for both dynamic and static seals. When compared with an O-ring seal, these seal types may show one or more design disadvantages which might be overcome by use of an O-ring. As an aid in assessing the relative merits of and O-ring, below table lists several of the important factors that must be considered in the selection of any effective seal geometry.