O-ring Seal Applications - Other Seal Application Types

Rotary Applications

In rotary O-ring sealing applications, the O-ring continuously moves against the same portions of the shaft. Heat due to friction is continuously generated in 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 (see Table E, Dynamic Piston and Rod Seal Gland Dimensions). 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:

• Measures should be taken to reduce heat buildup:
  • Provide absolute minimal squeeze, as little as .002 inch 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 250 psi).
• 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.
• 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.
• Relative motion must occur exclusively between the O-ring I.D. 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 .001 Inch.
  • Finish of the moving surface contacting the O-ring should not exceed 16 RMS- 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 Applications
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 elastomers, epichlorohydrin and butyl have the lowest permeability, followed by fluorocarbons, polyurethanes, nitrites, heoprenes, polyacrylates, and SBR. The fluorosilicones 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 being sealed. For the majority of applications, the rate of gas permeation through the O-ring is inconsequential (a few standard cubic centimeters per year) and standard groove dimensions are applicable. Where gas pressure exceeds 500 psi, and pressure is released after 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. 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 seal 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 being 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. The 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. Ethylene-propylene has provided superior performance due to its low stress relaxation, high temperature resistance, and overall reliability Silicone 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 approximately 10 +/-2%,
• Pulley grooves should be round, of depth and radius equal to the radius of the O-ring cross section.
• Pulley diameter (at bottom of groove) should be no less than four times the O-ring cross section diameter

Grommets
Grommets can be designed, developed, and manufactured using a variety of materials for a wide range of applications. They can be an essential yet versatile machine component. Grommets may serve as vibration isolators or as feet or bumpers. They can serve as pipe protection seals for rods, wires, and cables when passing through rigid surfaces or as sleeved grommets for surface protection. Grommets may also be used as blanking grommets for plugging, dust and moisture protection or as fasteners when used in conjunction with a screw. Rubber grommets have also been successfully used for insulation, as cord guards, electrical bushings, for mounting holes and as extended neck types.

Bushings
Bushings can also be used for a variety of purposes because they are available in so many different shapes, sizes, and materials. Bushings can be applied as shock absorbers, vibration inserts or insulators, as well as protectors for wires, pipes, tubing and rods. Bushings are also used on mechanical equipment such as on gears, cams, pulleys, conveyors, fans, and hubs to effortlessly mount all kinds of components.

Please contact us to learn more about our vacuum and pneumatic seals, bushings or Grommets.