Machinery's Handbook, 31st Edition
2688 O-RINGS O-Ring Lubrication.— Dynamic seals typically rely on system fluid to lubricate the seal as it rubs against the facing part. Installation instructions for O-rings in dynamic applica- tions often suggest coating the O-ring in either the process fluid if applicable, or an accept - able seal lubricant. Lubrication issues can arise when sealing a dry, volatile, or crystal- lizing fluid. In pneumatic applications, lubrication is provided either by using lubricated air or with periodic lubrication of the seals. Reciprocating seals maintain lubrication film thickness best at high piston velocities. Gland Surface Quality.—All gland surfaces should be free of burrs, nicks, and scratches; the chosen machining method should not leave tool marks that cut across the gland. To seal properly, the gland and O-ring surfaces must be clean of debris. Surface quality and cleanliness matter most with small cross section O-rings and/or stiff materials. In a static seal, an O-ring made from softer material can be used to compensate for a rough gland. It should be noted that the two systems of measurement recommend nonequivalent maxi- mum surface roughness average (Ra) for various cases, as follows: Face seals, sealing surfaces: SAE: 16 μin. ISO: 1.6 μm Face seals, contact wall of groove: SAE: 32 μin. ISO: 1.6 μm Face seals, non-contact wall of groove: SAE: 63 μin. ISO: 1.6 μm Piston and rod seals, sealing surfaces: SAE: Liquids: 32 μin. (Gases: 16 μin.) ISO: Dynamic: 0.4 μm (Static: 1.6 μm) Piston and rod seals, groove walls: SAE: Dynamic: 32 μin. (Static: 63 μin.) ISO: 1.6 μm Surface finishes of less than 5 μin. (0.13 μm) Ra are too smooth to ensure long seal life because they wipe too cleanly, causing the O-ring to wear against the housing in the absence of a lubricating film. The best quality surfaces are honed, burnished, or hard chromium-plated. Soft and stringy metals, such as aluminum, brass, bronze, Monel, or free machining stainless steel, should not be used in contact with moving seals. O-Ring Installation.— During installation, O-rings must not traverse across sharp edges or external threads. All edges encountered by the O-ring must have chamfers or radii. In radial glands, a generous lead-in chamfer of 15 to 20 degrees with a rounded edge is necessary to prevent damage, although an installation mandrel still may be needed in some cases. Never twist or roll an O-ring, and pay special attention during installation of O-rings with a large ratio of inner diameter to cross-sectional diameter. Avoid stretching the inner diameter of an elastomeric O-ring more than 50 percent, and certainly not more than 100 percent during installation. Rigid O-rings normally should not be stretched, though some materials may allow gentle expansion through applied heat for installation purposes. The preferred bore materials are steel and cast iron, and pistons should be softer than the bore to avoid scratching them. The bore sections should be thick enough to resist expan sion and contraction under pressure so that the radial clearance gap remains constant, reducing the chance of damage to the O-ring by extrusion and nibbling. Some compatibil ity problems may occur when O-rings are used with plastics parts because certain com pounding ingredients may attack the plastics, causing crazing of the plastics surface. O-Rings Used as Drive Belts.— O-rings are frequently used as driving belts in round bottom or V-grooves with light tension for low-power drive elements. Special compounds are available with high resistance to stress relaxation and fatigue for these applications. Best service is obtained in drive belt applications when the initial belt tension is between 80 and 200 psi and the initial installed stretch is between 8 and 25 percent of the circum- ferential length. Most of the compounds used for drive belts operate best between 10 and 15 percent stretch, although polyurethane has good service life when stretched as much as 20 to 25 percent.
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