The Effects of Friction

Breakout friction is an important consideration in intermittently moving applications. It can cause excessively high hydraulic pressures to develop. This pressure can tear portions of the seal that adhere to the gland wall when machine movement has been stopped for an extended period.

Once a system is up and running, the designer must then consider seal running friction as a potential source of problems. In continuously moving applications, excessive running friction can cause heat to develop, which results in O-ring swell. Once swelling occurs more heat is generated from increased friction which causes additional swelling and seal failure. High running friction, in combination with high system pressures, may also produce excessive wear in soft metal parts.

Critical Operating Environmental Factors: Methods Used to Control Friction

• Squeeze: Both running and breakout friction are reduced when squeeze is reduced.
• Durometer (Hardness): Breakout friction decreases with decreasing hardness. Running friction decreases with increasing hardness.
• Cross Section: O-rings with smaller cross sections tend to produce less friction.
• Lubrication: Seal adhesion can be minimized by the use of lubrication. Compatibility between the elastomer and lubricant should be predetermined to avoid seal shrinkage or swelling.
• Compound Additives: Rubber can be compounded with additives such as oils, graphite, Teflon, etc. to lower the coefficient of friction.
• Gland Machining: An optimum finished surface of 8 to 16 RMS will help to control friction. Finishes below 5 RMS will not hold the lubricant because it eliminates micropores.
• Groove Width: By increasing the groove width, the seal will be allowed more room to expand perpendicular to the compressive force.
• Material: Materials vary in their friction characteristics. For example, Teflon has a very low coefficient of friction.
• Pressure: Decrease system pressure to reduce the amount of running friction.