Machine Design

Handbook guides O-ring design

O-rings are stationary against gland surfaces and slide against shaft surfaces. O-rings tend not to roll in normal operation because fluid pressure forces a larger area of the O-ring against the gland than against the shaft.

The Parker O-Ring Handbook is a hardcover manual with 11 chapters detailing O-ring design. The chapters range from tutorial to purely technical in nature. For instance, technical chapters provide in-depth coverage on topics such as chemical compatibility, O-ring specifications, and sizes. The compatibility-tables chapter is actually one 53-page table listing nine common O-ring seal materials and how they react to the roughly 2,000 media listed in the table. Their compatibility is classified as satisfactory, fair, doubtful, and unsatisfactory.

More tutorial chapters include O-ring elastomers, applications, and static and dynamic sealing. The chapters cover topics such as selecting the correct O-ring material, installing O-rings, the effects of friction on O-rings, and avoiding O-ring failure.

One type of failure seen on reciprocating O-rings is called spiral failure. When it occurs the seal looks to be cut about halfway through the cross-section in a spiral pattern. Oddly enough, the O-ring usually seals satisfactorily until it completely breaks or separates.

Conditions that cause segments of the ring to slide and others to simultaneously roll produce spiral failure. A small amount of twisting is not detrimental. When excessive, it leads to spiral failure.

In a properly designed system, the O-ring slides during all but a small fraction of any reciprocating stroke. The seal does not normally roll or twist for several reasons. First, hydraulic pressure produces a greater holding force against the larger surface area of static components than it does against sliding surfaces.

O-rings also do not twist because running friction is lower than breakout friction so once motion begins O-rings move with relative ease. Additionally, the torsional resistance of the O-ring tends to resist twisting, and the smoother finish of the sliding surface, compared to the groove surface finish, produces less friction.

True spiral failure occurs after the seal excessively twists, but does not break, and then is subject to relatively high pressure. Fluid pressure forces the twisted seal into the sharp corner at the clearance gap. Stress above the elastomer's elastic limit then causes a rupture adjacent to the clearance gap, and slight flexing apparently causes the rupture to penetrate about halfway through the cross-section. When the O-ring is removed from the gland it returns to its original shape and the rupture appears as a tight spiral around the cross section.

According to the handbook, the three most common factors that contribute to spiral failure are low stroke speeds, lack of lubrication, and pressure differential and direction. Usually several factors combine to produce spiral failure.

One of the primary causes of spiral failure is reciprocating speeds below one fpm. At this slow speed, it appears that the sliding or running seal friction is high and comparable to breakaway friction. Extreme twisting occurs on low or balanced pressure components, such as hydraulic accumulators, in relatively few (about 200) cycles if the temperature is above 100°F. O-ring seals are not recommended, therefore, for speeds less than one fpm when pressure differentials are below 400 psi. If system pressure slowly diminishes, as through slow valve leaks, and a sealed piston moves slowly through a cylinder, spiral failure is likely. The obvious remedy here is to provide good system maintenance to prevent slow leaks, or exhaust the system after each work day.

The lack of lubrication on a surface exposed to the atmosphere is another prime contributor to spiral failure, particularly in applications with stroke lengths greater than 6 in. Remedies include using lubricated wiper rings, applying a grease that will not evaporate, lubricating metal surfaces prior to assembly, and using low-friction metal or surface plating.

Spiral failure is also more likely to occur if pressure and seal friction both act in the same direction than if they oppose each other. In other words, seals in a pump are more likely to spiral than are seals in an actuator. Normally O-rings will not twist when the pressure differential across the seal is greater than 400 psi.

Other common causes of failure are squeeze, groove shape, operating temperature, stroke length, and gland surface finish. The handbook explains these causes and recommends how to avoid them. It also points out that spiral failure is not limited to O-ring seals. Square, delta, four-leaf clover, and other cross sections can also fail by twisting. However, although other seal designs leak excessively when twisted, O-rings usually seal until they fail completely.

This information supplied by Parker Hannifin Corp., O-Ring Div., Lexington, Ky.

TAGS: Hydraulics
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