What’s the Difference Between O-Rings and Guide Rings?

O-rings and guide rings are both used on hydraulic cylinders, but they handle different tasks. O-rings are often the first choice for engineers because of their flexibility and performance in static and semi-dynamic applications, and nearly every sealing configuration in hydraulic cylinders require wear rings. Ultimately, the application and its specifications determine if an O-ring or a guide ring should be used.

O-ring Basics
O-rings are used to seal all kinds of devices to keep them operating and safe. They do so by keeping necessary fluids in hydraulic systems and unwanted fluids and contaminants out. O-rings are inexpensive to make, can be used in a wide variety of applications and are easy to use; these traits have made O-rings the most widely used seal for hydraulic applications.

Before O-rings were invented, machine makers crafted seals and gaskets out of cloth, leather and animal guts. The first O-ring was patented in 1848 by British inventor Alonzo B. Woodcock. Nearly, a century later, in 1938, a U.S. patent filed by Danish-born machinist Niels Christensen initiated their widespread use.

Today, every industrial segment uses O-rings, ranging from individual seals for repairs and maintenance to energizing elements for polytetrafluoroethylene (PTFE) seals in hydraulics.

O-rings are fitted between two components as a primary or secondary seal. As the primary seal, O-rings typically seal in two directions. They are functional immediately after installation and they prevent leaks initially by compressing the O-ring with the hardware. System pressure then adds to and completes the total sealing force. As secondary seals, O-rings are part of a sealing system with a main seal that adds strength or media resistance.

The important O-ring dimensions are its inner diameter and cross section.

O-rings are predominantly primary seals in static sealing applications, and the direction of the sealing force defines the O-ring as either axial or radial. O-rings used as axial seals, which can have pressure applied from inside or outside the seal, are common in flanges and cover plates.

Radial seals are for inner sealing when the sealing surface is at the inside diameter of the O-ring, and outer sealing when the sealing surface is at the outside diameter of the O-ring. Piston and rod sealing applications often use radial seal O-rings.

O-rings are suitable for static sealing if the seal’s contact area remains stationary. They are also suitable for moderate dynamic sealing conditions if the sealing surface only moves or turns occasionally. In fully dynamic applications where the contact area moves, O-rings typically serve as energizing elements for a main seal that is often non-elastic but more wear-resistant, and offers lower friction properties.

O-rings are the most common seals and often an engineer’s first choice based on their flexibility and performance in static and semi-dynamic applications. Ultimately, the application and its specifications determine if an O-ring should be used.

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