back up (parbak) o-rings

		Back-Up Rings Cross Refrence			8-XXX			Parbak Back-Up Sizes			Parbak Vs PTFE and Leather back-up

Parker’s Parbak® back-up rings help prevent extrusion in high pressure service and compensate for loose fitting parts. The use of loose fitting parts makes for cost reduction in the machining of unit components.

In double acting seal assemblies, a Parbak is required on both sides of the O-ring. It is cheap insurance to use two Parbaks even in single acting installations. At assembly, it is too easy to place a single Parbak on the wrong side of the O-ring. By specifying one on each side of the O-ring, there will be one on the low pressure side, where it is needed, and the extra Parbak does no harm. Parbaks are quick and easy to assemble, minimizing assembly costs, and they cannot fall out of the O-ring groove. Besides their advantages as anti-extrusion devices, Parbak rings help trap lubricant, preserving the O-ring and reducing friction. The standard sizes are listed in Table.

Parbak Elastomer Back-Up Rings
Hard rubber back-up rings combine most of the best features of both leather and PTFE anti-extrusion devices. Although no industrial or military standards have been issued for rubber back-up rings, they have been in use for a number of years. These are special devices designed to satisfy a specific problem. Parbaks in Parker Seal Group’s standard nitrile compound, N1444-90, are generally usable through a temperature range of -40°C to 121°C (-40° to 250°F). Hardening of this material due to high or low temperatures often improves performance as a back-up ring.

Features of Parbak Rings
1. Elastic memory permits Parbak rings to be stretched into place for assembly without  
    preconditioning or cutting.
2. Continuous construction prevents damage to the O-ring seal.
3. Lubrication is enhanced by rubber which absorbs system fluid and does not plate out on
     rubbing surfaces.

Back-Up (Parbak) O-Rings Compound Information
Some back-up ring materials tend to leave deposits in the micro fine grooves of the surface on which they rub. An ultra smooth, wax-like surface results. Because an O-ring may wipe all lubrication from such a surface, reverse stroking is dry and greatly reduces seal life. Parbaks of N1444-90 and other rubber compounds solve this problem. They do not leave a deposit on the metal surface, thus lubrication remains. The standard compound for Parbaks is N1444-90. Careful engineering and research has produced N1444-90 which has the best combination of characteristics for the majority of back-up ring installations — broad temperature range, proper hardness, long sealing life, and resistance to a great number of fluids. It is resistant to nearly all hydraulic fluids except certain non-flammable types such as Skydrol. It is also resistant to air and water. Functional tests have proven that millions of cycles can be obtained with Parbaks, showing their tremendous superiority over the older types of back-up rings. In addition to N1444-90, Parker Seal has developed other compounds for installations requiring special characteristics. Present capabilities include service at continuous temperatures as high as 204°C (400°F).

Parker Parbak 8-Series Dimensions
Parbaks will stretch up to 50%, and are quickly and easily installed. Advantages of the contour design are obtained regardless of how Parbaks are installed. They may be installed with the concave face in either direction, toward or away from the O-ring.