Back-up rings are one way to protect the O-ring and prevent extrusion in both static and dynamic applications. A back-up ring is a relatively hard, high shear modulus material placed in a seal groove between the O-ring and the clearance gap (i.e. on the seal’s low-pressure side). The back-up acts as a support or buttress for the O-ring even as it blocks the gap into which the pressurized seal might otherwise extrude. As an added bonus, the back-up ring also reduces friction and wear by trapping lubricant in the vicinity of the seal.

In some cases, 90-durometer O-rings are used to resist extrusion. Be aware, however, that sealability often suffers even as friction and wear increase with use of such a hard material. A resilient 70-durometer nitrile seal with a 90-durometer or harder back-up ring is preferable. Bi-directional pressure will necessitate back-ups on both sides of the O-ring. Use of two back-ups (even if pressure is acting from only one side) also eliminates the possibility of installing a single back-up on the wrong side of the O-ring. Performance differences for seals with and without a back-up ring are shown in Figure 77. As you can see, use of a back-up ring has clear advantages.

Table 39 is included here as a general guide to help you determine if back-ups are necessary in a given application. Using the fluid pressure in your system (scaled along the left of the graph in psi and along the right in MPa) and the diametral clearance of your design (scaled along the bottom in inches/millimeters), you can plot a point on the graph field. Using the line that corresponds to the durometer of your O-ring (e.g. 70 Shore A) as a reference point, you then note on which side of this durometer line the plotted point falls. If it falls to the left of the line, extrusion is not anticipated and no back-ups are necessary. If the point falls to the right, however, extrusion will likely be a problem and back-ups are recommended.

Though the information in Table 39 is useful, there are a number of things to keep in mind when consulting this data. First, these are general curves based on medium shear modulus hydrocarbons (such as nitrile and EPDM). Higher shear modulus materials (such as polyurethane) will offer increased extrusion resistance, thus shifting the curve upward (as indicated by the yellow shading).

For example, a 70-durometer polyurethane can withstand much higher pressures than a lower shear modulus 70-durometer nitrile. Also, these curves are not applicable for silicone, fluorosilicone, and other low shear modulus materials. Second, these curves assume a moderate system temperature (160° F). As temperature increases, the tendency of most elastomers is to soften and thus be more easily extruded. Third, these curves also assume that the total diametral gap is concentric. If eccentricity occurs (due to severe side-loading or misalignment), the gap on one side will become larger, making extrusion on that side more likely.

Again, it is always a good idea to use back-ups with O-rings (or other seals) in pressures exceeding 1,500 psi or in designs featuring large clearance gaps. The additional cost of back-ups is small in comparison with the cost of tighter machining tolerances, and back-ups easily pay for themselves by both improving seal performance and prolonging seal life.

R.L. Hudson & Company offers back-up rings in a wide array of materials, including contoured hard rubber (nitrile or fluorocarbon/Viton®), polyurethane, Hytrel®, PTFE (Teflon®), and engineering plastics. Polyurethane’s inherent toughness, ability to withstand high pressure, and resistance to extrusion make it an excellent material for back-ups. Hytrel® is a thermoplastic elastomer combining toughness, resilience, and chemical resistance.

As shown in Figures 78 through 80, Teflon® back-up rings come in three basic types: solid (uncut), single turn (also known as scarf cut, or split), and multi-turn (spiral cut). In addition to PTFE, back-ups are also available in PEEK (another high performance plastic) and in nylon (an engineering plastic).

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