Backup rings are used to support O-rings and to prevent O-ring deformation and resultant leakage. Two types of backup rings are used in naval aircraftTeflon single and double spiral. Teflon rings are made from a fluorocarbon-resin material, which is tough, friction-resistant, and more durable than leather. Precautions similar to those applicable to O-rings must be taken to avert contamination of backup rings an  damage to hydraulic components. Teflon backup rings may be stocked in individual sealed packages similar to those in which O-rings are packed, or several may be installed on a cardboard mandrel. If unpackaged rings are stored for a long period of time without the use of mandrels, a condition of overlap may develop. To eliminate this condition, stack Teflon rings in a mandrel of a diameter comparable to the desired diameter of the spiral ring, Stack and clamp the rings with their coils flat and parallel. Then place the rings in an oven at a

Figure 2-47.-O-ring installation.

Figure 2-48.Teflon backup ring damages caused by improper handling.

maximum temperature of 350F for a period of approximately 10 minutes. The rings are then removed and water quenched.

NOTE: After this treatment, rings should be stored at room temperature for a period of 48 hours prior to use.

IDENTIFICATION. Backup rings are not color coded or otherwise marked and must be identified from package labels. Backup rings made from Teflon do not deteriorate with age, and are unaffected by any other system fluid or vapor. They tolerate temperature extremes in excess of those encountered in high-pressure hydraulic systems. The specification number of a backup ring can be found on the package label. This specification number is followed by a dash (-) and a number. The number following the dash indicates the size. In some cases, this number is directly related to the dash number of the O-ring for which the backup ring is intended to be used. For example, the single spiral Teflon ring, MS28774-6, is used with MS28775-006 O-ring; and the double spiral Teflon ring, MS28782-1, is used with the AN6227B-1 O-ring.

INSTALLATION. Care must be taken during the handling and installation of backup rings. If possible, backup rings should be inserted by hand and without the use of sharp tools. The Teflon backup rings must be inspected prior to reuse for evidence of compression damage, scratches, cuts, nicks, and fraying conditions, as shown in figure 2-48. To install the Teflon backup ring (fig. 2-49), the following steps should be used.

1. Examine the fitting groove for roughness that might damage the seal.

2. Position the jam nut well above the fitting groove, and coat the male threads of the fitting sparingly with hydraulic fluid.

3. Install the backup ring in the fitting groove, and work the backup ring into the counterbore of the jam nut.

4. Install the gasket in the fitting groove against the backup ring.

5. The jam nut is then turned down until the packing is pushed firmly against the threaded portion of the fitting.

6. Install the fitting into the boss, and turn until the packing has contacted the boss. (The jam nut must turn with the fitting.)

7. Hold the jam nut and turn the fitting an additional one-half turn.

8. The fitting is then positioned by turning it not more than one turn.

9. Hold the fitting in the desired position, and turn the nut down tight against the boss.

When Teflon spiral rings are being installed in internal grooves, the ring must have a right-hand spiral. View A of figure 2-50 shows the method used to change directions of the spiral. The Teflon ring is then stretched slightly prior to installation into its groove. While the Teflon ring is being inserted in the groove, rotate the component in a clockwise direction. This action will tend to expand the ring diameter and reduce the possibility of damage to the ring.

When Teflon spiral rings are being installed in external grooves, the ring should have a left-hand spiral. As the ring is inserted into the groove, rotate the component in a clockwise direction. This action will tend to contract the ring diameter and reduce the possibility of damage to the ring.

Figure 2-49.-Properly Installed gasket and backup ring.

Figure 2-50.Installation of Teflon backup rings (internal).

Backup rings may be installed singly, if pressure acts only upon one side of the seal. In this case, the backup ring is installed next to the O-ring, opposite the pressure force. See view A of figure 2-51. When dual backup rings are installed, the split scarfed ends must be staggered, as shown in view B of figure 2-51. View C of figure 2-51 shows an improper dual ring installation.