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Housing design and dimensions

Housing dimensions for static radial sealing

O-rings for static (non-moving) sealing can be used in a wide variety of applications and arrangements. The most common arrangement in hydraulic cylinder applications is static radial sealing between coaxial cylindrical parts. The O-ring is installed in a housing that is machined either as an outside groove (fig. 1) or an inside groove (fig. 2) in one of the two cylindrical parts.

The housing dimensions for static radial sealing O-rings are listed in the size lists.

Housing groove edges

All housing groove edges should be smoothed and rounded off (fig. 1, fig. 2) to r = 0,1 to 0,2 mm (0.004 to 0,008 in.).

Lead-in chamfers

All edges and openings through which the O-ring has to pass during the assembly should have appropriate lead-in chamfers and should be well rounded off (table 1). The chamfers facilitate assembly and protect the O-ring from damage during the installation process. The O-ring and all surrounding parts should be well lubricated before assembly, preferably with the same fluid as used in the hydraulic system, ensuring compatibility with seals and cylinder components.

Extrusion gaps and back-up rings

The size of the permissible extrusion gap (→ Gap extrusion) depends mainly on the seal material, temperature and operating pressure. Harder materials (→ Materials) provide a certain resistance to gap extrusion. When the permissible extrusion gap for the pressure and temperature in application is exceeded, back-up rings may be used to prevent the seal pressing into the gap and causing extrusion damage and possibly even premature failure. Figure 3 shows the O-ring behaviour at different operating pressures and conditions. In applications where the O-ring is exposed to pressure from one side only, the back-up ring is installed at the zero pressure side. For an O-ring exposed to pressure from both sides, a back-up ring is installed on both sides.

For the extrusion gaps, SKF recommends machining the fits according to the following tolerance classes:
  • f8and H9 for diameters up to 120 mm
  • f7and H8 for diameters larger than 120 mm

Diagrams 1 to 3 provide guideline values for the maximum extrusion gap in relation to the operating pressure for standard O-rings without back-up ring (diagram 1) and different back-up ring materials and sizes (diagram 2 and diagram 3). These guideline values are based on extensive tests conducted in SKF laboratories at 90 °C (195 °F) and 100 000 pressure pulses. However, other factors such as temperature and fluid can influence these guideline values and should be considered.

For additional information about O-ring and back-up ring materials, refer to Materials and about extrusion, refer to Gap extrusion.

Housing groove width

To accommodate the additional back-up rings, the O-ring groove width L needs to be increased to L1 for one back-up ring or L2 for two back-up rings (fig. 3). The groove width dimensions are listed in the size lists.
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