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Gap extrusion

The process by which seal material is forced into the clearances between components is called gap extrusion. The dimension of this clearance gap is referred to as the extrusion gap, or “e-gap” (fig. 1).
The resistance of a given seal component to gap extrusion is mainly determined by the material composition and quality. Materials of greater hardness and stiffness typically also have improved resistance to extrusion. Therefore, full-face anti-extrusion or back-up rings of materials harder than the seal material may be used to prevent seal extrusion into the e-gap (fig. 2).
Pressure is the main driver of extrusion, but the e-gap size and application temperature are also major factors. Diagram 1 shows the pressure resistance of different materials as a function of temperature. The values were measured on an SKF test rig. The tests were carried out with a rectangular sample, dimensions 38,7 x 49 x 5 mm under static pressure and an extrusion gap of 0,3 mm. The pressure values were taken at an extrusion length of 0,5 mm (fig. 1). While these sample values illustrate the differences in extrusion resistance for standard grades of typical seal materials, there are many variations of each basic composition that impact the extrusion resistance of seals. In addition, the profile design and the seal friction affect extrusion. For maximum allowable pressure, temperature and e-gap of each seal profile, refer to the profile data for each profile in the relevant sections.
The maximum e-gap in a hydraulic cylinder cylinder occurs when the cylinder components are at the maximum radial misalignment of components. This misalignment is affected by:
  • external forces acting upon the cylinder assembly (e.g. acceleration forces, frictional moments from rotation of cylinder end connections)
  • the weight of the cylinder components (especially when used horizontally)
  • deformation of cylinder components (rod flexing, guide ring radial deformation under force)
  • the tolerance stack up of multiple cylinder components
Therefore, it is necessary to calculate the e-gap at the maximum misalignment at minimum material conditions of the cylinder and guide components.
For rod seals, the maximum e-gap should be calculated with the following conditions (fig. 3):
  • guide ring groove at maximum diameter D
  • rod at minimum diameter d
  • guide ring cross section at minimum thickness t (considering tolerances and any radial deformation of the guide ring under load)
  • rod seal housing throat at maximum diameter h
For piston seals, the maximum e-gap should be calculated with the following conditions (fig. 4):
  • bore at maximum diameter D
  • guide ring groove at minimum diameter d
  • guide ring cross section at minimum thickness t (considering tolerances and any radial deformation of the guide ring under load)
  • piston seal housing at minimum outside diameter OD
The maximum allowable e-gap is provided in the profile data for each rod seal and piston seal profile in the relevant section. The e-gap can be kept within these limits by specifying and controlling the tolerances of dimensions described above and shown in fig. 3 and fig. 4.
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