BS ISO 7425-1-2021 pdf free download – Hydraulic fluid power cylinders – Dimensions and tolerances of housings for elastomer- energized, plastic-faced seals Part 1: Piston seal housings.
5.2Axial length
For axial lengths for the nominal piston diameters, see Table 3.
lf the extrusion gap exceeds the capabilities of the plastic-faced material to bridge such a gap, length Lz2should be selected and back-up rings (anti-extrusion rings) used (see Clause 7).
5.3 Radial depth
5.3.1 For radial depths, see Table 3.
NOTE This document includes an alternative seal housing radial depth for bore diameters where D z 25 mm.
5.3.2 The larger radial depths shall be selected where higher stresses or wider tolerances are involved.Consultation with the manufacturer, however, is recommended when making the appropriate selection.
6Dimensions and tolerances
Seal housing dimensions and tolerances shall be selected from Table 3.
7Extrusion gap
7.1 The extrusion gap (g, see Figure 1) is determined by the bore diameter and adjacent metalcomponents on either side of the seal.
NOTE1 The extrusion gap varies considerably depending upon use or non-use of non-metallic bearingson the piston.
NOTE2Maximum value for the extrusion gap is achieved when the piston is in contact with one side of thecylinder or bearing (i.e.D-d3 – see Figure 1).
NOTE3 The extrusion gap is widened by the expansion of the cylinder due to internal pressure.
7.2 lt is recommended that details concerning the extrusion gap and need for non-extrusion rings bethe subject of consultation between the housing designer and seal manufacturer.
8Surface finish
8.1General statement
The surface roughness of the seal housing and any mating part has a significant impact on the life andsealing performance of the seal.
Where surface roughness measurements are taken, it is recommended that instruments complyingwith ISO 3274, including an electric wave filter, be used.
8.2Sliding and static sealing surfaces
8.2.1Unless otherwise agreed, the roughness values shall be in accordance with Table 1.
8.2.2 Unless otherwise agreed the material ratio, Rmr, of housing surfaces that are in mating contactwith the seal should be between 50 % and 80 % at a profile section level (RSc) of 25 % of Rz, from areference level, CO , of 5 % Rmr (in accordance with ISo 4287:1997,4.5.4).
8.2.3 For some seal designs, a minimum surface roughness of 0,1 um Ra can be required for the slidingsealing surface as the surface otherwise can be too smooth to provide adequate lubrication for the seal.
9Lead-in chamfer
9.1 To protect the seal from damage during assembly,lead-in chamfers are required, either as anintegral part of housing components or on separate assembly tools.
9.2 Reference shall be made to Figure 1 for typical locations of lead-in chamfers when these are part ofthe housing component.
9.3The minimum axial length of lead-in chamfers, whether on housing components or assembly tools,is given in Table 2.
10 ldentification statement
Use the following statement in test reports, catalogues and sales literature when electing to complywith this document:
“Piston seal housing for elastomer-energized，plastic-faced seals selected in accordance withISO 7425~-1,Hydraulic fluid power cylinders –Dimensions and tolerances of housings for elastomer-energized, plastic-faced seals – Part 1: Piston seal housings.”BS ISO 7425-1 pdf download.