BS ISO 7425-1:2021 pdf free download.Hydraulic fluid power cylinders — Dimensions and tolerances of housings for elastomer energized, plastic-faced seals.
1 Scope
BS ISO 7425-1 specifies the dimensions and associated tolerances for a series of hydraulic cylinder piston seal housings to accommodate elastomer-energized, plastic-faced seals used In reciprocating applications.
BS ISO 7425-1 does not stipulate details of seal design, since the manner of construction of seals varies with each manufacturer. The design and material of the seals, and any associated anti-extrusion components, are determined by conditions such as temperature and pressure.
BS ISO 7425-1 only applies to the dimensional characteristics of products manufactured in accordance with this document. It does not apply to their functional characteristics.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of BS ISO 7425-1. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 3320. FluId power systems and components — Cylinder bores and piston rod diameters and area ratios
— Metric series
lSO42li2i9Z Geometrical Product Specifications (GPS) — Surface texture: Profile method — Terms. definitions arid surjiice texture parameters
ISO 5598. Fluid power systems and components — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in I59 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
5.2 Axial length
For axial lengths for the nominal piston diameters, see Table 3.
lithe extrusion gap exceeds the capabilities of the plastic-faced material to bridge such a gap, length L2
should 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 BS ISO 7425-1 includes an alternative seal housing radial depth for bore diameters where D  25mm.
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.
6 Dimensions and tolerances
Seal housing dimensions and tolerances shall be selected from Takle3.
7 Extrusion gap
7.1 the extrusion gap (,g, see Figure 1) is determined by the bore diameter and adjacent metal components on either side of the seal.
NOTE I The extrusion gap varies considerably depending upon use or non-use of non-metallic bearings on the piston.
NOTE 2 Maximum value for the extrusion gap is achieved when the piston Is in contact with one side olthe cylinder or be.aring (I.e. L)-d3 — see FIgure 1).
NOTE 3 The extrusion gap is widened by the expansion olthe cylinder due to internal pressure.
7.2 It is recommended that detaik concerning the extrusion gap and need for non-extrusion rings be
the subject of consultation between the housing designer and seal manufacturer.
8 Surface finish
8.1 General statement
The surface roughness of the seal housing and any mating part has a significant impact on the life and sealing performance of the seal.
Where surface roughness measurements are taken, it is recommended that instruments complying with ISO 3274. IncludIng an electric wave filter, be used.
8.2 Sliding and static sealing surfaces
8.2.1 Unless otherwise agreed, the roughness values shall be in accordance with Tahiel.
8.2.2 Unless otherwise agreed the material ratio, Rmr, of housing surfaces that are in mating contact with the seal should be between SO % and 80 % at a proIile section level (R6c) of 25 % of Rr. from a reference level, CU. of 5% Rmr (in accordance with ISO 42871997. 4.5.4).
8.2.3 For some seal designs, a minimum surface roughness oIO,l pm Ra can be required for the sliding sealing surface as the surface otherwise can be too smooth to provide adequate lubrication for the seal.
8.2.4 Exceptional service conditions can necessitate the selection of other grades of surface roughness, in which case they should be subject to agreement between manufacturer and user.
8.2.5 All surfaces against which a seal operates should be free from chatter marks and scores along the operating axis of the seal.
9 Lead-in chamfer
9.1 To protect the seal from damage during assembly, lead-in chamfers are required, either as an integral part of housing components or on separate assembly tools.
9.2 Reference shall he made to Figure 1 for typical locations of lead-in chamfers when these are part of the housing component.
9.3 The minimum axial length of lead-in chamfers, whether on housing components or assembly tools, is given in Table 2.
10 Identification statement
Use the following statement in test reports, catalogues and sales literature when electing to comply with BS ISO 7425-1:
‘Piston seal housing for elastomer-energized, plastic-faced seals selected in accordance with ISO 7425-1. Hydraulic fluid power cylinders — Dimensions and tolerances of housings for elastomerenergized, plastic-faced seals — Part 1: Piston seal housings.”