ISO 7425-2:2021

Overview

ISO 7425-2:2021 - "Hydraulic fluid power cylinders - Dimensions and tolerances of housings for elastomer‑energized, plastic‑faced seals - Part 2: Rod seal housings" specifies the dimensional characteristics and associated tolerances for hydraulic cylinder rod seal housings used with elastomer‑energized, plastic‑faced seals in reciprocating applications. The standard covers housing geometry, groove and chamfer dimensions, surface finish requirements and extrusion‑gap considerations. It does not define seal design, materials or functional performance - those remain the responsibility of seal manufacturers and system designers.

Key topics and technical requirements

• Dimensional series and tolerances: standardized groove diameters, axial lengths, radial depths and corner radii are provided (see Table 3 in the document) to ensure interchangeability of housings and seals.
• Surface finish: requirements for sliding and static sealing surfaces (Ra, Rz and material‑ratio Rmr); where relevant, a minimum Ra of 0.1 µm may be specified to maintain lubrication and seal life. Recommended Rmr for mating surfaces is between 50 % and 80 % at a profile section level Rδc = 25 % of Rz.
• Extrusion gap (g): defined as the clearance difference between housing and rod diameters and influenced by coaxiality; designers should consult seal manufacturers about anti‑extrusion devices.
• Lead‑in chamfer: minimum axial chamfer lengths for rod ends or assembly tools are specified to protect seals during assembly (see Table 2).
• Run‑out and edge treatment: sharp edges and burrs shall be removed; maximum run‑out tolerance is noted (Y = 0.05).
• Reference measurements and symbols: includes surface texture parameters (Ra, Rz, Rmr), and calls out measurement methods (in line with ISO 4287 and ISO 3274).

Practical applications and users

Who benefits from applying ISO 7425‑2:2021:

• Hydraulic cylinder designers and OEMs aiming for interchangeability and reliable sealing performance.
• Seal manufacturers and suppliers to ensure housings match standard grooves and tolerances.
• Quality, inspection and manufacturing engineers for production tolerancing and surface‑finish control.
• Procurement and maintenance teams specifying replacement parts or verifying compatibility.

Applying the standard reduces seal failure risk, simplifies supplier interchangeability, and clarifies housing-to-seal interface requirements for reciprocating hydraulic systems.

Related standards

• ISO 7425 series (other parts for piston seal housings)
• ISO 3320 (rod/piston diameter series)
• ISO 4287, ISO 4288, ISO 1302, ISO 3274 (surface texture and measurement)
• ISO 5598 (fluid power vocabulary), ISO 883 (tooling), ISO 286‑2 (tolerance classes)

Keywords: ISO 7425-2:2021, hydraulic rod seal housings, dimensions and tolerances, elastomer‑energized plastic‑faced seals, extrusion gap, surface finish, lead‑in chamfer.