SKF HMV 138/233983 - O-Ring for HMV 138 Hydraulic Nut

O-rings are one of the most common sealing solutions. SKF supplies O-rings in a wide range of sizes and different materials, which make them appropriate for a wide variety of operating conditions and applications. They are easy to install and they enable a simple and cost effective seal housing design.

O-rings maintain sealing contact force by radial or axial deformation in the seal housing between two machine components. The most important criteria that influence the maximum operating pressure at which O-rings in static radial sealing can be used are the following:

• Extrusion gap
• Material
• Sealed fluid
• Temperature

Under specific conditions, there is a risk for gap extrusion. Back-up rings prevent O-rings from gap extrusion in static radial sealing. O-rings are used in a wide variety of applications sealing various media. This catalogue focuses on sealing systems for hydraulic cylinders. Therefore, this chapter and provided recommendations apply to static sealing of common mineral-based hydraulic fluids.

Designations

The designations of SKF O-rings (table 1) in both metric and inch sizes contain their dimensions in metric units.

The designations of SKF back-up rings (table 2) in metric sizes contain their dimensions in metric units, whereas those in inch sizes contain their dash-number.

The product tables for both are provided in inch and metric sizes. If applicable, the unique dash-numbers for O-ring sizes in accordance with standards AS586 and ISO 3601 are also listed there.

Materials

The O-rings listed in this catalogue are made of nitrile rubber (NBR) with 70 shA hardness as standard. This is the most common O-ring material and hardness used in hydraulic cylinder applications. On request, SKF can supply alternative hardnesses such as 80 shA or 90 shA. However, SKF generally recommends choosing O-rings with 70 shA and combining them with one or two back-up rings. At operating temperatures above 100 °C (210 °F), fluorocarbon rubber (FKM) or hydrogenated nitrile rubber (HNBR) can be an appropriate materials, depending on the fluid. SKF back-up rings listed in this catalogue are made of polyurethane (TPU). On request, SKF can supply alternative materials and various hardness grades. Common back-up ring materials are listed in the designation system (table 2).

Standards and sizes

Dimension standards

SKF can supply O-rings in a wide range of sizes in accordance with various O-ring standards. Table 3 provides the most common national and international O-ring standards and their relevant sizes.

Tolerance standard

SKF supplies all O-rings with dimensional tolerances in accordance with ISO 3601-1 Class B (table 4). They are suitable for any elastomer material provided that appropriate tooling is used, whereas the tooling most commonly used is based upon the shrinkage of nitrile rubber (NBR) with 70 shA hardness.

Inside diameter/cross section proportion

O-rings used in more demanding applications, such as those with higher operating pressures or larger misalignments, may require larger cross sections. SKF recommends inside diameter ID and cross section proportions as provided in diagram 1.