Surface Hardness

Dynamic Sealing Surface Hardness

Seal components normally have physical contact with their dynamic and static mating surfaces during most of the operating time. The physical contact on the dynamic surface can be

• boundary-near full physical contact between the seal lip contact surface and mating surface with boundary friction

• mixed -certain physical contact with lubrication between with mixed friction

• lubricated – lubrication fluid film going to separate the seal lip from the dynamic mating surface with lubricated friction

The relative motion and friction generate wear on the seal’s dynamic lip contact surfaces and on the mating surface as well. The speed of wear out of the seal lip contact area and/or wear out of the mating surface is going to determine the
sealability, service life of the seal and equipment. That is why wear resistance of the seal material and mating surfaces will have influence on the equipment reliability.

A harder mating surface allows for the use of higher wear-resistant seal material that will increase both the seal and hardware service lives. Softer surfaces require the use of lower wear-resistant seal materials that will not
damage the mating hardware surface, but it will generally reduce seal life. Harder mating surface hardness provide better resistance against mechanical impacts and damages too, having positive impact on cylinder and seal life time.

In the case of harder surfaces the adhesion between the metal component and the seal is reduced which is going to results in longer service life for the seal. A balance between seal and equipment surface materials should be met to
provide optimum overall sealing performance and to ensure that the seal is going to be sacrificed and not the metal hardware.

Below chart shows some of the common construction materials that are used for the manufacturing of cylinder’s and reciprocating equipment’s components in various industries, applications. This gives an overview about their average
hardness value ranges what should be taken into consideration as guideline only.

Recommended minimum mating surface hardness is dependent on operating conditions

• velocity (reciprocating speed)

• system pressure

• tribological conditions (fluid, viscosity, temperature)

working environment, further utilized seal materials, seal design and the seal systems. Generally, ow-speed and/or low-pressure applications require lower surface hardness while high-speed and/or high-pressure applications require
higher surface hardness.

The recommended absolute minimum surface hardness for reciprocating applications is Harness Rockwell C 35. In order to achieve optimum performance, especially in heavy-duty hydraulic applications or in harsh
environment, the recommended minimum surface hardness value is 48-50 Hardness Rockwell C (for rod, ram).

Seal and metal hardware performance can be improved by the use of certain plating, coating on the piston rod, press ram. Plating, coating can help to increase the hardness surface over 60 Hardness Rockwell C, improving wear and
corrosion resistance and sliding properties, reducing friction.

Further improvement can be achieved by applying hard chrome plating on hardened rod surface (for example induction hardening), where the hardened surface has a minimum. 48-50 Hardness Rockwell C value depth minimum is 1,5-2,5 mm

Below table gives an overview about the most common coating and plating methods, which are used in fluid power applications.