Greases and Lubricating Oils for Pneumatic Valves
A durable, low-friction lubricating film

The lubrication of pneumatic valves is based on two fundamental principles:

• Reduction of friction between the valve body and dynamic gaskets
• Compensation for the decay of the sealing effect due to gasket deterioration

Ring seals in pneumatic valves play the fundamental role of isolating pressure chambers in a dynamic environment that is unbalanced, tending towards higher operating frequencies and speeds along with shorter strokes than in pneumatic cylinders.

Although compressed air circuits are equipped with an oil lubrication system (to help reduce sliding friction of air shut-off and actuating components), market demands are geared more towards greater durability and reliability, with particular attention to the choice of lubricating grease to be used between the gasket and valve body.

In the absence of a homogeneous lubricating film, O-rings, seals and gaskets deteriorate rapidly. For this reason, it is necessary to encase them with a lubricating grease designed to:

• Limit static and dynamic friction coefficients between the seal and valve body
• Facilitate insertion into the housing without damaging the sealing profiles
• Resist leaching processes generated by pressure differences
• Help to hold the seal in place
• Improve operating efficiency by limiting air leakage

The guarantee of smooth valve operation can only be achieved by checking the compatibility of the lubricant and seal material.

The rubber-metal coefficient of friction is amongst the highest there is. Whilst this condition, together with the hardness value of the gasket, is set by the designer to ensure the "sealing effect", it also raises a potential threat of wear.

The choice of seal-body lubricating grease in a pneumatic valve is subject to an analysis of the application context and the tribological system as a whole, involving:

• Operating temperature range
• Type of fluid intercepted (such as oxygen)
• Incidental contact with food (NSF certification H1)
• Possible incompatibilities with the service (such as for valves used in the painting industry) or with silicone greases
• Type of gasket materials

The most commonly-used materials for the manufacture of gaskets in pneumatic valves are chosen according to the service and application context, such as:

• NBR (nitrile rubber)
• HNBR (hydrogenated nitrile)
• PUR (polyurethane)
• EPDM
• FKM (Fluorinated)
• SIL (Silicone)

The risks of interaction between lubricant and seal should always be assessed beforehand to prevent seals from losing their original mechanical properties.

This presentation deals with the case of EPDM seals in contact with mineral oils and PAO and the catastrophic consequences that this combination can generate.

Special attention must also be paid to NBR gaskets, which are usually suitable for contact with mineral lubricating greases or PAO. However, there are some formulations that tend to increase their volume and hardness, causing valves to block!