Avoiding rod seal leakage

The causes of rod seal failure are many and various, but they are all avoidable, as this design brief from Parker Hydraulics explains

Damaged piston rods or rod bearings are the most common cause of seal failure. The usual causes of such damage are poor alignment between the cylinder and its load or a bent piston rod.

Cylinders are designed to provide linear force and motion to a guided load. The path which the guided load travels must not impose significant eccentric load on the rod or the piston rod bearing, or the bearing surfaces will be damaged and fluid leakage and reduced bearing life will result. The maximum thrust force that a cylinder should provide is limited by its road diameter and overall length. In long stroke cylinders care should be taken to ensure that the piston rod is capable of handling the thrust forces generated in the application and that the cylinder is adequately supported. Note also that a head end mounting style will provide greater column strength than a cap end mounting, due to the smaller distance between mounting points.

Contaminated fluid can also cause premature rod seal failure. Abrasive particles suspended in the fluid can damage the seal and the piston rod surface, while airborne contamination can be drawn into a cylinder via a faulty wiper seal.

Water is a common contaminant in mineral oil systems, affecting the lubricity of the fluid and causing some of the most widely used seal materials to age harden at temperatures above 65 degrees C. Polyurethane seals are subject to hydrolysis effects in high water-based fluids at temperatures above 50 degrees C, leading to a loss of hardness and tensile strength.

NOT TO BE OVERLOOKED

Air is often overlooked as a fluid contaminant, but aerated oil can cause physical damage to piston rod seals. Pressure shocks in systems with high cycling speeds can cause air bubbles to become highly charged with heat energy. This is particularly pronounced in vertical rod up applications where a rapid increase in hydraulic pressure can cause intense, localised heating of the bubbles at the lip of the primary seal. The presence of air in the fluid can also intensify the transmission of vibration which, in turn, can lead to other forms of system failure.

Extreme temperature applications pose two challenges. First, the temperature itself may limit the choice of materials and geometries in order to avoid hydrolysis of the seal compound. Second, the fluids used in such applications often have less lubricity than mineral oils.

Some fluids react with seal materials and attack them chemically. Fluid manufacturers often list common seal materials as `compatible’ without defining the suitability of the material for a dynamic seal. It may be acceptable for a static seal such as a cylinder body `O-ring’ to swell by 10%, but, if a dynamic rod seal swells by 10%, the result can be a dramatic rise in friction and accelerated wear. One common mineral oil-based fluid has an additive that shrinks a widely used seal material by 10%.

High pressure leaks are rarely seen with the latest polyurethane seals. With seals of other materials, high pressure leakage is usually the result of inadvertent pressure intensification across the piston. A severe meter-out flow restriction at the head end of a cylinder with an oversized piston rod can expose the rod seal to a back pressure equal to twice the system pressure. Continued operation under these conditions can cause rapid seal wear due to excess friction, extrusion of the primary seal, or in extreme cases, failure of the rod cartridge or the retainer.

INFORMATION: Parker Hydraulics Tel: 01923 492000