New surface-inspection techniques improve hydraulic cylinder rods and seals

Rod seals on hydraulic cylinders may have the toughest job in fluid power. They have to contain fluid — often pressurized to several thousand psi — while letting the rod effortlessly move in and out. And they have to minimize friction, heat buildup, and wear despite fast-moving rods and extreme forces.

Engineers at major seal manufacturers have met this challenge with designs that harness pressure to prevent leakage and use advanced materials that resist abrasion, extrusion, harsh fluids, and temperature extremes.

As well-engineered as these seals are, however, they are only as good as the components they work with — in this case the cylinder rod. Too smooth and surfaces won’t hold a lubricant film, and seals overheat and fail. Too rough and seals quickly wear and tear apart.

Surface quality, therefore, plays a major role in proper sealing and dramatically affects seal life and performance. Engineers at Simrit, Elgin, Ill., have identified inspection parameters that accurately evaluate rod-surface finish in hydraulic applications and developed guidelines for manufacturers and users. These factors could lead to modernized standards for counter-surface preparation that extend the life of seals and cylinders.

Evaluating surfaces
Traditionally, sealmakers have relied on three factors to measure surface finish and define sealing requirements: average roughness, R_a, the maximum peak-to-valley roughness, R_max., and average peak-to-valley roughness,R_z. Although easy to measure and based on industry standards, these traditional factors are insufficient to accurately evaluate surface quality. Here’s why:

• R_a does not provide the needed detail of a surface. Two surfaces with entirely different roughness profiles can have the same R_a value.
• R_max is measured across a standard length, so it does not indicate a skew toward peaks or valleys.
• R_z uses measurements across five equal segments of standard length, so it can miss problem areas because averaging masks extremes.

Although these factors are important, they do not adequately classify surface abrasiveness and how well it holds lubricant. And as demands for better hydraulic performance, life, and warranty expectations continue to increase among users, manufacturers need more useful guidelines for good surfaces. To do this, more-extensive definitions of surface finishes are required. Here’s how.