Surface finishes that seal the deal

Hydraulic cylinders and similar fluid power devices are integral to our lives. We find them on bulldozers, aircraft, automobiles and lawnmowers. Regardless of the application, performance and reliability of the fluid power system is dependent on effective seals.

Experts will tell you that preventing leaks in dynamic hydraulic systems not only requires a good seal it also requires a good sealing surface. For this reason, the surface finish of components that mate with hydraulic seals must have proper surface finish.

Sealing surfaces must have finishes that are fine enough to prevent high pressure oil from passing between the seal and surface of the sealed component but also rough enough to allow proper lubrication. If the surface is too rough or the lay of (directional orientation) the tool marks is incorrect, fluids can pass between seal and component. Conversely, should the surface be too fine, then there is no way to retain lubrication and the seal will be abraded leading to failure.

Static and dynamic sealing surfaces typically have surface finish requirements between 8 μin and 12 μin. Common machining operations like reaming, turning and boring cannot reliably achieve these finish requirements. Therefore, abrasive operations like honing and grinding are the most common machining processes used to finish sealing surfaces.

Many applications like hydraulic cylinder rods are also hardened and have coatings like hard chrome applied to enhance wear resistance. These factors also preclude the use of non-abrasive machining.

The most common finishing process for sealing surfaces is grinding.

The most common finishing process for sealing surfaces is grinding. Component geometry, production volumes and component size are going to dictate the process that is used. Surface finish requirements and material properties will dictate the size, shape and type of abrasive wheel that is selected.

In repair shops, tool rooms or specialty applications when volumes are low a shop most often relies on processes like cylindrical grinding and possibly tool post grinding on a lathe to finish components.

In high volume shops centerless grinding might be the first choice for parts like rods for hydraulic cylinders. Centerless grinding allows the part to flow through the machine from end to end. This through feed process is unique to centerless grinding and is exceptionally fast. Centerless grinding also produces very round parts and very fine finishes.