How piston-rod coatings affect hydraulic seals

From aerospace equipment to earthmoving machines, hard-chrome plating on piston rods has been used for decades to protect hydraulic cylinders against corrosion and wear. Stricter environmental regulations and the demand for longer service life, however, have led cylinder manufacturers to develop alternative piston-rod coatings in recent years. But how do these new surface treatments affect seal performance, and is there an ideal coating for all applications?

Seal performance
Friction between a cylinder rod and its seals, and the resulting wear, has a crucial influence on the efficiency and service life of hydraulic cylinders. As long as the hydraulic pump generates sufficient power, seal friction is often overlooked with respect to the hydraulic system’s efficiency. In such cases the main concern is that the cylinder seals and wipers prevent leaks and keep dirt out of the system — even under harsh conditions such as when excavating a riverbed.

But the frictional properties of piston seals and piston-rod seals are becoming increasingly important to the operators of modern fluid-power systems. High static and dynamic friction accelerate wear, decrease the efficiency of the entire hydraulic system, and lead to undesirable stick-slip and high breakaway forces after prolonged rest — the so called “Monday-morning effect.” In addition, unsuitable surfaces can cause seals to squeak or creak and make equipment noisy.

Hard-chrome plating
There are two knocks against hard-chrome-plated piston rods. One involves service life. The surface quality or roughness of hard-chrome-plated piston rods changes over time. The surface is typically smoothed which, counterintuitively, increases friction on the piston-rod seals or causes microcracks and abrades the chrome tips on the rod surface. Long score marks also form across the entire running surface. These changes hurt the entire tribological system and can lead to equipment failure.

The other shortcoming of hard-chrome plating relates to the environment. Hexavalent chromium is toxic and heavily regulated by the U. S. Environmental Protection Agency (EPA). It is a human carcinogen and the EPA considers it a hazardous air pollutant under the Clean Air Act, a hazardous substance under the Clean Water Act, and a hazardous waste under the Resource Conservation and Recovery Act. By-products of the plating process cannot be discarded into wastewater