A new whitepaper and webinar in the Trelleborg Sealing Solutions Technical Library explores coatings to replace Hexavalent Chromium for countersurfaces in hydraulic applications.
The countersurfaces in hydraulic systems play a vital role in ensuring the efficiency and service life of hydraulic systems. REACH directives have called for the removal of Hexavalent Chromium as a coating, resulting in alternative materials being explored. However, surface finish characteristics for other coatings are very different from Hexavalent Chromium and require additional experimentation.
A huge variety of applications rely on the integrity of its hydraulics. From prosthetic limbs and aerospace landing gear, to forklift trucks and wind turbines; these feats of engineering would not be possible without harnessing the mechanical properties of fluids. A better understanding of the forces and physics at work can improve these systems.
“Studying the way a system’s surfaces interact when in motion can help to optimize performance and system life,” says Mandy Wilke, Senior Manager Global Technical Management. “The seals, hydraulic fluids and finishes must all be examined in combination to determine the lubrication and therefore the tribological environment.”
With Hexavalent Chromium, a two-dimensional model of the piston’s surface finish was considered sufficient for analysis, and the interactions within the system are well documented and understood. But for the newer coatings, a more complete picture is required. “Traditional two-dimensional modelling is no longer sufficient in our opinion, so we’ve been experimenting both practically and theoretically with adapting two-dimensional and three-dimensional models,” continues Mandy.
“Coatings are applied to improve the friction characteristics of the piston, minimizing wear and maximizing performance. These work in tandem with the seals to provide better function, hence why we provide roughness recommendations for counter surfaces. Hexavalent Chromium, though it worked well to reduce friction, is the most toxic form of chromium – a known carcinogen with a hazardous production process.”
Now manufacturers are using a range of different coatings. These include High-velocity oxygen-fuel (HVOF) and laser deposition welding with nickel or cobalt-based alloys. The two-dimensional models are still useful for these alternative coatings, but differences in surface topography lead to significant changes in the fluid film being created. “These differences mean the ‘general definitions’ of roughness are not as useful, so we also look at the material ratio curve, reduced peak height, ]reduced valley depth and core region, as well as the standard two-dimensional variables, to give a more accurate overview of the surface,” says Mandy.
“Our testing shows that by understanding the three-dimensional topography of a sealing surface, we can better tune the system’s performance, especially when combined with Lubrication Management principles, significantly reducing wear and increasing system life.”