Surface Deep

Date: 31.03.20
Surface Deep

As chrome is replaced by alternatives, the issues related to surface technology in hydraulic cylinders are becoming more complex. Traditional 2-D descriptions of surfaces need to be adapted to optimize the operation of fluid power systems. We talk to Mandy Wilke, Senior Manager Global Technical Management, to find out what support is being provided in this area. Knowledge about surfaces and its impact on sealing behavior is important for the optimal sealing performance in fluid power applications, such as cylinders. - Donna Guinivan

TRELLEBORG SEALING SOLUTIONS IS AN EXPERT

in fluid power and we’ve been leading the way in this field for more than 60 years,” says Mandy. “Perhaps it’s not the most glamorous of disciplines, but one where real impact can be made through optimum specification. It has been slow moving in terms of new developments, but lately with the advent of Lubrication Management and artificial intelligence, that’s beginning to change.”

Complex surface finish issues 

An issue that has been a recent concern is the necessary move from chrome on rod and piston surfaces to alternative coatings.

“The most common coating used on hydraulic piston rods was chrome,” continues Mandy. “This coating material is based on Hexavalent Chromium (CR6) and in 2017, under the REACH directive, restrictions were placed on the use of any coating or plating that utilized Hexavalent Chromium (CR6).”

Hexavalent Chromium is the most toxic form of chromium. It is a known human carcinogen, and the toxic waste produced from the Hexavalent Chromium bath during electroplating is deemed a hazardous waste material. It is included in the list of chemicals that have been identified as substances of very high concern, the use of which is restricted and can only be used under strict authorization.

Optimizing surface finishes

“Alternative processes are now being used to coat the piston rod, such as HVOF and laser deposition welding or cladding,” says Mandy. “Optimization of surface finishes with these alternatives is complex and it has not been addressed by many seal suppliers. These continue to give guidance based on the two-dimensional surface finish characteristics used to describe conventional chromium layers.

“The surface finish achieved with chrome, though, cannot usually be achieved with alternative coatings using common coating methods. The alternative coatings have a fundamentally different topography. This leads to significant changes in film formation when compared to chrome.

“That means, if the surface finish recommendations used for chrome are used for alternative coatings, hydraulic systems may more quickly reach their performance limits in terms of leakage and wear.” 

Specification support 

Trelleborg Sealing Solutions has invested in research on how to optimize surface finishes with the coatings and treatments that are replacing chrome. It means moving from simple two dimensional surface descriptions to more detailed descriptions specific to the alternatives.

“We’ve undertaken significant work in this area and are supporting our customers in ensuring their seals are properly specified for their hydraulic applications. A new whitepaper, An Introduction to Counter Surfaces, is now available, as well as an online webinar. These can act as a primer, a base to continue discussions with our technical people specific to an application,” concludes Mandy.

WHAT IS REACH?

The REACH (Registration, Evaluation, Authorisation and restriction of Chemicals) directive is aimed at providing a high level protection of both human health and the environment from potentially harmful chemicals.

ALTERNATIVE COATINGS

Due to the requirement to eliminate Hexavalent Chromium (CR6) from hydraulic systems, alternative processes are now being used to coat the piston rod. Types of coatings include:

High-Velocity Oxygen-Fuel (HVOF)

HVOF is a thermal spray system utilizing the combustion of gases, such as hydrogen or a liquid fuel, such as kerosene. Fuel and oxygen mix and atomize within the combustion area. The HVOF coating, generally of tungsten carbide or chromium carbide, is characterized by a comparatively dense layer structure with layer thicknesses of about 0.1 mm to 0.3 mm, dependent on
coating material.

Laser deposition welding or cladding

This coating technology achieves layer thicknesses of between 0.2 mm and 4 mm, or more, and usually uses nickel and cobalt-based alloys, steels and carbide blends as coating materials. These layers may be much softer compared to the HVOF layers and chrome layers, depending on the alloy used.

WHAT IS TRIBOLOGY?

Tribology is the science and engineering of interacting surfaces in relative motion and includes the study and application of the principles of friction, lubrication and wear.

The functionality of tribological systems is known to depend on the proper tuning of the tribo-partners.Relative to hydraulic sealing systems, these are the seal, the hydraulic fluid and the mating surface. If the correct tuning is achieved, then friction and wear can be minimized, thereby maximizing the performance and the life of hydraulics.