By Todd Lewis
A hydraulic cylinder is a mechanical actuator that gives a linear force to a load by converting energy stored in pressurized hydraulic fluid. It is also known as a linear hydraulic motor. It consists of a cylinder barrel, in which a piston rod can be pushed by hydraulic pressure to generate a force to move a load.
Applying a coating to the piston rod helps to maximize performance and minimize wear. The choice of coating for a hydraulic cylinder is influenced by various factors, including the environment it will be used in, the fluid it will interact with, the expected lifespan, and the cost of applying the coating.
A hydraulic cylinder rod coating, like a weld overlay or plasma spray coating, protects exposure to environmental, chemical, thermal, mechanical, and physical conditions by reducing the risk of abrasion, adhesion, corrosion, wear, high temperature, and pitting. These coatings increase cylinders' service life, thus maintaining productivity and operational accuracy while avoiding costly repairs.
Below are the most common coating approaches used in the hydraulics industry:
Hard chrome plating is an electrolytic method using chromic acid-based electrolytes to plate cylinders. It is a beneficial technique due to its properties of low friction, wear, and corrosion resistance, and can be used to protect against abrasion and provide attractive finishing with excellent adhesion.
This plating method can be implemented on various surfaces, including cast iron, different types of stainless steels, brasses, bronzes, aluminum alloys, nickel-based alloys, and titanium alloys. The thickness of the chrome layer it produces can range anywhere from 2 to 250 µm.
Laser cladding is a technique that employs a high-powered laser beam as a heat source for melting coating material and applying it on a base metal or substrate. This process can use a variety of nozzle shapes. When the laser beam from the nozzle interacts with various powder mixtures, the substrate surface is loaded with a powder coating layer.
In laser cladding, different types of lasers, for example, diode lasers, CO2 lasers, and fiber lasers can be utilized. Laser coating can also be utilized in pump and turbine components, cylinders and rolls, engine exhaust valves, sealing joints, and more.
Thermal spraying is a popular method for coating objects, including printing presses, food machinery, mining and quarrying equipment, earthmovers, valves, and aerospace turbine components.
It employs various subcategories - high-velocity oxyfuel technique, arc spraying, flame spraying, and plasma spraying - and utilizes metals, ceramics, or plastics as the coating material. This method entails melting the appropriate material with a heat source and then spraying it onto the desired surface.
Hydraulic cylinder weld overlay is a cost-effective procedure to enhance the corrosion resistance and strength of the metal foundation. In this process, a weld overlay of metal or alloy is melted and added to the existing metal structure.
This protects the outer surface from deterioration and failure while also repairing any cavitation or other imperfections on the inner cylinder walls.
Liquid nitrating is a popular black color coating used to protect subsea hydraulic systems in vessels, rigs, ships, and other machines.
This surface treatment provides the material with a thin, uniform, and highly resistant iron nitride layer, making it resistant to pitting corrosion and bacterial corrosion.
The material is submerged into an electrically controlled liquid tank by cleaning and super-polishing, helping protect the system from wear and tear and providing electrical insulation and chemical shielding.
Ceramic plating is widely used in the automotive, aerospace, and military industries to reduce wear and friction and increase heat shielding on key components, including cylinder heads, pistons, intake manifolds, and piston skirts.
To apply a ceramic hydraulic cylinder rod coating, the surface must be treated with a smoothing agent or sandblasted before being heated in an oven to reduce its porosity. Once prepped, a gravity-fed spray gun can apply titanium or tungsten ceramic coating.
Alternative coatings for a hydraulic cylinder tie rod, such as HFC (hard film coating), TPU (thermoplastic polyurethane), and PTFE (polytetrafluoroethylene), increase efficiency, provide corrosion and wear resistance, and reduce friction. Each coating has unique properties and must be selected based on its performance in corrosion protection, wear reduction, and friction reduction.
*Rmr determined ar cut depth C = 0,25 x Rz relative to reference line Cref = 5% for soft coatings and chrome
Cref 2% = for nitrated and hardened Cref = 0% for ceramic coating
Source: Fluid Power Journal
Selecting the right hydraulic cylinder rod coating can be tricky due to the several types of coatings available. Knowing the desired environmental conditions, piston configurations, the fluid used, the cylinder's operation and lifetime, and the associated cost can help narrow your options.
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