The piston rod of a hydraulic cylinder operates both inside and outside the barrel, and consequently both in and out of the hydraulic fluid and surrounding atmosphere.
Coatings
Wear and corrosion resistant surfaces are desirable on the outer diameter of the piston rod. The surfaces are often applied using coating techniques such as Chrome (Nickel) Plating, Lunac 2+ duplex, Laser Cladding, PTA welding and Thermal Spraying. These coatings can be finished to the desirable surface roughness (Ra, Rz) where the seals give optimum performance. All these coating methods have their specific advantages and disadvantages. It is for this reason that coating experts play a crucial role in selecting the optimum surface treatment procedure for protecting Hydraulic Cylinders.
Cylinders are used in different operational conditions and that makes it a challenge to find the right coating solution. In dredging there might be impact from stones or other parts, in salt water environments there are extreme corrosion attacks, in off-shore cylinders facing bending and impact in combination with salt water, and in the steel industry there are high temperatures involved, etc. There is no single coating solution which successfully combats all the specific operational wear conditions. Every technique has its own benefits and disadvantages.
Length
Piston rods are generally available in lengths which are cut to suit the application. As the common rods have a soft or mild steel core, their ends can be welded or machined for a screw thread.
Distribution of forces on components
The forces on the piston face and the piston head retainer vary depending on which piston head retention system is used.
If a circlip (or any non preloaded system) is used, the force acting to separate the piston head and the cylinder shaft shoulder is the applied pressure multiplied by the area of the piston head. The piston head and shaft shoulder will separate and the load is fully reacted by the piston head retainer.
If a preloaded system is used the force between the cylinder shaft and piston head is initially the piston head retainer preload value. Once pressure is applied this force will reduce. The piston head and cylinder shaft shoulder will remain in contact unless the applied pressure multiplied by piston head area exceeds the preload.
The maximum force the piston head retainer will see is the larger of the preload and the applied pressure multiplied by the full piston head area. The load on the piston head retainer is greater than the external load, which is due to the reduced shaft size passing through the piston head. Increasing this portion of shaft reduces the load on the retainer.
Side loading
Side loading is unequal pressure that is not centered on the cylinder rod. This off-center strain can lead to bending of the rod in extreme cases, but more commonly causes leaking due to warping the circular seals into an oval shape. It can also damage and enlarge the bore hole around the rod and the inner cylinder wall around the piston head, if the rod is pressed hard enough sideways to fully compress and deform the seals to make metal-on-metal scraping contact.
The strain of side loading can be directly reduced with the use of internal stop tubes which reduce the maximum extension length, leaving some distance between the piston and bore seal, and increasing leverage to resist warping of the seals. Double pistons also spread out the forces of side loading while also reducing stroke length. Alternately, external sliding guides and hinges can support the load and reduce side loading forces applied directly on the cylinder.
