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“For high-precision components like hydraulic cylinders, a smoother surface isn’t always better. Too smooth can actually shorten the life of the seal.”
We all know that hydraulic components require very high manufacturing precision. As the actuator in a hydraulic system, hydraulic cylinders naturally have very high requirements. High precision doesn’t necessarily require excellent surface roughness, or in other words, smoother is not necessarily better.
01. Why is a rough piston rod surface bad?
We all know that hydraulic cylinders perform reciprocating linear motion during operation, and the piston frequently extends out of the cylinder during operation. Dust is unavoidable in industrial applications. When dust and other debris adhere to the piston rod, the dust seal removes them. When the piston rod retracts into the cylinder, the dust and other impurities on the piston rod mix with the oil. Over time, the oil becomes increasingly contaminated with impurities. This impurity-laden oil acts like an abrasive, constantly grinding the seals during operation. This damages the seals of other components in the hydraulic system, as well as the seals of the hydraulic cylinder itself. As the grinding time increases, the seals wear more and more, gradually causing leaks.
02. Why does a smooth piston rod also shorten the life of the seal?
If the piston rod is too smooth, or if the surface roughness of the piston rod is too low, the hydraulic oil that lubricates the sealed piston rod will be scraped away. Without the lubricating effect of the lubricant, the friction coefficient between the piston rod and the seal will increase. This friction generates more heat and increases wear, ultimately leading to premature seal failure and leakage.
03. So what’s the perfect surface roughness?
Typically, we use Ra and Rz to evaluate the surface roughness of parts. However, when it comes to piston rod surface roughness, we need to introduce an additional surface roughness parameter, Rmr.
The support length ratio Rmr(c) of a profile is the ratio of the solid material length Ml(c) of the profile at a given horizontal position c to the evaluation length.
The surface microscopic profile at different Rmr values is shown in the figure below.
The higher the Rmr value, the flatter the micro-profile surface, that is, the fewer peaks, or the narrower the gaps. This surface can store oil without adhering to large particles of dust and other impurities, and there are no peaks that cause significant wear to the seals. Such a surface is beneficial to extending the life of the seals.
04. How can we achieve the surface roughness that is beneficial to sealing?
Rolling is performed after grinding. Grinding can obtain the required Ra value, while rolling can obtain a satisfactory Rmr value. Rolling can also enhance the fatigue resistance of the part surface.
Therefore, selecting the appropriate piston rod surface roughness is one of the important factors to ensure that the hydraulic cylinder seal will not fail prematurely. Generally, the recommended piston rod surface roughness is Ra0.2~Ra0.8, Rmr70%. Of course, it needs to be adjusted and verified according to the actual situation and the type of seal selected.
