English 
Spanish 
In recent years, with advancements in processing technology, the roughness requirements for the inner bore of hydraulic cylinder barrels and the outer surface of piston rods have been steadily increasing. The typical technical requirement for the inner bore roughness is Ra 0.4, but currently, after precision honing or roller burnishing, the inner bore roughness is around Ra 0.2, or even lower; the technical requirement for piston surface roughness has been adjusted from the original Ra 0.4 to within Ra 0.2, and currently, after chrome plating and surface polishing, the roughness is generally around Ra 0.1.
This improvement in roughness has a very significant impact on improving the quality and cleanliness of hydraulic cylinders.
We can understand this from the following key aspects, or visit jwcylinder.com to know more:
I. Ensuring the Reliability and Lifespan of Seals
Hydraulic systems rely on seals (such as O-rings, Glyd rings, and U-cup seals) to prevent high-pressure hydraulic fluid leakage.
Microscopic leakage channels: If the surface is rough, microscopic peaks will prevent complete contact with the seal, forming tiny leakage channels. Under high pressure (tens of megapascals), hydraulic fluid will leak through these channels.
Seal wear: The sharp peaks of a rough surface act like “sandpaper,” continuously scratching, cutting, and wearing down the relatively soft sealing material (such as polyurethane and rubber) during the reciprocating motion of the piston rod. This not only leads to premature leakage but also generates wear debris, contaminating the hydraulic fluid.
Lubricating film maintenance: Both excessively smooth and excessively rough surfaces are detrimental to lubrication. Surfaces within the Ra 0.4-0.2 range allow for effective contact with the seal while also retaining a small amount of oil film, forming boundary lubrication and reducing dry friction.
Friction: The rougher the surface, the more severe the interlocking of microscopic peaks and valleys on the actual contact area, resulting in greater sliding friction.
“Stick-slip” phenomenon: At low speeds, rough surfaces are more likely to cause unstable motion, resulting in a jerky “stick-slip” phenomenon, affecting control accuracy. Smooth surfaces (low Ra value) ensure smooth motion, improving mechanical efficiency and responsiveness.
III. Enhanced Wear Resistance and Corrosion Resistance
Wear Resistance: A smooth surface reduces contact pressure and microscopic cutting with seals and guide bushings, and is also less prone to fatigue spalling, thus greatly improving the wear life of the cylinder barrel and piston rod.
Corrosion Resistance: Rough surfaces have more valleys and cracks, which easily accumulate moisture and corrosive media, and are not easily completely covered by protective layers (such as chromium plating). A smooth surface, on the other hand, forms a denser, continuous protective layer, resulting in better corrosion resistance.
IV. Preventing Oil Contamination and Protecting the System
As mentioned in point 1, metal wear debris and seal wear debris generated by rough surfaces directly enter the hydraulic oil. These particles are the “natural enemy” of hydraulic systems and will:
- Scratch precision components (such as pumps and valves).
- Clog throttling orifices and small gaps.
- Accelerate oil degradation.
Maintaining the smoothness of key friction pairs is an important means of controlling contamination at the source.
V. Subtle Differences in Requirements for Cylinder Barrel and Piston Rod
Although both require high surface finish, the focus is slightly different:
5.1 Cylinder Bore:
Usually requires Ra within 0.4. This is because the inner wall of the cylinder barrel is the static or low-speed sliding surface of the seal, and the machining is difficult (deep hole machining).
Its “honing crosshatch” structure is particularly important. An ideal surface not only has a low Ra value but also features a uniform, intersecting crosshatch pattern, which serves to store lubricating oil, disperse wear, and accommodate tiny impurities.
①~④ are examples of surface roughness morphology.
④ shows the surface roughness morphology after roller burnishing treatment. The peaks (protrusions) of the surface roughness are flattened through plastic deformation. An oil film forms in the valleys (depressions), reducing wear and improving durability.
5.2. Piston Rod Outer Diameter:
The requirements are usually more stringent, reaching Ra 0.2 – 0.1 or less. This is because the piston rod is an exposed moving sealing surface and operates in a harsher environment.
It needs to extend and retract frequently, is exposed to the air, and may be contaminated with dust and moisture. Therefore, it must not only be smooth but also possess extremely high hardness (usually surface-plated with hard chrome) and resistance to wear and corrosion. Polishing to a mirror finish (Ra 0.1 or lower) minimizes the adhesion of external contaminants and protects the sealing ring.
This represents the optimal balance between processing costs, performance requirements, and reliability. Below this value (e.g., Ra 0.8), the sealing effect and lifespan will significantly decrease; pursuing a higher value (e.g., Ra 0.05) will drastically increase processing costs, while the marginal benefit of performance improvement is limited, and it may even be difficult to form a stable oil film due to the excessively smooth surface.
- Key factors in selecting a hydraulic cylinder
- Working principles and internal structures of 7 types of hydraulic cylinders
- Why can throttle valve for oil inlet throttling be larger than throttle valve for oil return throttling when the hydraulic cylinder speed is the same?
- 15 taboos in hydraulic cylinder design! A must-read for engineers: From anti-instability to explosion-proofing!
- Working principle and structural analysis of a complete set of hydraulic cylinders
- Technical requirements of each component of hydraulic cylinder
- Hydraulic cylinder diagram of various types and structures
- How much do you know about hydraulic cylinders? Detailed explanation of the structure and main parameters of hydraulic cylinders!
- Understand the structure of common hydraulic cylinders in one article
- Illustration! Hydraulic cylinder fixing method and installation precautions!