Honing removes material from the inner surface of the pipe by grinding it in a rotating motion with abrasive stones. Using the appropriate stones, the surface finish can be refined to meet the tolerances required for sealing. If the inner surface is scratched or overly rough, piston seals will wear out more quickly. That’s why inspecting and finishing the inner surface is a critical part of any repair process. In addition to removing scratches, honing also corrects shape deviations such as ovality and barrel-like distortion.
Benefits of Honing
A brand-new replacement tube can be extremely expensive, especially when it’s a custom part of a unique system. In some cases, hydraulic cylinder tubes used in the mining or process industries can approach six-figure price tags. If the tube can be restored to working condition through honing, it’s well worth firing up the machine. Even if the process takes several days, honing is often a cost-effective alternative to ordering a new replacement tube.
Honing allows us to locate deviations in the inner surfaces of the pipe
The honing equipment we use features a digital instrument cluster that monitors the pressure applied to the stones, allowing us to measure any deviations in the pipe using calibrated measuring tools.
Honing is a critical step in the finishing process after welding. During welding, the shape of the cylinder tube can change due to thermal expansion and shifts in internal stresses. If these changes aren’t addressed during the repair, the cylinder tube may become oval or slightly “shrunk,” which puts excessive strain on the piston support rings. Honing also effectively removes various types of oxidation from inside the cylinder.
Surface Quality and Size Classes
Honing creates a crosshatch pattern on the inner surface of the pipe due to the abrasive motion of rotating stones. Although this crosshatch may appear rough to the naked eye, its surface roughness is within the required tolerances to prevent seal wear. We measure the surface roughness of the honed pipe using a surface roughness tester. With roughing stones, we achieve a roughness of Ra 1.0–1.2, and with finishing stones, Ra 0.2–0.4.
With our honing equipment, we can hone cylinder tubes with diameters ranging from Ø50 mm to 450 mm. In terms of length, we can handle up to 5 meters in a single pass, or up to 10 meters by turning the tube.
Correctable shape defects
In serviced cylinders, ovality and barrel-shaped deformation of the tube often cause issues, as these shape defects immediately affect the sealing performance of piston seals. Honing can effectively correct minor shape deviations, helping to restore the tube to the required tolerances for reliable seal function.
Challenges
In theory, honing can slowly wear down the cylinder tube from the inside out—even to the point of failure. In practice, it’s the piston seals that define the limits. If too much material is removed, the tube’s inner diameter increases and the piston seals can no longer hold pressure. In cases where the tube blank is hard to source, the tube itself is valuable, or timelines are tight, the piston can be fill-welded or a new piston can be machined. In these situations, the seal dimensions will differ from the original, so this needs to be considered for future maintenance. In most cases, re-machining the piston is significantly cheaper than replacing the entire cylinder tube.
Example images of the tube before and after honing
The cylinder tube of the excavator in question had scratches detectable with a fingernail running along nearly the entire length of the bore. Without honing, it would be difficult to accurately determine the tube’s shape—even with measurements. These longitudinal scratches could lead to internal leakage along the scratch path, and over time, continuous operation would wear a groove into the Teflon piston seal, eventually allowing oil to leak to the opposite side of the piston.
The small scratches were removed with a 12-minute cleaning and honing.
