As a product group, cylinders are almost as common as pumps and motors combined. They are less complicated than other types of hydraulic components and are therefore relatively easy to repair. As a result, many hydraulic equipment owners carry out cylinder repairs themselves.
The following is a guide to carrying out effective repairs to hydraulic cylinders. The extent of the repair work that can be carried out in-house depends on the extent of wear or damage to the cylinder and how well-equipped your repair shop is. As with any repair, the economics of repairing a cylinder ultimately depend on the cost and availability of a new one.
Disassembly and Inspection
Typically, a cylinder will be removed for repair due to either external or internal leakage. Close inspection of the cylinder parts, particularly the seals, after disassembly can reveal problems that may not otherwise be obvious.
If the piston seal is badly distorted, eroded or missing, the barrel is oversized or has bulged in service. In this case, the barrel or the complete cylinder should be replaced. Replacing the piston seal without replacing the barrel should be considered a short-term fix and will require comprehensive maintenance at the earliest opportunity.
If the rod seal is badly distorted, usually the guide bush is excessively worn or the rod is bent. Both cases result in the weight of the rod riding on the seal which causes failure. Replacing the rod seal without identifying and rectifying the cause of the problem is a short-term fix.
Check the rod for cracks using dye penetrant at all points where its cross-section changes. Inspect the chrome surface of the rod. If the chrome looks dull on one side and polished on the opposite side, the rod is bent. Rod straightness should be checked when a cylinder is being repaired. This is done by placing the rod on rollers and measuring the run-out with a dial gauge (Figure 1). Position the rod so that the distance between the rollers (L) is as large as possible and measure the run-out at the midpoint between the rollers (L/2).
In most cases, bent rods can be straightened in a press. It is sometimes possible to straighten rods without damaging the hard-chrome plating; however if the chrome is damaged, the rod must be either rechromed or replaced.
If the chrome surface of the rod is pitted or scored, the effectiveness and service life of the rod seals will be reduced. Minor scratches in the chrome surface can be polished out using a strip of fine emery paper in a crosshatch action. If the chrome is badly pitted or scored, the rod must be either rechromed or replaced. Machining a new rod from hard-chrome plated round bar is usually the most economical solution for small-diameter rods.
In cylinders used in light-duty applications, it is common for the rod to be supported directly on the head material, which is usually aluminum alloy or cast iron. A metallic or nonmetallic guide bush (wear band) is fitted between the rod and the head, in applications where there are high loads on the rod. If a cylinder is fitted with a bush between the rod and the head, it should be replaced as part of the repair.
If the rod is supported directly on the head, use an internal micrometer or vernier caliper to measure the head’s internal diameter. Take measurements in two positions, 90 degrees apart, to check for ovalness. The inside diameter of the head should not exceed the nominal rod diameter plus 0.004 inch. For example, if the nominal diameter of the rod is 1.5 inches then the inside diameter of the head should not exceed 1.504 inches. If the head measures outside this tolerance, it will allow the rod to load the rod seal, resulting in premature failure of the seal. Therefore, the head must be sleeved using a bronze bush or be replaced with a new head, machined from a similar material.
Minor scoring on the lands of the seal grooves inside the head is not detrimental to the function of the cylinder, as long as the maximum diameter across the lands does not exceed the nominal rod diameter plus 0.016 inch. For example, if the nominal diameter of the rod is 1.5 inches then the inside diameter of the head, measured across the lands of the seal grooves, should not exceed 1.516 inches. If the seal lands measure outside this tolerance, the service life of the rod seal will be reduced. Therefore, the head must be replaced with a new head, machined from a similar material.
Inspect the barrel for internal pitting or scoring. If the barrel is pitted or scored, the effectiveness and service life of the piston seal will be reduced. Therefore, the barrel must be honed to remove damage or be replaced. On small diameter barrels, pitting or scoring less than 0.005 inch deep can be removed using an engine-cylinder honing tool. The barrel must be honed evenly along its full length.
The maximum bore diameter for standard-size piston seals is the nominal bore diameter plus 0.010 inch. For example, if the nominal bore diameter of the barrel is 2.5 inches then the maximum size after honing should not exceed 2.510 inches. This size should be checked at several points along the barrel, using an internal micrometer.
If scoring or pitting is still present at 0.010 inch oversize, the barrel must be honed further to accommodate oversize seals or be replaced. Manufacturing a new barrel from honed tubing is usually the most economical solution for small-diameter cylinders.
Large diameter, inch-size cylinder barrels can be salvaged by honing either 0.030 inch or 0.060 inch oversize and fitting the corresponding oversize piston seals. Oversize seals for metric-size cylinders have limited availability and therefore it is not always possible to salvage metric-size barrels by fitting oversize seals.
The pistons of cylinders used in light-duty applications are usually machined from aluminum alloy or cast iron and operate in direct contact with the cylinder bore. Minor scoring on the outside diameter of the piston is not detrimental to the function of the cylinder, as long as the minimum diameter of the piston is not less than the nominal bore diameter minus 0.006 inch. This can be checked using an external micrometer. For example, if the nominal diameter of the barrel is 2.5 inches then the minimum piston diameter would be 2.494 inches. If the piston diameter measures outside this tolerance, it must be replaced with a new piston machined from a similar material.
Nonmetallic wear bands are fitted between the piston and barrel, in applications where there are high loads on the rod. If the cylinder is fitted with piston wear bands, these should be replaced as part of the repair.
When ordering seals from a seal supplier, avoid the common practice of measuring the old seals. Seals can either shrink or swell in service, or an incorrect seal may have been installed previously. To ensure that you are supplied with the correct seals, measure all seal grooves with a vernier caliper and give this information to your seal supplier.
Thoroughly clean all parts in a petroleum-based solvent and blow-dry using compressed air. Coat all parts with clean hydraulic fluid during assembly. Prior to installing seals, ensure that the seal grooves are clean and free from nicks and burrs. Avoid using a screwdriver or other sharp object when installing seals, as this can result in damage to the seal. After the cylinder has been assembled, plug its service ports to prevent ingress of moisture or dirt.