Hydraulic cylinders are critical to your productivity, and when one fails you need it repaired quickly and accurately. The repair process for a hydraulic cylinder may seem rather straightforward, but there are certain important aspects that must be kept in mind for the repair to be done correctly the first time.
Preparing to Repair a Hydraulic Cylinder
The first step in preparing to disassemble a hydraulic cylinder is to clean off the unit to prevent the unnecessary incursion of dust, oil, grease, and similar debris into the cylinder unit. Also keep in mind that repairs need to be performed in a clean space if at all possible; while there are times when emergency repairs may need to be performed in the field, it is always preferable to take care of maintenance and repair in an area as free of dust and moisture as possible.
Once the unit is clean, the hoses can be disconnected and both the ports and hoses should be plugged securely. From that point, you can open the cylinder ports and allow the hydraulic fluid to drain.
Hydraulic Cylinder Repair
There are different types of cylinders, and the repair process depends on which type is involved. The most common types of cylinders are either wire ring or threaded head (and in the case of a threaded head cylinder, there will be either a set screw or a locking ring). Note that, during the repair process, all components should be cleaned in a petroleum-based solvent, blown dry via compressed air and, before they are installed, coated with hydraulic fluid.
Disassembling a Wire Ring Cylinder
For a wire ring cylinder, the rod assembly should be retracted and the external steel wire ring removed. A mallet and punch can then be used to push the head into the cylinder tube until the internal groove is exposed and the internal wire ring is moved into a position to be removed.
The seal replacement kit for the cylinder includes a plastic removal ring, which should be straightened and then inserted in the internal groove with the feathered end pointing into the tube itself. If the ring is not entirely within the groove before the rod is pulled out, it may get stuck and become trapped between the tube and the head (which is certainly something to be avoided). If the ring is not in the groove, it may be necessary to remove it and try again.
This step is followed by extending the rod in order to pull the head out of the tube, then completely removing the rod. If you try to force the rod out when resistance is met, the ring could jam and the cylinder may be damaged. Once the rod has been removed, the removal ring should also be removed and now you can remove the locknut, piston, and head/gland. From there you can access the seals, which should all be inspected and replaced.
Note that it is extremely important that the interior surface of the cylinder tube be closely inspected along with the piston and rod. These and other polished parts need to be visually checked for the presence of burrs, scratches, or scuffs.
Repairing a Wire Ring Cylinder
Next the piston and locknut should be replaced. Torque the locknut to the appropriate value based on its size. The band should be installed so it compresses the inner wire ring on the head assembly — it is extremely important to ensure that the band clamp cam is not over the top of the ring gap. Once the band clamp is in place, it should be tightened to ensure the wire ring is seated fully. To allow it to slide during the final assembly stage, loosen it about ½ turn.
Note that during the reassembly process related to the head/gland you should leave the outer o-ring dual seal loose on the rod for it to be installed at a later step. The cylinder tube and piston seals should now be lubricated using hydraulic oil and the piston inserted into the tube. The cylinder head should be tapped in the tube until the band clamp slides over and the wire ring is located inside the tube. Loosen and remove the clamp.
Now the o-ring dual seal can be installed. Once that is complete, you should tap the head until the end of it is flush with the tube. This step is extremely important: this allows the inner wire ring to snap into position within the internal cylinder groove. If this is not accomplished, then as soon as pressure is applied to the cylinder, the head and rod assembly will come out of the tube. Finally, you should pull the rod out so the external wire ring groove (in the cylinder head) is exposed so the ring can be installed.
Repairing Typical Threaded Head Cylinder (Set Screw)
For threaded head cylinders, disassembly begins with loosening the set screw and the end cap. From there, the piston, rod, and gland assemblies can be removed. You can remove the piston from the rod assembly by loosening the lock nut. Next, slide off the end cap and gland assembly. Once that is complete, remove the seals and inspect the parts, including polished parts such as the cylinder tube, rod, and piston.
After the seals have been replaced, reinstall the rod through the gland assembly and end cap, then secure the piston to the rod using the lock nut. After lubing the seals and inside of the tube with hydraulic oil, the piston, gland, end cap, and rod can be installed. You should make sure the cylinder body is gently secured in a vise and use a slight rocking motion to get the components inserted. Before installing the cylinder end-cap, use Loctite anti-seize on the threads, then tighten the cylinder, and set screw to recommended torques.
Common Aspects of Hydraulic Cylinder Repair
The details for repairing threaded head cylinders differ in certain respects from the process for a wire ring cylinder. There are, however, certain things that both procedures have in common. For example, if you find it difficult to push the cylinder back in due to pressure, it is highly likely that the input valves need to be slightly loosened. Also, polished surfaces do need to be inspected carefully, seals should always be replaced once they have been removed, and specific torque values need to be used when tightening critical components. Finally, incorrectly reassembling the hydraulic cylinder or failure to correctly install seals can result in serious, expensive damage.
Key Things to Look For During Disassembly
Remember that the rod should always be carefully inspected to determine if it is bent or if the surface is scratched. If the rod is bent, it either needs to be straightened or a new one needs to be fabricated. If the piston seal is eroded, misshapen, or missing, then there is an issue with the tube that likely requires replacement.
The rod seal should also be checked for distortion, which normally is a sign of a bent rod or a worn-out guide bushing. If the inside of the tube is scratched, scored, or pitted, it will need to be honed or, if the damage is too deep into the surface, replaced.
If a leak is present and there seem to be no problems with the shaft or cylinder fore, then the issue is likely a damaged seal. However, keep in mind that scratches and gouges on the surface where the seal is installed against or upon which it rests can lead to leaks as well. And when seals are installed, they must be handled carefully.
Going Beyond Cylinder Repair
When inspecting the components for damage, it is important not just to address the damage (i.e., polishing or resurfacing) or replace the parts involved but troubleshoot why the damage occurred in the first place. If a repair is made without tracking down what caused the problem in the first place, given enough time the repair will simply have to be repeated. An experienced technician can recognize the signs of common problems and see to it that the source of the problem is addressed in addition to the necessary repairs being performed.
The hydraulic cylinder repair process involves attention to detail and careful visual inspection — both of which often require several years of experience to master. The majority of repairs, such as honing tubes and fabricating rods, are best performed by professionals. In addition, professionals have the skill to troubleshoot the root cause of the damage so that it can be addressed.
At MAC Hydraulics, hydraulic cylinder repair is one of the many services that we offer. We can install new seals, hone tubes, polish rods, and manufacture replacement components for your hydraulic cylinders, regardless of the brand. Our state-of-the-art facilities support tube, rod, and cylinder fabrication as well as machining and welding. In addition, we offer 24-hour cylinder resealing along with a pick-up/delivery service. If you have hydraulic cylinders in need of repair, contact MAC Hydraulics today for quick, efficient, high-quality service.