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Hydraulic Cylinder Assembling
I. Assembly Work
All tools, gages, cleaning oil, and utensils must be prepared and ready for assembly.
All parts must be deburred, deflashed, and thoroughly cleaned to remove all dirt.
II. Assembly Key Points
When assembling a hydraulic cylinder, first install the seals for each component separately into their respective components, then proceed from the inside out. During installation, pay attention to the following points:
- 1. Do not damage the seals. When assembling the seal ring, be careful not to allow it to be scratched by burrs or sharp corners, especially seals with lips and new coaxial seals. If there are vent holes or oil holes on the cylinder inner wall, inspect and remove any burrs.
Intersecting areas of the cylinder with oil ports and vent holes should be plugged with a soft material before installing the piston assembly to prevent scratches and rupture of the seal when passing through these openings. Also, inspect any surfaces that contact or rub against the seal ring. If any scratches are present, regrind and repair them. If the seal ring passes through a threaded area, apply a layer of sealing tape to the threads and apply some grease to the tape before installation.
During the assembly of the hydraulic cylinder, use washing oil or gasoline to clean all parts, and then apply some grease on the inner surface of the cylinder and the sealing ring. This will not only make it easier to install the sealing ring, but also protect the sealing ring from damage during assembly, which is more effective.
- 2. Be sure not to misalign the seal ring installation direction and avoid twisting during installation.
- 3. After assembling the piston rod and piston, use a dial indicator to measure their coaxiality and straightness along their entire length, ensuring that the deviations are within the allowable range.
- 4. Before assembly, move the piston assembly within the hydraulic cylinder. Ensure it moves freely, without any resistance or unevenness. Only then can final assembly begin.
- 5. If there is any obstruction during assembly of parts such as the guide sleeve and cylinder head, do not force or knock them. Be sure to identify the cause and eliminate the problem before reassembling.
- 6. When tightening the cylinder head fastening screws, apply force diagonally and evenly, ensuring that the piston rod moves freely and evenly throughout its full range of motion. After tightening all parts, it is best to re-tighten them with a torque wrench to achieve the appropriate tightening torque and consistent torque values.
Disassembling and inspection of Hydraulic Cylinder
I. Sequence of Disassembly
When disassembling a hydraulic cylinder, pay attention to the following:
- 1. First, activate the hydraulic system and use hydraulic pressure to move the piston to a position suitable for disassembly.
- 2. Before disassembly, release the relief valve to reduce the pressure in the hydraulic circuit to zero. Then, disconnect the power supply to stop the hydraulic system.
- 3. To analyze the force applied to the hydraulic cylinder and help identify any faults or damage, mark and record the characteristics and installation positions of key components, such as the cylinder barrel tube, piston rod, piston, and guide sleeve, before disassembling the cylinder.
- 4. To remove the hydraulic cylinder from the equipment, first remove the inlet and outlet piping and loosen all connectors and mounting bolts at the piston rod end. During disassembly, be careful not to damage the piston rod threads, port threads, or the piston rod surface. For example, improper knocking or sudden dropping can damage the threads or scratch the piston rod surface. Therefore, extreme caution is required during disassembly.
- 5. Due to the different structures and sizes of hydraulic cylinders, the disassembly sequence varies slightly. Generally, loosen the end cap fastening threads or connecting rod first. Then, remove the end cap, piston rod, piston, and cylinder barrel tube in that order. When removing the piston and piston rod, do not force them out of the cylinder barrel tube to avoid damaging the inner surface.
II. Parts Inspection
After disassembling the hydraulic cylinder, first perform a visual inspection of each component. Based on experience, you can determine which parts can continue to be used and which require replacement or repair.
- 1. Cylinder barrel tube Inner Surface
Shallow linear or dot-like scratches on the cylinder inner surface are acceptable and pose no practical risk. However, deep longitudinal scratches will not prevent leakage even with a new piston seal. The inner bore must be ground, and correction can be done with fine sandpaper or an oilstone. If the longitudinal scratches are deep and cannot be corrected, brush plating or other methods must be used to repair or replace the cylinder.
- 2. Slide Surfaces of the Piston Rod
If longitudinal scratches or nicks develop on the piston rod sliding surface, which slides against the piston rod seal, the identification and treatment methods are the same as those for the cylinder bore inner surface. However, the piston rod sliding surface is typically hard chrome plated. If a portion of the plating peels off due to wear, forming longitudinal scratches, external oil leakage at the piston rod seal can be significant. The old plating must be removed, re-chrome plated, and polished. The chrome plating thickness should be approximately 0.05mm.
- 3. Sealing
The piston seal and piston rod seal are critical components for preventing internal and external oil leaks in hydraulic rods. When inspecting the seals, first check the seal lip for damage and the friction of the seal friction surface. If the seal lip has minor scratches or minor wear is observed, it is best to replace the seal. Seals that have hardened and become brittle due to long-term use should also be replaced.
- 4. Inner surface of the piston rod guide bush
Some scratches will not hinder operation. However, if the uneven wear depth exceeds 0.2-0.3mm, the guide bush (guide sleeve) should be replaced.
- 5. Piston Surface
Minor scratches on the piston surface will not affect operation. However, if the scratch is less than 0.2-0.3mm deep, the piston should be replaced. Also, check for cracks in the piston caused by impact with the end cap or internal pressure. If any, the piston must be replaced, as cracks could cause internal oil leakage. Also, check the seal groove for damage.
Assembly of the Sealing
I. Guide to Assembly
- 1. The seal must be installed on the preload side, facing the pressure direction;
- 2. The cylinder body and piston rod must have push-in chamfers that meet the requirements;
- 3. Sharp edges must be deburred and rounded or chamfered;
- 4. Threads, guide ring grooves, etc. should be covered, because the seal cannot be pushed through grooves, drilled holes or rough surfaces;
- 5. Any dust, debris or other foreign particles must be carefully removed;
- 6. Before installation, the cylinder, piston rod and seals should be oiled or greased, and consideration should be given to matching the sealing medium;
- 7. Tools with sharp edges should not be used;
- 8. PTFE sealing rings can easily expand and return to their original shape in heated oil or hot water (about 80~120℃);
- 9. If the seal lip needs to pass through the oil pressure hole, use a plastic stick to gently push the lip buckle to prevent the chamfer of the hole from damaging the lip. All holes on the cylinder must be chamfered.
II. Installation Method
Open-groove installation requires no tools.
- (1) Installation of piston rod seals in closed grooves
- 1. Clean and oil all seal bases, seals and installation tools;
- 2. Place the O-ring into the groove (do not twist it);
- 3. Compress the PTFE sealing ring into a kidney shape and avoid forming a pointed bend. Place the compressed PTFE sealing ring into the groove and gently flatten it by hand;
- 4. Push the recovery spindle to the seal and gently rotate it. Leave it for 1 to 5 minutes before removing it.
- (2) Installation of piston seals in closed grooves
- 1. Clean and oil all sealing substrates, seals and installation tools;
- 2. Place the O-ring into the groove (do not twist it);
- 3. Install the bolt head into the guide sleeve, and then install the guide sleeve onto the piston;
- 4. Install the PTFE sealing ring on the guide sleeve and insert the push rod into the guide sleeve;
- 5. Put on the nut and lock the bolt, and gradually push the PTFE sealing ring into the groove;
- 6. Remove the guide sleeve;
- 7. Push the correction sleeve to the seal. Rotate the correction sleeve at the same time and keep it from being removed for about 1 to 5 minutes.
Assembly of O-Ring
When installing the O-ring, pay attention to the following:
- 1. Ensure that all edges or transition holes have been chamfered or blunted and burrs removed;
- 2. Check whether there are any defects on the sealed surface;
- 3. Remove all processing residues;
- 4. The threads on the installation path need to be covered with protective sleeves to prevent the sharp corners of the threads from scratching the O-ring;
- 5. In order to facilitate installation, you can apply lubricating grease to the O-ring installation surface;
- 6. When installing manually, do not use sharp tools, but try to use tools effectively to ensure that the O-ring is not twisted;
- 7. Do not overstretch the O-ring;
- 8. O-rings made of sealing tape are prohibited from being stretched at their connections.
Basic structure diagram of hydraulic cylinder
