The most common faults of hydraulic cylinders during use are weakness and creeping. There are many reasons for the weakness and creeping of hydraulic cylinders, such as seal wear, cylinder wear, inner wall scratches, inner wall corrosion, piston or piston rod scratches, etc. Today, let’s learn about the faults, causes and repair methods of hydraulic cylinders.

01. The piston rod cannot move

(I) Insufficient pressure

1. The oil does not enter the hydraulic cylinder

• 1.1. The reversing valve does not change direction. Check the reason for the reversing valve not changing direction and eliminate it;
• 1.2. The system does not supply oil. Check the cause of the failure of the hydraulic pump and the main hydraulic valve and eliminate it.

2. Although there is oil, there is no pressure

• 2.1. The system has a fault, mainly the pump or the overflow valve has a fault. Check the cause of the failure of the pump or the overflow valve and eliminate it;
• 2.2. The internal leakage is serious, the piston and the piston rod are loose, and the seal is seriously damaged. Tighten the piston and the piston rod and replace the seal.

3. The pressure does not reach the specified value

• 3.1. The seal is aging and ineffective, the seal lip is installed upside down or damaged, replace the seal and install it correctly;
• 3.2. The piston ring is damaged, replace the piston ring;
• 3.3. The system set pressure is too low, readjust the pressure until it reaches the required value;
• 3.4. The pressure regulating valve is faulty, check the cause and eliminate it;
• 3.5. The flow through the throttle valve is too small. When the leakage in the hydraulic cylinder increases, the flow is insufficient, resulting in insufficient pressure. The flow through the throttle valve must be greater than the leakage in the hydraulic cylinder.

(II) The pressure has reached the requirement but it still does not move

1. Problems with the hydraulic cylinder structure

• 1.1. The piston end face is close to the cylinder end face, and the working area is insufficient, so it cannot be started. An oil groove should be added on the end face to allow the working fluid to flow quickly into the working end face of the piston;
• 1.2. The one-way valve circuit on the cylinder with a buffer device is blocked by the piston, and the inlet and outlet positions of the cylinder should be staggered with the piston end face.

2. The piston rod moves “unevenly”

• 2.1. The clearance between the cylinder and the piston, the guide sleeve and the piston rod is too small. Check the clearance and grind it to the specified value;
• 2.2. The clearance between the piston rod and the cloth-reinforced bakelite guide sleeve is too small. Check the clearance, scrape the guide sleeve hole, and reach the required clearance;
• 2.3. The hydraulic cylinder is poorly assembled (such as poor coaxiality between the piston rod, piston and cylinder head, and poor parallelism between the hydraulic cylinder and the workbench). Reassemble and install. Unqualified parts should be replaced to check the cause and eliminate it.

3.Reasons caused by the hydraulic circuit are mainly that the oil in the back pressure chamber of the hydraulic cylinder is not connected to the oil tank, the throttle port of the speed regulating valve on the return oil line is adjusted too small, or the reversing valve connected to the return oil does not work.

02. The piston rod cannot move

(I) Severe internal leakage

1. The seal is seriously damaged, replace the seal;

2. The viscosity of the oil is too low, replace the hydraulic oil with appropriate viscosity;

3. The oil temperature is too high, check the cause and eliminate it.

(II) Excessive external load

1. Design error, the pressure is too low, replace the components after calculation, and increase the working pressure;

2.The process and use errors cause the external load to be larger than the preset value, and use it according to the specified value of the equipment.

(III) The piston moves with “stiffness”

1. Poor precision, the taper and roundness of the cylinder bore are out of tolerance, check the size of the parts, and replace the parts that cannot be repaired;

2. Poor assembly quality

• 2.1. The coaxiality between the piston, piston rod and cylinder head is poor, so reassemble as required;
• 2.2. The parallelism between the hydraulic cylinder and the workbench is poor, so reassemble as required;
• 2.3. The clearance between the piston rod and the guide sleeve is too small, so check the clearance, scrape the guide sleeve hole, and achieve the required clearance.

(IV) Dirt enters the sliding part

1. The oil is too dirty, filter or replace the oil;

2. The dust ring is damaged, replace the dust ring;

3. It is not cleaned or dirt is brought in during assembly, disassemble and clean, and pay attention to cleaning during assembly.

(V) The speed of the piston drops sharply during the end stroke

1. The throttle port of the buffer regulating valve is adjusted too small. When entering the buffer stroke, the piston may stop or the speed drops sharply. The opening of the buffer throttle valve should be adjusted appropriately and can play a buffering role;

2. The throttle hole diameter in the fixed buffer device is too small, and the throttle hole diameter should be appropriately increased;

3. The gap between the fixed buffer throttle ring and the buffer plunger on the cylinder head is too small, and the gap should be appropriately increased.

(VI) The piston is found to slow down or stop when moving halfway

1. The inner diameter of the cylinder is poorly processed and the surface is rough, which increases the internal leakage. Repair or replace the cylinder;

2.The cylinder wall is swollen. When the piston passes through the enlarged part, the internal leakage increases. Replace the cylinder.

03. The piston rod cannot move

(I) The piston moves “unsteadily”

1. Poor precision, cylinder bore taper and roundness are out of tolerance, check part size, and replace irreparable parts;

2. Poor assembly quality

• 2.1. Poor coaxiality between piston, piston rod and cylinder head, reassemble as required;
• 2.2. Poor parallelism between hydraulic cylinder and workbench, reassemble as required;
• 2.3. The clearance between piston rod and guide sleeve is too small, check the clearance, scrape the guide sleeve hole to achieve the required clearance.

(II) Air enters the cylinder

1. For a new hydraulic cylinder, a repaired hydraulic cylinder or a cylinder that has been downtime for too long, there is air in the cylinder or the exhaust in the hydraulic cylinder pipeline is not completely exhausted. The cylinder moves back and forth with a large stroke without load until the air is exhausted.

2. Negative pressure is formed inside the cylinder, and air is sucked in from the outside. First, seal the joint surface and joints with grease. If the air absorption situation improves, tighten the fastening screws and joints.

3. The volume of the pipeline from the cylinder to the reversing valve is much larger than the volume inside the hydraulic cylinder. When the hydraulic cylinder is working, the oil in this section of the pipeline is not completely exhausted, so it is difficult to exhaust the air. You can add an exhaust valve at a high point in the pipeline close to the hydraulic cylinder. Unscrew the exhaust valve, and the piston moves multiple times under full stroke. After the air is exhausted, close the exhaust valve.

4. The pump sucks in air (hydraulic pump failure);

5. Air is mixed in the oil (hydraulic pump failure).

04. Buffer device failure

1) Excessive buffering

1. The throttle opening of the buffer regulating valve is too small. Adjust the throttle opening to a suitable position and tighten it;

2. The buffer plunger is “stuck” (such as the gap between the plunger head and the buffer ring is too small, the piston is tilted or eccentric), disassemble and clean it to increase the gap appropriately, and unqualified parts should be replaced;

3. There is dirt between the plunger head and the buffer ring, remove the burrs and clean it;

4. The gap between the plunger head and the bushing of the fixed buffer device is too small, and the gap should be increased appropriately.

II) Failure of buffering

1. The buffering regulating valve is in the fully open state, adjust it to the appropriate position and tighten it;

2. The inertial energy is too large, and a suitable buffer mechanism should be designed;

3. The buffering regulating valve cannot be adjusted, repair or replace it;

4. The one-way valve is in the fully open state or the one-way valve seat is not tightly closed. Check the size, replace the cone valve core or steel ball, replace the spring, and repair it;

5. The seal on the piston is damaged. When the pressure in the buffer chamber increases, the working fluid flows back from this chamber to the working pressure side, so the piston does not slow down. Replace the seal;

6. There are scratches on the inner surface of the plunger head or bushing. Repair or replace it;

7. The buffer ring embedded in the cylinder head falls off, and a new buffer ring is replaced;

8.The length and angle of the buffer plunger cone are not suitable and need to be corrected.

(III) “Crawling” occurs in the buffer stroke section

1. Poor processing, such as the verticality of the cylinder head and piston end face does not meet the requirements, the gap between the piston and the cylinder barrel is uneven over the entire length, the cylinder head and the cylinder barrel are not concentric: the inner diameter of the cylinder barrel and the center line of the cylinder head have a large deviation, the verticality of the piston and the nut end face does not meet the requirements, causing the piston rod to bend, etc., carefully check each part, and unqualified parts are not allowed to be used;

2.Poor assembly, such as the hole where the buffer plunger and the buffer ring match is eccentric or tilted, etc., reassemble to ensure quality.

05. External leakage

(I) Poor assembly

1. The end cover is installed skewed during the assembly of the hydraulic cylinder, and the piston rod is not concentric with the cylinder barrel, making it difficult for the piston rod to extend, accelerating the wear of the seal. Disassemble and inspect, and reassemble;

2. The hydraulic cylinder and the worktable guide surface are poorly parallel, making it difficult for the piston to extend, accelerating the wear of the seal. Disassemble and inspect, reinstall, and replace the seal;

3. Seal installation errors, such as seal scratches, cuts, reverse installation of the sealing lip, lip damage or incorrect shaft chamfer size, wrong or missing seals, replace and reinstall the seal;

4. The sealing gland is not installed properly

• 4.1. The gland is installed with deviations, reinstall;
• 4.2. The fastening screws are unevenly stressed, reinstall, tighten the screws to make them evenly stressed;
• 4.3. The fastening screws are too long, so that the gland cannot be tightened. Select the screw length reasonably according to the depth of the screw hole.

(II) Seal quality problems

1. The storage period is too long, and the seals are naturally aged and fail

2. Poor storage, deformation or damage

3. Poor performance of the rubber material, oil resistance or poor compatibility between the rubber material and the oil

4. Poor product quality, incorrect dimensions, and tolerances that do not meet the requirements

In the above cases, the sealant needs to be replaced

(III) Poor processing quality of piston rods and grooves

1. The piston rod surface is rough, the chamfer of the piston rod head does not meet the requirements or is not chamfered, the surface roughness should be Ra0.2μm, and the chamfer should be as required;

2. The groove size and accuracy do not meet the requirements

• 2.1. There are errors in the design drawings, and the grooves are designed according to relevant standards;
• 2.2. The groove size processing does not meet the standards, check the size, and correct it to the required size;
• 2.3. The groove accuracy is poor, there are many burrs, correct and remove the burrs

(IV) The viscosity of the oil is too low

1. The wrong oil is used, replace it with a suitable oil;

2. The oil has other brands of oil infiltrated into the oil, replace it with a suitable oil.

(V) The oil temperature is too high

1. The resistance of the hydraulic cylinder oil inlet is too large, check whether the oil inlet is unobstructed;

2. The ambient temperature is too high, take heat insulation measures;

3. The pump or cooler has a fault, check the cause and eliminate it.

(VI) High-frequency vibration

1. The fastening screws are loose, and the screws should be tightened regularly;

2. The pipe joints are loose, and the joints should be tightened regularly;

3. The installation position moves, and the installation screws should be tightened regularly

(VII) Piston rod strain

1. The dust ring is aging, failed, and sand chips and other dirt are intruded. Clean and replace the dust ring, and repair the strain on the piston rod surface;

2. The fit between the guide sleeve and the piston rod is too tight, causing the active surface to overheat, causing the chrome layer on the piston rod surface to fall off and be strained. Check and clean, and use a scraper to scrape the inner diameter of the guide sleeve to achieve the fit clearance.

Contact: Nancy Zhu, Sales manager, JW GROUP.

Email: nancy@jwgroup.cc

Web: https://jwcylinder.com

Mobile/Whatsapp:+86 15902166721

HYDRAULIC CYLINDER and CNC parts specialist

JW GROUP is an integrated steel product group, products include: hydraulic cylinder, hydraulic cylinder spare parts, drilling forging parts, pneumatic actuator-scotch yoke, CNC machinery parts etc.