Have you ever encountered a hydraulic cylinder moving slowly in a stationary state? The stability of hydraulic cylinders affects production efficiency and the operator’s safety. If you are a maintenance engineer or equipment manufacturer, it is necessary to understand the causes of hydraulic cylinder drift and how to repair and prevent it.
What is Hydraulic Cylinder Drift?
Hydraulic cylinder drift refers to the situation where the piston rod of a hydraulic cylinder moves slowly and uncontrollably without any operation or receiving a control signal.
Signs of Hydraulic Cylinder Drift
Automatic Load Dropping
After the hydraulic equipment stops operating and enters the pressure holding state, the heavy objects, mechanical arms or working platforms supported by the hydraulic cylinders will slowly and continuously descend under the influence of gravity.
Even if the device does not receive any action commands, the hydraulic cylinder cannot lock the load position or maintain the original height stably.
Piston Rod Movement under No Load
Under no external load, the piston rod of the hydraulic cylinder will still experience a slight displacement.
Abnormal Temperature Rise in the Cylinder Barrel
When the hydraulic cylinder drifts, abnormal heating often occurs on the surface of the cylinder barrel.
System Pressure Drops under the Pressure Holding State
When the hydraulic cylinder is in a pressure holding state, you may find that the reading on the pressure gauge cannot stabilize at the set value, but gradually decreases.
Hydraulic Cylinder Shaking
During the process of locking and maintaining a stationary position, the hydraulic cylinder may experience slight shaking, which cannot maintain stability. This is also a common early characteristic of hydraulic cylinder drift.
The Causes of Hydraulic Cylinder Drift
Piston Seal Failure
The piston seal is a key component that separates the pressure chambers on both sides of the hydraulic cylinder. Long term friction, high temperature hardening, material aging, and lateral compression can cause seal deformation. At this time, the hydraulic oil on the high-pressure side will flow along the gap to the low-pressure side.
This will cause the pressure on both sides of the piston to gradually become balanced. The hydraulic cylinder fails to remain stationary and experiences a gradual position deviation under the influence of the load.
Piston Rod Seal Failure
The failure of the piston rod seal can cause hydraulic oil to flow out of the cylinder through the gap between the piston rod and the cylinder head. When the piston rod seal is damaged, the internal oil volume of the system decreases, and the pressure supporting the load also drops. As a result, the hydraulic cylinder moves under the load.
Hydraulic Cylinder Damage
When hard particle impurities are mixed into the hydraulic oil, or when the piston rod is impacted and causes eccentric operation, scratches or rust may appear on the inner wall of the cylinder.
This will create a tiny leakage channel between the piston seal and the cylinder wall, causing the high-pressure oil to leak and thereby causing hydraulic cylinder drift. The damaged area will also damage the sealing surface of the sealing component, leading to greater leakage.
Hydraulic Valve Damage
There is a small gap between the spool and valve body of the slide valve directional valve to make sure smooth movement of the valve core. Long-term use will cause the sealing surface between the valve core and the valve body to wear, and the gap between them to increase.
At this point, even if the valve is in the locked state, the pressure oil will still slowly flow back to the oil tank or enter the other side of the oil chamber through a small gap, causing pressure changes on both sides of the hydraulic cylinder and resulting in hydraulic cylinder drift.
The balance valve, hydraulic lock and one-way valve are important components for maintaining the pressure holding state of the hydraulic cylinder. These components rely on conical seals or valve seats to block the oil passage.
When oil sludge accumulates at the valve core, causing the valve to fail to close tightly or the valve seat sealing surface to wear due to hard impurities, the locking function of these valves will fail. High pressure oil may flow in the opposite direction through the valve, causing the oil in the pressurized oil chamber to drain.
Hydraulic Oil Contamination
Solid particles, moisture, and air in hydraulic oil can accelerate seal wear. Solid particles will grind the seal surface and the mating surface of the valve seat. Moisture can cause metal parts to rust and reduce the lubricity of the oil. Mixing air into hydraulic oil can cause cavitation and damage to metal surfaces.
When the oil temperature rises abnormally and the viscosity of the hydraulic oil decreases, the oil fluidity increases, making it easier for the oil to flow through extremely small clearance gaps and causing leakage.
Installation Deviation and Lateral Load
If there are problems such as misalignment, loose installation base or deformed support during the installation of the hydraulic cylinder, the piston rod will be subjected to lateral loads.
Lateral force can cause the piston rod to bend and deform, damaging the concentricity between the piston and the cylinder barrel, and causing uneven force on the sealing components and increased local wear.
Long term exposure to lateral loads can also cause wear on the guide sleeve and uneven wear on the piston rod, expand the sealing gap, resulting in oil leakage and hydraulic cylinder drift.
How to Solve Hydraulic Cylinder Drift?
Locate the Source of the Fault
Firstly, you need to determine whether the fault is caused by the internal components of the oil cylinder or the control circuit. After running the hydraulic cylinder to the middle of the stroke and bearing the load, use a high-pressure plug or shut-off valve to isolate the hydraulic cylinder from the hydraulic circuit.
If the hydraulic cylinder stops drifting after the isolation, this indicates that the fault is caused by control components such as the directional valve and the balance valve, or by pipeline leakage. If the hydraulic cylinder still moves, the fault is caused by damage to the seals or cylinder barrel inside the hydraulic cylinder.
Replace the Seals and Repair the Surface
When you determine that there is an internal leakage in the hydraulic cylinder, you need to disassemble the cylinder and replace the hardened, aged and worn-out seals. The material of the sealing parts needs to be compatible with the type of hydraulic oil and the working temperature.
At the same time, check the inner wall of the cylinder. Minor scratches can be eliminated through honing and polishing treatment. If the scratch is deep, you need to replace the cylinder to prevent the new seal from quickly wearing out and failing.
Check the Control Components
For hydraulic cylinder drift caused by control circuits, you can disassemble and inspect the directional valve. If the valve core is worn or the fit clearance is too large, it needs to be repaired or replaced.
For hydraulic locks, balance valves and other components, clean the impurities and oil sludge at the valve core, and repair or replace the damaged valve seat sealing surfaces.
Purify Hydraulic Oil
Polluted hydraulic oil will accelerate the seal wear and may damage the cylinder barrel and the valve body. You need to check the cleanliness of the hydraulic oil. Replace the heavily polluted hydraulic oil.
Check and maintain the cooling device to ensure that the system oil temperature is controlled within the normal range. Prevent hydraulic oil from leaking through the fitting clearance due to low viscosity.
Fix the External Leakage Points
Check for oil leakage in pipeline joints, flanges, piston rod seals, and other areas. Replace the worn-out seals and tighten the joints.
Correcting Deviations
Re-adjust the installation position of the hydraulic cylinder, and secure the base bolts. Make sure that the cylinder body remains coaxial with the direction of load movement to avoid additional lateral forces on the piston rod.
System Exhaust and Pressure Adjustment
After the maintenance is complete, start the equipment and repeatedly operate the hydraulic cylinder to its stroke limit. This can eliminate residual air in the system and avoid pressure fluctuations caused by air.
Set the system pressure within the rated range by detecting and adjusting the overflow valve through a pressure gauge. Then you can conduct a pressure holding test to confirm stable pressure and no displacement of the piston rod. Finally, the hydraulic cylinder drift is resolved.
How to Prevent the Hydraulic Cylinder Drift?
Install the Load Holding Valve
Installing load holding valves such as pilot operated check valves and balance valves in hydraulic systems is a common method to prevent hydraulic cylinder drift. These valves can lock the internal oil of the hydraulic cylinder when the control valve is in the neutral position, maintaining a stable load position.
Standardize the Installation of Hydraulic Cylinders
When installing the hydraulic cylinder, make sure that the cylinder body is coaxial with the direction of the load movement. You can use floating joints and universal couplings to compensate for installation errors and avoid additional lateral forces on the piston rod.
Tighten the base bolts evenly and strictly follow the specified torque to prevent deformation of the cylinder body from affecting the sealing effect. Regularly check the alignment status of the installation base, pin shafts and load connection points, and replace the worn pin shafts and bushings.
Strengthen the Protection of the Piston Rod
In harsh working conditions, you can install a protective cover on the piston rod to isolate dust and impurities in the environment and prevent them from entering the cylinder body as the piston rod extends and retracts.
Regularly inspect the surface of the piston rod. If you find scratches or bumps, repair them promptly to prevent the damaged surface from wearing out the seal during the reciprocating motion. At the same time, promptly clean the surface stains of the piston rod to keep it clean and smooth.
Use Appropriate Seals
You need to choose the appropriate sealing material based on the working temperature, oil type, and working pressure of the hydraulic system. For example, under high temperature conditions, you can choose a fluororubber material.
Use specialized tools for installation to avoid scratches and deformation of the seals. Based on the operating hours of the equipment, you need to replace the seals in time before they reach the fatigue limit.
Keep the Hydraulic Oil Clean
You can install a multi-stage filtration device. Install hydraulic filters at the oil tank suction port, pressure oil circuit, and return oil circuit, and replace the filter elements regularly. You can regularly check the particle size, moisture content, and viscosity of the hydraulic oil based on its usage time.
Develop a Maintenance Plan
Every day, you can observe whether there are scratches on the piston rod surface. Check whether there are any leakage marks on the pipeline joints and flanges.
Every month, you can measure the clearance between the cylinder barrel and the piston. Check the wear condition of the guide sleeve. Test the performance of the hydraulic lock and the balance valve.
You can disassemble and maintain the hydraulic cylinder once a year. Replace the seals that have reached their service life and clean the impurities inside the hydraulic valve.
FAQ
Is the Hydraulic Cylinder Drift Allowed to Exist?
Yes. There are small gaps between seals and valves, making it difficult for hydraulic systems to achieve complete leak free operation. Industrial standards and equipment manufacturers will specify the allowable drift range per unit time based on operating conditions and equipment types.
What is Thermal Drift？
Thermal drift refers to the phenomenon where hydraulic oil undergoes thermal expansion and contraction, causing hydraulic cylinders to produce a similar drift.
After the hydraulic cylinder stops working, the hydraulic oil in the chamber will undergo volume changes due to the variation of environmental temperature, which will cause the piston to move slightly.
Will the Hydraulic Cylinder Drift during Horizontal Installation?
Yes. When there is valve leakage or seal leakage in the hydraulic system, the forces on both sides of the piston will be uneven. Even if the hydraulic cylinder is installed horizontally, it can still cause drift.
Is Hydraulic Cylinder Drift Related to Piston Rod Size?
Yes. Hydraulic cylinders with larger piston rod diameters are easier to drift. The compressed areas on both sides of the piston are not equal. When there is an internal leakage, it is more likely to generate unbalanced forces, which will cause the hydraulic cylinder to move slowly.
Will the New Hydraulic Cylinder Experience any Drift?
The newly installed hydraulic cylinder may also experience slight drift. This is usually caused by incomplete air discharge from the system and the seals are not fully broken-in. After the equipment runs for a period of time, the seals break-in, the air in the system is exhausted, and the drift will disappear.
If hydraulic cylinder drift still exists or worsens, you need to check the system for internal leaks or installation issues.
Final Thoughts
Hydraulic cylinder drift can affect the equipment’s operational accuracy and production safety. Timely handling of hydraulic cylinder drift problems and taking preventive measures can help you reduce failure losses and extend equipment lifespan. If you need high-quality hydraulic cylinders, please feel free to contact us.
