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The seal of the hydraulic cylinder is the core link to ensure its normal operation. Failure of the seal will directly lead to performance degradation, reduced efficiency and even equipment damage. The following is a detailed explanation of the failure phenomenon, common causes and preventive measures:
1.Typical phenomena of sealing failure
The phenomenon of hydraulic cylinder seal failure can be judged by appearance observation, performance changes or abnormal performance, mainly including the following categories:
1. External leakage
The most intuitive phenomenon is that hydraulic oil seeps out from the sealing mating surface, common locations:
The joint surface between the cylinder barrel and the end cover (static seal failure);
The fitting point between the piston rod and the guide sleeve (dynamic seal failure, oil stains can be seen on the surface of the piston rod, or even dripping);
Auxiliary sealing parts such as drain valves and joints (seals are aging or loose).
2. Internal leakage
The hydraulic oil in the high-pressure chamber leaks into the low-pressure chamber through the sealing gap between the piston and the cylinder barrel (no external oil stains, but the performance is significantly reduced), which manifests as:
Insufficient output force of the hydraulic cylinder (such as “unable to push” when pushing heavy objects);
Slower telescopic speed (especially the difference between no-load and full-load speed is too large);
Unstable movement (such as “creeping”, “stuck”, or automatic retraction when stopped).
3. Intrusion of pollutants
After the seal fails, external dust, moisture, and particulate impurities enter the cylinder through the gap, resulting in:
Hydraulic oil contamination (turbid oil, precipitation);
Increased wear on the inner wall of the cylinder barrel and the surface of the piston rod (scratches and rust);
Abnormal noise in the system (impurities stuck and increased friction).
2. Common causes of seal failure
Seal failure is often caused by the seal itself, installation process, working conditions or improper maintenance, as follows:
1. Seal problems
- Material mismatch
The seal material is incompatible with hydraulic oil (such as mineral oil, water-based fluid), temperature (such as aging of nitrile rubber at high temperature) or medium (such as special hydraulic oil containing acid and alkali), resulting in swelling, hardening or softening.
- Performance overrun
The actual working pressure exceeds the rated pressure of the seal (such as low-pressure seals are used in high-pressure systems, resulting in “squeezing out”); or the temperature is too high (such as when approaching 100°C, ordinary rubber seals age quickly).
- Quality defects
The seal has bubbles, flash, cracks, etc. during production, or the dimensional tolerance exceeds the standard (such as a thin cross-section resulting in poor sealing).
2. Installation process defects
- Installation damage
During installation, the seal is scratched by groove burrs, sharp edges of the piston rod or rough tools (such as “cut edges” on O-rings and torn lips on U-rings).
- Poor groove processing
The seal groove dimensions (depth, width) are out of tolerance (such as too shallow resulting in insufficient seal compression, too deep resulting in “loose” seals); or there are burrs and scratches on the groove surface, which wear the seal.
- Assembly misalignment
The seal is twisted during installation (such as the Y-ring is installed in the wrong direction), causing the seal lip to fail to fit the sealing surface.
3. Increased friction and wear
- Mating surface damage
The chrome plating on the piston rod surface falls off, and the inner wall of the cylinder is rusted or scratched (such as being hit by a hard object), forming a “sharp” surface that cuts the seal during reciprocating motion.
- Impurity wear
Metal particles, dust and other impurities are mixed into the hydraulic oil, and enter the sealing surface as the piston rod expands and contracts, wearing the seal lip like “sandpaper”.
4. Working conditions and environmental factors
- Pressure shock
When the hydraulic cylinder starts, stops or changes direction, the pressure rises suddenly (if there is no buffer device), causing the seal to be subjected to excessive force instantly, and fatigue fracture after repeated impact.
- Temperature abnormality
The ambient temperature is too high (such as near a heat source) or the hydraulic oil has poor heat dissipation, causing the seal to age at high temperature (such as hardening and loss of elasticity).
- Frequent reciprocating
Long-term high-frequency movement (such as more than 10 reciprocating times per minute) causes the seal to generate heat and fatigue due to continuous friction, and gradually lose its sealing ability.
5. Improper maintenance
- Oil contamination
If the hydraulic oil is not replaced for a long time, or the filtration accuracy is insufficient, there will be too much water and oxidation products in the oil, which will corrode the seals and increase wear.
- Failure to repair in time
The seals have shown slight aging and wear (such as surface cracks), and they have not been replaced in time, resulting in the expansion of failure.
3. Preventive measures for sealing failure
In view of the above reasons, prevention can be carried out from four aspects: selection, installation, maintenance and working condition control:
1. Reasonable selection of seals
- Select materials according to system parameters (pressure, temperature, medium): for example, nitrile rubber (oil resistant) for low pressure (<10MPa), polyurethane (anti-extrusion) for high pressure (>30MPa), and fluororubber (aging resistant) for high temperature (>120℃).
- Control the size tolerance of the seal: ensure that it matches the groove (for example, the compression of the O-ring is generally 10%-20% of the cross-sectional diameter), and avoid being too loose or too tight.
2. Standardize the installation process
- Pretreatment
The sealing groove needs to be deburred and polished (surface roughness Ra≤0.8μm); the piston rod and cylinder end need to be rounded (avoid sharp edge scratches).
- Special tools
Use plastic or copper tools to install the seal. Do not use metal tools such as screwdrivers and pliers to directly contact the seal lip.
- Installation inspection
Ensure that the seal is not twisted and the direction is correct (such as the U-ring opening facing the high-pressure chamber), and apply a small amount of hydraulic oil to the sealing surface for lubrication.
3. Strengthen daily maintenance
- Oil management
Regularly (such as every 3-6 months) check the hydraulic oil contamination, replace it as needed (or filter it to NAS level 8 or below), and avoid excessive moisture and particles.
- Surface inspection
Check the piston rod surface for scratches and rust, and the inner wall of the cylinder for wear every week. If problems are found, repair them in time (such as chrome plating to repair the piston rod).
- Replace parts regularly
According to the working frequency, seals should be replaced every 1-2 years (or 1000 hours of operation) (even if there is no obvious damage on the appearance) to avoid aging failure.
4. Optimize working conditions
- Control pressure and temperature
The system pressure does not exceed the seal rating (a relief valve can be installed to limit the peak value); the oil temperature is controlled at 15-60℃ (a cooler or heater is installed).
- Reduce shock
Install a throttle valve or buffer valve at the inlet and outlet of the hydraulic cylinder to reduce the pressure shock during startup and reversing.
- Protect against external impurities
Install a dust cover (such as a telescopic protective cover) on the exposed part of the piston rod to prevent dust and rain from directly contacting the seal.
