In the hydraulic system, the hydraulic cylinder is the key actuator that converts hydraulic energy into mechanical energy. The performance of the hydraulic cylinder directly affects the working efficiency and stability of the system. Internal leakage in the hydraulic cylinder is one of the common faults, which can cause pressure loss, slow movement, reduced system efficiency, and even cause safety accidents. Therefore, it is very important to accurately detect the internal leakage in the hydraulic cylinder. This article will introduce the test method of hydraulic cylinder internal leakage in detail to help technicians find problems in time and take effective measures.

01. Causes of internal leakage in hydraulic cylinder

Internal leakage of hydraulic cylinders mainly occurs between the piston and the inner wall of the cylinder, at the seal between the piston and the piston rod, and at the one-way valve on the piston (if any), and components. Under normal circumstances, the seal fills the fitting gap through its own elastic deformation to prevent the hydraulic oil from flowing from the high-pressure chamber to the low-pressure chamber. However, when the seal is aged, worn, or damaged, or the fitting gap increases due to manufacturing errors or long-term use, the hydraulic oil will bypass the seal and leak from the high-pressure chamber to the low-pressure chamber, forming an internal leakage.

02. Internal leakage test conditions

1. Ambient temperature: The test should be carried out at the specified ambient temperature, generally (20±5)℃. Too high or too low temperature will affect the viscosity of the hydraulic oil, and thus affect the accuracy of the test results.
2. Hydraulic oil: Use the same brand of hydraulic oil as the actual working of the hydraulic cylinder, and ensure that the oil cleanliness meets the requirements to avoid impurities from damaging the test results and the hydraulic cylinder. Before the test, the temperature of the hydraulic oil should be checked to keep it in the normal operating temperature range (generally 30-50℃).
3. Hydraulic cylinder status: The hydraulic cylinder under test should be in good condition, without external leakage, mechanical damage and other faults that affect the test. Ensure that the hydraulic cylinder is firmly fixed during installation to avoid vibration and displacement during the test.

03. Hydraulic cylinder internal leakage test method

(I) Static test method
Test steps:
1. Connect the oil inlet port at one end of the hydraulic cylinder to the output pipeline of the hydraulic pump, and the oil outlet port at the other end to the oil tank through a pipeline, and install a flow meter at the oil outlet port. Close other irrelevant oil circuits.
2. Start the hydraulic pump and slowly adjust the pressure to gradually increase the pressure in the hydraulic cylinder to 125% of the rated working pressure, and maintain the pressure for a certain time (generally 5-10 minutes). During the pressure maintenance process, observe whether the pressure gauge reading is stable. If the pressure drops too quickly, it means that there may be a large internal leakage.
3. Use a stopwatch to record the volume of leaked oil passing through the flow meter within a certain time (for example, 30 seconds or 60 seconds), and calculate the leakage per unit time according to the formula. The leakage calculation formula is:
Q=V/t, where
Q is the leakage (L/min),
V is the volume of leaked oil during the recording time (L), and
t is the recording time (min).
Result judgment:
Compare the calculated leakage with the allowable leakage standard specified by the hydraulic cylinder manufacturer. If the actual leakage is less than or equal to the allowable leakage, it is considered that the internal leakage of the hydraulic cylinder is within the normal range; if the actual leakage exceeds the allowable value, it indicates that there is an internal leakage fault in the hydraulic cylinder and further inspection and repair are required.

(II) Dynamic test method
Test steps
1. Connect the hydraulic cylinder and the hydraulic system in normal working state to ensure that all components are installed correctly and the oil circuit is unobstructed.
2. Start the hydraulic system and make the hydraulic cylinder reciprocate within the specified stroke range. The movement speed and load should be close to the actual working conditions.
3. During the movement of the hydraulic cylinder, observe the pressure fluctuation of the system pressure gauge. If the pressure drops significantly and is unstable under no load or light load, or there is a phenomenon of excessive pressure shock during reversing, there may be an internal leakage problem.
4. In order to detect internal leakage more accurately, pressure sensors can be installed at the oil inlet and outlet of the hydraulic cylinder respectively to monitor the pressure changes of the two chambers in real time. By analyzing the pressure difference change curve over time, the degree and location of the internal leakage can be judged.
Result judgment
Make a comprehensive judgment based on the pressure fluctuation and the pressure difference change curve. If the pressure fluctuation exceeds the normal range and the pressure difference gradually increases during the movement, it means that the internal leakage is more serious; if the pressure fluctuation is small and stable, and the pressure difference remains basically unchanged, the internal leakage is less serious or there is no internal leakage.

(III) Pressure drop test method
Test steps
1. Close the oil inlet and outlet at both ends of the hydraulic cylinder respectively to keep the hydraulic cylinder in a closed state.
2. Fill a chamber of the hydraulic cylinder with hydraulic oil of a certain pressure (generally 80% – 90% of the rated working pressure) and record the initial pressure value P1 at this time.
3. After keeping it for a period of time (for example, 10 minutes), read the pressure value P2 of the chamber again.
Result judgment
Judge the internal leakage according to the pressure drop value. The pressure drop value ΔP=P1−P2. If ΔP exceeds the specified allowable pressure drop value (the value is provided by the hydraulic cylinder manufacturer or determined according to industry standards), it means that there is an internal leakage in the hydraulic cylinder; if ΔP is within the allowable range, the internal leakage is normal.
The hydraulic cylinder internal leakage test method introduced above can accurately detect the internal leakage of the hydraulic cylinder, providing a strong basis for the maintenance and troubleshooting of the hydraulic system. In actual work, technicians should select appropriate test methods according to specific circumstances and conduct tests in strict accordance with the operating procedures to ensure the stable operation of the hydraulic system.
The above content covers the common methods and key points of hydraulic cylinder internal leakage testing. If you need more technic support of hydraulic cylinder welcome contact us!

 

Article from 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.