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“Hydraulic Cylinders – Test Methods” standard describes the operational and leakage tests under low pressure:
What is hydraulic cylinder leakage under low pressure?
How to test for leakage in a hydraulic cylinder under low pressure?
Operation and Leakage Test at Low Pressure
(1) Procedure
Adjust the test system pressure and flow rate so that the hydraulic cylinder under test starts under no-load conditions, moves at a speed within the design range, and reciprocates through its full stroke at least 3 times, expelling air from each working chamber. Each time, pause at the end of the stroke for at least 10 seconds. When testing large-diameter hydraulic cylinders, the pause time should be extended.
For cylinder barrel bore diameter greater than 32 mm, test at a pressure of 0.5 MPa (if the hydraulic cylinder does not start, gradually increase the pressure until it starts).
For cylinder barrel bore diameter not greater than 32 mm, test at a pressure of 1 MPa (if the hydraulic cylinder does not start, gradually increase the pressure until it starts).
(2) Visual Inspection
During the test, the following should be checked:
a) Whether there is vibration or crawling during movement;
b) Measurement of the stroke;
c) Whether there is leakage at the piston rod seal;
d) Whether there is leakage at all static seals;
e) Whether there is leakage at the throttle valve and/or cushioning elements installed on the hydraulic cylinder;
f) Whether there is leakage at the welds of the hydraulic cylinder under test;
g) When the hydraulic cylinder under test has cushioning (e.g., with a cushioning throttle valve, the throttle valve opening should be slightly opened), verify whether there is a cushioning effect.
We will analyze the leakage under low pressure:
I. What is hydraulic cylinder leakage under low pressure?
Hydraulic cylinder leakage under low pressure refers to the abnormal loss of hydraulic fluid inside (internal leakage) or outside (external leakage) the hydraulic cylinder when the system operating pressure is low.
1. The term “low pressure” here refers to:
When the cylinder barrel bore diameter is greater than 32 mm, the test is conducted at a pressure of 0.5 MPa (if the tested hydraulic cylinder does not start, the pressure is gradually increased until it starts).
When the cylinder barrel bore diameter is not greater than 32 mm, the test is conducted at a pressure of 1 MPa (if the tested hydraulic cylinder does not start, the pressure is gradually increased until it starts).
2. The difference from high-pressure leakage:
High-pressure leakage: This mainly tests the strength, deformation, and extrusion resistance of the seals under high pressure. The seals may be “squeezed” into gaps under high pressure and damaged, leading to leakage.
Low-pressure leakage: This primarily tests the sealing effect and self-sealing capability between the sealing element and the mating surface. Under low pressure, the elasticity and pre-tightening force of the sealing element itself are crucial in preventing leakage.
II. Types and Causes of Low-Pressure Leakage
1. Internal Leakage
Definition: Hydraulic fluid leaks from one chamber (high-pressure chamber) to another chamber (low-pressure chamber) of the hydraulic cylinder.
Location: Between the piston seal (such as Glydring, Stepseal, U-ring, etc.) and the inner wall of the cylinder barrel.
Causes:
Improper seal selection: Some high-pressure seals from certain brands are prone to leakage problems at low pressure. Therefore, it is essential to consult with professional technical personnel from the seal manufacturer before selection. Additionally, some seals may exhibit “air leakage but no oil leakage” during air bleeding of the hydraulic cylinder.
Improper seal installation or damaged seals: The seal lip is scratched during installation, or the seal is twisted or rolled, destroying its sealing geometry.
Cylinder inner wall damage: The inner wall of the cylinder has pitting, scratches, or wear, forming grooves or pits. At low pressure, the seal cannot fill these grooves, allowing the hydraulic fluid to pass through.
Improper clearance: The clearance between the piston and the cylinder barrel is too large. At low pressure, the seal cannot effectively “block” this wide gap.
2. External Leakage
Definition: Oil leaks from the inside of the hydraulic cylinder to the external environment.
Location of occurrence:
Piston rod seal (main seal), static seal between the guide bushing and the cylinder barrel/piston rod, various threaded connections and welds.
Causes:
Piston rod surface damage, seal lip damage, improper seal installation (e.g., seal installed backward), welding defects, etc.
III. How to Test for Hydraulic Cylinder Leakage?
Testing is divided into internal and external leakage tests, with methods ranging from simple to professional.
3.1. External Leakage Test
1. Visual Inspection:
Method: Carefully observe all possible leakage points during hydraulic cylinder operation and while stationary: piston rod, guide bushing, end cap joints, pipe joints, and welds.
Judgment: Any oil drops, oil film, or wet oil traces indicate external leakage. Only a very thin film of oil is allowed on the piston rod for lubrication; no oil drops should be present.
2. White Paper/Tissue Test:
Method: Wipe the suspected leakage area with a clean piece of white paper or tissue.
Judgment: If there are noticeable oil stains on the paper, it indicates external leakage. This method is very effective for detecting small leaks.
3.2. Internal Leakage Test (Key and Difficult Part)
Method 1: Pressure Drop Method (Static Pressure Holding Test): This method determines internal leakage by observing the speed and magnitude of the pressure drop.
Operating Steps:
1. Preparation and Connection: Move the hydraulic cylinder piston rod to either the fully extended or fully retracted end position (usually the rodless end). Connect a high-precision pressure gauge to the oil port of the rodless chamber.
2. Pressurization: Slowly inject hydraulic oil into the rodless chamber using a hydraulic pump or manual pump until the pressure reaches the required test pressure.
3. Pressure Holding and Observation: Stop pumping oil, close the shut-off valve (if installed), and start timing. Closely observe the pressure gauge needle.
4. Judgment Criteria:
Normal: The pressure gauge needle remains basically stable for several minutes or even longer, or drops very slowly (within the specified allowable leakage range).
Internal leakage is present: The pressure gauge needle shows a continuous and significant drop. The faster the drop, the greater the amount of oil leaking through the piston seal into the rod end chamber, indicating more severe internal leakage.
Method Two: Observation Method at the Rod-Side Port (Dynamic Leakage Test): This method is more intuitive as it involves directly observing the leaked oil.
Operating Steps:
1. Preparation: Similarly, move the hydraulic cylinder piston rod to the fully extended position (i.e., the rodless chamber is filled with oil, and the rod-side chamber space is minimized). Loosen or remove the pipeline or connector at the rod-side port, and place a container below the port.
2. Pressurization: Pressurize the rodless chamber to the rated working pressure.
3. Observation and Judgment:
Normal: Almost no oil flows out from the rod-side port, or only a very small amount of leakage occurs (for seals that allow a small amount of internal leakage, such as rubber seals, there may be some wetting).
Internal Leakage Present: Continuous and steady oil flow is observed at the rod-side port. The greater the flow rate (e.g., a continuous stream rather than drips), the more severe the internal leakage.
Note: The piston must be moved to the end position to ensure that the rodless chamber and the rod-side chamber are effectively separated by the piston. If stopped in the middle position, the test will be invalid.
Low-pressure leakage in a hydraulic cylinder is an indicator of its overall health, particularly the integrity of its sealing system.
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