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“Hydraulic Cylinders” standard specifies requirements for the durability of hydraulic cylinders. What exactly is hydraulic cylinder durability, and how is it tested? Let’s try to analyze this.
Durability
1. Double-acting hydraulic cylinders: When the piston stroke L ≤ 500mm, the cumulative stroke shall be ≥ 100km; when the piston stroke L > 500mm, reversing is allowed at a stroke of 500mm, and the cumulative number of reversals N shall be ≥ 200,000.
2. Single-acting hydraulic cylinders:
a) Piston-type single-acting hydraulic cylinders: When the piston stroke L ≤ 500mm, the cumulative stroke shall be ≥ 100km; when the piston stroke L > 500mm, reversing is allowed at a stroke of 500mm, and the cumulative number of reversals N shall be ≥ 200,000.
b) Plunger-type single-acting hydraulic cylinders: When the plunger stroke L ≤ 500mm, the cumulative stroke shall be ≥ 75km; when the plunger stroke L > 500mm, reversing is allowed at a stroke of 500mm, and the cumulative number of reversals N shall be ≥ 150,000.
3. Multi-stage telescopic single and double-acting hydraulic cylinders: When the cylinder stroke L ≤ 500mm, the cumulative stroke shall be ≥ 50km; when the cylinder stroke L > 500mm, reversing is allowed at a stroke of 500mm, and the cumulative number of reversals N shall be ≥ 100,000.
4. After the durability test, the increase in internal leakage shall not be greater than twice the specified value, and the parts shall not show abnormal wear or other forms of damage.
More details check JW CYLINDER: jwcylinder.com
I. What is Hydraulic Cylinder Durability?
Hydraulic cylinder durability refers to the ability of a hydraulic cylinder to continuously maintain its designed functions and performance parameters (such as output force, speed, and sealing) under specified operating conditions and maintenance cycles. It is usually quantified in the following two ways:
1. Number of cycles: The number of “extension-retraction” cycles completed by the hydraulic cylinder. For example, the durability of a hydraulic cylinder in a piece of construction machinery might require 500,000 cycles.
2. Operating time: The total number of hours the hydraulic cylinder can operate normally under constant or alternating load. For example, a hydraulic cylinder in an injection molding machine might require 10,000 hours of trouble-free operation.
The core of durability is “wear” and “fatigue.”
The essence of durability testing is to simulate the wear and fatigue processes that a hydraulic cylinder may experience throughout its design life, mainly including:
• Wear and aging of seals: Wear of piston seals and rod seals is the main cause of internal and external leakage.
• Wear of guide bushings and bearings: This can cause sticking, jamming, or uneven wear during hydraulic cylinder movement.
• Wear and corrosion of the piston rod surface: Especially for piston rods exposed to harsh environments, surface damage can directly compromise the rod seal.
• Wear and scratches on the inner wall of the cylinder barrel: This affects the lifespan of the piston seal and leads to increased internal leakage.
• Fatigue of welds and threaded connections: Under long-term alternating loads, structural components may develop fatigue cracks.
• Overall aging of other components: such as buffer valves and dust seals.
II. How to Test the Durability of a Hydraulic Cylinder?
The durability testing of a hydraulic cylinder needs to be conducted on a specialized test rig, following strict testing standards. The goal of the test is to verify whether the hydraulic cylinder can operate reliably within its designed lifespan under accelerated conditions.
The following are the standard durability testing procedures and key elements:
1. Test Preparation
• Test sample: The hydraulic cylinder to be tested.
• Test rig: Including a hydraulic power unit, a loading device (such as another opposing hydraulic cylinder), a sturdy base and frame.
• Measurement sensors:
• Pressure sensors: To monitor the pressure in the inlet and return ports.
• Temperature sensors: To monitor the oil temperature.
• Displacement sensors/encoders: To measure the displacement and speed of the piston rod.
• Flow meter (optional): For accurate measurement of internal leakage.
• Force sensor (optional): To directly measure the output force.
• Data acquisition system: Used to continuously record all sensor data.
2. Test Procedure
Core idea: Simulate actual working conditions, but usually with higher frequency and load for accelerated testing.
Step 1: Initial Performance Testing
Before the durability test begins, a comprehensive performance test of the hydraulic cylinder must be conducted as “baseline data.” This includes:
• Internal leakage test: Measuring the leakage through the piston seal at the rated pressure.
• External leakage inspection: Checking for leakage at all static and dynamic seals.
• Load efficiency test (optional): Measuring its mechanical efficiency.
• Pressure test: Conducting a pressure test to check for permanent deformation or leakage in the cylinder body, welds, and other strength components.
• Full stroke operation: Checking for smooth operation and any jamming.
Step 2: Durability Cycle Testing
This is the core part of the test. The hydraulic cylinder is mounted on the test rig, connected to the test system, and typical test parameters are set:
• Test pressure: Usually the rated pressure of the hydraulic cylinder, sometimes including short-term peak pressure cycles.
• Test stroke: Usually full stroke, or a specific stroke simulated according to the actual application.
• Test speed: Set according to the actual application, usually a higher average speed to accelerate the test.
• Load direction: Simulating actual force conditions (such as thrust, tension, or alternating force). • Fluid temperature: Maintain within the specified range (e.g., 50°C ± 4°C). High temperatures will accelerate the aging of seals and the deterioration of the hydraulic fluid.
• Number of cycles/time: Continue operation until the predetermined number of cycles is reached or until the hydraulic cylinder malfunctions.
III. A typical test cycle is as follows:
1. The hydraulic cylinder accelerates from the retracted position to the test speed.
2. It extends at a constant speed until the end of the stroke is reached.
3. There may be a brief pressure hold at the end of the stroke (simulating working conditions).
4. The direction is reversed, and it accelerates to the test speed in the retraction direction.
5. It retracts at a constant speed until the starting position is reached.
6. This cycle is repeated.
A method for manufacturing a durability testing device for a cab tilting hydraulic cylinder.
IV. Intermediate and Final Testing
• Periodic Interruption Checks: After completing a certain number of cycles (e.g., 50,000 cycles), pause the test, repeat the internal and external leakage tests from Step 1, and record the data. This helps track the trend of performance degradation.
• Final Testing: Stop the test when the predetermined total number of cycles is reached, or when the hydraulic cylinder performance parameters (mainly internal leakage) exceed the allowable standards. Conduct a comprehensive final performance test, identical to the initial test.
V. Result Evaluation and Failure Determination
After the test is completed, evaluate the durability by comparing the initial, intermediate, and final data:
• Pass: After completing all predetermined cycles, the hydraulic cylinder’s internal leakage is still within the standard limits, there is no external leakage, all functions are normal, and there is no damage to the main parts.
• Failure: Failure is determined if any of the following occurs before reaching the predetermined number of cycles:
• Excessive internal leakage: This is the most common failure mode.
• Unrepairable external leakage: Such as continuous oil leakage at the rod seal.
• Part damage: Such as severe wear of the piston rod, cylinder barrel scoring, weld cracking, etc.
• Loss of function: Such as inability to operate normally, buffer failure, etc.
VI. Key Considerations for Testing
• Simulating Real Operating Conditions: The most effective durability tests should simulate the load curves, speed curves, and operating environment of the hydraulic cylinder in real equipment as closely as possible.
• Fluid Cleanliness: The cleanliness of the hydraulic fluid in the test system must be strictly controlled (typically requiring ISO 4406 18/16/13 or cleaner), as contamination is a major cause of hydraulic cylinder wear.
• Alignment and Installation: Proper alignment must be ensured during installation to avoid lateral forces, otherwise it will greatly accelerate uneven wear of seals and guide bushings, leading to inaccurate test results.
• Comprehensive Test Report: The report should detail the test conditions, all performance data, any abnormal phenomena, and failure modes and root cause analysis.
Hydraulic cylinder durability testing is a systematic, accelerated life validation process. By simulating harsh but realistic working conditions, it exposes potential design flaws, manufacturing process issues, and material weaknesses in the hydraulic cylinder in a short period of time, making it a crucial step in ensuring product reliability and quality.
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