Hydraulic Cylinder Testing
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Pressure Test Steps
All hydraulic cylinders go through full pressure tests before shipment. The test step by step: run without load to release air, check movement at low pressure, then do high-pressure strength test and rated pressure leak test. We watch welds, seals and joints for oil leaks and other problems.
1. Low-pressure No-load Air Venting
For the first step of pressure testing, we carry out low-pressure no-load air venting on the cylinder. Both standard and custom hydraulic cylinders require this check. First, we operate the test bench and feed low-pressure hydraulic oil into the cylinder. We let the cylinder fully extend and retract 3 to 5 times. This repeatedly pushes trapped air out of the cylinder. When bubbles no longer come out continuously from the oil ports, we can confirm most trapped air is cleared. We use low pressure here. It stops high-pressure impact from damaging newly fitted seals, and lets air vent out easily. During this step, we watch the piston rod as it moves. We check if its extension and retraction run smoothly, and look for jamming or strange noises. If we spot any abnormality, we stop the test immediately to find the problem.
After we finish venting and confirm everything looks good, we top up hydraulic oil in the cylinder. We fill the oil to the specified level to ensure the cylinder has enough oil. Meanwhile, we visually check oil ports, connectors and air release valves to make sure no seepage or oil leakage appears at these spots.
2.Low-pressure Hold Test
Once low-pressure no-load venting is complete, we move to the low-pressure hold verification. This test checks whether the piston rod extends and retracts smoothly. We set the pressure at 0.5–1.5 MPa and let the cylinder fully extend and retract 2 to 3 times. At this point, we mainly check the breakaway pressure. This is the lowest oil pressure needed to push the stationary piston rod out. If we see it running high, we stop and take a look. That tells us there's too much friction inside the cylinder. Most of the time, that's because the seals are too tight or the parts aren't lined up right. When the cylinder moves back and forth at low pressure, we also check the piston rod for sticking, jitter, creeping, odd noises.
When the movement test is done, we hold the same pressure for 1 to 2 minutes. Then we inspect every connection point such as cylinder wall, cylinder head, oil port fittings for oil seepage or leakage. If the test doesn't pass, we go through everything one by one. We first look for damage on the seals. Then we check the concentricity between piston rod and cylinder barrel, to make sure the rod is not fitted off-center. Once we find the root cause, we fix it and run the test again.
3. High-pressure Strength Test
After passing the low-pressure hold test, we move on to the high-pressure strength test which is 1.5 times the rated pressure. The purpose of this test is to check the structural strength of the cylinder parts and make sure they won't get damaged under high pressure. First, we increase pressure to the set value slowly. This is critical, because sudden pressure spikes will damage the cylinder from impact. Once we hit the target pressure, we hold for a minimum of 2 minutes on standard products. For marine cylinders, we follow the drawing requirements and can hold up to 5 minutes. At this point, we check key spots like welds, cylinder barrel, cylinder bottom, cylinder head, oil ports, fittings and tie rods. This is to find out if there is any oil leak.
High pressure test is risky, we use remote control through the whole pressure test. We also fit a protective guard around the cylinder before test to keep our operators safe. At the same time, we collect pressure data and keep records of the pressure-time curve. Our test standard requires no visible oil leaks. It also means that the cylinder must not have permanent deformation or burst. If we see any of these problems, we stop right away. We will not move forward until all issues are fixed. That is a hard requirement.
4.Rated Pressure Hold Test
Once the high-pressure strength test passes, we move on to the fourth step of pressure testing: the rated pressure hold test. This step checks internal leakage at the piston and tells us whether the cylinder seals work properly. We will raise the pressure to the cylinder's rated pressure and hold it for at least 1 to 2 minutes. We test the rod-side chamber and rodless chamber separately, because the two chambers carry different loads and have different sealing conditions.
There are two ways we check for internal leakage. One is to watch for piston rod movement. If oil is creeping from one chamber to the other, the rod will start to drift slowly. The other is use measuring cup to collect leaking. We measure the oil volume and compare it against standard to judge if leakage is passes. In practice, we use stricter internal leakage limits for high-grade engineering and marine cylinders than standard cylinders. For example, standard cylinders allow up to 0.5 mL/min leakage, but marine and high-grade ones must stay under 0.1 mL/min as required by drawings. If leakage goes over maximum allowed in the drawing, we will take the cylinder apart, replace the seals and do the test again. During the test, we note pressure drop, leakage volume, oil temperature and seal batch numbers. We keep these test data so we can check product quality when needed.
5. Comprehensive Operating Check
After we finish the four previous pressure tests, we will do overall check on the cylinder under working conditions. This step is part of final inspection. We watch the whole time while the cylinder extends and retracts. We look at the welds, seal areas, and oil port fittings for any sign of seepage. And also watch how the cylinder works to look for creeping, strange noise, shaking and any other problems. We check by eye and record the full test on video. The key point is we don’t just shoot short clips. We record the complete working condition test from start to finish and save the data for later review, this let customers can also check it. To meet our test requirements, the cylinder must have no oil seepage at any point. The rod moves smoothly, with no strange noise or shaking.
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