Hydraulic Cylinder Testing

Hydraulic Cylinders - How to test them

The traditional way of testing the piston seals of a double acting cylinder is to stroke the cylinder in and out a couple of times, then stop at either end and pressurise the cylinder for a period of time to check for leakage. This is called ‘end-of-stroke’ bypass testing. The trouble with ‘end-of-stroke’ bypass tests is that, as the term says, you are only testing the seals when they are at each end of the cylinder tube where the least amount of wear and tear happens. The test needs to be done in the centre of the tube where the most damage, caused by ballooning, occurs. Figure 1 below shows an exaggerated view of a ballooned cylinder. The tube literally blows up like a balloon. When the piston is at either end, the seals are effective and a bypass test at this position will give acceptable results. However, as can be seen, at the mid position, the test results won’t be so desirable. Anything more than 0.010” difference in the diameter of the tube will see the seals leak. The best way to test the piston seals of a double acting cylinder is to carry out a ‘mid-stroke’ bypass test. The main problem with this type of test is how to control the cylinder. You need to contain the cylinder mechanically to hold the piston at the centre position. As even a small cylinder can develop 6 to 10 tonnes of force this becomes a very dangerous exercise, and with a large cylinder it becomes impractical. What we need is a test that holds the piston, at any position we choose, hydraulically. This is called hydrostatic testing and the circuit in figure 2 shows how it is done. This uses the pressure intensification effect to hold the piston at any point along the tube to test the integrity of the seals. The one danger with this type of test is that if the pressure is directed into the piston end of the cylinder, with valve (1) closed, then the pressure developed in the rod end of the cylinder will be much greater to the point of catastrophic failure. To prevent this happening it is important to use a pressure relief valve (5) in the rod end circuit. This can be set to the pressure rating of the cylinder so that the tests can be carried out safely. The full test procedure is detailed below.

Test Procedure

1. Secure the cylinder so that it is under control.  2. Fill the cylinder with clean oil on both side of the piston. 3. Connect the circuit in figure 2 and open both ball valves (1) and (2). 4. Stroke the cylinder back and forth multiple times with directional control valve (6) to remove all the air from both sides. 5. With the rod fully extended, pressurise the ‘A’ side of the circuit with directional control valve (6), to move the piston to the middle of the cylinder, then close valve (2). 6. Operate control valve (6) again to increase the pressure on the rod side to the cylinder test pressure. This can be done by either adjusting relief valve (5) or adjusting the pressure supply to control valve (6). 7. Gauge (3) should now read the test pressure. Gauge (4) will read a lower pressure due to the difference in the surface areas each side of the piston. This is explained in detail here... 8. Close off valve (1) and return control valve (6) to the centre position. Monitor the pressure readings of gauges (3) and (4) over a period of time, say 15 minutes to see if there is any differential pressure change. If we use our cylinder example described in the article cylinder dangers, with a piston size of 3” and a rod size of 2”, then with a pressure of 3000psi on the rod size, we should read 1714psi on the piston side. If this differential pressure is not maintained over a 15 minute period then this indicates a problem with the piston seals or the cylinder tube.

Danger

Never ever adjust relief valve (5) to above the pressure rating of the cylinder under test. If the control valve (6) is moved to supply system pressure to the piston side of the cylinder with valve (1) closed then pressure intensification will cause the pressure in the rod side to climb above a safe level. Failure of the cylinder and/or personal injury could result. Always wear recommended personal-protective equipment. Always test by pressurising the rod side of the cylinder. Remember, at Central Qld Hydraulics, we take any old cylinders that are preserved well, replace their damaged parts to make them go on doing their outstanding work as new cylinders.