How To Test A Hydraulic Cylinder

The conventional way of testing the integrity of the piston seal in a double-acting cylinder is to pressurize the cylinder at the end of stroke and measure any leakage past the seal. This is commonly referred to as the "end-of-stroke bypass test". The major limitation of the end-of-stroke bypass test, is that it generally doesn't reveal ballooning of the cylinder tube caused by hoop stress.

In a previous post, I described the danger associated with the intensification of pressure in a double-acting hydraulic cylinder. In this post, I will explain how to use the intensification effect to test the integrity of the piston seal in a double-acting hydraulic cylinder. But before attempting this test procedure, it is absolutely essential that the danger associated with pressure intensification in a hydraulic cylinder is fully understood. Therefore, read this article FIRST! The conventional way of testing the integrity of the piston seal in a double-acting cylinder is to pressurize the cylinder at the end of stroke and measure any leakage past the seal. This is commonly referred to as the "end-of-stroke bypass test". The major limitation of the end-of-stroke bypass test, is that it generally doesn't reveal ballooning of the cylinder tube caused by hoop stress. The ideal way to test for ballooning of the cylinder tube is to conduct a piston-seal bypass test, mid-stroke. The major difficulty with doing this is that the force developed by the cylinder has to be mechanically resisted, which in the case of large diameter, high-pressure cylinders is impractical. However a mid-stroke bypass test can be conducted hydrostatically using the intensification effect. The necessary circuit is shown in Figure 1 below.

1. Secure the cylinder with its service ports up.
2. Fill both sides of the cylinder with clean hydraulic fluid through its service ports.
3. Connect ball valves (1) and (2), gauges (3) and (4), relief valve (5) and directional control valve (6) as shown in Figure 1.
4. With ball valves (1) and (2) open, stroke the cylinder using the directional control valve (6) multiple times to remove all remaining air from both sides of the cylinder - take care not to 'diesel' the cylinder.
5. Position the piston rod mid-stroke and close ball valve (2).
6. With the adjustment on the relief valve (5) backed out, direct flow to the rod side of the cylinder.
7. Increase the setting of relief valve (5) until the cylinder's rated pressure is seen on gauge (3).
8. Close ball valve (1) and center directional control valve (6). Note: it is assumed that the hydraulic power unit used to conduct the test has its own over-pressure protection - not shown in Figure 1.
9. Record the respective pressure readings on gauges (3) and (4) and monitor any change over time.

If the ratio of effective area between the piston and rod side of the cylinder is 2:1, then if the rod side of the cylinder has been pressurized to 3,000 PSI, gauge (2) on the piston side should read 1,500 PSI. If the differential pressure across the piston is not maintained, this indicates a problem with the piston seal or tube.

Under no circumstances should flow be directed to the piston side of the cylinder with ball valve (1) closed. Failure of the cylinder and/or personal injury could result. When conducting this or any other hydrostatic (pressure) test, always wear appropriate personal-protection equipment.

REMEMBER: pressure intensification in a double-acting hydraulic cylinder is a potentially dangerous phenomenon. And failing to consider its implications can be a costly mistake. To discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get "Six Costly Mistakes Most Hydraulics Users Make. And How You Can Avoid Them!" available for FREE download here.