Testing Hydraulic Cylinders
In a previous article, I described the danger associated with the intensification of pressure in double-acting hydraulic cylinders. In this article, I will explain how to use the intensification effect to test the integrity of the piston seal in a double-acting cylinder. Before attempting this test procedure, it is absolutely essential that the danger associated with pressure intensification in a 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 "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 as a result of under designed cylinder wall thickness or reduction of wall thickness through excessive honing. 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.
Figure 1. Hydraulic cylinder test circuit.
The procedure for conducting the test is as follows:
- Secure the cylinder with its service ports up.
- Fill both sides of the cylinder with clean hydraulic fluid through its service ports.
- Connect ball valves (1) and (2), gauges (3) and (4), relief valve (5) and directional control valve (6) as shown in Figure 1.
- 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.
- Position the piston rod mid-stroke and close ball valve (2).
- With the adjustment on the relief valve (5) backed out, direct flow to the rod side of the cylinder.
- Increase the setting of relief valve (5) until the cylinder's rated pressure is seen on gauge (3).
- 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.
- 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.
Safety is paramount
Under no circumstances should flow be directed to the piston side of the cylinder with ball valve (1) closed. Failure of the cylinder and personal injury could result. When conducting this or any other hydrostatic (pressure) test, always wear appropriate personal-protection equipment.
If you enjoyed this article, you'll love Brendan Casey's Inside Hydraulics newsletter.
It gives you real-life, how-to-do-it, nuts-and-bolts, hydraulics know-how ? information you can use today.
Listen to what a few of his subscribers have to say:
Can't Put It Down
?I get magazines and e-mails like this all the time. I never find time to read them. I decided to read Issue #30 and I couldn't put it down. I'll make time from now on.?
Richard A. Shade, CFPS
Project Engineer (Hydraulic Design)
JLG Industries Inc.
So Valuable It Earned Me A Raise
?The knowledge I've gained from this newsletter has been so valuable it has earned me a raise!?
Heavy Equipment Mechanic
Sharpe Equipment Inc.
Love It - Keep Them Coming
?I just love this newsletter. As a Hydraulics Instructor for Eaton, I make copies and distribute them to my students as I address various topics... Keep 'em coming.?
Michael S Lawrence
Eaton Hydraulics Inc.
Here's a sample of what's covered in this powerful newsletter: troubleshooting, contamination control, component repair and testing, preventative maintenance, failure analysis, and much, much more!
To get a FREE subscription to the Inside Hydraulics newsletter, fill out this
form - don't forget to capitalize the first letter of your name - and hit 'SUBSCRIBE NOW!'
This is a private mailing list that will never
be sold or given away for any reason.
You can also unsubscribe at anytime.