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Hydraulic Cylinders - How to test them

Before we start testing cylinders we need to understand just where a cylinder can leak. We will only be looking at double acting cylinders here because they are the most common on earthmoving equipment and stationary machinery. There are only two places a double acting cylinder can leak if we ignore fittings and connections, which are technically not part of the cylinder. There is the very evident leakage at the gland seal, the hole that the rod goes in and out of, which is unmistakable. But it’s the hidden leak at the piston seals inside that causes the most trouble with cylinder action. The common way of testing the piston seals of a double acting cylinder is to stop the piston at either end of the stroke, remove the hose at that end and check for leakage. This is full-stroke bypass testing. The trouble with this test is that 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 around the centre of the barrel where the problem, caused by barrel ballooning, occurs. Figure 1 below shows a badly ballooned cylinder. The barrel balloons out until the seals no longer make contact with the inner bore allowing oil to bypass. When the piston is at either end, the seals are in contact with the barrel bore and a bypass test at this position will be acceptable. However, somewhere around the centre position, the test results show leakage. Anything more than 0.010” increase in the diameter of the tube will allow the seals to leak. So we need to get the piston somewhere in the middle position and devise a test there, that will indicate leakage past the seals. The easiest and most effective way of doing this is with, what I call the “Rod Extension Test”. This test can even be done insitu on the machine. Quick and simple to do yet just as effective as on the test bench. Figure 2 shows the set-up with a good cylinder. The piston is in the centre of the barrel. There is a ball valve in the piston end outlet port, which is closed. The supply line into the rod end port is open and a pressure relief valve (PRV) is connected to prevent the cylinder from over pressurising. This pressure build-up or multiplication is explained here... The theory of this test is, you put pressure in the rod end of the cylinder which pushes the piston against the oil in the piston end of the cylinder until you get pressure equalization. As the pressures equalize, the piston comes to a stop. Now pressure equalization does not mean equal pressure. If you put a gauge in each end you would find that the piston end is at a lower pressure than in the rod end. This is because of the different surface areas on each side of the piston. The bottom side of the cylinder, or the piston side as it is called, has the full surface area of the piston available. The top side, or the rod side of the cylinder, has the surface area of the rod subtracted from it so it is much smaller in surface area. So if, when calculated, the piston side had 10 square inches of surface area and the rod side had only 6 square inches, 10 square inches of the piston minus 4 square inches of the rod, then a force of 100 psi on the rod side would only require 60 psi on the piston side to equalize the pressure. 100 pounds per square inch acting on 6 square inches is 600 pounds of pressure. However, at the piston end 10 square inches of surface area only requires 60 pounds per square inch to make up the 600 pounds of pressure to stop the piston from moving. None of this matters however for the test to be successful. But it is important to understand that if you happened to put pressure in the piston end of the cylinder with the ball valve at the rod end closed then the process is reversed. This is what would happen... 100 pounds per square inch acting on 10 square inches is 1000 pounds of pressure. But then the rod side would have to produce 167 pounds per square inch of pressure to equalize the 2 sides. This may not sound too bad but if you remember that most earthmoving equipment operate with over 2000 psi of pressure you would have 20,000 pounds of pressure on the piston side. And the rod side would develop 3,333 PSI of pressure which could be above the burst pressure of the cylinder. It is very important to understand this, as serious injury could result from this test if you don’t. Now you don’t actually need the ball valve in the rod end of the cylinder. I’ve just included it and gone through all this explanation just in case someone does. And it is usually a part of test bench equipment. If the test is being performed on the machine then you would not use it. All you would need to do is install a ball valve on the piston side of the cylinder. Now for the test. In Figure 2 applying pressure to the rod side of the cylinder while closing the valve at the piston side port tests the gland seal and the piston seal at the same time. As the pressure pushes against the piston forcing it down and pressurizing the opposite end, it is also applying the same pressure on the gland nut pressure seal. So if the seal is damaged you will see oil coming from the wiper seal that the rod slides through. At the same time pressure is being applied to the piston seals which are being pushed out against the cylinder walls. If the piston seal is not damaged it will seal against the cylinder walls and prevent the piston, and therefore the rod, from moving, once pressure equalization has been achieved. The cylinder in figure 2. has passed our test. But why would it not pass? Figure 3. shows why. Figure 3 shows the old familiar ballooned cylinder from Figure 1 above, under our test. Here we have a greatly exaggerated view of piston seals that don’t even reach the cylinder walls. And one seal seams to be shredded. In this situation, pressure applied to the rod end supply port will not push the piston back with the full force of the oil. Some of the pressure will bypass the piston seals and join the pressure on the other side of the piston. This joining of pressure will create enough force to overcome the pressure left on the rod side and so move the rod out of the cylinder at a pace proportional to the condition of the seals. The full test procedure is detailed below.

Test Procedure

1. Secure the cylinder so that it is under control. If it is still on the machine, it is under control.  2. Fill the cylinder with clean oil on both side of the piston. 3. Connect a suitable ball valve to the piston end of the cylinder. 4. Stroke the cylinder back and forth multiple times, with the ball valve open, to remove all the air from both sides. 5. With the rod fully extended, pressurise the rod end supply port to move the piston to the middle of the cylinder, then close the valve on the piston side port. 6. Measure how far the rod is extended then apply pressure to the rod end port and watch the rod. 7. If oil flows from the wiper seal then the pressure seal in the gland is leaking. 8. If the rod moves out from the cylinder, then the piston seals are leaking. 9. If the rod moves in, then the ball valve you have installed at the other end is leaking. 10. You don’t need a lot of pressure to find a leak. Seals will tend to leak at lower pressures rather than higher. This is because the higher the pressure, the harder the seal lip is pressed against the cylinder wall and so, seals better. Note: It will not work the other way. Applying pressure to the piston end port with a ball valve on the rod end port will not move the rod in to indicate seal bypass. Because of the greater volume of oil on the piston side it cannot physically move into the lower volume side of the rod end. So don’t try it, it’s very dangerous.

Danger

Never ever adjust relief valve (PRV) if used, to above the pressure rating of the cylinder under test. 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.
Danger flag cyl1 cyl2 cyl3 excavator

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Hydraulic Cylinder Testing

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page back
Home Products Filtration Service & Repairs Hire Distributorships Quality & Safety Profile Contact Us headerline Flare
Copyright © 2013, Central Queensland Hydraulics Pty.Ltd. - Contact our Webmaster
Oil drop

Hydraulic Cylinder Testing

Phone us
Call us today for more information 07 4952 5621
cyl1 cyl2 cyl3
Find us here...

Hydraulic Cylinders - How to test them

Before we start testing cylinders we need to understand just where a cylinder can leak. We will only be looking at double acting cylinders here because they are the most common on earthmoving equipment and stationary machinery. There are only two places a double acting cylinder can leak if we ignore fittings and connections, which are technically not part of the cylinder. There is the very evident leakage at the gland seal, the hole that the rod goes in and out of, which is unmistakable. But it’s the hidden leak at the piston seals inside that causes the most trouble with cylinder action. The common way of testing the piston seals of a double acting cylinder is to stop the piston at either end of the stroke, remove the hose at that end and check for leakage. This is full- stroke bypass testing. The trouble with this test is that 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 around the centre of the barrel where the most damage, caused by ballooning, occurs. Figure 1 below shows a badly ballooned cylinder. The barrel balloons out until the seals no longer make contact with the inner bore allowing oil to bypass. When the piston is at either end, the seals are in contact with the barrel bore and a bypass test at this position will be acceptable. However, somewhere around the centre position, the test results show leakage. Anything more than 0.010” increase in the diameter of the tube will allow the seals to leak. So we need to get the piston somewhere in the middle position and devise a test there, that will indicate leakage past the seals. The easiest and most effective way of doing this is with, what I call the “Rod Extension Test”. This test can even be done insitu on the machine. Quick and simple to do yet just as effective on the test bench. Figure 2. Shows the setup with a good cylinder. The piston is in the centre of the barrel. There is a ball valve in the piston end outlet which is closed. The supply line into the rod end port is open and a pressure relief valve (PRV) is connected to prevent the cylinder from over pressurising. This pressure build-up or intensification is explained here... The theory of the test is, you put pressure in the rod end of the cylinder which pushes the piston against the oil in the piston end of the cylinder until you get pressure equalization. As the pressures equalize, the piston comes to a stop. Now pressure equalization does not mean equal pressure. If you put a gauge in each end you would find that the piston end is at a lower pressure than in the rod end. This is because of the different surface areas on each side of the piston. The bottom side of the cylinder, or the piston side as it is called, has the full surface area of the piston available. The top side, or the rod side of the cylinder, has the surface area of the rod subtracted from it so it is much smaller in surface area. So if, when calculated, the piston side had 10 square inches of surface area and the rod side had only 6 square inches, 10 square inches of the piston minus 4 square inches of the rod, then a force of 100 psi on the rod side would only require 40 psi on the piston side to equalize the pressure. 100 pounds per square inch acting on 4 square inches is 400 pounds of pressure. However, at the piston end 10 square inches of surface area only requires 40 pounds per square inch to make up the 400 pounds of pressure to stop the piston from moving. None of this matters however for the test to be successful. But it is important to understand that if you happened to put pressure in the piston end of the cylinder with the ball valve at the rod end closed then the process is reversed. This is what would happen... 100 pounds per square inch acting on 10 square inches is 1000 pounds of pressure. But then the rod side would have to produce 250 pounds per square inch of pressure to equalize the 2 sides. This may not sound too bad but if you remember that most earthmoving equipment operate with over 2000 psi of pressure you would have 20,000 pounds of pressure on the piston side. And the rod side would develop 50,000 pounds of pressure which is probably above the burst pressure of the cylinder. It is very important to understand this, as serious injury could result from this test if you don’t. Now you don’t actually need the ball valve in the rod end of the cylinder. I’ve just included it and gone through all this explanation just in case someone does. And it is usually a part of test bench equipment. If the test is being performed on the machine then you would not use it. All you would need to do is install a ball valve on the piston side of the cylinder. Now for the test. In Figure 2. applying pressure to the rod side of the cylinder while closing the valve at the piston side port tests the gland seal and the piston seal at the same time. As the pressure pushes against the piston forcing it down and pressurizing the opposite end, it is also applying the same pressure on the gland nut pressure seal. So if the seal in damaged you will see oil coming from the wiper seal that the rod slides through. At the same time pressure is being applied to the piston seals which are being pushed out against the cylinder walls. If the piston seal is not damaged it will seal against the cylinder walls and prevent the piston, and therefore the rod, from moving, once pressure equalization has been achieved. The cylinder in figure 2. has passed our test. But why would it not pass? Figure 3. shows why. Figure 3. shows the old familiar ballooned cylinder from Figure 1. above, under our test. Here we have a greatly exaggerated view of piston seals that don’t even reach the cylinder walls. And one seal seams to be shredded. In this situation, pressure applied to the rod end supply port will not push the piston back with the full force of the oil. Some of the pressure will bypass the piston seals and join the pressure on the other side of the piston. This joining of pressure will create enough force to overcome the pressure left on the rod side and so move the rod out of the cylinder at a pace proportional to the condition of the seals. The full test procedure is detailed below.

Test Procedure

1. Secure the cylinder so that it is under control. If it is still on the machine, it is under control.  2. Fill the cylinder with clean oil on both side of the piston. 3. Connect a suitable ball valve to the piston end of the cylinder. 4. Stroke the cylinder back and forth multiple times, with the ball valve open, to remove all the air from both sides. 5. With the rod fully extended, pressurise the rod end supply port to move the piston to the middle of the cylinder, then close the valve on the piston side port. 6. Measure how far the rod is extended then apply pressure to the rod end port and watch the rod. 7. If oil flows from the wiper seal then the pressure seal in the gland is leaking. 8. If the rod moves out from the cylinder, then the piston seals are leaking. 9. If the rod moves in, then the ball valve you have installed at the other end is leaking. Note: It will not work the other way. Applying pressure to the piston end port with a ball valve on the rod end port will not move the rod in to indicate seal bypass. Because of the greater volume of oil on the piston side it cannot physically move into the lower volume side of the rod end. So don’t try it, it’s very dangerous.

Danger

Never ever adjust relief valve (PRV), if used, to above the pressure rating of the cylinder under test. 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.
Danger flag excavator