A hydraulic ram, also known as a displacement cylinder, is a single-acting hydraulic cylinder where fluid pressure acts on the rod’s cross-section, meaning it lacks a piston (see Figure 1).

During the Automotive Lift Repair Tampa Florida inspection, I found no obvious cause of failure with the rod seal (U-cup type) or the rod’s surface. I then asked the operator to describe the leak. He reported noticing a continuous trickle of hydraulic fluid running down the outside of the ram during his morning checks.

Further investigation revealed that the current operator had only recently been assigned to the machine. When I asked him about his shutdown procedure, he mentioned that he always relieved the hydraulic pressure by removing the load from the ram after shutting down. This practice seemed to explain the leak issue.

Automotive Lift Repair Tampa Florida Hydraulic ram or displacement cylinder.

To function properly, a U-cup seal depends on hydraulic oil pressure to keep the seal lips pressed against the rod and seal groove (see Figure 1 inset). By releasing the load-induced pressure from the hydraulic ram after shutdown, the seal is effectively de-energized. As a result, any subsequent increase in the oil volume due to thermal expansion can cause fluid to leak past the seal. This slow loss of oil prevents the seal from being adequately energized, allowing the leak to persist until the fluid temperature and volume stabilize.

I advised my client that the practice of unloading the hydraulic ram after shutdown was likely causing the leak. To address the issue, they could either discontinue this practice or switch to an energized U-cup seal, which includes an O-ring in the ‘U’ to ensure the seal lips are consistently energized.

A straightforward Automotive Lift Repair Tampa Florida bench test can be used for this purpose, but it involves increasing the pressure within the cylinder. While the test is safe if you understand pressure intensification in a hydraulic cylinder, it can be very dangerous if you don’t. In this post, I’ll outline the risks associated with pressure intensification in a double-acting cylinder, and in my next post, I’ll describe the test procedure.

The force produced by a Automotive Lift Repair Tampa Florida hydraulic cylinder is determined by the equation F = p x A, where F is force, p is pressure, and A is area. In a typical double-acting cylinder, the areas on the piston and rod sides are different, leading to unequal forces. For instance, if the rod side has half the effective area of the piston side, it will produce only half the force for the same pressure.

Rearranging the formula, we get p = F/A, meaning pressure equals force divided by area. If the rod side must resist the force from the piston side but has only half the area, it requires twice the pressure. Therefore, if the piston side is pressurized to 3,000 PSI, the rod side would need 6,000 PSI to produce an equal force. This explains why pressure intensification can occur in a double-acting cylinder. Be aware that pressurizing a cylinder rated for 3,000 PSI to 6,000 PSI can lead to severe consequences. For a clearer understanding of pressure intensification, watch this 6-minute video.

If, for any reason, the piston side of a double-acting cylinder is pressurized while the rod side is blocked from releasing fluid, pressure will intensify on the rod side until the forces balance out or the cylinder fails catastrophically. For example, one of our members shared this scenario:

In this case, the gland blew out due to pressure intensification caused by a blockage between the rod side and the tank, exacerbated by the extreme cold. The hydraulic oil had thickened below its pour point, preventing proper flow.

The traditional method for testing the integrity of the piston seal in a double-acting cylinder involves pressurizing the cylinder at the end of the stroke and measuring any leakage past the seal. This is commonly known as the “end-of-stroke bypass test.”

However, the end-of-stroke bypass test has a significant limitation: it typically does not detect ballooning of the cylinder tube caused by hoop stress. To test for ballooning, a piston-seal bypass test should be performed mid-stroke. The challenge with this is that the force generated by the cylinder needs to be mechanically resisted, which is impractical for large-diameter, high-pressure cylinders.

Instead, a mid-stroke bypass test can be carried out hydrostatically using the effect of pressure intensification. The necessary test circuit is illustrated in Figure 1 below.

Hydraulic Cylinder Test Circuit  

Automotive Lift Repair Tampa Florida Test Procedure

1. Secure the Cylinder: Position the cylinder with its service ports facing upward.

2. Fill the Cylinder: Add clean hydraulic fluid to both sides of the cylinder through the service ports.

3. Connect the Circuit: Set up the circuit by connecting ball valves (1) and (2), gauges (3) and (4), relief valve (5), and directional control valve (6) as shown in Figure 1.

4. Remove Air: Open ball valves (1) and (2), and stroke the cylinder multiple times using the directional control valve (6) to expel all air from both sides of the cylinder—avoid causing cavitation.

5. Position the Automotive Lift Repair Tampa Florida Piston Rod: Move the piston rod to mid-stroke and close ball valve (2).

6. Adjust the Relief Valve: Back out the adjustment on relief valve (5), and direct flow to the rod side of the cylinder.

7. Increase Pressure: Raise the relief valve (5) setting until gauge (3) shows the cylinder’s rated pressure.

8. Close Ball Valve (1): Close ball valve (1) and center the directional control valve (6). Note: Ensure the hydraulic power unit used for the test has its own over-pressure protection, which is not depicted in Figure 1.

9. Record and Monitor: Record the pressure readings on gauges (3) and (4) and monitor any changes over time.

If the effective area ratio between the piston and Automotive Lift Repair Tampa Florida rod side of the cylinder is 2:1, and the rod side is pressurized to 3,000 PSI, gauge (4) on the piston side should read 1,500 PSI. A failure to maintain this differential pressure indicates a potential issue with the piston seal or cylinder tube.