A client recently sought my expertise to investigate a seal failure on a hydraulic ram. This ram had been removed from a hydraulic lift due to a leaking rod seal. However, upon examination, both the rod seal and the rod’s surface appeared to be in good condition.
As a reminder, a Car Lift Repair Tampa Florida hydraulic ram, also known as a displacement cylinder, is a single-acting hydraulic cylinder where fluid pressure acts on the cross-section of the rod, lacking a piston (refer to figure 1).
Despite thorough inspection of the Car Lift Repair Tampa Florida rod seal (of U-cup type) and the rod’s surface, no evident cause of failure was found. I then asked the operator to describe the leak’s nature. He explained that during his morning checks, he consistently observed a trickle of hydraulic fluid along the outside of the ram.
Further inquiry revealed that the current operator had only recently taken charge of the machine. I inquired about his shutdown procedure, and he mentioned always relieving the hydraulic pressure (unloading the load off the hydraulic ram) before shutting down for the night. This revelation provided a plausible explanation for the persistent leak.
For a U-cup seal to function effectively, it relies on hydraulic pressure to energize the seal lips against the rod and seal groove (refer to figure 1 inset). Releasing the load-induced pressure from the hydraulic ram post-shutdown effectively de-energizes the seal. Consequently, a gradual increase in oil volume within the ram, possibly due to thermal expansion, can cause fluid to seep past the seal. This gradual oil loss hinders the buildup of sufficient pressure to effectively energize the seal, thus allowing the leak to persist until fluid temperature, and consequently volume, stabilizes within the Car Lift Repair Tampa Florida hydraulic ram.
I informed my client that the practice of unloading the hydraulic ram post-shutdown was likely the cause of the leak. Consequently, there were two potential solutions: cease the practice or switch to an energized U-cup seal profile (incorporating an O-ring within the ‘U’ to positively energize the seal lips).
Confirmation of the root cause came when, without altering the seal profile, discontinuation of the practice of unloading the Car Lift Repair Tampa Florida hydraulic ram after shutdown successfully eliminated rod seal leakage. However, it’s essential to note that leaving loads suspended on hydraulic equipment may pose safety risks in certain scenarios. Therefore, it’s advisable to conduct a safety risk assessment on a case-by-case basis before adopting this practice. Additionally, to avoid other costly mistakes with hydraulic equipment, it’s crucial to explore further insights.
One of the common questions I receive is, “What’s the most effective method for assessing the integrity of the piston seal in a double-acting hydraulic cylinder?”
There exists a straightforward bench-test for this purpose, involving the amplification of pressure within the cylinder. In this discussion, I’ll outline the risks associated with pressure amplification in a double-acting cylinder, followed by an explanation of the testing procedure in my subsequent post.
The force generated by a hydraulic cylinder is a result of the combination of pressure and area (F = p x A). In a typical double-acting cylinder, the effective area, and consequently the force generated by the piston and rod sides of the cylinder, are unequal. Therefore, if the rod side of the cylinder possesses half the effective area of the piston side, it will generate half the force of the piston side for the same pressure.
This equation can be rearranged as p = F/A, meaning pressure equals force divided by area. If the rod side of the cylinder needs to counteract the force exerted by the piston side with only half the area, it requires double the pressure. Thus, if the piston side is pressurized to 3,000 PSI, a pressure of 6,000 PSI will be necessary on the rod side to produce an equivalent force. This phenomenon illustrates why pressure amplification can occur in a double-acting cylinder. It’s crucial to note that pressurizing a cylinder rated at 3,000 PSI to 6,000 PSI can lead to severe consequences. For a better comprehension of pressure amplification in a hydraulic cylinder, I recommend watching this 6-minute video.
If, for any reason, the Car Lift Repair Tampa Florida piston side of a double-acting cylinder is pressurized while simultaneously preventing fluid from escaping the rod side, pressure will intensify in the rod side of the cylinder until forces are balanced or the cylinder undergoes catastrophic failure. Consider a recent scenario described by one of our members:
“During extremely cold weather, approximately minus 36 degrees Celsius, we encountered an issue with a hydraulic cylinder exposed to temperatures around minus 10 degrees Celsius. While attempting to remove a pin, our supervisor activated the pump and engaged the lever. Suddenly, the gland end of the cylinder ruptured. This particular cylinder had a 7.5-inch diameter and operated at a pressure of 2,500 PSI.”
The gland failure in this hydraulic cylinder was a result of pressure amplification caused by a blockage between the rod side of the cylinder and the tank, exacerbated by the cold conditions. As the ambient temperature dropped below the hydraulic oil’s pour point, hindering its flow, pressure intensified, leading to the gland failure.
In a prior post, I discussed the hazards associated with pressure intensification in a double-acting hydraulic cylinder. In this subsequent post, I will elucidate how to leverage the intensification effect to assess the integrity of the piston seal in such cylinders. However, before embarking on this testing procedure, it is paramount to fully comprehend the peril linked with pressure intensification in hydraulic cylinders. Hence, it is imperative to read this article beforehand.
The conventional method for evaluating the Car Lift Repair Tampa Florida piston seal’s integrity in a double-acting cylinder involves pressurizing the cylinder at the end of the stroke and monitoring any leakage past the seal, commonly known as the “end-of-stroke bypass test.”
However, the primary drawback of the end-of-stroke bypass test is its limited ability to detect ballooning of the cylinder tube caused by hoop stress. To effectively assess cylinder tube ballooning, a piston-seal bypass test conducted mid-stroke is ideal. Nonetheless, implementing this test presents a significant challenge: mechanically resisting the force generated by the cylinder, particularly in the case of large-diameter, high-pressure Car Lift Repair Tampa Florida cylinders, is impractical.