A popular disbelief involving Rotary Hydraulic Ram is that if the piston seal is leaking, the cylinder will drift. While a leaking piston seal could possibly be the root cause of cylinder drift, the physics involved are frequently misinterpreted. Fact is, f the piston seal is entirely removed from a double-acting cylinder, the cylinder is full of oil as well as the ports are plugged, the cylinder will hold its load indefinitely, unless the rod-seal leaks.
In this condition, because of the unequal volume on either side of the piston, fluid pressure equalizes and also the cylinder becomes hydraulically locked. Once this happens, the cylinder can move only if fluid escapes from the cylinder using the rod seal or its ports.
Exceptions to the Rule – The two main exceptions for this theory. The very first is a double-rod cylinder (Figure 1) where volume is equal for both sides in the piston. The next exception involves a lot hanging over a double-acting cylinder (Figure 2). In this particular arrangement, the volume of pressurized fluid on the rod side can easily be accommodated on the piston side. But because the cylinder drifts, a vacuum will experience on the piston side because of unequal volumes, and depending on the weight from the load, this vacuum may eventually bring about equilibrium that arrests further drift.
This is simply not the conclusion in the cycle, but it’s essential to at the very least grasp this theory before continuing. Notwithstanding these two exceptions, when a double-acting cylinder’s service ports are blocked by way of a closed-to-actuator spool (Figure 3), as well as the piston seal does bypass, pressure will ultimately equalize for both sides in the cylinder. At this stage, a hydraulic lock is effected and no further drift can take place, unless fluid is able to escape from the cylinder or cylinder circuit.
Loss in Effective Area – Due to the loss in effective area as a result of pressure now acting on the rod-side annulus area, the static pressure inside the cylinder must increase to back up the identical load. Remember, force created by a cylinder is really a product of pressure and area. For example, if the load-induced pressure on the piston side from the cylinder was 2,000 PSI and zero on the rod side if the directional control valve closed, assuming no leakage beyond the spool, the equalized pressure might be 3,000 PSI depending on the ratio of the piston and annulus areas.
New call-to-action – Now consider what can happen if the circuit includes a service port relief valve (Figure 4) set at 2,500 PSI. As pressure equalizes over the piston seal as well as the increasing static pressure on the piston side in the Hydraulic Drive System reaches the cracking pressure in the port relief, however the cylinder will still not retract. A similar situation can occur in circuits using a load control (counterbalance) valve installed. In this circuit, shown in Figure 5, the directional control valve has a float center spool (service ports A and B ready to accept tank).
As previously mentioned, when the piston seal leaks, unequal volumes of oil on the rod and piston sides of the cylinder indicates hydraulic lock may prevent any noticeable drift. But once again, due to the lack of effective area as a result of the identical pressure now acting on the piston and rod-side annulus areas, the static pressure in the cylinder must increase to support the identical load.
The magnitude with this pressure increase depends on the ratio in the cylinder’s piston and annulus areas. If the rise in static pressure exceeds the set maximum load of the counterbalance valve, the valve will open allowing oil from the piston side in the cylinder to circulate for the tank and also the cylinder to retract.
Diagnosing Cylinder Drift – Therefore, while the real cause from the problem in both examples is the leaking piston seal, the physics is fundamentally distinct from the general belief. And if the theory is understood, a pressure gauge can be a useful tool for establishing the reason for cylinder drift. In either of those examples, in the event the cylinder is drifting however, there is no equalization of pressure throughout the piston seal, the directional control valve or load control valve will be the source of rldvub problem.
Exactly what is the maximum pressure range for the application? Keep in mind pressures can vary greatly depending on the specific job the system is performing. Cylinders are rated both for nominal (standard) pressure and test pressure to take into account variations. System pressure should never exceed the nominal rated design pressure from the cylinder.
Push or pull-or both (double acting)? The reply to this question might require Tow Truck Hydraulic Cylinde if the hydraulic product is doing “double duty.” (Single-acting cylinders extend the piston under hydraulic pressure; double-acting cylinders extend and retract the piston under pressure.) In a push application, it is extremely important to size the rod diameter properly to prevent rod buckling. In a pull application, it is important to size the annulus area (piston diameter area minus the rod diameter area) correctly to maneuver the burden in the rated design pressure of the cylinder.
Yongxiang Hydraulic Equipment Co., Ltd locates in Yangzhou, CN, and it covers an area of 143,500 square feet. Winning customer trust with innovation, Yongxiang has been aiming to provide customers with safe and reliable hydraulic products, services, on-time delivery, and customer satisfaction while ensuring employee safety, fostering employee relations and driving efficiency improvements.
Jiangsu Yongxiang Hydraulic Equipment Co. Ltd
Factory: Wujian Industrial Park, Jiangdu District, Yangzhou, CN
Office: 3107# No.2 Building, Global Financial Center, Wenchang East Road, Yangzhou, CN
E-mail: [email protected]