A well known disbelief involving Double Acting Hydraulic Ram is that if the piston seal is leaking, the cylinder will drift. While a leaking piston seal could possibly be the cause of cylinder drift, the physics involved are often misunderstood. Simple truth is, f the piston seal is completely removed from a double-acting cylinder, the cylinder is filled with oil and also the ports are plugged, the cylinder will hold its load indefinitely, unless the rod-seal leaks.
Within this condition, because of the unequal volume on each side in the piston, fluid pressure equalizes as well as the cylinder becomes hydraulically locked. Once this takes place, the cylinder can move only when fluid escapes from the cylinder via the rod seal or its ports.
Exceptions for the Rule – The two main exceptions to this theory. The very first is a double-rod cylinder (Figure 1) where volume is equal for both sides of the piston. The 2nd exception involves a load hanging over a double-acting cylinder (Figure 2). Within this arrangement, the amount of pressurized fluid on the rod side may be easily accommodated on the piston side. But because the cylinder drifts, a vacuum will develop on the piston side due to unequal volumes, and depending on the weight in the load, this vacuum may eventually lead to equilibrium that arrests further drift.
This may not be the conclusion from the cycle, but it’s essential to at least grasp this theory before continuing. Notwithstanding those two exceptions, if a double-acting cylinder’s service ports are blocked by way of a closed-to-actuator spool (Figure 3), and also the piston seal does bypass, pressure will ultimately equalize for both sides of the cylinder. At this time, a hydraulic lock is effected without any further drift can occur, unless fluid is permitted to escape from the cylinder or cylinder circuit.
Loss in Effective Area – As a result of loss in effective area due to pressure now acting on the rod-side annulus area, the static pressure in the cylinder must increase to aid the same load. Remember, force designed by a cylinder is really a product of pressure and area. As an example, when the load-induced pressure on the piston side in the cylinder was 2,000 PSI and zero on the rod side if the directional control valve closed, assuming no leakage past the spool, the equalized pressure might be 3,000 PSI depending on the ratio in the piston and annulus areas.
New call-to-action – Now consider so what can happen if this type of circuit has a service port relief valve (Figure 4) set at 2,500 PSI. As pressure equalizes over the piston seal and also the increasing static pressure on the piston side of the Anti Corrosion Fiberglass Push Rod reaches the cracking pressure in the port relief, though the cylinder will still not retract. A comparable 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, if the piston seal leaks, unequal volumes of oil on the rod and piston sides from the cylinder indicates hydraulic lock may prevent any noticeable drift. But once again, because of the loss in effective area due to exactly the same pressure now acting on the piston and rod-side annulus areas, the static pressure inside the cylinder must increase to support the identical load.
The magnitude with this pressure increase depends on the ratio of the cylinder’s piston and annulus areas. If the increase in static pressure exceeds the set maximum load of the counterbalance valve, the valve will open allowing oil through the piston side in the cylinder to flow to the tank and also the cylinder to retract.
Diagnosing Cylinder Drift – Therefore, while the root cause from the symptom in both examples will be the leaking piston seal, the physics is fundamentally distinct from the typical belief. And if the idea is understood, a pressure gauge can be quite a great tool for establishing the cause of cylinder drift. In either of those examples, when the cylinder is drifting there is however no equalization of pressure over the piston seal, the directional control valve or load control valve is the source of rldvub problem.
What exactly is the maximum pressure range for your application? Remember pressures can vary greatly greatly depending on the specific job the device is doing. Cylinders are rated both for nominal (standard) pressure and test pressure to take into account variations. System pressure must not exceed the nominal rated design pressure of the cylinder.
Push or pull-or both (double acting)? The reply to this inquiry might require Power Pack Hydraulic Pump when the hydraulic system 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 essential to size the rod diameter properly in order to avoid rod buckling. In a pull application, you should size the annulus area (piston diameter area without the rod diameter area) correctly to go the stress in the rated design pressure in 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