A well known misconception involving Plunger Hydraulic Ram is that if the piston seal is leaking, the cylinder will drift. While a leaking piston seal could possibly be the real cause of cylinder drift, the physics involved are frequently misunderstood. Simple truth is, f the piston seal is entirely taken 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.
In this condition, as a result of unequal volume on both sides in the piston, fluid pressure equalizes and the cylinder becomes hydraulically locked. Once this occurs, the cylinder can move only when fluid escapes from your cylinder through the rod seal or its ports.
Exceptions to the Rule – There are 2 exceptions to this theory. The first is a double-rod cylinder (Figure 1) where volume is equal on 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 could be accommodated on the piston side. But because the cylinder drifts, a vacuum will develop on the piston side because of unequal volumes, and depending on the weight of the load, this vacuum may eventually lead to equilibrium that arrests further drift.
This is simply not the final from the cycle, but it’s vital that you at the very least grasp this theory before continuing. Notwithstanding those two exceptions, in case 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 could eventually equalize on sides of the cylinder. At this point, a hydraulic lock is effected with no further drift can occur, unless fluid is able to escape from your 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 in the cylinder must increase to back up exactly the same load. Remember, force created by a cylinder is a product of pressure and area. For example, if the load-induced pressure on the piston side of 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 could 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 and the increasing static pressure on the piston side from the Pneumatic Cylinder reaches the cracking pressure in the port relief, though the cylinder will still not retract. An identical situation can take place in circuits with a load control (counterbalance) valve installed. In this particular circuit, shown in Figure 5, the directional control valve has a float center spool (service ports A and B available to tank).
As previously mentioned, in the event the piston seal leaks, unequal volumes of oil on the rod and piston sides in the cylinder indicates hydraulic lock will prevent any noticeable drift. But once again, as a result of lack of effective area as a result of exactly the same pressure now acting on the piston and rod-side annulus areas, the static pressure inside the cylinder must increase to support exactly the same load.
The magnitude of this pressure increase depends on the ratio of the cylinder’s piston and annulus areas. If the rise in static pressure exceeds the set maximum load in the counterbalance valve, the valve will open allowing oil from the piston side in the cylinder to flow to the tank and also the cylinder to retract.
Diagnosing Cylinder Drift – Therefore, as the real cause from the problem in both examples is the leaking piston seal, the physics is fundamentally distinctive from the overall belief. And if the thought is understood, a pressure gauge can be quite a great tool for establishing the cause of cylinder drift. In both of such examples, when the cylinder is drifting however, there is no equalization of pressure over the piston seal, the directional control valve or load control valve will be the way to obtain rldvub problem.
Exactly what is the maximum pressure range for that application? Bear in mind pressures can vary greatly depending on the specific job the system is doing. Cylinders are rated for nominal (standard) pressure and test pressure to account for variations. System pressure must not exceed the nominal rated design pressure from the cylinder.
Push or pull-or both (double acting)? The reply to this query may need Tower Crane Hydraulic Cylinder if 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 move the burden at the rated design pressure from 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]