A popular misconception involving Short Hydraulic Ram is that if the piston seal is leaking, the cylinder will drift. While a leaking piston seal could be the cause of cylinder drift, the physics involved are often misunderstood. Fact is, f the piston seal is completely removed from a double-acting cylinder, the cylinder is filled with oil as well as the ports are plugged, the cylinder will hold its load indefinitely, unless the rod-seal leaks.
In this particular condition, due to the unequal volume on either side of the piston, fluid pressure equalizes and also the cylinder becomes hydraulically locked. Once this takes place, the cylinder can move only if fluid escapes from the cylinder using 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 on both sides of the piston. The next exception involves a lot hanging on a double-acting cylinder (Figure 2). In this particular arrangement, the amount of pressurized fluid on the rod side can easily be accommodated on the piston side. But since the cylinder drifts, a vacuum will experience on the piston side as a result of unequal volumes, and depending on the weight of the load, this vacuum may eventually result in equilibrium that arrests further drift.
This may not be the conclusion from the cycle, but it’s vital that you at least grasp this theory before continuing. Notwithstanding these two exceptions, if a double-acting cylinder’s service ports are blocked with a closed-to-actuator spool (Figure 3), as well as the piston seal does bypass, pressure could eventually equalize on both sides in the cylinder. At this stage, a hydraulic lock is effected and no further drift can happen, unless fluid is able to escape from the cylinder or cylinder circuit.
Lack of Effective Area – Because of the loss in effective area because of pressure now acting on the rod-side annulus area, the static pressure within the cylinder must increase to back up exactly the same load. Remember, force developed by a cylinder is a product of pressure and area. As an example, if the load-induced pressure on the piston side of the cylinder was 2,000 PSI and zero on the rod side once the directional control valve closed, assuming no leakage past the spool, the equalized pressure may be 3,000 PSI depending on the ratio of the piston and annulus areas.
New call-to-action – Now consider so what can happen if the circuit has a service port relief valve (Figure 4) set at 2,500 PSI. As pressure equalizes across the piston seal and also the increasing static pressure on the piston side of the Hydraulic Press Machine 100 Tons reaches the cracking pressure from the port relief, nevertheless the cylinder will still not retract. A similar situation can happen in circuits with a load control (counterbalance) valve installed. In this circuit, shown in Figure 5, the directional control valve includes 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 from the cylinder indicates hydraulic lock will prevent any noticeable drift. But once again, as a result of lack of effective area because of the identical pressure now acting on the piston and rod-side annulus areas, the static pressure inside the cylinder must increase to back up the same load.
The magnitude of 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 from the counterbalance valve, the valve will open allowing oil from the piston side from the cylinder to flow towards the tank as well as the cylinder to retract.
Diagnosing Cylinder Drift – Therefore, whilst the real cause of the problem in both examples will be the leaking piston seal, the physics is fundamentally distinctive from the overall belief. And if the thought is understood, a pressure gauge can be a useful tool for establishing the reason for cylinder drift. In both of these examples, in the event the cylinder is drifting however, there is no equalization of pressure over the piston seal, the directional control valve or load control valve is definitely the supply of rldvub problem.
What exactly is the maximum pressure range for the application? Bear in mind pressures can vary greatly greatly depending on the specific job the system is doing. Cylinders are rated for both 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 solution to this question may require Prestressed Hydraulic Lifting Jack 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 very essential to size the rod diameter properly in order to avoid rod buckling. In a pull application, it is important to size the annulus area (piston diameter area without the rod diameter area) correctly to maneuver the load on 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
E-mail: [email protected]