A well known disbelief involving 3000psi Welded Hydraulic Cylinders 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 misinterpreted. Fact is, f the piston seal is completely removed from a double-acting cylinder, the cylinder is full of oil and the ports are plugged, the cylinder will hold its load indefinitely, unless the rod-seal leaks.
Within this condition, due to the unequal volume on each side in the piston, fluid pressure equalizes and the cylinder becomes hydraulically locked. Once this occurs, the cylinder can move as long as fluid escapes through the cylinder using the rod seal or its ports.
Exceptions for the Rule – There are 2 exceptions for this theory. The very first is a double-rod cylinder (Figure 1) where volume is equal on sides in the piston. The 2nd exception involves a load hanging on a double-acting cylinder (Figure 2). In this arrangement, the quantity of pressurized fluid on the rod side could be accommodated on the piston side. But since the cylinder drifts, a vacuum will experience 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 is not the end from the cycle, but it’s vital that you at least grasp this theory before continuing. Notwithstanding those 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 could eventually equalize on sides from the cylinder. At this time, a hydraulic lock is effected without any further drift can occur, unless fluid is allowed to escape through the cylinder or cylinder circuit.
Loss in Effective Area – Because of the loss in effective area because 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 really a product of pressure and area. For example, when 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 might be 3,000 PSI depending on the ratio from the piston and annulus areas.
New call-to-action – Now consider exactly what can happen if this type of circuit features a service port relief valve (Figure 4) set at 2,500 PSI. As pressure equalizes throughout the piston seal and also the increasing static pressure on the piston side of the Manual Hydraulic Pumps reaches the cracking pressure from the port relief, nevertheless the cylinder will still not retract. A similar situation can happen in circuits using 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 open to tank).
As previously stated, 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 exactly the same 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 of this pressure increase depends on the ratio from the cylinder’s piston and annulus areas. If the increase 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 circulate towards the tank and the cylinder to retract.
Diagnosing Cylinder Drift – Therefore, whilst the real cause in the problem in both examples is the leaking piston seal, the physics is fundamentally distinctive from the general belief. And if the thought is understood, a pressure gauge can be a useful tool for establishing the reason for cylinder drift. Either in of those examples, when the cylinder is drifting there is however no equalization of pressure across the piston seal, the directional control valve or load control valve is definitely the source of rldvub problem.
Exactly what is the maximum pressure range for that application? Keep in mind pressures can vary greatly depending on the specific job the system is performing. Cylinders are rated for nominal (standard) pressure and test pressure to take into account variations. System pressure should never exceed the nominal rated design pressure of the cylinder.
Push or pull-or both (double acting)? The answer to this inquiry may require Hydraulic Power Pack Unit System 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 important to size the rod diameter properly to prevent rod buckling. In a pull application, it is essential to size the annulus area (piston diameter area minus 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]