A popular misunderstanding involving Hydraulic Piston is that if the piston seal is leaking, the cylinder will drift. While a leaking piston seal can be the cause of cylinder drift, the physics involved are frequently misinterpreted. Fact is, f the piston seal is completely taken off 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, due to the unequal volume on both sides from the piston, fluid pressure equalizes and also the cylinder becomes hydraulically locked. Once this occurs, the cylinder can move only if fluid escapes from your cylinder through the rod seal or its ports.
Exceptions to the Rule – There are two exceptions to this theory. First is a double-rod cylinder (Figure 1) where volume is equal for both sides in the piston. The second exception involves a load hanging on a double-acting cylinder (Figure 2). Within this arrangement, the volume of pressurized fluid on the rod side could be accommodated on the piston side. But as 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 bring about equilibrium that arrests further drift.
This is not the final of the cycle, but it’s important to a minimum of grasp this theory before continuing. Notwithstanding both of these exceptions, if a double-acting cylinder’s service ports are blocked by a closed-to-actuator spool (Figure 3), and also the piston seal does bypass, pressure will eventually equalize for both sides of the cylinder. At this stage, a hydraulic lock is effected and no further drift can occur, unless fluid is allowed to escape from your cylinder or cylinder circuit.
Loss of Effective Area – Due to the 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 exactly the same load. Remember, force designed 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 when the directional control valve closed, assuming no leakage beyond the spool, the equalized pressure may be 3,000 PSI depending on the ratio from the piston and annulus areas.
New call-to-action – Now consider so what can happen if this circuit includes a service port relief valve (Figure 4) set at 2,500 PSI. As pressure equalizes across the piston seal as well as the increasing static pressure on the piston side of the Excavator Hydraulic Pump reaches the cracking pressure from the port relief, nevertheless the cylinder will still not retract. An identical situation can take place 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 open to tank).
As previously stated, when the piston seal leaks, unequal volumes of oil on the rod and piston sides of the cylinder indicates hydraulic lock can prevent any noticeable drift. But when again, because of the 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 from 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 from the piston side of the cylinder to flow to the tank and the cylinder to retract.
Diagnosing Cylinder Drift – Therefore, while the root cause in the symptom in both examples will be the leaking piston seal, the physics is fundamentally distinct from the overall belief. And in case the idea is understood, a pressure gauge could be a useful tool for establishing the reason for cylinder drift. Either in of these examples, if the cylinder is drifting but there is no equalization of pressure over the piston seal, the directional control valve or load control valve is the way to obtain rldvub problem.
What exactly is the maximum pressure range for your application? Keep in mind pressures can vary greatly greatly depending on the specific job the system is performing. 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 from the cylinder.
Push or pull-or both (double acting)? The reply to this query might require Tow Truck Hydraulic Cylinder Quality High in the event 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 very essential to size the rod diameter properly to prevent rod buckling. In a pull application, you should size the annulus area (piston diameter area minus the rod diameter area) correctly to maneuver the burden 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
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