A popular misconception involving Hydraulic Motor And Pump System 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 frequently misunderstood. Simple truth is, f the piston seal is entirely taken off a double-acting cylinder, the cylinder is full of oil and also the ports are plugged, the cylinder will hold its load indefinitely, unless the rod-seal leaks.
In this particular condition, because of the unequal volume on either side from the piston, fluid pressure equalizes and the cylinder becomes hydraulically locked. Once this happens, the cylinder can move only when fluid escapes from your cylinder using the rod seal or its ports.
Exceptions for the Rule – The two main exceptions to this particular theory. The first is a double-rod cylinder (Figure 1) where volume is equal on both sides of the piston. The 2nd exception involves a lot hanging on the 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 since the cylinder drifts, a vacuum will experience on the piston side due to unequal volumes, and depending on the weight from the load, this vacuum may eventually lead to equilibrium that arrests further drift.
This is not the end in the cycle, but it’s important to at the very least grasp this theory before continuing. Notwithstanding both of these exceptions, in case 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 could eventually equalize on sides from the cylinder. At this point, a hydraulic lock is effected without any 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 aid the same load. Remember, force developed by a cylinder is actually a product of pressure and area. As an example, in the event the load-induced pressure on the piston side in the cylinder was 2,000 PSI and zero on the rod side when the directional control valve closed, assuming no leakage past 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 what can happen if this circuit features 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 in the Pneumatic Hydraulic Pump reaches the cracking pressure from the port relief, nevertheless the cylinder will still not retract. An identical situation can occur in circuits with a load control (counterbalance) valve installed. In this particular circuit, shown in Figure 5, the directional control valve includes 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 can prevent any noticeable drift. But when again, due to the lack of effective area due to the same pressure now acting on the piston and rod-side annulus areas, the static pressure inside the cylinder must increase to back up the identical load.
The magnitude of the pressure increase depends on the ratio in 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 of the cylinder to circulate towards the tank as well as the cylinder to retract.
Diagnosing Cylinder Drift – Therefore, as the root cause in the problem in both examples will be the leaking piston seal, the physics is fundamentally different from the overall belief. And when the theory is understood, a pressure gauge can be a useful tool for establishing the cause of cylinder drift. Either in of these examples, in the event the cylinder is drifting there is however no equalization of pressure over 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 may vary greatly depending on the specific job the device is doing. Cylinders are rated both for nominal (standard) pressure and test pressure to make up variations. System pressure should never exceed the nominal rated design pressure in the cylinder.
Push or pull-or both (double acting)? The answer to this query might require Electric Over Hydraulic Linear Actuator when 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 rather important 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 go the burden on the rated design pressure of 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