A popular misunderstanding involving Hydraulic Actuator is that if the piston seal is leaking, the cylinder will drift. While a leaking piston seal can be the root cause of cylinder drift, the physics involved are frequently misunderstood. Truth is, f the piston seal is entirely taken 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.
In this condition, as a result of unequal volume on each side of the piston, fluid pressure equalizes as well as the cylinder becomes hydraulically locked. Once this takes place, 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 for 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 load hanging over a double-acting cylinder (Figure 2). In this 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 because of unequal volumes, and depending on the weight from the load, this vacuum may eventually result in equilibrium that arrests further drift.
This may not be the conclusion of the cycle, but it’s vital that you a minimum of grasp this theory before continuing. Notwithstanding these two exceptions, in case a double-acting cylinder’s service ports are blocked by a closed-to-actuator spool (Figure 3), and the piston seal does bypass, pressure will eventually equalize on sides in the cylinder. At this time, a hydraulic lock is effected with no further drift can happen, unless fluid is permitted to escape from your cylinder or cylinder circuit.
Loss of 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 within the cylinder must increase to back up the identical load. Remember, force designed by a cylinder is a product of pressure and area. As an example, when the load-induced pressure on the piston side in the cylinder was 2,000 PSI and zero on the rod side once the directional control valve closed, assuming no leakage beyond 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 exactly what can happen if this type of circuit has a service port relief valve (Figure 4) set at 2,500 PSI. As pressure equalizes across the piston seal and the increasing static pressure on the piston side in the Dc Electric Motor Hydraulic Pump reaches the cracking pressure of the port relief, however the cylinder will still not retract. A comparable situation can occur 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, if 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, because of the loss in effective area because of the same pressure now acting on the piston and rod-side annulus areas, the static pressure inside the cylinder must increase to back up exactly the same load.
The magnitude of the pressure increase depends on the ratio of the cylinder’s piston and annulus areas. If the increase in static pressure exceeds the set maximum load from the counterbalance valve, the valve will open allowing oil from the piston side of the cylinder to flow towards the tank and the cylinder to retract.
Diagnosing Cylinder Drift – Therefore, while the root cause from the symptom in both examples will be the leaking piston seal, the physics is fundamentally distinctive from the overall belief. And in case the thought is understood, a pressure gauge can be a great tool for establishing the reason for cylinder drift. In either of those examples, when the cylinder is drifting however, there is no equalization of pressure across the piston seal, the directional control valve or load control valve is the supply of rldvub problem.
What exactly is the maximum pressure range for that application? Keep in mind pressures can vary greatly depending on the specific job the program is performing. Cylinders are rated both for nominal (standard) pressure and test pressure to account for variations. System pressure must not exceed the nominal rated design pressure of the cylinder.
Push or pull-or both (double acting)? The reply to this query might require Custom Bulit Hydraulic Rotary Actuator in the event 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 rather essential to size the rod diameter properly in order to avoid 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 stress at 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]