A popular misunderstanding involving Lift Hydraulic Cylinder 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 often misinterpreted. Fact is, f the piston seal is entirely taken off a double-acting cylinder, the cylinder is filled with oil and also the ports are plugged, the cylinder will hold its load indefinitely, unless the rod-seal leaks.
Within this condition, as a result of unequal volume on both sides of the piston, fluid pressure equalizes and also the cylinder becomes hydraulically locked. Once this takes place, the cylinder can move only when fluid escapes through the cylinder using the rod seal or its ports.
Exceptions for the Rule – There are 2 exceptions for this theory. The first is a double-rod cylinder (Figure 1) where volume is equal for both sides of the piston. The next exception involves a load hanging over 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 because the cylinder drifts, a vacuum will experience on the piston side as a result of unequal volumes, and depending on the weight in the load, this vacuum may eventually result in equilibrium that arrests further drift.
This may not be the final in the cycle, but it’s important to a minimum of 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 take place, unless fluid is allowed to escape from the cylinder or cylinder circuit.
Lack of Effective Area – As a result of loss in effective area because of pressure now acting on the rod-side annulus area, the static pressure within the cylinder must increase to support the same load. Remember, force developed by a cylinder is actually a product of pressure and area. For instance, when the load-induced pressure on the piston side from 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 might be 3,000 PSI depending on the ratio of 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 over the piston seal as well as the increasing static pressure on the piston side from the Tie Rod Hydraulic Press Cylinder reaches the cracking pressure of the port relief, though the cylinder will still not retract. An identical situation can happen in circuits using a load control (counterbalance) valve installed. In this circuit, shown in Figure 5, the directional control valve features a float center spool (service ports A and B ready to accept 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 may prevent any noticeable drift. But once again, as a result of lack of effective area due to the same pressure now acting on the piston and rod-side annulus areas, the static pressure within the cylinder must increase to aid exactly the same load.
The magnitude with this 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 from the counterbalance valve, the valve will open allowing oil from the piston side of the cylinder to circulate towards the tank and the cylinder to retract.
Diagnosing Cylinder Drift – Therefore, while the root cause of the problem in both examples is definitely the leaking piston seal, the physics is fundamentally distinctive from the general belief. And in case the theory is understood, a pressure gauge can be quite a useful tool for establishing the main cause of cylinder drift. In either of such examples, if the cylinder is drifting but there is no equalization of pressure throughout the piston seal, the directional control valve or load control valve is definitely the source of rldvub problem.
What is the maximum pressure range for the application? Remember pressures can vary greatly greatly depending on the specific job the system does. 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 question may require Hydraulic Breaker Power Pack Station 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 very vital that you 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 without the rod diameter area) correctly to go the burden 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
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