A popular disbelief involving Snow Plow Hydraulic Cylinder is that if the piston seal is leaking, the cylinder will drift. While a leaking piston seal could be the real cause of cylinder drift, the physics involved are frequently misinterpreted. Truth is, f the piston seal is entirely taken from 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.
Within this condition, as a result of unequal volume on each side in the piston, fluid pressure equalizes and also the cylinder becomes hydraulically locked. Once this takes place, the cylinder can move only when fluid escapes from the cylinder via the rod seal or its ports.
Exceptions towards the Rule – There are two exceptions for this theory. The very first is a double-rod cylinder (Figure 1) where volume is equal for both sides in the piston. The 2nd exception involves a load 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 as 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 lead to equilibrium that arrests further drift.
This is not the conclusion from the cycle, but it’s important to a minimum of 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), and also the piston seal does bypass, pressure will ultimately equalize on sides from 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.
Loss in Effective Area – As a result of loss in effective area as a result of pressure now acting on the rod-side annulus area, the static pressure in the cylinder must increase to support exactly the same load. Remember, force created by a cylinder is a product of pressure and area. For example, when the load-induced pressure on the piston side in the cylinder was 2,000 PSI and zero on the rod side if 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 so 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 also the increasing static pressure on the piston side of the Air Driven Hydraulic Pump reaches the cracking pressure of the port relief, nevertheless the cylinder will still not retract. A comparable situation can occur in circuits using a load control (counterbalance) valve installed. Within this circuit, shown in Figure 5, the directional control valve has a float center spool (service ports A and B ready to accept tank).
As previously mentioned, when the piston seal leaks, unequal volumes of oil on the rod and piston sides of the cylinder indicates hydraulic lock may prevent any noticeable drift. But once again, due to the loss in effective area because of exactly the same pressure now acting on the piston and rod-side annulus areas, the static pressure inside the cylinder must increase to aid the same load.
The magnitude with this pressure increase depends on the ratio in the cylinder’s piston and annulus areas. If the rise in static pressure exceeds the set maximum load of the counterbalance valve, the valve will open allowing oil through the piston side from the cylinder to flow to the tank as well as the cylinder to retract.
Diagnosing Cylinder Drift – Therefore, whilst the root cause in the problem in both examples is the leaking piston seal, the physics is fundamentally distinct from the typical belief. And when the theory is understood, a pressure gauge can be quite a useful tool for establishing the reason for 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 supply of rldvub problem.
Exactly what is the maximum pressure range for your application? Remember pressures can vary greatly depending on the specific job the device 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 in the cylinder.
Push or pull-or both (double acting)? The reply to this inquiry might require Tow Truck Cylinder in the event the hydraulic method 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 vital that you size the rod diameter properly to avoid rod buckling. In a pull application, you should size the annulus area (piston diameter area without the rod diameter area) correctly to maneuver the burden at the rated design pressure of the cylinder.
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Jiangsu Yongxiang Hydraulic Equipment Co. Ltd
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