A well known disbelief involving Refractory Hydraulic Ram 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 misinterpreted. Truth is, f the piston seal is completely removed from 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 particular condition, because of the unequal volume on both sides in the piston, fluid pressure equalizes and the cylinder becomes hydraulically locked. Once this takes place, the cylinder can move as long as fluid escapes through the cylinder using the rod seal or its ports.
Exceptions towards the Rule – There are two exceptions for this theory. First is a double-rod cylinder (Figure 1) where volume is equal on sides in the piston. The second exception involves a lot hanging on the double-acting cylinder (Figure 2). Within this 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 develop on the piston side as a result of unequal volumes, and depending on the weight of the load, this vacuum may eventually bring about equilibrium that arrests further drift.
This is simply not the end of the cycle, but it’s important to at the very least grasp this theory before continuing. Notwithstanding those two exceptions, when a double-acting cylinder’s service ports are blocked with a closed-to-actuator spool (Figure 3), and also the piston seal does bypass, pressure could eventually equalize on both 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 due to pressure now acting on the rod-side annulus area, the static pressure within the cylinder must increase to back up exactly the same load. Remember, force developed by a cylinder is actually a product of pressure and area. As an example, when the load-induced pressure on the piston side of 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 in the piston and annulus areas.
New call-to-action – Now consider what can happen if this type of circuit has a service port relief valve (Figure 4) set at 2,500 PSI. As pressure equalizes over the piston seal and also the increasing static pressure on the piston side of the Robot Arm Air Cylinder Components reaches the cracking pressure from the port relief, nevertheless the cylinder will still not retract. A similar 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 available to tank).
As previously stated, if the piston seal leaks, unequal volumes of oil on the rod and piston sides of the cylinder indicates hydraulic lock will prevent any noticeable drift. But once again, as a result of lack of effective area because of the same pressure now acting on the piston and rod-side annulus areas, the static pressure within the cylinder must increase to back up the identical load.
The magnitude of the pressure increase depends on the ratio of the cylinder’s piston and annulus areas. If the rise in static pressure exceeds the set maximum load from the counterbalance valve, the valve will open allowing oil from the piston side from the cylinder to flow towards the tank as well as the cylinder to retract.
Diagnosing Cylinder Drift – Therefore, while the cause of the symptom in both examples will be the leaking piston seal, the physics is fundamentally different from the overall belief. And in case the theory is understood, a pressure gauge can be a useful tool for establishing the cause of cylinder drift. In both of these examples, when 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 supply 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 system is performing. Cylinders are rated for nominal (standard) pressure and test pressure to account for 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 query may need Lifting Car Jack 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 rather vital that you size the rod diameter properly 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 move the burden on the rated design pressure from the cylinder.
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