A well known misunderstanding involving Forklift Hydraulic Cylinder is that if the piston seal is leaking, the cylinder will drift. While a leaking piston seal could possibly be the real cause of cylinder drift, the physics involved are frequently misinterpreted. Simple truth is, f the piston seal is completely taken 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.
Within this condition, as a result of unequal volume on each side in the piston, fluid pressure equalizes as well as the cylinder becomes hydraulically locked. Once this occurs, the cylinder can move as long as fluid escapes from the cylinder via the rod seal or its ports.
Exceptions to the Rule – The two main exceptions to this particular theory. The first is a double-rod cylinder (Figure 1) where volume is equal on sides of the piston. The second exception involves a load hanging on a double-acting cylinder (Figure 2). In this arrangement, the quantity of pressurized fluid on the rod side could be accommodated on the piston side. But because the cylinder drifts, a vacuum will experience on the piston side because of unequal volumes, and depending on the weight in the load, this vacuum may eventually lead to equilibrium that arrests further drift.
This is simply not the end of the cycle, but it’s essential to a minimum of grasp this theory before continuing. Notwithstanding those two exceptions, in case a double-acting cylinder’s service ports are blocked by a closed-to-actuator spool (Figure 3), as well as the piston seal does bypass, pressure could eventually equalize for both sides of the cylinder. At this point, a hydraulic lock is effected without any further drift can happen, unless fluid is able to escape from your cylinder or cylinder circuit.
Loss in Effective Area – As a result of loss in effective area because of pressure now acting on the rod-side annulus area, the static pressure inside the cylinder must increase to support the identical load. Remember, force designed by a cylinder is actually a product of pressure and area. For instance, if 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 in the piston and annulus areas.
New call-to-action – Now consider exactly what can happen if the 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 from the Single Acting Hydraulic Press Cylinder reaches the cracking pressure of the port relief, nevertheless the cylinder will still not retract. An identical situation can take place in circuits using a load control (counterbalance) valve installed. Within this circuit, shown in Figure 5, the directional control valve features a float center spool (service ports A and B open to tank).
As previously mentioned, in the event the piston seal leaks, unequal volumes of oil on the rod and piston sides in the cylinder indicates hydraulic lock may prevent any noticeable drift. But when again, because of the loss in effective area due to exactly the same pressure now acting on the piston and rod-side annulus areas, the static pressure within the cylinder must increase to support exactly the same load.
The magnitude of this 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 of the cylinder to circulate towards the tank and the cylinder to retract.
Diagnosing Cylinder Drift – Therefore, whilst the real cause from the problem in both examples is definitely the leaking piston seal, the physics is fundamentally different from the general belief. And if the thought is understood, a pressure gauge could be a great tool for establishing the reason for cylinder drift. In both of these examples, in the event 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 supply of rldvub problem.
What is the maximum pressure range for your application? Keep in mind pressures can vary greatly depending on the specific job the program does. 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 from the cylinder.
Push or pull-or both (double acting)? The reply to this question might require Mini Hydraulic Power Pack Unit when 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 prevent rod buckling. In a pull application, you should size the annulus area (piston diameter area minus the rod diameter area) correctly to move the burden on the rated design pressure from the cylinder.
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