Why Does Hydraulic Cylinder Leak?
Why Does Hydraulic Cylinder Leak?
Why Does Hydraulic Cylinder Leak?
A common misconception about hydraulic cylinders is that if the piston seal is leaking, the cylinder will drift. While a leaking piston seal is the primary cause of cylinder slippage, the law of physics involved is often misunderstood.
The fact is that if the piston seal is completely removed from a double acting hydraulic cylinder, the cylinder is filled with oil and the ports are clogged, the Piston rod seal will hold the cylinder load indefinitely unless it leaks.
In this case, due to the unequal volume on both sides of the piston, the fluid pressure is equalized and the cylinder is hydraulically locked. When this occurs, the cylinder can only move if fluid escapes from the cylinder through the rod seal or its bores.
Exceptions to the Rule
There are two exceptions to this theory. The first is a double-shaft cylinder in which the volume is equal on both sides of the piston.
The second exception involves a load suspended from a double-acting cylinder. In this arrangement, the volume of pressurized fluid on the rod side can be easily accommodated on the piston side. However, as the cylinder drags, a vacuum will form on the piston side due to unequal volumes, and depending on the weight of the load, this vacuum can eventually cause an equilibrium that stops further drag.
This is not the end of the cycle, but it is important to at least grasp this theory before continuing.
Despite these two exceptions, if the service ports of a double-acting cylinder are blocked by a spool closed to the actuator (Figure 3) and the piston seal bypasses, eventually the pressure on both sides of the cylinder will equalize. At this point, hydraulic lock is achieved and no further drift occurs unless fluid is allowed to escape from the cylinder or cylinder circuit.
Effective Area Loss
Due to the loss in effective area due to the pressure now acting on the ring area on the rod side, the static pressure in the cylinder must increase to support the same load. Remember, the force developed by the cylinder is a product of pressure and area.
For example, if the load-induced pressure on the piston side of the cylinder is 2,000 PSI and zero on the rod side when the directional control valve is closed, the equalized pressure, assuming no leakage from the spool, could be 3,000 PSI, depending on the ratio. piston and ring areas.
Now consider what might happen if this circuit had a service port relief valve (Figure 4) set at 2,500 PSI. When the pressure across the piston seal equalizes and the increased static pressure on the piston side of the cylinder reaches the cracking pressure of the port discharge, but the cylinder will still not retract.
A similar situation may occur in circuits where a load control (counter-balancing) valve is installed. In this circuit shown in Figure 5, the directional control valve has a float center spool (service ports A and B are open to the tank).
As mentioned before, if the piston seal is leaking, an uneven amount of oil on the rod and piston sides of the cylinder will indicate that the hydraulic lock will prevent any visible slippage. But once again, due to the loss of effective area as a result of the same pressure acting on the ring areas on the piston and rod sides, the static pressure in the cylinder must increase to support the same load.
The magnitude of this pressure increase depends on the ratio of the piston and ring areas of the cylinder. If the increase in static pressure exceeds the set maximum load of the balancing valve, the valve will open and allow oil from the piston side of the cylinder to flow into the tank and into the cylinder.