There are lots of influences of spool valve configuration in the cylinder drift warranted discussion. The most common reason for cylinder drift is the DCV controlling it. Closed to actuator spools almost always leak pressure to both service ports. So, if you will effectively plug the actuator lines with gauges you can see something like 30% to 50% of the “P” line pressure in the actuator lines. It can create problems with closed center cylinder spools (all ports blocked have been blocked in the center position) when the pump is not at all unloaded. So many people who are working with the hydraulic spool valves are raising different questions in different ways. Some person posed the question in this way like they want to know about the horizontally mounted cylinder powered by a pressure compensated pump at 3,000 PSI and using all the ports blocked center condition valve. They have the question whether it can cause the cylinder to drift?
So, you can even bet it will. As you may probably gathered that it has nothing to do with the integrity of the cylinder’s pistol seal about which we are talking about in last week’s post. It is just because of the radial clearance of the spool because radial clearance is mainly required for the spool to slide in its bore; the design of this valve is not leak free. We can say it in another way as even when a port in a spool valve is closed off – a small portion of leakage should be expected.
On the other hand, another main valve design used in hydraulics is the poppet type where the valve ‘poppet’ closes against a seat IS generally considered as leak free. So, if the valve is closed and the poppet and as well as its seat are all in good condition, then there will no leakage across the valve’s ports. But there is an important exception to this rule. Slip-in-cartridge valves which are also called logic elements are a type of poppet valve commonly found in today’s hydraulic systems. In fact you can configure a logic element for flow in two directions which will be only leak free in one direction.
Getting back to cylinder drift mainly caused by the radial clearance of the directional control valve spool, if the pump can’t be unloaded then the float center spool with load holding check or counterbalance valve is the typical solution.
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