Hydraulic Uncompensated Four-way Control Valve Characteristic
Figure 1.11 illustrates the valve reaction forces for an uncompensated four-way valve. It can be noticed that the forces become very large for the higher flows and at large valve openings. AB the valve spool is moved to even larger openings approaching over one-half of the port area in the sleeve the reaction forces then tend to reverse and become negative.
This steady-state reaction force is the result of the fluid as it has passed through the restricting orifice. For very small openings of the orifice, the direction of this flow is about a 68 to 70 degree angle from the bore of the valve sleeve as indicated on Fig. 1.12. For small orifice openings, the following equation gives the approximate reaction forces in terms of flow and orifice area:
where F = reaction force on spool
p = mass density of fluid,
Q = fluid flow, in.
A = area of orifice opening,
The reaction forces on the valve spool will be approximately the same if the flow is reversed through the orifice. The above equation does not apply for large openings of the orifice since the angle of the flow from the orifice approaches 90°; also other restrictions to the flow tend to nullify the reaction forces.
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