Directional Control Valves Classification by Spool Types
The directional control valves in hydraulic systems must work such that when the spools are in a neutral position, the fluid flow patterns meet the purpose of the systems, in addition to causing reversible motion of the hydraulic cylinders and motors.
For example, take the “three-position” valve in Table 4-1. This closed-center valve (all ports are closed when the spool is in the neutral position) locks the cylinder at its
position. The pressure in the pump line is maintained at a preset level for the relief valve or the variable pumps, and other systems can be operated as desired. On the other hand, the center-bypassed valve (ports P and T are open when the spool is in the neutral position) unloads the pump line while locking the cylinder, which is desirable for energy saving: lower heat generation and reduced pump load.
However, the overall performance of the hydraulic system should be taken into consideration to choose the best valves for it. Table 4.2 lists common spool types with their functions and usage. Note that the numbers in the double quotation marks “**” may vary according to valve manufacturers.
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