Page 185 - The Mechatronics Handbook

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P C

D D

P B
x

P A

d

FIGURE 10.3 Operation principle of a puppet type cartridge valve.
The spring force biases the poppet towards closing. When the poppet is in the closed position, the
spring force reaches its minimum value. The force increases as the poppet lifts to open the flow passage.
F S = kx 0 + x) (10.12)
(
The steady-state flow force tends to open the poppet in this valve. The flow force is a function of the
flow rate and fluid velocity passing through the valve orifice.
F F = rqv cos a (10.13)

The flow control characteristics of a spool valve are similar to those of a cartridge valve and can be
described using an orifice equation. The only difference is that spool valve flow-passing area is determined
by its wet perimeter, w, and spool displacement, x.

2
q = c d wx ---∆P (10.14)
r
If the orifice is formed by the edge of the spool and the valve body, the wet perimeter is w = πd. If the
orifice is formed by n slots cut on the spool and the perimeter of each slot is n, the corresponding wet
perimeter is w = nb. The orifice coefficient for a spool valve normally uses c d = 0.60∼0.65.
The forces acting on the spool also include the pressure, spring, and flow forces (Merritt, 1967). The
pressure force is either balanced on the spool, because of its symmetric structure in a direct-actuator
valve (actuated by a solenoid directly), or the pressure force to actuate the spool movement in a pilot
actuated valve. The spring force tends to keep the spool in the central (neutral) position and can be
described using Eq. (10.12). The flow forces acting on the spool can be calculated using Eq. (10.14). The
flow velocity angle, α, is normally taken as 69°.

10.4 Hydraulic Pumps

Principles of Pump Operation
The pump is one of the most important components in a hydraulic system because it supplies hydraulic
flow to the system. Driven by a prime mover, a hydraulic pump takes the fluid in at atmospheric pressure
to fill an expanding volume of space inside the pump through an inlet port and delivers pressurized

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