Page 478 - Mechatronics with Experiments

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Q2
Spring 2
T
Spool 2
Q Spring C (Poppet spool)
Q
T
Ideal
Spring 1 Actual
Poppet
Q1
Main Sealed
relief B fitting
line
A Spool 1
T Leakage
P
line
P P
Line flow direction relief line
P
line
Line flow direction
(a) (b) (c)
FIGURE 7.46: Pressure relief valve: (a) direct acting, (b) indirect acting, (c) line pressure, set
relief pressure, and flow rate to tank relationship in steady state (ideal case and actual case
shown).
overcome the preload of spring 1 to open the main relief line. When the poppet valve opens,
the line pressure is relieved to the tank not through the poppet valve orifice, but through a
separate orifice on the side. The poppet section has a small (or almost zero) leakage flow to
tank. Compared to the direct acting relief valve, the indirect acting valve operation is less
sensitive to flow rate, is more stable, but has a slightly slower response due to the orifice.
It is not unusual that a direct acting relief valve can operate in a high frequency open and
close motion due to pressure spikes in the line, hence resulting in valve failures in a matter
of minutes in large power applications.
Poppet valves are commonly used as pressure regulators or pressure relief valves
(Figure 7.47). The main components of a poppet valve are: poppet (spherical ball or conical
shaped), seat, valve body, and poppet actuator which may include a spring. Poppet valves
are easy to manufacture, have lower leakage and are insensitive to clogging by dirt particles
compared to other types of spool valves. As a result, poppet valves are often used in load-
holding applications to minimize leakage and hence movement of the load. Depending on
the flow rate supported by the valve, it may be actuated directly by a solenoid for small
flow rate valves or via a secondary pilot stage amplification of actuation force for large
flow rate valves. Poppet valves are used as screw-in cartridge valves with manifold blocks.
A poppet valve performance is rated in terms of its maximum operating pressure (p max ),
rated flow through the valve at a given pressure drop across it (Q at Δp ). In recent years,
v
r
poppet valves have been developed as proportional flow control valves. However, since
the valve actuation force directly acts against the dynamic flow forces, the dynamic stable
control of the valve is a difficult task. The cross-sectional shape of the poppet and its seat
make a significant difference in flow forces and proportional flow metering ability of a
poppet valve.
Pressure reducing valves maintain an output pressure which is lower than the input
pressure by venting the excess flow to tank or limiting the orifice area based on desired and
actual output pressure sensing. Figure 7.48 shows the basic design, symbol, and steady-
state input–output relation of the valve. When the output pressure is less than the output
pressure set by the spring, the spool does not move. Output pressure is very close to the
input pressure.

p out ⋅ A out < F spring,0 (7.170)