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a balanced distribution of supply pressure between valve and load is possible in servo valve
applications. In proportional valve applications, especially in large construction equipment,
the pressure drop across the valve can be much less than 1∕3 of the supply pressure. In
such applications, precision motion control is not critical. It is desirable that more of the
hydraulic power is delivered to the load and less is lost in the process of metering it. If the
valve is sized too large and the pressure drop across it is not large enough, the flow will not
be modulated well until the valve almost closes. As a result, resolution of the motion control
will be low. If the valve is too small, it will not be able to support the desired flow rates or
the pressure drop across it will be too large. The valve is the flow metering component. It is
desirable to have a good resolution in metering the flow. In general, the higher the metering
resolution required, the higher the pressure drop that must exist across the valve. Higher
pressure drops lead to higher losses and lower efficiency. Therefore, metering resolution
and efficiency are two conflicting variables in a hydraulic control system. A good design
targets an acceptable metering resolution with minimal pressure drop.
There are two main variables which determine the component size requirements:
required force and speed. Another way of stating that is required pressure and flow rate.
Based on these two requirements, supply (pump) and actuator (cylinder and motors) are
sized. The valve, which modulates and directs the flow, is sized to handle the flow rate and
pressure required. The control question in hydraulic systems always involves the control of
flow rate and direction, and/or the pressure.
In general, the load speed requirement dictates the flow rate, and the load force/torque
requirement dictates the operating pressure. Consider a cylinder as the hydraulic actuator. A
given load force/torque requirement can be met with different pressure and cylinder cross-
sectional areas. For instance, in order to provide a certain force, many pressure and cylinder
area combinations are possible, F = p ⋅ A. Similarly, a given load speed requirement can
be met with different combinations of flow rate and cylinder cross-sectional areas, Q =
V ⋅ A. The trend in industry is to use higher operating pressures which result in smaller
components. However, high pressure circuits require more frequent maintanence and have
a lower life cycle.
The application requirements typically specify the load conditions in terms of three
variables: the no-load maximum speed (w for a rotary actuator or V for a translational
nl
nl
actuator) and speed at rated load (w at T or V at F ). For rotary and linear system the
r
r
r
r
specifications are, respectively,
{w ,(w , T )} or {V ,(V , F )} (7.258)
r
r
nl
nl
r
r
Recall that the relationships between the hydraulic variables and mechanical variables
for the actuator (hydraulic rotary motor or hydraulic linear cylinder) are,
w = Q∕D or V = Q∕A (7.259)
m c
T =ΔP ⋅ D or F =ΔP ⋅ A (7.260)
L m L c
where ΔP is the pressure difference between the two sides of the actuator (two sides of the
L
cylinder), D is the hydraulic motor displacement (volumetric displacement per revolution),
m
A is the cross-sectional area of the cylinder (assuming it is a symmetric cylinder).
c
When the load force (or torque) is specified, the total force output from the actuator
should include the force to be exerted (F ) on the external load (i.e., pressing force in
ext
a press or testing application), plus the force needed to accelerate the total moving mass
(cylinder piston, rod, and load inertia, m ⋅ ̈ x), and finally some force needed to overcome
friction, (F ),
f
F = F = m ⋅ ̈ x + F + F ext (7.261)
L
f