Tech Briefs Magazine - October 2021 - MD-8

For example, there are valves
operated by proportional solenoids
that do not internally
close the loop around the
spool. They are used for
acceleration and metering
applications.
flow code) allow system designers to
achieve regenerative function without
the need to add additional valves
to the circuit.
Other valves close the loop in -
ternally around the position of
the spool but can be integrated
into a closed-loop system. The error
in the system is measured by a transducer;
i.e. a position or speed sensor on an
actuator, a pressure transducer, or a flow
meter for even greater accuracy. Elec -
trohydraulic servo valves operating in
closed-loop control systems are designed
to use low power and mechanical feedback
to provide precise control.
Parameters that must be considered
when specifying open and closed-loop
control systems include:
* Hysteresis: The difference in measured
output between increasing and
decreasing command.
* Step response: The time required from
initial command to when the valve stabilizes
at the desired output.
* Frequency response: The maximum
speed at which a valve can operate with
accuracy.
* Internal leakage: Bypass flow inherent
to spool valves due to mechanical
clearances.
* Flow capacity: The amount of fluid
that can pass through the valve.
4. Sizing Spools
Proportional valve spools are typically
rated for a nominal flow at a differential
pressure of 10 bar, while servo
valve spools are typically rated for a
nominal flow at a differential pressure
of 70 bar. Equal metering spools provide
symmetrical flow to each work
port. This can be useful when driving a
motor or a double-rod cylinder with
equal effective areas. Equal metering
spools will result in reduced speed during
retracting of a single-rod cylinder
due to the differential area between
the rod and the piston.
Ratio spools provide asymmetrical
flow between the work ports; the most
commonly used is a 2:1 ratio design.
When used to drive a 2:1 ratio cylinder,
for example, equal speed will be
achieved between extension and re -
traction of the cylinder due to the
matching of the imbalanced areas. It is
8
Cov
ToC
The VW Series directional control valves are
on/off direct-operated hydraulic valves.
recommended to size proportional
valves as small as possible to control
the load. To maintain control, back
pressure against the load must be
exerted at all times. A general rule of
thumb is to select a spool that will use
90-95 percent of the maximum flow
rating. Selecting a spool with too large
of a flow capacity can result in instability
of the system.
5. Sealing Compounds
When selecting the compound for
the elastomeric seals in any directional
control valve, consult the manufacturer's
resources for fluid and compound
compatibility information. Standard
industrial applications using mineral oil
will typically use a nitrile seal, which is
also recommended when controlling
water-glycol.
Applications involving elevated temperatures
or less commonly used fluids
may utilize one of many grades of fluorocarbon
seal. When in doubt, consult
the factory for assistance in selecting a
sealing compound.
6. Regenerative Circuits and
Hybrid Functions
A regenerative circuit routes fluid
evacuated from the rod end of a cylinder
back to the piston end instead of to
the tank for accelerated extension of
the actuator. The use of a regenerative
circuit can allow a system designer to
use a smaller pump to achieve design
requirements when rapid movement is
needed in only one direction. Regenerative
directional control valves for
on/off and proportional control (R
Hybrid regenerative valves offer
system designers the ability to enable
regenerative control through a separate
electrical signal than the command
signal. When utilizing re -
generative control, force is
sacrificed for speed. The hy -
brid function (Z flow code)
enables designers to select
between the standard hy -
draulic function to build
force and regenerative function
to accelerate the load
quickly.
7. Mounting Patterns
Mounting configurations for electrohydraulic
valves are guided by NFPA/
ISO standards - D03, D05, D07, D08,
and D10 series designations will indicate
compliance with the standards.
Pilot-operated valves are more stable
across a wide range of flows and enable
systems to operate with greater flow
capacity. Often, the hydraulic pilot
pressure used to control the main stage
spool provides greater force than that
of a solenoid on a directional drive
valve, resulting in a more predictable
performance for the user.
Conclusion
There are many resources available to
design engineers when specifying components
for systems: reference sheets,
calculators, configuration tools, and
more. However, there is no substitute
for experience and deep application
knowledge.
Take advantage of the " been there
and seen that " experience that your supplier's
applications engineers can offer.
Not only do they understand how the
components their company offers perform
but they also have the benefit of
having helped others in the same situation
correct design mistakes and they
have problem-solved for some of the
most unique applications.
This article was written by Matthew Davis,
product sales manager; Mitch Eichler, applications
engineer; and Tom Gimben, product
sales manager at Parker Hannifin Corp.,
Hydraulic Valve Division, Elyria, OH. For
more information, visit http://info.hotims.
com/79418-283.
Motion Design, October 2021
http://info.hotims.com/79418-283 http://info.hotims.com/79418-936
Tech Briefs Magazine - October 2021

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