Specifications

1.Power Units
Pumps:
Constant supply pressure is preferred with minimum
variation. Use accumulators with variable displacement
pressure compensated pumps. Fixed displacement pump:
constant pressure with use of accumulator is an option.
– If more than one critical system is fed from one pump,
isolate each system with check valves and accumulators
(avoids cross-talk).
– Reservoir breather: 3 to 5 micron air filter preferred
with capacity appropriate to fluid displacement.
– Temperature and pressure should be closely controlled
if good long term control accuracy is critical.
– Fluid flowing over a relief valve represents wasted energy.
2.Piping and Fittings
Do not use pipe dope. (It contains fine, hard to filter, particulate.)
Use TFE tape when necessary. Do not use pipe or pipe fittings.
– Use only correct tube cutting tools, no hacksaw.
Deburr if necessary.
– Cold bending preferred.
– Descale after hot bending and welding.
Rotating joints can generate contamination.
– Flexible lines: if unavoidable use teflon, nylon or
thermoplastic lined hoses rather than rubber (neoprene)
which eventually shed particles. Place flex lines before filter,
not after.
– Use O-ring fittings rather than tapered pipe type. If pipe
fittings cannot be avoided, use Teflon tape.
3. Filtration
The Moog filtration philosophy is summarized as follows:
– Use a 10 to 15 micron absolute non-bypass high pressure
filter just before the Servo or Proportional Valve.
– Use a 3 micron low pressure filter in the return line,
if possible.
– Use a 3 to 5 micron low pressure filter in an off-line
filtration loop.
– Recirculate oil in reservoir more than 5 times per hour.
This is justified on the bias that:
(i) The Servo or Proportional Valve can accept the odd
particle up to 25 microns.
(ii) It is neither practical nor economical to try to clean
the oil with a small, relatively expensive, high pressure
element.The cheaper, low pressure element is many
times larger and has the potential to filter continually
and under more ideal conditions. (Steady flow and
lower velocities increase filtration efficiency.)
– In the case where large changes of oil volume in the
reservoir occur, as with a single ended hydraulic cylinder,
it is suggested that a 3 micron low pressure element be
used as an air breather.
– Always use dirt alarms/pressure switches to enable
changing of elements at correct intervals.
– Use cheaper low-pressure flushing elements to flush the
system on start-up – remember that new oil is “dirty oil,”
having picked up contaminant in transit and packaging.
– The tank volume should be flushed through the filter at
least 50 times, changing the element when indicated by the
pressure switch (contaminate alarm), or until the system
has operated 6 to 8 hours without the need for a flushing
element change.
4. Servo and Proportional Valve –
Characteristics of Major Importance:
– Frequency response (time constant)
– Threshold (resolution)/hysteresis
4.1 Placement:
– Mount as near as possible to the actuator to reduce
the entrapped oil volume. Oil is compressible and can
often limit servo response.
– Flexible lines between valve and actuator can be
rarely justified. As a rule of thumb they decrease
stiffness to one-third of the volume that they contain.
Additionally, they produce contamination which must
pass through the valve. Use only nylon, teflon or
thermoplastic lined hose.
4.2 Sizing:
– Select the valve size to obtain between 1/4 and
1/3 system pressure (P
S
) drop across the valve at
maximum velocity. If the drop across the valve is too
small, then a flow change will not take place until the
valve is nearly closed.
– Remember: to control flow the valve must drop
pressure across itself. Too large a valve is a waste, or
worse than that, it lowers system resolution.
PRACTICAL CONSIDERATIONS WHEN LAYING OUT
ELECTROHYDRAULIC CONTROL SYSTEMS
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