(reveiw) Hydraulic System with tandem center solinoid for control
hi i am fairly new to the hydraulic business and need to create a
hydraulic lift that has warm up loop that also raises the lift at the
speed i choose based on the power given to the pump. and lowers slowly
with a regulator.
i made a schematic of the way i think it is supposed to be done.
First off the flow control will be better between the direcional valve and the cylinder.
Secondly,
are you sure about the transitional flow paths of the directional
valve. I am not sure that there are valves that block all ports as the
move from the normal position to the working position.
Because I am lazy and because it will help you to understand I suggest you research the following...
1) Counterbalance valves
2) Overcentre valves
3) Pilot operated check valves
All
of the aboove will hold the load while the directional valve moves. The
spool in the directional valve will allow oil from the cylinder to
escape to tank as the it moves and the cylinder will drop slightly and
there will be a noticeable bang as the valve moves. Any of the above
valves will do what you need, but with varying degrees of success.
As
you have drawn the system, any pressure generated at the flow control
will be seen at the directional valve and the leakage across the spool
could mean that the cylinder extends during the warm up.
The
directional valve would be better with the P line blocked and A & B
lines connected to T in the normal position. This will keep stop
pressure build up in the cylinder during the warm up.
The pump
could then be run against a releif valve or orifice to warm the oil. The
size of the orifce can be calculated as...(metric units)
Flow(LPM)/Area(mm^2)x 1.44 = square of the pressure drop(BAR)
The answer squared is the pressure drop in BAR.
The temperature differential(deg's C) across the valve/orifice is equal to...
(5.7 x Pressure Drop)/100
It
is worth noting that warm up cycles only warm the reservoir oil. The
oil in the line and the cylinder will remain cold until it reaches the
reservoir. When the warm oil is pumped out to the cylinder it will cool
rapidly unless the the oil is worked hard to keep the temperature up.
I hope that helps a little.
Throttle Valves are commonly called Flow Controls when they have a
Bypass Check Valve and Needle Valves when they control Flow in both
directions are unnecessary in your circuit unless there is a need for
variable speed. Even then I would use a Variable Volume Pump and set
speed by changing pump flow. A lot less energy waste and woiuld
eliminate the need for a Heat Exchanger.
Also, you seem to
indicate the cylinder only needs the 750# of force for lifting so a
Regeneration circuit would set speed nearly the same in both directionsa
with the same flow.
Anytime I am asked to design a Fluid Power
Circuit I ask a lot of questions up front so the simplest, least energy
wasting and less expensive circuit can be designed.
You can see
some questions I always ask for a simple circuit design situation in my
basic book in the "Formula" Section on pages 14-15. Pages 16-17 are for
sizing a Pneumatic circuit.
These two sizing examples are not
all inclusive but give a general way to approach a given situation. With
comtinued experiennce and a full understanding of the components
available you can do some real energy saving circuits.
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