Tuning early model electrohydraulic control systems On machines with
proportional control flow and or pressure there is a relationship between the
operator controls and the hydraulic system. The most common problem seen in the
field is the lack of linearity between these two controls being at minimum or
maximum and the hydraulic responding accordingly.
For simplicity sake we will cover one of the work horses of the industry the
“Cincinnati Milacron T-Line Toggle with a PC3 control”. This press is no
longer in production due to the improvement of electrohydraulic control systems
and the introduction of Milacron’s VISTA line and newer style of machines. But
because of the number of them in the field and the presses that were similar to
them we will cover them here.
We will cover here the pressure control. In your prints you should find a sheet
detailing the pots that control the “zero” and the “gain”. These must be
set before any other adjustments can be made. It is necessary to realize the
zero and gain will effect the minimum and maximum of all pressure related
functions that are controlled by the valve driver in question. There are however
two distinct differences that need to be realized, those are “on board” or
factory set adjustments and settings that are adjustable by the setup personnel
which are typically Digital Thumb Wheel (DTW) and / or rotary or slide
potentiometer adjustments that give a feed back signal to the controller
proportional to the amount that we want the sequence to operate at. For example
the high injection pressure and high injection speed are adjustable by the setup
personnel. And while the low injection pressure is adjustable by setup personnel
the low injection speed is not. Instead it is adjustable only on the valve
driver board on a factory set adjustment since the part is by theory
volumetrically filled by high injection and only packed out to obtain the
correct weight or dimensional requirement by low injection (yes some machines
use up to 10 states of pack and 10 states of hold pressure but these are all
typically at the same volume since the part is by theory filled out). We may go
further into proper part filling and or machine cycles if the readers of this
article wish. Give us some feed back as to what you would like to see in future
articles. Well lets push on.
With the injection screw in the forward position, the DTW settings at maximum,
and a gauge monitoring the injection pressure press the injection forward switch
and record the current settings of the machine. (On some machines you may have
to move the high injection override switch out of the way to prevent the machine
form going to low injection pre sets.) For example if you show maximum pressure
at 70% of the DTW setting and anything above the 70% setting maintains the same
pressure reading, and below 20% of the DTW setting you stay as low as the
pressure will go you only have adjustment over a 50 point range. This means that
with 2000 PSI as the nominal you will get a 40 psi differential for every click
of the DTW versus an ideal setting of the zero and gain where the differential
sees a 2000 PSI range over a 99 point range which equates to a 20.2 PSI per
click of the DTW. It is important to record the original differential so the
setup personnel can know what effect their adjustments will have on the machine.
MAKING THE ADJUSTMENTS
To set the pressure valve driver that controls the injection end of the machine
it is best to use the HIGH INJECTION DTW settings. Place the machine in a manual
NOTE : It is very important to follow all factory safety recommendations in
operating your equipment. Protective gear when purging ect.........
With the screw bottomed out you should read maximum injection pressure on the
gauge. If you don’t see the required pressure begin turning in the gain
adjustment until the pressure is obtained. If you cannot obtain it you will need
to determine why? For example worn pump, weak reliefs, ect.......We may cover
these in later articles if needed.
If you are already at or close to the recommended setting you will need to
reduce the gain until the pressure just starts to drop. Now go to the a 0 DTW
setting. You should see as low a pressure as obtainable. This is often not zero
but more likely 50 to 75 psi. Zero would be nice but unobtainable with many
older systems. If you can get to 100 Psi at zero call it good and go on unless
you absolutely have to have very low pressure on injection (below 100 psi). As
you could guess if your pressure is high trim the zero pot down, If it is low
trim it higher. If it is where you think it should be you still need to take it
one way or another to ensure it is just starting to increase pressure as you
increase milliamps to the valve itself. As an example a Vickers CGE 02 or CGE 06
is operated at maximum of 400 milliamps. It usually reaches its maximum at about
385 milliamps. Also it only starts to close the metering orifice off at about 65
or 70 milliamps. (This can be measured by taking one of the two wires going to
the valve loose and putting a Digital Volt Meter or DVM in series and dialed in
to read 0 to at least 400 milliamps). If you are reading say 2000 psi but your
valve driver has an output of 420 milliamps you can reduce the setting of the
DTW by probably 10 or 15 clicks before you will see any change in the actual
pressure. If you instead you dial the gain into 385 milliamps or so at 2000 PSI
you should see an immediate reduction in pressure in proportion to the amount
the DTW is reduced. Hopefully it will be linear. If it is not exact do not be
greatly concerned. Remember the goal is to be able to adjust the machine
“some” with every click of the DTW. Of all the machines I have seen in the
field very few have had much more than 80 clicks for the entire adjustment range
of 2000 PSI.
When you adjust one be sure to check the other since they are almost always
proportional to one another. Once you have the GAIN and ZERO set we can go on to
the settings that are factory set knowing that any adjustment made by the DTWs
that are controlled by the valve driver we have just tuned will have a minimum
value equal to X and a maximum value equal to Z. And if we have done our job
well it will be adjustable with every click of the DTWs proportional to the
maximum pressure obtainable divided by the number of clicks on the DTW. For
example 2000 psi divided by 99 clicks equals 20.2 psi per adjustment increment.
You can now move on to the factory settings and dial them in per print.
Typically you will have the following settings as a general rule of thumb.
After finishing with the pressure you can move on
to the speeds. It should be noted that the Vickers CGE has a pre filter that
should be checked for cleanliness before setting the ranges.
Also the Dynex valve used for speed control on the earlier T-Lines (PJ-5000
series) has a prefilter in it by the force motor.
On a final note be sure to record all before and after settings, date, time, and
technician doing the work. This extremely useful when troubleshooting in the
future. Good record keeping is essential to long term preventive maintenance.