Well, here I am again, talking about the so very important matter of
adjusting correct neutral swashplate position in closed circuit pumps.
This third article concludes the "finding zero" series, so if you
hadn't read the first two articles, you should do so before reading
this one.
Although nailing "zero" position of the swash plate
is not a hard task, you must understand that there is no such thing as
a "universal adjusting technique". Steps you take to correctly
adjust "the neutral" will vary greatly with the pump's control type and
servo system design. You must understand in detail the
function of the control module used, and apply a good share of common
sense before going into any adjustments, bearing in mind that there are
situations when a correct null adjustment is impossible without
disassembling the pump and/or replacing parts. And if you come across a
pump or a control you are not familiar with, your best strategy is to
look up technical info before performing any adjustments, at
least when the pump is mounted on a machine (if you have it in your
workshop, mounted on your test bench, and time isn't an issue, you may
indulge yourself into tampering with them adjusting screws and backengineering, just
don't forget - you broke it - you bought it!)
The key words are - knowing of operation and common sense!
For example, in pumps that have only one centering system, namely
mechanical (like Rexroth A4VG pumps with DA control),
there will be no need to install an
external servo-pressure bypass, as the control directional vale does it
for you when the solenoids have no signal current. In such systems the
adjustment is dead straightforward - there is only one adjusting screw
to adjust, although in some cases you might want to check the
servo-pressures to see if the control directional valve is successfully bypassing
the servo-cylinder. Let's not forget that there are controls which
pressurize servo-cylinder sides, like the MS control,
or that roportional non feedbak displacement controls
have no hydraulic zero as well... Then there are pumps that have no centering systems at
all (e.g. special HP pumps, which are now Bondioli & Pavesi, with
free-floating servo-cylinder), and, obviously, when someone asks you to
adjust the null in one of those, you should tell him to go take a hike.
Adjustment procedures ("by-the-book" ones)
will have predictable results only in case of pumps that have no or
little wear. However, a technician should be ready to face a pump which
is, let us say, "exhausted yet still kicking", marked by a significant
play in the centering mechanisms, often both mechanic and hydraulic. In
such cases, when an overhaul is not an option and the machine's
operation is indisputably necessary, a technician should know that,
most probably, the null will remain drifting, and the best thing that
can be achieved is a close approximation to the so wanted zero, with
the machine still (occasionally) running away... In such cases there isn't much you can do but
explain to your client all the benefits of the
drifting null condition, and also that you won't be held responsible for
whatever the damage the machine might (will) cause in the future.
In the first part I mentioned that I didn't like the
"flow symmetricity" null adjusting technique. I still do not like it,
but I must admit that in some cases it is the only technique
to adjust the null correctly. As a rule, it happens with
severely worn hydraulic positioning systems, which is often the case of
small medium duty closed loop pumps, like Sauer series 40
with manual displacement controls. When these get worn out, there is
a HUGE play of the control lever between the centering springs,
and in this case there's no use in looking for the right position of
the adjustment bracket to equal the lines pressures - the null will
drift because the control lever and the attached control spool are
loose. In this case, your best reference will be the maximum flow, and
you will have to lock the bracket in a position, where the pump will
present equal maximum flows in both directions, and forget about the
exact null (sad but true...). In most cases this is not a big problem,
actually, because on the actual machine the control joysticks, that are
attached to the pump's control lever, are centered by their own set of
springs, thus "overriding" whichever null position play the control
might have.
As you can see, hydrostatic transmission null
adjustment involves various distinctive techniques, but some previous study
and a little bit of sound judgment makes the job a hundred times
easier. Watch you step, though! Most of the times these procedures
are simple, yet they should never be applied without complete understanding of the theoretical part behind the pump's operation!
I TRULY hope that this short series made the sensitive null adjustment matter a little bit clear-er..
Should you have any questions, please, do not hesitate to ask.
