The following article is the second part of the "finding zero" series.
First of all, let us recapitulate part one.
- Closed loop pumps null adjustment is important
- The easiest way to check correct null position is through comparing work-lines
pressures with the machine stopped and brakes applied (alternatively -
pump lines closed on a test rig)
- Closed loop pumps, that have proportional displacement feedback controls, have two "zeroing" systems - mechanical and hydraulic
- The best way to adjust these systems is one at a time - first mechanical, then hydraulic
- To eliminate influence of the hydraulic zero and adjust the mechanical zero you must by-pass the servo-cylinders
- Next time you ask your boss for a raise, blink an eye and casually
remind him of the "boxter incident", then wish him good luck with his
new sports car...(don't know what I'm talking about? Read part 1 of the series for a guaranteed raise!!!)
Now - an imaginary example: You are asked to
"take a look" at the closed loop propelled machine,
that just can't stay put. First thing you do is check the type of pump
and control, and find that it's electric proportional feedback displacement control.
Then you either mount two pressure gauges in both sides of the loop and
apply the parking brake, or lift the machine off the ground (I didn't
mention this technique in the first part, but, despite all the work it
might give, it is
the safest way, as you can tweak the transmission all you like without
the risk of passing over someone's foot). Then you install the by-pass
valve in the servo-cylinder test ports using appropriate fittings. The machine
is heated to a normal working temperature and is at high idle. You
activate (open) the by-pass and adjust the mechanical zero by turning
the adjusting screw till the wheels stop (pressures level out), then
you turn the screw in one direction and notice its position when the
wheels start to move, then you turn it in the opposite direction and
again notice its position when the wheels start to rotate in the
opposite direction, then you lock it in the middle position. Then you
close the by-pass and make sure the control is getting no signal
(or simply remove the control signal connector).
Then you adjust the hydraulic zero, trying again to lock
the adjustment in the middle of the deadband. Then you
reconnect the signal wire and check if the null position remains
unchanged after stroking the pump several times in both directions. And
- you're done!
If you can't imagine what a by-pass connection looks like take a look at this picture and this short video.
OK then, now we know how to adjust correct null position
of hydrostatic transmission pumps equipped with proportional
displacement controls, but so far we have been looking into pumps, that
have only one
mechanical null adjustment. In such designs, the mechanical freeplay of
the pre-tensioned springs is next to none and is either adjusted during
the assembly (like in this example, older A4V pumps) or isn't adjustable at all (example,
newer models). Such servo-mechanisms, when assembled or machined
correctly, have practically zero freeplay to cause unstable null
condition, and the spring pre-tension creates servo-pressure deadband,
defining the minimum servo-pressure necessary to make the
servo-cylinder and the swash-plate move. This condition is a guarantee
that even when there are small servo-pressure fluctuations, caused by
the hydraulic positioning system, the swashplate will remain in fixed
position.
However, there are servo-mechanisms, that use
threaded servo-cylinders to position the servo-piston and the attached
pre-tensioned springs. Such designs, when adjusted incorrectly, can
allow for a certain swashplate freeplay. For example, in this type of servo-mechanism, or this
(Sauer Danfoss H1 series), one can unscrew the servo-cylinders and the
swashplate will no longer be held in place between the two
pre-tensioned springs, becoming loose. The hydraulic null will
still hold the swashplate in neutral position, but it will be far less
stable (drifting null), especially if the hydraulic positioning system
has wear, as there will be no servo-pressure deadband. Furthermore,
incorrect position of the servo-pistons can cause incorrect
displacement values, especially when the servo-pistons are too much in.
The correct servo-cylinder position for such systems is when both the cylinders are touching the pre-tensioned springs with the swashplate in neutral
position. Neither too much in, nor too much out - just touching - zero
freeplay, servo-pressure deadband defined by the springs pre-tension,
ideal null conditions created - the only thing to adjust now would be
the hydraulic null.
Obviously, the best way to adjust the correct servo-piston position in
such pumps is doing it during the assembly and may require special
tools, but what if a mistake was made during the assembly,
or, which is normally the case, someone had tampered with the
adjustments? Using the by-pass technique will be useless, as the
freeplay makes the hydraulic null system the only one responsible
for positioning the swashplate, and even when we by-pass it, the
swashplate still remains in neutral due to the centering forces of
the rotary group - pistons acting on the swashplate keep it in
neutral position.
The following procedure is what I
usually use in such cases. First of all, when I suspect that
servo-cylinders were tampered with, I never know whether they
were screwed in or screwed our, so I unscrew both of the servo-cylinders, leaving intentionally the servo-piston loose. Then
I block the pump's outlets by closing the restrictor (on a test bench)
or applying brakes (in the field). At this point the swash plate
position is defined by the hydraulic null. Then, if the null is
off-center (which I can see by the unequal lines pressures), I bring it
back to center. Now the fun part begins, because people, who
aren't familiar with this technique, get quite
puzzled on seeing the adjustment procedure. The next thing I do is turn the hydraulic null adjustment in
one direction to the point when it causes the pressure in one of the
lines to rise slightly. Just a little bit, maybe 10 bars or so, only enough to make sure the swashplate is starting
to tilt. When possible, it's useful to read servo-pressures at this
point.
The servo pressure differential created by the intentional off-center adjustment must be minimal. Then I screw-in the opposite to the slightly pressurized
servo-cylinder till the loop sides' pressures level out and lock the
cylinder in this position. The small servo-pressure differential
guarantees that the pre-tensioned spring touches the
servo-cylinder, but the pressure is not enough to compress the
pre-tensioned spring, which means that I bring the swashplate to
neutral by pushing on the pre-tensioned spring without compressing it
and thus bring the servo-cylinder to the desired position - touching
the servo-piston, with the swasplate in neutral position! Next - I do
the same to the other side. (Actually, the other side is a little
bit easier to adjust, as the swashplate is already in neutral, and
one of the cylinders is locked in the correct position. All I
have to do is to touch the servo-piston spring with the other
cylinder, which is easy to feel - the moment it hits the
pre-tensioned spring is indicated by the increase in torque). That's it!
So very easy. Still in doubt? Check this diagram.
If you do this once, you'll learn it for life.
By the way - the necessary servo-pressure differential can
alternatively be created by supplying the pump with a minimum input
signal, which can easily be done at a test bench but, in most cases,
is impossible to perform in the field.
The most important thing is to understand
how the above described technique works, rather than blindly follow the
instructions.
Well, that is it for this part. There's still
a couple of worth mentioning closed loop null adjustment questions,
which will be discussed in part 3, but even at this point I am dead
certain that IH readers will never let their machinery "escape on its
own!"
