This post is about Kawasaki K3V/K5V series displacement controls - more precisely about how to transform a common negative displacement control (pilot pressure up - displacement down) into a positive displacement control (pilot pressure up - displacement up).
Kawasaki K3V series is an old passion of mine. If I
was asked to choose the best open circuit pump in the world - my
subconscious would definitely make me say Kawasaki K3V before my
subjective brain would have any chance to think it over. The reason for
this is obvious to everybody who worked with these pumps - they are
super-robust, which makes them last, and last, and last... Another good
reason is economic - original and after-market spares are readily
available everywhere, even complete units can be easily and relatively
cheaply bought on the market without delays.
I like seeing (and using) these pump in industrial
applications - when you create a powerpack that uses this pump you get
two things - one - you get a machine with pump that never breaks, and
two - when it breaks - you have all the necessary parts or a complete
unit available virtually "on every corner". The perfect union of
technology and business in one double pump package.
Most of the pumps sold on the "white market" have negative
displacement controls (coupled, of course, with this or that variation
of torque limiter), which is due to the fact that these modules were
designed to work in excavators with excess flow orifice controls
Japanese brands like so much. But what if you need a positive flow
control? Is there a need to order a new displacement control? The
answer is no. A negative displacement control can be easily transformed
into a positive one, and here is how:
If you look at the control's design,
you'll see that its function is based on one main spool, which directly
controls pressure at the larger side of the servo-cylinder (the smaller
one is always connected to the servo-pressure feed), by connecting it
either to tank or to the servo-pressure source. The swashplate position
feedback link is connected to the main spool on one side, and to the
servo-piston on the other side. The two control levers act on the
feedback link through connecting pins and by this a closed loop control
is achieved, because moving the feedback link causes the main spool to
move as well. The levers are connected to pilot spools in such a way
that one of them provides the displacement control, and the other -
torque limiting function.
Now let's look closer at the displacement control
link - it has a fixed pivot point, which is a pin connected to one of
the two cylindrical inserts. On older pumps the cylinder part with the pin used to have a tapered thread entrance,
while the other one (the one that didn't have a pin) was flat. On newer
models, or copies, this feature seems to have vanished, which is sad,
because one glance at the control could give you an instant idea about
its arrangement. It is the position of this pivot point in relation to
the control spool (which moves always in the same direction when acted
upon by the pilot pressure) that defines whether the main spool will
move left or right, or, in other words, whether the displacement will
increase or decrease!
So, there you have it - to change the pump's displacement
control from negative to positive all you need to do is to move the
pivot point. Again - older original pumps had the levers already
prepared for the transformation, with both pivot holes in them. Newer
models (or copies) often have only one hole (here
you can see the original Kawasaki lever next to the one from a decent
quality copy), in this case you'll have to drill your own hole, which
is perfectly doable. The link is hardened, so you might want to use a
carbide bit for this, for a K3V112 size the distance between the
centers of the holes is 18 mm. On this picture
you can see the same lever already machined - the hole skewed a little
to the right, but the test showed the control functioning 100 percent
OK, the only thing that had to be adjusted was the control threshold
adjustment.
Now you know what to do when you want your K3V with
a positive control, and only have negative controls in stock.
Oh yes, one more thing, now you can (when you're
lucky to bump into one of them original older K3Vs) have a quick glance
at the control module (and note where the cylinder piece with taper
is), and casually drop - "this is clearly a _____ (negative or
positive) displacement control..." - to the astonishment and
amazement of the less hydraulic-controls-conscient standers-by...
