Home>> Back-Engineer This!>> K3V112 - Transforming Negative Displacement Control Into Positive>>
InsaneHydraulics - Sergiy Sydorenko 2009-2011 All Ridghts Reserved
The Simple Test
The Most Basic Basics
Let's Talk Hydraulics
Back-Engineer This!
News Archive
HomeIntroductionThe Simple TestBack-Engineering?The Most Basic BasicsLet's Talk HydraulicsBack-Engineer This!BattlefieldKaboomLibraryNews ArchiveBla-BlaimerContactsGuestbook |
    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...
K3V technical information
Kawasaki K3V displacement control schematics
Kawasaki K3V displacement control
Kawasaki K3V displacement control pivot pin
Kawasaki K3V displacement control pivot pin
Kawasaki K3V displacement control, main spool link
Main spool link - before machining
Main spool link - after machining
Machined link in place