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     From my point of view, the most beautiful and perfect hydraulic circuit have been and will always be a closed loop hydrostatic transmission. Simple, at first glance, and yet so complicated, when you look into it... Today, hydrostatic transmission is, probably, the best if not the only solution, when a infinitely controlled high force two-way motion is required to be delivered in a compact package, especially when large amounts of power are involved, with the most common application being industrial vehicle propulsion.

     Although hydrostatic transmissions are no news, and there are many mechanics out there advertising extreme readiness to tweak your closed loop on your first demand, I still see many of those pumps come to my shop off-centered. And, to top it up, I often get to see intuitive, yet incredibly nonsensical techniques used to "find the damned zero", showing complete misunderstanding of how this or that swashplate servo-mechanism works and what must be done to "make it stop!" The following series is, therefore, my timid attempt to approach the very important question of adjusting neutral swashplate position in closed loop pumps.

     Obviously, this is a very important adjustment, as nobody likes heavy machinery moving on its own, unless, of course, we're talking about a 30 ton forest crane with nobody inside the cabin, slowly crawling over the new boxter your boss bought, right after telling you that "these are tuff times and the raise will have to wait...", giving you the perfect opportunity to blame the cruel hydrostatic malfunction with an expression of utter sorrow and condolence on your face, while doing your very best not to giggle in his face... (Gone a little bit off-topic here...)

     I must warn you that extreme care must be taken when dealing with hydrostatic transmission neutral position adjustments. If you are new to this, you should NOT do it. Although setting your machine in uncontrolled motion can be fun to watch, it most surely won't be fun to be responsible for, so, if you're not sure whether you are up for the task, call someone who is. And read the disclaimer once again.

    Now that we're done with the safety drilling, let's go ahead and talk about them transmissions.

     Let's ask ourselves - where do the "crooked" transmissions come from? Well, the sad truth about it is the fact, that most closed loop pumps don't go off center "by themselves", but are the result of "craftsmen intervention", normally performed "to improve something". The pumps that do go off-center by themselves either were left with adjustment screws unlocked, or have gained a whole lot of internal clearances due to wear, and, in the latter case, aren't null-adjustable any more...  By what I've seen so far, the most common reason for this or that hydrostatic transmission present the "drifting null" symptom, is the previously performed overhaul, because almost always disassembling and reassembling a pump shifts the correct null, and then the mechanic simply skips/screws up the adjusting procedure after/during the mounting process.

     Before going into any adjusting techniques we must define which is the best way to check if the null swashplate position is set correctly. I personally consider the best and easiest way to be the comparison of the work-lines' pressures, which, with the machine stopped (needle valve closed on a test bench) shouldn't differ more than a couple of bars. The null check up and adjustments should be performed at normal system working temperature and at high idle. It is very important to make sure that the pump goes back to zero pressure differential after stroking and achieving maximum system pressures in both directions, and takes the same amount of time to destroke in both directions. It is also important to verify control sensitivity - the response to the input signal should be symmetrical for both sides, but this is not imperative and depends greatly on the type of control applied. In case of closed loop pumps driven by noiseless electric motors you can actually hear the pump go off-center, something an experienced mechanic can use as a reference, but checking those two gauges is still necessary.

     It is also possible to check if the pump is off-null by reading the charge pressure. This technique can only be used on a closed loop circuits equipped with loop flushing system. Any unwanted pressure differential will trigger the loop flushing spool, which will cause the charge pressure to drop. The advantage of this method is the use of only one pressure gauge and, if you use an analogical needle type gauge, the pressure drop can be seen from a distance. However this technique will NOT work for all circuits, as the charge pressure drop, depending on the type of valves used in the flushing block, can be hardly noticeable.

    There's also the flow measuring technique. I don't like it and  don't use it, but I have seen some mechanics stick to it, that's why I am describing it here. The method is used during bench test adjustments , and its essence is to adjust a pump in a way that the flow is the same in both directions. It might seem as a good technique, as the same flow is supposed to mean the same swahplate angle, but practice often shows poor final results. The main reason for it is the fact that in most situations the flow metering system of a test bench will present unequal accuracy when measuring forward and reverse flow. Then, the mechanical clearances in the swashpalte servo-mechanism often allow for certain assymetry of the swashplate maximum angles. Therefore, when comparing forward and reverse flows, a certain margin of flow reading error will be present. Even on quality test benches the flow reading difference can go as high as 5 percent. In such cases bringing the two flows to the same value equals creating the floating zero condition on purpose. Some might argue that advantage of such adjustment would be the same operation speed in both directions, but what is more important? Making sure that your loader has top speed of exactly 28.5 km per hour in both directions, although sometimes "creeps" a little? Or has top speeds of 30 km/h forward and 28km/h reverse, and hasn't got a "mind of its own"? Flow comparison is necessary, (especially for certain types of servo mechanisms, which will be discussed further) but finding reasonable flow asymmetry is normal in all closed loop pumps, that is why it can not be used as the main point of reference for null adjustment.

      Now that we know that the best way to check the correct neutral swashplate position is the lines pressure comparison, let us look closely into typical closed loop controls and servo-mechanism designs, and the best ways to null them.

     The most common type of a closed loop pump control (and the one that gives most "zeroing troubles") is the proportional displacement control (hydraulic or electric). The main reason for that is the fact that in most closed loop pumps, that use this type of controls, there are two independent systems responsible for the swashplate positioning - mechanic (springs) and hydraulic (servo pressures).

    Mechanic positioning is done by means of one spring (example), two or more springs using the same design principle (example), two independent separated pre-loaded springs (example, feel the nostalgia),  or one or two pre-loaded springs, constructively placed inside the servo-piston (example, used on Danfoss H1 series), which, mechanically, is still the same separate spring design only much more compact.

    Hydraulic positioning is done by the proportional displacement control, defining the position of the swashplate by means of controlling the servo-pressures.

    Both of the systems have deadbands, and when they don't overlap, an unstable null condition is created, because the hydraulic positioning system overrides the mechanic thus forcing the springs, but, like any hydraulic (closed loop feedback) positioning system, it is subject to small fluctuations (originated by a number of things, like charge pressure deviations, shaft rotation speed, vibrations, case pressure fluctuations, etc..) which  result in unwanted swashplate movements and, consequently, cause the null to drift. The only way to create a stable neutral is to make sure that both mechanical and hydraulic positioning deadbands overlap symmetrically.

    This can ONLY be achieved by adjusting one system at a time and ONLY in the following order -  first - the mechanical zero, second - the hydraulic zero. If a proper tooling is available, correct mechanical zero can be adjusted during the assembly, if not, you will have to do it during testing. The most common course of action would be to eliminate the action of the hydraulic  positioning system by by-passing the servo-cylinders (the by-pass hose should be as short as possible to minimize the pressure drop. I use a ball valve in the middle, to conveniently turn the by-pass on and off) and adjusting the mechanical zero first. Then, after the mechanical zero is adjusted (note that the best way is to always look for the middle position of the adjusting screw within the deadband, not just the first position when both of the outlet pressures level out), you close the by-pass and adjust the hydraulic zero, again looking for the middle position within the deadband. Skipping this procedure, and trying to adjust the neutral position with both positioning systems active gives poor results, and resembles trying to win a lottery - highly improbable (although not impossible).

     Note that for some pumps adjusting the correct mechanic neutral position of the swashplate will be something more than simply turning an adjustment screw. For Sauer Danfoss series 90 pumps, for example, the mechanic zero is defined by the position of the round lateral cap guiding the centering T-bar (drawing), which has small rotational play. To adjust it you must loosen (just a little) the six holding screws and then put it in the correct position by hitting on the machined pit.

     Again, some series 90 pumps use an extra spring to hold the swashplate in neutral (like this). This design is used to create stronger forces in the servo mechanism. The ONLY way to precisely adjust the correct mechanical zero in such pumps would be adjusting it during the assembly process.

     The described technique of servo-cylinder by-pass works well on closed loop pumps that have spring centering systems with fixed swashplate free play (usually insignificant, existing only due to small clearances of the servo-mechanism). But what about servo systems that can allow, when set incorrectly, a significant swashplate free play. For example, on pumps that use this servo-mechanism design, or this, a "fiddler" can unscrew the servo-cylinders and leave the swashplate loose. If hydraulic zero is set correctly, there will be no pressure differential in the outlets, as the reaction forces of the rotary group will keep the loose swashplate centered, but the response of the control will be asymmetrical, and, in case of worn displacement control, small fluctuations, which in case of correctly adjusted mecanical zero would have been absorbed by the springs,  may cause the null to drift.

   Obviously, a different adjustment technique must be used for these pumps, which, along with other null-adjustment-related stuff will be discussed in Part 2.
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