Insane Hydraulics
Bold and audacious blog about fluid power
For me, the most beautiful and perfect hydraulic circuit has always been and forever will be the closed-loop hydrostatic transmission. Simple, at first glance, and yet so complex at the same time. The best and most compact solution for infinitely controlled bi-directional hydraulic motion, with industrial vehicle propulsion being, by far, its most common application.
Hydrostatic transmissions are old technology, and many shops will readily tweak your closed-loop on your first demand, yet I still come across off-centered closed-loop pumps more often than I would like to. And, to top it off, I sometimes even see intuitive, yet incredibly nonsensical techniques used to "catch the null", showing a complete misunderstanding of how a given servo-mechanism works and what must be done to "make it stop!" This series is, therefore, my attempt to shine some light on the very important matter of adjusting the neutral swashplate position of closed-loop pumps.
Nobody likes heavy machinery moving on its own. Unless, of course, we're talking about a runaway forest crane slowly bulldozing the shiny new SUV your boss bought right after telling you that "these are tough times and the raise will have to wait..." But, in all seriousness - correct null adjustment of a closed-loop transmission is (quite literally) vital.
I must warn you that extreme care must be taken when dealing with hydrostatic transmission neutral adjustment. If you are new to this, you should not do it! Period. Don't even think about it! Setting your machine in uncontrolled motion may be (arguably) fun to look at, but I guarantee you that it won't be fun to be responsible for! If you're not sure whether you are up for the task, call someone who is. And if you're brave enough to take any of my advice - re-read the disclaimer before you reach for the spanners.
Now that we're done with the safety drill, let's go ahead and talk about them pumps.
Let's ask ourselves - where do the "crooked" transmissions come from? From what I've seen so far, the most common reason for a hydrostatic transmission to exhibit the "drifting null" symptom is the recent overhaul, during which the "overhauler" skipped or simply screwed up the adjusting procedure. Of course, a pump with million hours can "wear itself out of null" or even become not "null-adjustable", but the sad truth is that most closed-loop pumps don't go off-center "by themselves", but are the result of "craftsmen intervention". Which is a good thing, because it's something you can fix.
Before doing anything else we must define a way to determine when the swashplate is at null. For me - the best way is the comparison of pressures in the loop legs, which shouldn't differ by more than a couple of bar with no control input and the actuator not moving:
Don't forget that the null check-up and adjustment 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 de-stroke in both directions. It is also important to verify control sensitivity - the response to the input signal should be symmetrical for both sides (although this may not be imperative and can depend on the type of control used). In the case of closed-loop pumps driven by relatively quiet electric motors, you can actually hear the pump go off-center as it builds pressure in one of the legs, something an experienced mechanic can use as a convenient reference, but checking those two gauges is still the best way to go.
It is also possible to check if the pump is off-null by reading the charge pressure:
This technique can only be used with closed-loop circuits equipped with a 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 fact that you only need one pressure gauge and, if you use an analog gauge, the needle drop can be easily seen from a distance. However, this technique will not work for all circuits, because the charge pressure drop depends on the type of valves used in the flushing manifold, and for certain arrangements can be hardly noticeable at all.
Now that we know how to check the correct neutral swashplate position, let us look into typical closed-loop controls and servo-mechanism designs, and the best ways to null them.
The most common type of closed-loop pump control is proportional displacement control (hydraulic or electric). It is also usually the one that gives the most "zeroing troubles" because it employs two independent and over-lapping systems for keeping the swashplate in the neutral position - the mechanical (springs) and the hydraulic (servo pressures).
Mechanical centering can come in many flavors. It can be achieved by means of a single spring,
two or more springs using the same principle,
two independent pre-loaded springs (feel the nostalgia!),
or one or two pre-loaded springs, constructively placed inside the servo-piston (used in Danfoss H1 series):
The hydraulic positioning of the swashplate is provided by the proportional displacement control, which defines the position of the swashplate through servo-pressures.
It is important to understand, that both of the systems have deadbands, and an unstable null condition is created when they don't overlap, because the hydraulic positioning system overrides the mechanic "forcing the springs" into the correct null, but, like any hydraulic 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 when the spring-centering mechanism is allowed to "push back to its own zero" 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, second - the hydraulic. If proper tooling is available, the correct mechanical null can be set during the assembly of the pump, if not, you will have to do it during the testing phase. The most common course of action would be to eliminate the action of the hydraulic positioning system by by-passing the servo-cylinders with an external hose and adjusting the mechanical zero first. The by-pass hose should be as short as possible to minimize a possible pressure drop. I use a ball valve in the middle of the line to conveniently turn the by-pass on and off:
Then, after the mechanical zero is set (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 loop leg pressures level out), you remove (close) the by-pass and adjust the hydraulic zero, again aiming to leave the adjustment in the middle position within the deadband. Skipping this procedure, and trying to adjust the neutral position with both positioning systems active produces unreliable results.
Note that for some pumps adjusting the correct mechanical neutral position of the swashplate can be more than simply turning an adjustment screw. In the Sauer Danfoss series 90 pumps, for example, the mechanical zero is defined by the position of the round side cover that guides the centering T-bar and has a small rotational play (part E10):
To adjust the null you must loosen the six screws and then turn the cover into the correct position by hitting on the machined pit (one of the rare cases you actually need a hammer to adjust a hydraulic setting).
Again, some series 90 pumps use an extra spring to hold the swashplate in neutral:
The external spring is used to create additional centering force in the servo mechanism. The best way to precisely set the correct mechanical zero in such a pump would be by adjusting it during the assembly.
The described technique (servo-cylinder by-passing) works well on closed-loop pumps that have mechanical centering systems with fixed swashplate free play (usually insignificant, existing only due to small clearances in 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 the hydraulic zero is set correctly, there will be no pressure differential in the loop, but the response of the control will be asymmetrical, and the displacement control fluctuations, which in the case of correctly adjusted mechanical zero would have been absorbed by the springs, will cause the null to drift.
Obviously, a different adjustment technique must be used for such pumps, which, along with other null-adjustment-related stuff is discussed in Part 2 of this series.