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    A classic here - Sauer Danfoss series 90 closed loop pump, very commonly used in mobile hydrostatic transmissions. This particular unit is a 100cc pump from a John Deere forester, I get those all the time, not because they break a lot, but because they are so widely used.

    Here you can see how the pump looks (AFTER it has been cleaned up, cause when they brought it to the shop it was completely covered with dirt).

   First of all let's discard the obvious dull stuff everybody knows about (pic2), here we have the shaft, the cylinder block, the pistons, swash plate, charge pump, servopiston and the servocylinders, and the rest of the crap (pic3) like bolts, nuts and what not every piston pump has.

   On the pic4 you can see already lapped piston shoes, along with one of several  small differences that always exist between the original and not original (made in China) parts. On the original block the brass sleeve is slightly immersed in the body, creating a 1 mm ramped entrance, while after-market blocks usually don't have it, which makes the "screw-in" mounting technique much harder, as there is no good guidance for the pistons to enter the cylinder block.

   Early series 90 came with an all-flat sealing surface of the cylinder block, while later models have the outer brass ring a little lower then the kidneys ring, which I am guessing makes it a better dynamic bearing. Anyway, my own experiments, which I am going to describe in a separate article, proved, actually, that lowering outer ring can difficult the block lift, so after lapping the sealing surface I sometimes lathe the outer ring down 0.05mm or so (pic5, pic6). By the way, some models carry the cylinder block that doesn't have any outer ring at all! I guess the engineers came to a conclusion that it was not really necessary, at least for that particular application.

    Now let's have a closer look at the end-cover (pic7). Here we can see a pretty standard for modern hydrostatic pumps housing of a gerotor-type charge pump, with easily identifiable suction and pressure sides. Note that in this model charge pressure relief valve connects directly to the pressure side, BEFORE the filter (filtering option can be disabled or altered for the use of an external filter, by the way) which means that if you measure the charge pressure from the port before the filter, you may get false reading. I have seen machines with a test coupling mounted at this spot. When the filter gets clogged, the transmission starts loosing charge pressure and cavitate and yet you still read the normal charge pressure in the "before" port. I have seen them filters clog pretty bad on these John Deere (Timberjack in the good old days...) foresters.
   
    The two measuring ports (pic8) might give you a crazy idea of continuous filter state monitoring, by measuring the delta P.

        On pic9 and pic10 you can see the inside of the back cover and its connections. Sauer produces many variations of these. Some, for example, don't have all of those internal passages, and are meant to be used in applications which don't require the pressure limiter function. The official Sauer Danfoss website provides complete  pdf catalogues with schematics for this model (which are an excellent learning material by themselves, by the way).

    Check out pic10, ever wondered what these check valves are for? Take a close look at the typical Sauer series 90 closed loop schematics. The multi-function valve acts as a sequence valve, so when a set pressure is reached the small poppet lifts, channeling oil to the servo piston opposite to the one tilting the swash plate at the moment - and this is the moment these check valves enter the game. If the flow from the multi-function valve is too violent these checks act as servo-pressure limiters (my guess is their cracking pressure is around 5-10 bar, so with a charge pressure of 25 bar they will open at 30-35 absolute) venting the excess to the lower pressure side, thus preventing the servo cylinder caps from bursting out.

    You can also see that if something gets stuck in one of these check valves the swash plate will remain tilted, as the charge pressure will pass through the check valve to the servo cylinder. I've seen it happen once.

     Existence of a pressure limiter function also explains the need for all those plugs and plug holes (pic10). As you know, depending on the shaft rotation, the same servo-cylinder movement will result in opposite oil flow directions . So if you have the plugs wrong, when the multifunction valve opens, instead of decreasing the displacement it will do just the opposite, sometimes with very hilarious consequences. You can also eliminate the pressure limiter function by plugging all the holes.

    Now let's take a look at the control part. Pic11 and pic12 show you how the charge pressure is fed to the proportional control. Note the small screens. One time I actually got one of these pumps with a small piece of plastic BEHIND the screen, which was acting as a check valve and a flow restrictor, causing slow and sluggish control operation.

    Pic13 shows you the proportional control detail. Green lines indicate the oil going in and out of the servo motor part. Although Sauer says we shouldn't open the servo-motor part, it is possible, IF you know what you are doing. I opened quite a few, most of the times to repair the results of somebody else's previous opening... Inside you will find a classic servo motor (you know, two nozzles, small flap in the middle..) with two independent coils. One of my "curious" clients switched the position of the permanent magnets by mistake, arranging them in a manner that one was canceling the magnetic field of the other, thus rendering the control completely inoperable. All the parts were there and in place, the coils were not damaged, the oil passages weren't clogged, and yet nothing worked.

    Notice that oil enters the servo motor through a small circular screen (pic14). It is actually very common for this screen to get clogged. You should check it every time you overhaul such a pump. Inspect the tank line orifices - they get clogged very often too! Big round empty orifice right in the middle of the pic14 would hold a check orifice in a standard pump. Its function is to restrict the oil coming INTO the control, but to open as a check valve when you have back flow from the control, which happens when the pressure limiter function kicks in and the swash-plate tilts toward the smaller displacement, thus creating the back flow by pushing the active servo-cylinder back (serves to make the limiter function faster).

    Pic14 shows you the feedback arrangement. Green lines show you the servo motor pressures acting on both sides of the main control spool. Note that all of those moving parts tend to gain excessive play with time - another thing to check.

   Take a look at the servo motor connectors (pic17). Remember that it has two independent coils? It means that you can arrange its connections in series or in parallel. You can actually use only one (any) coil to control the pump, changing the direction of the flow by changing the polarity of the current, or use each coil for one direction (John Deere's module does that). When you connect the coils in series or in parallel make sure to check the polarity, because one may cancel the other if connected improperly.

   Now let's take a closer look at the multi-function valves (pic18, pic19), the valve's cut view is here. Its actual parts are seen on pic24 and pic25. We know already that it acts as a sequence valve, channeling oil from the high pressure side to a servo piston. However when an unusually fast load increase occurs and the pressure differential of the oil flow through the poppet orifice (pic20) exceeds certain level, the poppet becomes a logic element piloted by the small sequence valve, lifting up and venting the high pressure to the low pressure side. The smaller the orifice of a logic element, the faster it will open, as the sufficient delta P will be created by a smaller flow. So the smaller the orifice of a multi-function valve poppet is, the more shock-absorbing qualities it will have (and more oil-heating also, as a downside). I have seen with my own eyes different sizes of those orifices in different pumps, AND they were marked by different numbers (pic21, pic22, pic23). However when I some time ago encountered a series 90 based hydrostatic circuit subject to high pressure peaks equipped with standard no1 valves (the ones that have a big orifice), and  contacted the local Sauer representative on the subject of replacing the standard valves with more shock absorbing ones, I was told, to my great surprise, that only one type of those valves existed. Please excuse me, but I don't see Sauer factory producing different orifice sizes by mistake, and having all the trouble of marking the valves. My guess is that, unfortunately, lack of communication does happen even among official brand representatives. Please correct me if I am wrong.

    Pic26 and pic27 show you the t-bar and the springs - the "mechanical zero" part. Always examine closely the t-bar guides, as sometimes they get pretty worn out. Note that the holes on the cover allow some rotational play (pic28), which lets you adjust the mechanical zero if necessary. To do so you should lighten the cover holding screws a little and turn it by hitting on the machined pit (pic29), you can actually do it when the pump is working. Soon I will be writing a more detailed article on centering closed circuit pumps, as I keep seeing all kinds of "incorrect" techniques used by many "artists" out there.

   There's not much science about these pumps, they are quite simple and complete pdf catalogues for this series are available online. My opinion about these pumps in one phrase? They are not bad. Reliable, after-market parts easily available, universal, but a little over-engineered. When I first saw a series 90, I thought to myself: wow! They used fifty pieces to do the same stuff Hydromatik does with ten. But the series HAS been improving greatly, it's true, and it surely proved to be a good pump in the field.

    Inevitably Sauer engineers came to the classic hydromatik-like design in their newer H1 series (none of those passed through my shop yet, at least until today, 1/03/2009).

    I have absolutely no doubts that Sauer series 90 pumps will be still widely used many years from now.