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    Have you ever seen a working hydraulic system without an oil tank? No? Well, I haven't seen one either... But what about a system that has a ve-e-ery small tank, like really tiny, fifty times smaller than the nominal pressure line oil flow rate? So small, that it would appear like there's no oil tank at all? Have you ever come across one of those? Impossible you say, my good sir? Read this article then, if you ever come across one of these, it might just save you from making a "faux pas"...

    The circuit I am going to talk about, is the good old closed loop transmission,  with one small detail - the oil tank and the pump case are the same thing. Well, it's true - sometimes a machine manufacturer opts for using the very pump's case as the oil tank, because it saves tons of space and makes the production cheaper. As a rule such systems have a very small external tank connected to the casing, and its function is to make sure the pump case is full, to compensate for the oil temperature expansion and to serve as a small extra amount of oil to maintain the transmission operational in case of an oil leak.

    Although this arrangement is still a basic closed loop transmission, there are several small but significant differences, which, if not taken under consideration, may lead to an "oups" or "I want my money back" situations, making it very important to be able to  recognize such pumps "on the spot".

    The first and most obvious difference between this special pump and a standard closed loop pump is the existence of an internal connection between the casing and the charge pump suction line, which is normally done through an extra passage in the end cover, like this one, for example (Sauer Danfoss series 90, mounted on a Claas harvester). Sometimes, during an overhaul,  a new end cover is needed, and in this case a standard end cover won't be the correct replacement choice. The pump will work, because there is a small quarter of an inch hose, connected between the standard suction port and the small expansion oil tank (picture), which will allow the transmission to function, but will also create poor suction conditions for the charge pump, compromising the life span of the whole transmission. In case of a larger series 90 pump (the ones I've seen so far all came from Claas harvesters), the charge suction passage goes through an opening in a special valve plate, and replacing it with a standard valve plate will likewise condition the charge pump suction line. Often (in the next paragraph you will see why) these pumps get damaged beyond economically justified repair, and need to be replaced.  It is highly unlikely that your local Sauer Danfoss dealer (or other brand dealer, although so far I've come across only two names that produce such closed loop pumps - Sauer Danfoss and Linde) will have this special pump in stock, that is why special care must be taken when replacing this pump with a standard unit - you will either have to machine the new end cover/ reuse the old end cover, or at least increase the suction line hose and fittings, and find a way to direct the charge flow through the radiator. Skipping this step may result in drastically reduced transmission life.

    Another peculiarity of such transmissions is their total dependence on the cooling system. In a typical hydrostatic transmission at least 25 percent of the input energy is transformed into heat. In case of a 60 kw input you will have to dissipate at least 15 kw of heat to maintain the oil temperature stable. Due to the fact that total oil volume is extremely small, any malfunction of the cooling system will result in instant overheating, and from what I've seen so far, will normally lead to a catastrophic failure. When repairing and re-commissioning such transmissions, special care must be given to make sure that ALL parts of the cooling system are functional. Often a manufacturer will use some kind of a thermal valve (like this wax motor, for example) to control the flow through the cooler, which is an absolute MUST to check whenever you overhaul such a transmission. You can find a description of this option for Sauer Danfoss series 90 here. The biggest problem arises when this thermo-valve doesn't close, resulting in permanent cooler bypass - you can see the real-life outcomes of this malfunction here and here. If you can't get a replacement wax motor for the cooler bypass valve, but still need to put the harvester back in service,  your best bet would be to block the bypass spool, thus directing all of the oil to the cooler. It is never a problem here in Portugal, due to the hot climate, but in cooler regions the thermo-valve will have to be replaced eventually.

    Next peculiarity is the charge oil filtration, which is an absolute must for such systems. Never, but never, you can skip replacing the filter with a quality element. A smart choice would be buying a cheaper replacement (in relation to the original brand) at a local Sauer representative, but I wouldn't advise toying around with cheaper unknown brand (and quality) filters. If the pump is equipped with the filter delta P monitoring system (like in this example), it should be kept clean, otherwise it will be no more useful then a  speedometer mounted under the hood...Hydraulic oil should be chosen in accordance to the normal system temperature in the region the machine will work. Don't go blindly for the machine's manual recommendations, more often than not it won't be the best choice.

    Due to the fact, that  such "compact" closed loop systems operate with an extremely reduced overall amount of hydraulic oil, they are much less forgiving than "normal" hydrostatic transmissions, and the stretch between the appearance of a malfunction and complete component destruction is extremely small. However, when repaired and maintained correctly, these systems have the life span equal to the one of  transmissions  operating with relatively large oil tanks - it all boils down to knowing the small differences between them and applying the knowledge correctly.
Special end cover, Sauer Danfoss series 90
Special end cover, Sauer Danfoss series 90
Thermo vale
Special manifold, filter and cooler by-pass
Special end cover, Sauer Danfoss series 90