Have you ever seen a hydraulic system without an oil tank? No? Well, I haven't seen one either...
But how about a system that has a very small tank, like really tiny, fifty times smaller than the nominal flow rate of the main pump? So small, that it would appear there's no oil tank at all? Impossible, say you? Don't say "impossible" till you finish reading this article!
The hydraulic system 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! That's right - sometimes, designers opt for using the pump's very case as the oil tank, because it makes the transmission super compact and makes the production cheaper. The only thing that can come close to being called an oil tank in such systems would be a small vessel, connected to the case via a small hose, and whose function is to compensate for the oil temperature expansion and, maybe, to serve as a small extra amount of oil to keep the transmission operational in case of an oil leak.
Although this arrangement is still a basic closed-loop, there are several small but significant differences in the pump design, which, if not taken under consideration, may lead to "D'oh!" or even "I want my money back" overhaul scenarios, making it very important to be able to recognize such pumps on the spot.
The first and most obvious difference between such a special pump and a standard closed-loop pump is the fact that the charge pump suction line is internally connected to the casing, which means that there should be a large boring in the end cover (for example, this Danfoss series 90 special end plate from a CLAAS harvester).
More often than not such end covers are manufactured exclusively for an OEM, and are not available trough standard parts program, which is why in case the end cover needs to be replaced during an overhaul, a stock end cover won't be the correct choice. The overhauled pump may even work perfectly fine on a test bench because the mechanics will use the standard suction port to feed the charge pump, and look the same "from the outside", but when mounted on the machine, it will fail either immediately or after a very short while.
In this particular transmission, for example, there is a small diameter hose connection between the standard suction port and the expansion tank. See how unusually small the fitting in the suction port is? An attentive mechanic would consider such a choke point to be an immediate "red flag" signaling that there's something "fishy" going on with the suction line, but when one is "on a tight schedule", most of the time what one does is take the small fitting out, put a "proper" one in - test it - put it back together as it was - and be done with it.
In case of larger series 90 pumps (the ones I've seen so far all came from CLASS harvesters), the special suction bore in the end plate is placed very close to the valve plate, and the valve plate has a cut-out that makes sure the suction is not obstructed. This is another "special design" part that if replaced with a standard one will likewise condition the charge pump suction.
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 Danfoss dealer (or another 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, which is why special care must be taken when replacing these pumps with standard units - you will either have to machine the new unit's end cover (alternatively - reuse the old end cover), or devise an "external workaround" - i.e. use the standard suction port and find a way to direct the charge flow through the radiator. Skip this step - and the transmission ends up in the Kaboom collection!
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. This means that in case of, say, a 60 kW input, the cooling system will have to be capable of dissipating at least 15 kW of heat to maintain the oil temperature stable. Since the total volume of oil is extremely small, any malfunction of the cooling system will result in instant overheating and, consequently, a catastrophic failure.
When repairing and re-commissioning such transmissions, special care must be taken 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) to control the flow through the cooler, which is an absolute MUST to check. 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 spectacular 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.
The 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. I would definitely advise against toying around with cheaper unknown brand (and quality) filters. If the filter manifold is equipped with a clogging indicator, the operators should be instructed to inspect it daily and keep it clean, otherwise, it is no more useful then a speedometer of a race car placed under the hood...
Another very important thing would be the hydraulic oil, which should be chosen in accordance with 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 loops 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. Only if repaired and maintained correctly that these systems have a chance of having the life span comparable to one of the transmissions operating with large oil tanks.