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
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.