The oil-circulation is well known from the classic converter-automatic. In the manual gearbox, there has always been oil, this however, although kept in movement by the largest or second largest gearwheel, is not the same as through a pump. This statement is however, only valid for motor car gearboxes. In truck gearboxes, pressure circulation lubrication is the standard.

Their pumps are driven directly through the converter casing or the crankshaft. Certain companies even mount another one on the output-side, to make it possible to tow the vehicle. The suction-filter is important for the oil circulation.

It becomes technically more complicated in the case of the automatic gearbox. Here there are sometimes, particularly in the early examples, a separate circulation with hydraulic oil, often driven by an electric pump. This first of all, makes it possible to operate the selector rods and somewhat later, also the clutch. It becomes complicated for the oil circulation, when this joins in, in the taking over of the tasks.

What we have here are considerable pressures, e.g., of up to 20 bar and because of the relatively small tolerances and the need not only for the switching- and the clutch slider, we also need the necessary pressure regulating valve and of course, the pump for better filtered oil. This is why, apart from the suction- there is also a pressure oil filter. As the name indicates, it is mounted on the pressure side of the pump.

Filters in the suction area may only have a fleece-mat that filters relatively roughly. If the oil is pressed through a filter, then the filtering can be finer. The oil-flow is thus, e.g., divided through a bypass valve, so that, in this case, we can speak of bypass filtering. The particularly filtered part of the oil is then also guided to the positioning elements which have a tight clearance.

The pressure filters in/on the gearbox have the most different forms. Inside them, paper, fleece or even fiber-glass can take over the actual task. Despite the temparatures, the casing can be made of plastic or it can be integrated into the gearbox without a casing at all. To change the suction filter, one mostly has to open the casing from below. 09/13