A method to reduce the rotary resistance at gear wheels of a gear box at a motor vehicle, and a gear box to carry out the method

Background to the invention

1. Technical field

The invention relates to a method for reducing the rotation resistance of gears in a gearbox for a motor vehicle, which gearbox comprises partly a plurality of pairs of mutually engaging gearwheels which constitute various gear combinations of the gearbox, and partly a direct drive between an input shaft and an output shaft of the gearbox, in which operating with gear combinations engaged involves maintaining in the gearbox an oil level such that a peripheral portion of lower gearwheels of the gear combinations is immersed in oil in an oil sump of the gearbox. The invention also relates to a gearbox which makes it possible to reduce the rotation resistance to gears in the gearbox during direct-drive operation.

2. State of the art In gearboxes of conventional types the lower gearwheels of the various gear combinations in the gearbox normally rotate with their periphery immersed in the oil in the oil sump of the gearbox housing in order, during operation, to lubricate and cool the loaded gearwheels of the various gear combinations. During direct-drive operation, however, i.e. when the input and output shafts of the gearbox are connected together with the transmission ratio 1:1, the lower gearwheels of the gear combinations are not loaded and need no direct lubrication and cooling. Keeping these gearwheels immersed in the oil in the oil sump entails energy consumption through splash losses. Since more than 90% of all driving involves direct-drive operation, these losses due to rotation resistance become substantial in the long term.

One way of reducing such splash losses has been to use a "dry sump" whereby oil is pumped from an underlying accumulator tank and is caused to spray the active

gearwheels, after which the oil dropping off can run back to the tank via bottomholes in the sump. This entails using an energy-consuming pump and special nozzles for spraying the gearwheels.

Summary of the invention

An object of the present invention is to propose a method of the kind mentioned in the introduction which makes it possible to avoid splash losses during direct-drive operation. To this end, the method according to the invention in its broadest meaning is distinguished by the features indicated in the independent claim 1. Splash losses in the gearbox can be reduced by temporarily lowering the oil level in the oil sump when direct-drive operation commences to a level at which the lower gearwheels of the gear combinations rotate with substantially no contact with the oil.

According to a preferred embodiment of the method according to the invention, the oil level lowering is effected by the lower gearwheels of the gear combinations being caused to propel oil from the surface region into an adjacent oil gathering tank, with momentary closure of the outlet leading back to the oil sump. This involves using the rotational energy of the lower gearwheels to move oil to a higher positional potential in a tank, with the possibility of the tank outlet being opened immediately upon reversion to operating with gear combinations, so that the oil can run spontaneously back to the sump and raise the oil level to the normal level for gear combinations.

Another object of the present invention is to provide a gearbox which makes it possible to implement this method with regulation of the oil level in the sump in different operating situations. To this end, the gearbox according to the invention is distinguished by the features indicated in the independent claim 4. When direct drive is engaged, using the rotating lower gearwheels of the gear combinations as conveying wheels for moving oil up from the sump to an inlet to an adjacent tank which has its outlet temporarily closed lowers the oil level in the sump so much that the lower gearwheels rotate with substantially no contact with the oil in the sump, thereby

reducing splash losses without having to supply further energy for pumping oil back to the sump when any of the gear combinations are activated.

Further features of the method and the gearbox according to the invention are indicated by the detailed description set out below and the dependent claims with reference to the attached drawings.

Brief description of the drawings

Fig. 1 is a schematic cross-sectional view through a gearbox according to the invention in a static position of rest;

Fig. 2 is a similar cross-sectional view in an operating state where direct drive has just been engaged; and

Fig. 3 depicts the gearbox in a subsequent operating state where the oil level in the oil sump has been lowered.

Detailed description of a preferred embodiment of the invention

The gearbox 10 according to the invention depicted in schematic cross-section in Figs. 1-3 comprises a housing 12 in which two parallel shafts which bear gearwheels, viz. an upper mainshaft 14 and a lower countershaft 16, are supported for rotation. The mainshaft 14, which is connectable to undepicted input and output shafts at the ends of the gearbox, bears a plurality of gearwheels for different gear combinations of the gearbox 10, one of which gearwheels is designated 18 and engages with an associated pinion mounted on the countershaft 16 and designated 20. The gearwheel pair 18, 20 depicted in the drawings thus constitutes one of a plurality of gear combinations of the gearbox 10.

The lower gearwheels 20 etc. are kept immersed a few centimetres deep in oil in an oil sump 22 in the bottom portion of the gearbox housing 12 in order, during operation, to

lubricate and cool the gearwheels 18, 20 of all the gear combinations. This immersion of the gearwheels causes splash losses when they rotate in the oil, which is also the case when direct drive is engaged in the gearbox, i.e. when power is transmitted directly between the input and output shafts at the transmission ratio 1 : 1 and there is therefore no load on the gearwheels of the gear combinations, which then do not need the lubrication and cooling which are required during operation with these gear combinations. In order to substantially reduce these splash losses when direct drive is engaged, the present invention proposes that the normal level Ni (Fig. 1) of the oil in the oil sump 22 be lowered to a level N ₂ (Fig. 3) which enables the lower pinions 20 etc. on the countershaft to rotate with, in principle, no contact with the oil in the sump 22, whereas the oil level is restored to the normal level Ni as soon as any of the gear combinations is engaged. To achieve this lowering of level advantageously from an energy point of view, it is proposed that when direct drive is engaged the lower gearwheels 20 etc. be used as conveying wheels to move oil up from the sump 22 to an adjacent tank 24 which has an inlet aperture 26 situated on the wall 28 of the housing 12 and exposed to oil splash during rotation of the gearwheels 20 in the direction illustrated in Fig. 2. To this end, a guide plate 30 may be suitably curved to facilitate the introduction of the oil into the tank 24. At the bottom of the tank 24 there is an outlet 32 which leads back to the tank 24, is situated above the free oil surface in the sump and has arranged in it an electrically operable valve 34 which can be kept open or closed depending on the respective operating situation. Alternatively, the outlet 32 may also be situated at a level below the free oil level in the sump 22 to lead oil accumulated in the tank 24 to the sump 22.

During operation with any of the gear combinations engaged in the gearbox 10, the valve 34 in the outlet 32 of the tank 24 is kept open so that the oil sprayed into the tank 24 can be led directly back to the sump 22 in order to maintain a correct oil level Ni therein. As soon as direct-drive operation is initiated, the valve 34 closes, whereupon the tank 24 begins to fill with the oil which the gearwheels 20 splash in via the inlet aperture 26 (see Fig. 2). The volume of the tank 24 is with advantage adapted, in principle, to the amount of oil which has to be transferred from the sump 22 to the tank

24 in order to lower the oil level to the level N ₂ at which the gearwheels 20 rotate with substantially no contact with the oil in the sump 22 (Fig. 3).

When thereafter a gear is engaged via any of the gear combinations of the gearbox, the valve 34 opens immediately in order to quickly restore the oil level Ni in the sump 22. To this end the valve 32 has a large outlet cross-section. To ensure trouble-free movement of oil back from the tank 24 to the sump 22 in cases where the outlet 32 is situated above the oil level in the sump 22, a shielding element 36 is arranged between the outlet 32 of the tank 24 and the lower gearwheels 20 etc. of the gear combinations.

The proposed embodiment of the gearbox according to the invention thus makes it possible, without supplying energy to, for example, an external oil pump, to temporarily lower the oil level while running in direct drive and hence reduce the splash losses in the gearbox.