The invention relates to a device for contra- rotating two concentric shafts, one of which is hollow in order to accommodate the other, by means of at least one drive shaft which is connected to a power source, said concentric shafts being connected to each other by means of a geared transmission. Such devices are currently widely used for the drives of ships and aircraft, where two propellers rotating in opposite directions and about the same axis of rotation are used.

The problem found with known drive de -ces is that they are complex, take up a large amount of space, and are relatively heavy.

The object of the invention is to produce a device of the type mentioned above, in which the above- mentioned disadvantages are avoided in an effective manner.

This object is achieved according to the invention in that the geared transmission comprises a double right-angle transmission formed by one or more crown wheels meshing with cylindrical pinions.

Hitherto people have had to rely on the use of bevel gears for the formation of a right-angle transmission. These bevel gears require an accurate axial adjustment between the gears meshing with each other, the tips of the cones having to meet accurately at one point. Moreover, axial forces are produced in this type of transmission and have to be absorbed by the bearings of the shafts. These disadvantages are avoided by using crown wheels instead of bevel gears, so that

the bearings of the shafts can be made simpler. Besides, the axial position of a pinion meshing with a crown wheel is not critical, and time-consuming adjustment operations are thus unnecessary. According to the invention, each of the concentric shafts is preferably rigidly connected to a crown wheel, and the drive shaft is rigidly connected to one of these crown wheels, and meshing with the two crown wheels are a number of pinions which are fitted in a freely rotatable manner at intervals in an annular holder which is connected to a fixed point in the environment. This design has the advantage that the power transmission between the two crown wheels takes place by means of a number of freely rotating pinions, the power to be transmitted being distributed virtually equally over the pinions, and each pinion transmitting only a part of this power from one crown wheel to the other. The result of this is that the load on the teeth of the two crown wheels is reduced considerably. The crown wheels can thus be made smaller and less sturdy. The holder with the freely rotating pinions can, for example, be fixed to the housing of the geared transmission or to another fixed point of the vessel or the like which is to be driven. Such a design with more than two pinions is virtually impossible to achieve in bevel gear transmissions, since the adjustment of more than two rotating conical pinions relative to each other would in this case come up against insurmountable problems. The invention is explained in greater detail with reference to the drawings, in whic r

Fig. 1 shows schematically a first embodiment of -the device according to the invention;

Fig. 2 shows schematically a second embodiment of the device;

Fig. 3 shows schematically a third embodiment of the device; and

Fig. 4 shows a variant of the embodiment according to Fig. 3. Fig. 1 shows a hollow shaft 1 with a shaft 2 running concentrically through it. These concentric shafts can each be connected to, for example, a propeller or driving screw which are driven in opposite directions. Such driving systems are currently in increasing use in aircrafts (prop-fan) and ships.

Each of the concentric shafts 1, 2 is provided with a cylindrical pinion 3, 4, said pinions having identical teeth and being meshed diametrically opposite each other with two crown wheels 5 and 6. The two crown wheels face each other with their teeth and rotate about a common axis 7, so that these crown wheels rotate in opposite directions of rotation.

In the embodiment shown in Fig. 1 a drive shaft 8 is connected in a rotation-free manner to the pinion 4 of the internal shaft 2. The drive shaft thus lies in line with the two concentric shafts 1, 2. It is, however, also possible to connect the drive shaft 8 in a rotation-free manner to one of the crown wheels 5, 6, this shaft then forming an angle of 90° with the concentric shafts 1, 2. It will also be clear that it is possible in principle to make do with only one crown wheel in the embodiment described above.

With such a design it is also difficult to drive the concentric shafts with power coming from different sources. Instead of with the pinion 4, the two crown wheels can also be driven directly by a different power source, in which case an accelerating transmission is obtained. It is also possible to drive the two crown wheels by means of pinions which are meshed with the two crown wheels, and each of which is connected in a

rotation-free manner to a drive shaft.

Fig. 2 shows a different embodiment of the invention, in which each of the concentric shafts is connected in a rotation-free manner to a crown wheel 9, 10. The two crown wheels face each other with their teeth and both rotate about the common axis of the concentric shafts. The drive shaft 8 is provided with a cylindrical pinion 11 which meshes with the two crown wheels 9, 10. Rotation of the drive shaft 8 causes the crown wheels 9, 10, and thus the concentric shafts, to rotate in opposite directions.

Fig. 3 shows schematically another embodiment of the device, in which each of the concentric shafts 1, 2 bears a crown wheel 9, 10 which is connected to it in a rotation-free manner. The drive shaft 8 is in this case also connected in a rotation-free manner to the crown wheel 10, and in practice the drive shaft 8 is integral with the concentric shaft 1. Disposed between the two crown wheels 9, 10 are a number of cylindrical pinions 12 which are fitted at intervals in a freely rotating manner in an annular holder 13. This annular holder 13 is connected to a fixed point in the environment such as, for example, the housing of the geared transmission. Four of such pinions 12 are present in the embodiment shown in Fig. 3. This means that approximately 50% of the power supplied through the shaft 8 passes through the shaft 1 to the front propeller 14. The remaining 50% is imparted via the crown wheel to the four pinions, so that each pinion transmits approximately 1/4 x 50 = 12.5% of the power to the crown wheel 9. The total power imparted to the crown wheel 9 is thus also approximately 50%, and it is subsequently conveyed via the hollow shaft 2 to the rear propeller 15.

It will be clear that any other desired number of pinions 12 can be fitted between the two crown wheels.

This design has the major advantage that the tooth loads of the two crown wheels are reduced considerably, so that these crown wheels can be made smaller, with the result that the overall radial dimensions of the geared transmission can be reduced.

Fig. 4 shows a part of a variant of the device shown in Fig. 3, in which the crown wheel 9 has a larger number of teeth than the crown wheel 10, so that the propeller 15 will rotate correspondingly more slowly than the propeller 14. This design is particularly simple to produce by means of crown wheels, and it need only be ensured that the pinions 12 are wide enough to permit them to mesh simultaneously with both the crown wheel 10 and the crown wheel 9. In conventional bevel gear transmissions it is impossible to make the concentric shafts rotate at different speeds, at least if the shafts of the pinions have to be in line with each other.

In Fig. 4 the annular holder 13 is also provided on the outer periphery with a spline joint 16 which is in mesh with a corresponding spline joint 17 of the fixed housing 18.

The main advantages of the design according to the invention compared with the conventional use of bevel gears are that the geared transmission is lighter and is of smaller radial dimensions. This is extremely important in the aircraft industry in particular. Besides, cylindrical pinions are very cheap to make, so that the possibly more expensive production of the crown wheels is amply made up for by this.