Epicvclic gearing

The invention relates to an epicyclic gearing, in particular for use in an engine starter for a motor vehicle.
It is known to use epicyclic gearings to adapt the rotational speed of one load shaft as a function of a driving or motor shaft.

Usually, the epicyclic gearing comprises a sun gear for example attached to an electric motor, three or four planet gears, regularly placed around the sun gear and meshing with said sun gear, and a circular ring gear, surrounding the precedent gears and meshing with the planet gears, which lay be attached to the load.

By adaptation of the respective diameters of the gears, a proportionality between the number of revolutions of the motor shaft and of the load shaft is reached. In the case of a starter, a reduction is reached to enhance the torque.

However, the high number of gears causes the epicyclic gears to be expensive, while their load such as in e.g. smaller private or utility vehicle starters, does not always require the high tolerances the gearings in use do feature.

In order to overcome the aforementioned drawbacks, the invention has for object an epicyclic gearing, in particular for a combustion starter, comprising:

- a sun gear,
- a first shaft, fastened to the sun gear and rotating with said sun gear,
- a ring gear, surrounding the sun gear,

characterized in that it further comprises two planet gears meshing with the sun gear, spaced from their neighbour by an approximately 180° rotation around the sun gear, and meshing with the ring gear.

The epicyclic gearing according to the invention may also present one or more of the following characteristics, taken separately and/or in combination.

The planet gears rotate each around a planet shaft carried by a common planet shaft carrier, and in that said planet shaft carrier comprises a central plate featuring an elongated central portion, two circular holes at the far ends of the central plate and two ring portions delimiting at least the radially most outward half of the circular holes.

Said central portion may have an elliptic shape, a convex polygonal shape, a rectangular shape, or a hexagonal shape. It may be made of a cut metal plate.

The planet gears comprise a cylindrical axis housing in which an axis shaft of the planet gear is destined to be inserted, and the axis housing comprises on its inner peripheral wall a lubricant guiding groove, extending axially from one side of the planet gear to the other. The lubricant guiding groove may tilted so as to form an Archimedes' screw to increase lubricant circulation. The planet shafts may then be solidly bound to the planet shaft carrier and the planet gears placed directly on said planet shafts so as to rotate on the planet shafts.

The invention also has for object the associated starter for a combustion engine, comprising an electric motor configured to set a first shaft in motion, characterized in that it comprises an epicyclic gearing as previously described.

The invention also has for object an epicyclic gearing, in particular for a combustion motor starter,
comprising:

- a sun gear,

- a first shaft, fastened to the sun gear and rotating with said sun gear,

- a ring gear, surrounding the sun gear,

characterized in that it further comprises two planet gears meshing with the sun gear, spaced from their neighbour by an approximately 180° rotation around the sun gear, and meshing with the ring gear.

Other characteristics and advantages of the invention will appear at the reading of the description of the following figures, among which:

- figure 1 is a partial exploded view of a starter comprising an epicyclic gearing according to the invention,

- figure 2 shows a planet shaft carrier according to the invention,

- figure 3 is an axial view of a central plate for the shaft carrier of figure 2,

- figure 4 is an axial view of a planet gear for an epicyclic gearing according to the invention.

On all figures, the same element is referred to with the same number.

On figure 1 is partially depicted an exploded view of a starter for a combustion engine comprising an epicyclic gearing 1. The shown epicyclic gearing 1 comprises a sun gear 3, a ring gear 5, and two planet gears 7.

The sun gear 3 is placed in the centre of the epicyclic gearing 1, and bound to a first shaft 8, for most encased in an electric motor 9 of the starter and hence only partially visible on figure 1. This first shaft 8 is set in motion by the electric motor 9. The two planet gears 7 are placed on opposite sides of the sun gear 3, at an angle of approximately 180° one from the next around the sun gear 3. The two planet gears 7 mesh with the sun gear 3, so that the rotation of the first shaft 8 causes rotation of the planet gears 7.

The planet gears 7 are in particular only two, with an angle of approximately 180° between each in rotation around the sun gear 3.

The ring gear 5 surrounds the meshing sun gear 3 and planet gears 7. It possesses gear teeth on the inside of the ring it forms, which mesh with the planet gears 7 on their side opposite to the region meshing with the sun gear 3, which is the radially most outward part of said planet gears 7.

When the sun gear 3 is rotated (via the first shaft 8 and motor 9), the rotational movement is transmitted to the planet gears 7 by their meshing. The planet gears 7 rotate both around themselves and around the sun gear 3 by their meshing with the ring gear 5, which remains stationary.

The reduction factor of the rotational speed is given by the ratio between the radii of the sun 3 and planet 7 gears. In particular, the planet 7 gears may be fashioned bigger in radius than in the state of art epicyclic gearings with three or more planet gears 7, since more free space is available per planet gear in the gearing 1. A larger range of reduction factors is thus available.

The planet gears 7 feature each a housing 11 in which is inserted a planet shaft 13. The planet shaft 13 is cylindrical and the corresponding housing 11 matches its form and dimensions.

The planet shafts 13 are carried by a planet shaft carrier 15.

A planet shaft carrier 15 is shown in more detailed fashion in figure 2, shown in perspective view.

The carrier 15 comprises a central plate 17, at the extremities of which are placed the planet shafts 13, inserted in holes 23 and protruding so that at assembling the planet shafts 13 merge in the housing 11 of the planet gears 7.

The planet shaft carrier 15 is bound in rotation with a second shaft 19 which is in this particular example the load shaft, as the motor shaft is the first shaft 8 connected on one hand to the sun gear 3 and on the other to the electric motor 9. Said motor 9 drives the first shaft 8 at a relatively high rotating speed, and the gearing 1 transmits the rotational movement with a reduced speed to the second shaft 19, but with an increased torque, for example to start a combustion engine.

Of course alternative embodiments may foresee that the second shaft 19 is the motor shaft, while the first shaft 8, bound to the sun gear 3, is the load shaft.

In figure 3 is represented a particular embodiment of the central plate 17 for the planet shaft carrier 15.

In this embodiment, the central plate 17 of the planet shaft carrier 15 essentially comprises an elongated central portion 21, here in elliptic shape. At the far ends of the central portion 21, two circular holes 23
are made, in which the planet shafts 13 merge.

Other embodiments foresee the use of convex elongated forms such as convex polygons, such as rectangles, hexagons and the like.

In order to increase the dispersion of the efforts during use of the epicyclic gearing 1, an important part, at least the radially further part, of the perimeter of the circular holes 23 is delimited by a ring portion 25, said ring portion 25 having a roughly constant thickness.

Such a central plate can in. particular be easily obtained by cutting or stamping a metal sheet or plate with the appropriate contour.

Experiments have shown that this particular form, with an important part of the perimeter of the circular holes 23 delimited by a constant thickness ring portion 25 show an improved resistance during stress tests since no particular point concentrates an important portion of the stress.

Figure 4 shows a particular embodiment of a planet gear 7 according to a particular aspect of the invention.

The planet gear 7 comprises here a set of peripheral grooves 27 on the inner peripheral wall of the housing 11 for the planet shafts 13.

The peripheral grooves 27 are here of triangular section, pointing outwards in radial direction. They extend on the whole axial length, from one side of the gear 7 to the other.

Said peripheral grooves act as lubricant pathway so as to allow a better lubrication of both the planet shafts 13 and the planet shaft carrier 15, which can otherwise, as experiments showed, block and damage the gearing 1.

A particular embodiment foresees to tilt the peripheral grooves 27, so that they form helical grooves. When the planet gear 7 rotates, the grooves 27 act as an Archimedes' screw to increase lubricant circulation.

With the increased lubrication it is possible to bind the planet shafts 13 solidly to the planet shaft carrier 15 and to place the planet gears 7 directly on the planet shafts 13 so as to rotate said planet gears 7 on said planet shafts 13 without further need of friction reducing sleeves placed in between.

In the particular case of smaller vehicles, such as small or compact vehicles, such as urban vehicles and compact family and compact utility vehicles, a three or more planet gear 7 epicyclic gearing 1 provides too much solidity, while a two planet gear 7 epicyclical gearing according to the invention is sufficient and potentially cheaper and lighter.

Since the two remaining planet gears 7 share the stress that would otherwise be shared between (usually) three or four planet gears 7, the improvements described above allow to maintain the functionalities of the gearing on the long term, even though the stress is now concentrated on a smaller number of planet gears 7.

1. Epicyclic gearing (1), in particular for a combustion motor starter, comprising:

- a sun gear (3),
- a first shaft (8), fastened to the sun gear (3) and rotating with said sun gear (3),
- a ring gear (5), surrounding the sun gear (3),
characterized in that it further comprises only two planet gears (7) meshing with the sun gear (3), around the sun gear (3), and meshing with the ring gear (5).

2. Epicyclic gearing (1) according to claim 1, characterized in that the planet gears (7) rotate each around a planet shaft (13) carried by a common planet shaft carrier (15), and in that said planet shaft carrier (15) comprises a central plate (17) featuring an elongated central portion (21), two circular holes (23) at the far ends of the central plate (17) and two ring portions (25) delimiting at least the radially most outward half of the circular holes (23).

3. Epicyclic gearing (1) according to claim 2, characterized in that the central portion (21) has an elliptic shape.

4. Epicyclic gearing (1) according to claim 2, characterized in that the central portion (21) has convex polygonal shape.

5. Epicyclic gearing (1) according to claim 2, characterized in that the central portion (21) has a rectangular shape.

6. Epicyclic gearing (1) according to claim 2, characterized in that the central portion (21) has a hexagonal shape.

7. Epicyclic gearing according to any of claims 2 to 6, characterized in that the central plate (17) is made of a cut metal plate.

8. Epicyclic gearing (1) according to any of the precedent claims, characterized in that the planet gears (7) comprise a cylindrical axis housing (11) in which planet shafts (13) of the planet gear (7) are inserted, and in that the axis housing comprises on its inner peripheral wall a lubricant guiding groove (27), extending axially from one side of the planet gear (7) to the other.

9. Epicyclic gearing according to claim 8, characterized in that the lubricant guiding groove (27) is tilted so as to form an Archimedes' screw to increase lubricant circulation.

10. Epicyclic gearing according to claim 8 or 9, characterized in that the planet shafts (13) are solidly bound to the planet shaft carrier (15) and in that the planet gears are placed directly on said planet shafts (13) so as to rotate on the planet shafts (13).

11. Starter for a combustion engine, comprising an electric motor configured to set a first shaft (8) in motion, characterized in that it comprises an epicyclic gearing (1), in particular for a combustion starter, comprising:

• a sun gear (3) fastened to the first shaft (8),
• a ring gear (5), surrounding the sun gear (3),

- two planet gears (7) meshing with the sun gear (3), spaced from their neighbour by an approximately 180° rotation around the sun gear (3), and meshing with the ring gear (5).

12. Epicyclic gearing (1), in particular for a combustion motor starter, comprising:

- a sun gear (3),
- a first shaft (8), fastened to the sun gear (3) and rotating with said sun gear (3),
- a ring gear (5), surrounding the sun gear (3),
- characterized in that it further comprises two planet gears (7) meshing with the sun gear (3), spaced from their neighbour by an approximately 180° rotation around the sun gear (3), and meshing with the ring gear (5).