Patent Description:
When the wheels change their rotational direction, i.e. perform a forward and backward motion and/or perform an oscillating movement between the two directions, the backlash between the gearings may cause an uneven motion profile of the wheels. This may lead to an unwanted wear of the wheels.

<CIT> discloses a planetary gear unit, which is considered by the examining division of the European Patent Office as falling within the wording of the pre-characterizing portion of claim <NUM>, and in which sliding resistance between a double helical gear and a peripheral member is reduced. The planetary gear unit comprises pinion gears individually having two rows of oppositely oriented helical gears in an axial direction; a first pushing member that elastically pushes at least one of the pinion gears toward in a predetermined axial direction; and a second pushing member that elastically pushes at least one of the remaining pinion gears in the opposite axial direction.

<CIT> discloses a planetary gear unit having a first and a second shaft, each with two spur planet wheels. The spur wheels mesh with toothed gears of the gear unit. One of the shafts is pre-tensioned in axial direction. The first shaft is pre-tensioned in the opposite direction from the second shaft. There may be a third shaft with spur wheels meshing with the toothed gears. One spur wheel of each shaft meshes with a first sun wheel, and a second spur wheel meshes with a second sun wheel. The gear unit is part of a rack-and-pinion hydraulic-steering gear.

It is thus an object of the present invention to reduce the backlash between the gearings of the wheels of a planetary gear.

This object is solved by a planetary gear according to claim <NUM>.

The planetary gear comprises at least a first hollow wheel with a helical gearing and a second hollow wheel with a helical gearing, wherein the first hollow wheel and the second hollow wheel are adapted to rotate in a first rotational direction and a second opposite rotational direction. The planetary gear further comprises a first double planet having a first wheel with a helical gearing meshing with the helical gearing of the first hollow wheel and having a second wheel with a helical gearing meshing with the helical gearing of the second hollow wheel.

Each tooth of each helical gearing of the hollow wheels and the double planets has a first flank and a second flank opposite to the first flank. A surface normal of the first flank is directed to the first rotational direction and a surface normal of the second flank is directed to the second rotational direction.

To minimize the backlash between the helical gearings of the wheels of the first double planet and the hollow wheels, a first force exerting element is provided. The first force exerting element exerts a first force in the first direction so that the first flank of a tooth of the helical gearing of the first wheel of the first double planet is in contact with the second flank of a tooth of the helical gearing of the first hollow wheel and so that the second flank of a tooth of the helical gearing of the second wheel of the first double planet is in contact with the first flank of a tooth of the helical gearing of the second hollow wheel. Due to this contact, a preload to the hollow wheels in the first rotational direction is provided. By pressing the first double planet into the direction of the first hollow wheel, the helical gearings of the first double planet are in contact with the helical gearings of the first and the second hollow wheel. Due to the preload, a contact between the helical gearings of the hollow wheels and the helical gearings of the wheels of the double planet exists already during start up. In addition, this contact can be kept by means of the first force exerting element during operation by maintaining the preload and thus, a backlash between the helical gearings may be avoided.

The first wheel and the second wheel of the first double planet are fixed in relation to each other. By this fixation, it is possible to exert the first force simultaneously to both wheels of the first double planet so that the first flank of a tooth of the helical gearing of the first wheel as well as the second flank of a tooth of the helical gearing of the second wheel of the first double planet is in contact with the second flank of a tooth of the helical gearing of the first hollow wheel and the first flank of a tooth of the helical gearing of the first hollow wheel, respectively.

In addition, the planetary gear further comprises at least one ratchet mechanism, for allowing a movement of the double planets in one direction, in particular wherein a movement in the other direction may be locked. The ratchet mechanism may be used for allowing the double planets to move in the direction of the first force and the second force (when using a second double planet as explained below), only, wherein the other direction is locked.

The ratchet mechanism may use force or form closure. Such a mechanism may be based on principle solutions like self-jamming cams or asymmetric teeth.

One of the hollow wheels may be implemented as an input which is typically used in planetary gear, i.e. an element which is driven by a shaft, wherein the other hollow wheel may act as output. As the first double planet is in contact with the first hollow wheel with the first wheel and is in contact with the second hollow wheel via the second wheel, the movement of the hollow wheel acting as input is transferred to the hollow wheel acting as output. Also, other combinations are possible. For example, a planetary carrier may be provided which acts as input. In addition, the planetary gear may comprise a sun wheel.

If only one double planet is used, the two hollow wheels may move in relation to each other. To avoid such a movement, the planetary gear may comprise a second double planet having a first wheel with a helical gearing meshing with the helical gearing of the first hollow wheel and a second wheel with a helical gearing meshing with the helical gearing of the second hollow wheel, and may comprise a second force exerting element exerting a second force in a second direction opposite to the first direction so that the second flank of a tooth of the helical gearing of the first wheel of the second double planet is in contact with the first flank of a tooth of the helical gearing of the first hollow wheel and that the first flank of a tooth of the helical gearing of the second wheel of the second double planet is in contact with the second flank of a tooth of the helical gearing of the second hollow wheel thereby providing a preload to the hollow wheels in the second rotational direction.

By using a first and a second double planet, the stability in the planetary gear may be improved. Using the second double planet, it may be avoided that the first double planet moves in relation to the first and the second hollow wheel. For this purpose, the first and the second double planet may also be coupled via a planetary carrier. The planetary carrier may act as an input for transferring a force from the shaft to the double planets and thus to the hollow wheels.

The backlash between the two hollow wheels and the double planets is avoided due to the usage of the preloaded double planets. Due to the preload of the second double planet which is in the opposite direction to the preload of the first double planet, backlash between the gearings may be avoided and further improved. In addition, the equal distribution of the force due to the two double planets may keep the hollow wheels in place.

According to a further embodiment, the planetary gear comprises an even number of double planets, wherein a first half of the double planets is preloaded in the first rotational direction and wherein a second half of the double planets are preloaded in the second rotational direction. By providing an even number of double planets, the first force and the second force may be equally distributed over the number of double planets. Also, the preload in the first rotational direction and the preload in the second rotational direction may be equally distributed.

In a preferred embodiment, the first half and the second half of the double planets are alternatingly arranged. This may further improve the equal distribution of the first and second force as well as of the preload in the first rotational direction and the preload in the second rotational direction.

According to a further embodiment, the first force and the second force are higher than an axial force being exerted by the meshing of the helical gearings of the double planets and the hollow wheels during rotation. By such a distribution between the first and second force and the axial forces, an elastic behavior of the system may be reduced. This means that it may be avoided that the double planets move in the first and second direction during operation of the planetary gear.

In the planetary gear, the first force and the second force are preferably equal. Also, when an uneven number of double planets is used, the total amount of the first force and the total amount of the second force are preferably equal.

According to a further embodiment, the force exerting element may be a spring element, a magnetic element and/or hydraulic element. Also, any other kind of force exerting element may be possible.

Further advantages and preferred embodiments are disclosed in the claims, the description and the figures. It should be further noted that a person skilled in the art may regard or use the presented features individually or combine the presented features otherwise than indicated without extending the scope of the invention.

In the following, the present invention will be described by means of embodiments shown in the figures. The shown embodiments are exemplarily, only, and are not intended to limit the scope of protection. The scope of protection is solely defined by the attached claims.

In the following same or similar functioning elements are indicated with the same reference numerals.

<FIG> shows a planetary gear <NUM>, which comprises a first hollow wheel <NUM> and a second hollow wheel <NUM>. The planetary gear <NUM> further comprises a first double planet <NUM>, a second double planet <NUM>, a third double planet <NUM>, and a fourth double planet <NUM>. The double planets <NUM>, <NUM>, <NUM>, <NUM> may be coupled by means of a planetary carrier (not shown), which may be arranged between the double planets <NUM>, <NUM>, <NUM>, <NUM>. The planetary gear <NUM> may also comprise only one double planet <NUM>, or any other number of double planets <NUM>, <NUM>, <NUM>, <NUM>. Furthermore, a sun wheel (not shown) in addition to both hollow gears may be used.

In the embodiment shown in <FIG>, the first hollow wheel <NUM> is greater than the second hollow wheel <NUM>. The first hollow wheel <NUM> may act as an input and the second hollow wheel <NUM> may act as an output. Also, other embodiments as well as proportions of the hollow wheels <NUM>, <NUM> and the double planets <NUM>, <NUM>, <NUM>, <NUM> are possible.

The double planets <NUM>, <NUM>, <NUM>, <NUM> each comprise two wheels <NUM>, <NUM>; <NUM>, <NUM>; <NUM>, <NUM>; <NUM>, <NUM>. The first wheels <NUM>, <NUM>, <NUM>, <NUM> are in contact with the first hollow wheel <NUM> and the second wheels <NUM>, <NUM>, <NUM>, <NUM> are in contact with the second hollow wheel <NUM>. Due to this contact between the double planets <NUM>, <NUM>, <NUM>, <NUM> and the hollow wheels <NUM>, <NUM>, a movement of the first hollow wheel <NUM> in a first rotational direction <NUM> or a second rotational direction <NUM> is transferred to the second hollow wheel <NUM> and vice versa. The rotational directions <NUM>, <NUM> of the hollow wheels <NUM>, <NUM> are indicated by arrows <NUM>, <NUM> in the figures.

<FIG> shows sectional views of each double planet <NUM>, <NUM>, <NUM>, <NUM> in combination with the hollow wheels <NUM>, <NUM>. As can be seen in <FIG>, the hollow wheels <NUM>, <NUM> and the double planets <NUM>, <NUM>, <NUM>, <NUM> each have a helical gearing <NUM>. Each tooth of each helical gearing <NUM> has a first flank and a second flank opposite to the first flank. A surface normal of the first flank is directed to the first rotational direction <NUM> of the first and second hollow wheel <NUM>, <NUM> and a surface normal of the second flank is directed to the second rotational direction <NUM> of the hollow wheels <NUM>, <NUM>.

When the wheels (the hollow wheels <NUM>, <NUM> and thus also the wheels <NUM>, <NUM>; <NUM>, <NUM>; <NUM>, <NUM>; <NUM>, <NUM> of the double planets <NUM>, <NUM>, <NUM>, <NUM>) change their rotational direction <NUM>, <NUM>, i.e. perform a forward and backward motion and/or perform an oscillating movement between the two rotational directions, a backlash between the gearings <NUM> may cause an uneven motion profile of the wheels.

To minimize the backlash between the helical gearings <NUM> of the wheels <NUM>, <NUM>; <NUM>, <NUM>; <NUM>, <NUM>; <NUM>, <NUM> of the double planets <NUM>, <NUM>, <NUM>, <NUM> and the hollow wheels <NUM>, <NUM>, one or more force exerting elements <NUM>, <NUM> may be provided. A first force exerting element <NUM> exerts a first force in a first direction and a second force exerting element <NUM> exerts a second force in a second direction opposite to the first direction. The first direction and the second direction are perpendicular to the first and the second rotational direction <NUM>, <NUM>.

In the embodiment shown in <FIG> and <FIG>, the first force is applied to the first and fourth double planet <NUM>, <NUM> and the second force is applied to the second and third double planet <NUM>, <NUM>. Due to the first force, the first flanks of teeth of the helical gearings <NUM> of the first wheels <NUM>, <NUM> of the first and fourth double planet <NUM>, <NUM> are in contact with the second flanks of teeth of the helical gearing of the first hollow wheel <NUM>. In addition, the second flanks of teeth of the helical gearings <NUM> of the second wheels <NUM>, <NUM> of the first and fourth double planet <NUM>, <NUM> are in contact with the first flanks of teeth of the helical gearing of the second hollow wheel <NUM>. Due to this contact, a preload to the hollow wheels <NUM>, <NUM> in the first rotational direction <NUM> is provided.

In addition, due to the second force, the second flanks of teeth of the helical gearings <NUM> of the first wheels <NUM>, <NUM> of the second and third double planet <NUM>, <NUM> are in contact with the first flanks of teeth of the helical gearing of the first hollow wheel <NUM>. In addition, the first flanks of teeth of the helical gearings <NUM> of the second wheels <NUM>, <NUM> of the second and third double planet <NUM>, <NUM> are in contact with the second flanks of teeth of the helical gearing of the second hollow wheel <NUM>. Due to this contact, a preload to the hollow wheels <NUM>, <NUM> in the second rotational direction <NUM> is provided.

By pressing the first and fourth double planet <NUM>, <NUM> into the direction of the first hollow wheel <NUM>, the helical gearings <NUM> of the first and fourth double planet <NUM>, <NUM> are in contact with the helical gearings of the first and the second hollow wheel <NUM>, <NUM>. By pressing the second and third double planet <NUM>, <NUM> into the direction of the second hollow wheel <NUM>, the helical gearings <NUM> of the second and third double planet <NUM>, <NUM> are in contact with the helical gearings of the first and the second hollow wheel <NUM>, <NUM>.

Due to this preload, a contact between the helical gearings <NUM> of the hollow wheels <NUM>, <NUM> and the helical gearings <NUM> of the wheels <NUM>, <NUM>; <NUM>, <NUM>; <NUM>, <NUM>; <NUM>, <NUM> of the double planets <NUM>, <NUM>, <NUM>, <NUM> exists already during start up. In addition, this contact can be kept by means of the force exerting elements <NUM>, <NUM> during operation by maintaining the preload and thus, a backlash between the helical gearings <NUM> may be avoided.

The force exerting elements <NUM>, <NUM> may be implemented for example as springs, magnetic elements, hydraulic elements or any other kind of element being able to exert a force to the double planets <NUM>, <NUM>, <NUM>, <NUM>. Preferably, the total amount of the first force exerted by the first force exerting elements <NUM> and the total amount of the second force exerted by the second force exerting elements <NUM> is equal.

Claim 1:
Planetary gear (<NUM>) comprising at least
a first hollow wheel (<NUM>) with a helical gearing (<NUM>) and a second hollow wheel (<NUM>) with a helical gearing (<NUM>), wherein the first hollow wheel (<NUM>) and the second hollow wheel (<NUM>) are adapted to rotate in a first rotational direction (<NUM>) and a second opposite rotational direction (<NUM>), and
a first double planet (<NUM>) having a first wheel (<NUM>) with a helical gearing (<NUM>) meshing with the helical gearing (<NUM>) of the first hollow wheel (<NUM>) and having a second wheel (<NUM>) with a helical gearing (<NUM>) meshing with the helical gearing (<NUM>) of the second hollow wheel (<NUM>), wherein the first wheel (<NUM>) and the second wheel (<NUM>) of the first double planet (<NUM>) are fixed in relation to each other,
wherein each tooth of each helical gearing (<NUM>) has a first flank and a second flank opposite to the first flank,
characterized by
a first force exerting element (<NUM>) exerting a first force in a first direction so that the first flank of a tooth of the helical gearing (<NUM>) of the first wheel (<NUM>) of the first double planet (<NUM>) is in contact with the second flank of a tooth of the helical gearing (<NUM>) of the first hollow wheel (<NUM>) and that the second flank of a tooth of the helical gearing (<NUM>) of the second wheel (<NUM>) of the first double planet (<NUM>) is in contact with the first flank of a tooth of the helical gearing (<NUM>) of the second hollow wheel (<NUM>), thereby providing a preload to the hollow wheels (<NUM>, <NUM>) in the first and the second rotational direction (<NUM>, <NUM>),
the planetary gear further comprising a ratchet mechanism for allowing a movement of the first double planet in the direction of the first force, wherein a movement in the other direction is locked.