Patent Description:
Gears in transmission systems are traditionally solid gears for maximum stiffness and increased strength. However, solid gears can be undesirable since they can significantly increase the weight of the gearing system resulting in reduced economy, and can be expensive to produce.

Gears typically experience both axial translation and rotational effects (i.e. bending of the outer rim) when loads are applied in use. This is because the force acting on the helical gear from an adjacent meshing gear includes an axial component. The axial component of the force can cause the gear to bend at the outer rim. This bending produces a rotation of the gear outer rim, about an axis tangential at the circumference of the rim, perpendicular to the gear main axis of rotation, at the point of contact with the mating gear. Typically in transmission applications even microscopic rotations (bending of the gears) can prevent teeth of the gear from meshing together with teeth of adjacent gears as intended, (non-conjugate meshing). This can increase gear local contact stress, transmission error and noise which can lead to premature gear failure.

Pure axial translation deflections of the gear rim generally do not have a detrimental effect on meshing of adjacent gears and so should have minimal effect on gear noise, local contact stress or wear.

Hollow gears have previously been designed to reduce weight of transmission systems. Such hollow gears have been used in relatively large gearing systems such as marine or wind turbine gears to reduce weight, and therefore cost. However, hollow gears can detrimentally affect the stiffness and strength of the gear compared to a solid gear, thereby increasing the risk of the wheel rim rotational effects and axial translations discussed above.

<CIT> relates to a differential gearing for a motor vehicle having a hollow driving gear.

<CIT> describes a composite and metal hybrid gear including a hub, a cylindrical rim, and a plurality of engagement features. <CIT> describes a gear of the prior art, on which the two part form of claim <NUM> is based.

According to the present invention there is provided a gear for use in a transmission system as defined in claim <NUM>. Embodiments of the invention are defined by the dependent claims.

Certain embodiments of the invention provide the advantage that the gear is more resistant to higher gear loading that is often associated with higher speed transmission systems.

Certain embodiments of the invention provide the advantage that the weight of a gear can be reduced compared to known gears.

Certain embodiments of the invention provide the advantage that rotational deflection of the gear rim, caused by axial forces, can be reduced compared to known gears.

Certain embodiments of the invention provide the advantage that wear of the gear teeth is reduced compared to known gears, resulting in an increased lifetime of the gear.

Certain embodiments of the invention provide the advantage that gear noise can be reduced in a transmission system compared to known systems.

<FIG> show an example of a gear <NUM>. The gear <NUM> is suitable for use in various transmission systems. The gear <NUM> is particularly useful in high speed transmission systems, in transmission systems under high loading or transmission systems where it is desirable to minimise weight. For example, the gear <NUM> may be used in a transmission system of a vehicle (e.g. a car, ship, helicopter), or a fixed machine requiring different rotational speeds and torques (e.g. a wind turbine). The gear <NUM> may be particularly useful for weight saving in transmission systems in the electric automotive industry where weight and performance are critical. Transmission systems are well known by those skilled in the art, so for brevity will not be described in detail.

The gear <NUM> includes an outer annular element <NUM>. The outer annular element <NUM> extends around the outer periphery of the gear <NUM>, forming an annular ring. The outer annular element <NUM> also extends in the axial direction to form a substantially cylindrical outer surface.

On the radially outer surface of the outer annular element <NUM> (i.e. on a peripheral surface of the gear <NUM>), there are a plurality of gear teeth <NUM>. The gear teeth <NUM> project radially outwardly from the radially outer surface of the outer annular element <NUM>, so that in use the gear teeth <NUM> can mesh with gear teeth of an adjacent gear or pinion. The gear teeth <NUM> are helical gear teeth, each forming a portion of a helix extending around the gear <NUM>. In other words, the gear teeth <NUM> are arranged helically on the radially outer surface of the outer annular element <NUM>. The gear teeth <NUM> are arranged parallel to each other and are equally spaced around the periphery of the gear <NUM>.

In this example, the gear is about <NUM> in diameter, having an axial thickness at the outer periphery of about <NUM>. The gear teeth <NUM> project radially outwardly about <NUM> from the radially outer surface of the annular element <NUM>. The gear teeth <NUM> are positioned at an angle of about <NUM>° from the axial direction of the gear, each of the teeth forming a portion of a helix extending around the gear <NUM>. The gear teeth <NUM> are arranged parallel to one another, with an equal pitch between each of the teeth of about <NUM>. The gear teeth <NUM> are formed integrally with the outer annular element <NUM>.

The gear <NUM> also includes an inner support element <NUM>. The inner support element <NUM> forms a radially inner portion of the gear <NUM> and is configured to support radially outer components of the gear (including the outer annular element <NUM>). The inner support element <NUM> is arranged coaxially with the outer annular element <NUM>, so that both the inner support element <NUM> and outer annular element <NUM> share the same axis of rotation.

The inner support element <NUM>, in this example, is an axially extending shaft. The shaft is substantially cylindrical and has a hollow bore in this example. In other examples a solid shaft may be used. The axis of rotation of the gear <NUM> extends longitudinally through the centre of the shaft.

First and second opposing side walls <NUM>, <NUM> connect the inner annular element <NUM> to the outer annular element <NUM>. Each side wall <NUM>, <NUM> extend from the outer annular element <NUM> to the inner support element <NUM>. The inner support element <NUM>, outer annular element <NUM> and first and second side walls <NUM>, <NUM> form an annular space <NUM> inside the gear <NUM>.

In cross-section, as shown in <FIG>, each of the first and second side walls <NUM>, <NUM> extend from the outer annular element <NUM> at an angle θ. The angle θ is greater than <NUM> degrees. The angle θ is the angle between the respective side wall and a radially inner surface of the outer annular element <NUM>.

In this example, the outer annular element <NUM> is substantially cylindrical and extends axially (i.e. in a longitudinal direction parallel to the axis of rotation of the gear). Thus, the angle θ may alternatively be defined as the angle between the side wall and the axial direction.

The first and second side walls <NUM>, <NUM>, in this example each extend from the outer annular element <NUM> at an angle θ of about <NUM>°. In this example, both side walls <NUM>, <NUM> are constantly inclined between the outer annular element <NUM> and the inner support element. In this example, the first and second side walls <NUM>, <NUM> are both substantially frusto-conical in shape.

Alternatively, the angle of the first and second side walls <NUM>, <NUM> may be measured with respect to a direction perpendicular to the rotational axis (A-B) of the gear <NUM>. Thus, the first and second side walls <NUM>, <NUM> extend from the outer annular element <NUM> at an angle greater than <NUM> degrees with respect to the direction perpendicular to the rotational axis (A-B) of the gear <NUM>. In the example shown in <FIG>, the first and second side walls extend from the outer annular element <NUM> at an angle of about <NUM>° with respect to the direction perpendicular to the rotational axis (A-B) of the gear <NUM>.

As seen best in <FIG> and <FIG>, the first and second side walls <NUM>, <NUM> each flare outwardly from the outer annular element <NUM> towards the inner support element <NUM>, so that the gear <NUM> is thicker (i.e. has a greater axial length) at the centre than at the outer periphery. That is, the axial length of the inner support element <NUM> is greater than an axial length of the outer annular element <NUM>.

In other examples, the angle θ at which each side wall <NUM>, <NUM> extends from the outer annular element <NUM> may aptly be up to <NUM>°, or between <NUM>° and <NUM>°, or between <NUM>° and <NUM>°, or between <NUM>° and <NUM>°. That is, the first and second side walls <NUM>, <NUM> may extend from the outer annular element <NUM> at an angle that is aptly up to <NUM>°, or between <NUM>° and <NUM>°, or between <NUM>° and <NUM>°, or between <NUM>° and <NUM>° with respect to the direction perpendicular to the rotational axis (A-B) of the gear <NUM>.

Aptly, both the first and second side wall <NUM>, <NUM> extend from the outer annular element <NUM> at the same angle (i.e. the first and second side walls <NUM>, <NUM> each have equal and opposite angles of inclination).

Aptly, at least one of the side walls <NUM>, <NUM> is constantly inclined between the outer annular element <NUM> and the inner support element <NUM> so that the side wall <NUM>, <NUM> meets the inner support element at an angle α that is equal to <NUM>° minus the angle θ. For example, where the angle θ is <NUM>°, the angle α is <NUM>°.

The angle of inclination of each of the first and second side walls <NUM>, <NUM> is aptly constant along their respective lengths between the outer annular element <NUM> and the inner support element <NUM>.

The thickness of each of the first and second side walls <NUM>, <NUM> is configured to optimise the rigidity of the gear <NUM>, whilst still maintaining a reduced weight compared to a solid gear. For example, a gear of <NUM> diameter that may be used in automotive applications may have first and second side walls <NUM>, <NUM> with a wall thickness of between <NUM> and <NUM>. Aptly, the wall thickness may be about <NUM>.

The first and second side walls <NUM>, <NUM> are rigidly connected to the inner support element <NUM> and the outer annular element <NUM> (e.g. via a weld). In other examples, the inner support element <NUM>, outer annular element <NUM> and first and second side walls <NUM>, <NUM> may be integrally formed (e.g. in a mould, or machined from a fabrication). In other examples, the side walls may be thermally expanded or shrunk to be fit and/or held in place with fixtures.

The components of the gear <NUM> (including the inner support element <NUM>, first and second side walls <NUM>, <NUM>, outer annular element <NUM> and gear teeth <NUM>) in this example are all formed from steel. In other examples, the outer annular element <NUM> could be made from a gear steel and the other components, <NUM>, <NUM> and <NUM>, could be made from cheaper steels or other materials. In another example each gear component could be made from any suitable materials, non-metallic or otherwise.

The gear <NUM> may be manufactured by first forming each of inner support element <NUM>, outer annular element <NUM> (including gear teeth <NUM>) and first and second side walls <NUM>, <NUM>. Each of these components may be cast, forged or machined from suitable steel using suitable tooling. The first and second side walls <NUM>, <NUM> are then rigidly attached via a weld to each of the inner support element <NUM> and the outer annular element <NUM> to form the gear <NUM> having an annular space <NUM>. Alternatively the outer annular element <NUM>, one side wall <NUM> and the inner support element <NUM>, can be machined from one solid piece as one component. Finally the remaining wall <NUM> can be welded in place. In some examples access holes, cut-outs or some other local features on the side walls <NUM> and <NUM> may also be provided.

In use, the gear <NUM> is arranged within a transmission system to mesh with one or more adjacent gears. The gear is therefore subject to gear loading forces from the adjacent gears. As discussed above, in general these forces can cause axial deflection and radial rotation of the gear annular element <NUM>.

Table <NUM> below illustrates the differences in axial radial rotation of different gear types. The first gear is a typical solid gear with parallel side walls, having a gear rim width (i.e. thickness at the outer periphery) of <NUM>. The second gear is a solid gear shaped with frusto-conical sides also having a gear rim width of <NUM>. As shown in the table, the solid conical sided gear is significantly heavier than the solid parallel sided gear. The third gear is a hollow gear shaped with frusto-conical side walls, as per the embodiment of the invention discussed above in relation to <FIG>. The hollow conical sided gear also has a rim width of <NUM> and has a mass about <NUM>% lighter than the solid parallel sided gear.

Each of the gears in Table <NUM> were subjected to typical and identical gear loading. As shown in the Table <NUM>, the hollow conical sided gear has a <NUM>% reduction in radial rotation compared to a solid parallel sided gear and a <NUM>% reduction in radial rotation compared to a solid conical sided gear. <FIG> illustrate the axial deflection effects of the hollow conical sided gear and the solid parallel sided gear respectively. <FIG> illustrate the radial rotation effects of the hollow conical sided gear and the solid parallel sided gear respectively. It is clearly shown in <FIG> that the hollow conical sided gear experiences significantly less radial rotation than the solid, parallel sided gear.

The rotational gear deflection causes each gear tooth, as it passes an adjacent gear tooth, to be misaligned, which results in unwanted premature tooth impacts, at tooth passing frequency. This often causes a higher frequency noise and also has a detrimental effect on the life of the gear.

This reduction in radial rotation of the hollow conical sided gear reduces misalignment of adjacent gear teeth, thereby reducing, non-conjugate local contact stress, transmission error and noise, thus reducing wear of the gears and therefore increasing lifetime of the gear.

Although the hollow conical sided gear has a slight increase in axial deflection, this is not critical as the linear translation does not generally result in misalignment of adjacent gear teeth, so does not have much of an effect on gear noise or wear.

Thus, as can be seen from Table <NUM>, the hollow conical sided gear presents significant weight saving advantages over the traditional parallel sided solid gear as well as having a significant reduction in radial rotation (deflection). Similar effects can be seen in other examples of gears described herein in which at least one of the first and second side walls extends from the outer annular element at an angle greater than <NUM> degrees (i.e. gears in which the first and second side walls extend from the outer annular element at an angle that is greater than <NUM> degrees with respect to the direction perpendicular to the rotational axis of the gear).

In some examples, the gear <NUM> may form part of a differential gear assembly. <FIG> shows a gear <NUM> including a differential gear housing assembly having a differential gear shaft component <NUM> as the inner support element. In this example, the first and second side walls connect to the differential shaft component <NUM>, containing the differential gears. The other components of the gear <NUM> may be configured to similarly to the gear <NUM> described in relation to <FIG>, so will not be described again in detail.

Although in the example described above both the first and second side walls each have equal and opposite angles of inclination, in other examples the first and second side walls may each have different angles of inclination. For example, the first side wall may extend between the outer annular element <NUM> and the inner support element <NUM> at an angle perpendicular to the axis of rotation of the gear <NUM>, whilst the second side wall may extend from the outer annular element <NUM> at an angle greater than <NUM>° (i.e. greater than <NUM> degrees with respect to the direction perpendicular to the axis of rotation of the gear). In another example both the first and second side walls may extend from the outer annular element at different angles greater than <NUM>°. For example, the first side wall may extend from the outer annular element at an angle of about <NUM>° (i.e. about <NUM> degrees with respect to the direction perpendicular to the axis of rotation of the gear), whilst the second side wall may extend from the outer annular element at an angle of about <NUM>° (i.e. about <NUM> degrees with respect to the direction perpendicular to the axis of rotation of the gear).

In some examples, the first and second side walls may not be fully frusto-conical. For example, one or both of the side walls may include additional features that interrupt the frusto-conical shape. The additional features may include, for example, a region of the side wall having a different angle of inclination, or grooves or protrusions in the surface of the side wall. In other examples, at least one of the first and second side walls may have a continuous gradient in the angle of inclination (e.g. starting from an angle of <NUM>° at the outer annular element and gradually increasing to an angle of inclination of <NUM>° at the inner support element).

As above, the inner support element is a shaft. In the example shown in <FIG>, in which, as above, the differential gear shaft component <NUM> is the inner support element, the first and second side walls each have a different angle of inclination. However, any angles of inclination as described above in relation to the example of <FIG> (including equal and opposite angles of inclination) may also be used.

Although the example described above relates to a specific size gear, the gear may be various sizes depending on the application. For example, the diameter of the gear may range from <NUM> to <NUM>. Suitable angles of inclination of the first and second side walls may vary between different size gears and will be within the range discussed above for all sizes. However, the thickness of the side walls may be greater for larger diameter gears, where the gear teeth geometry will also change.

For example, a gear suitable for use in a wind turbine may have a diameter of between <NUM> and <NUM>. The first and second side walls for a gear this size may be between <NUM> and <NUM>, and aptly around <NUM>. The gear teeth may project about the gear rim from the peripheral surface of the gear, and be spaced proportionate with the necessary increase in gear module.

With the above-described examples, the gear has a reduced weight compared to a solid gear, which can reduce costs of materials required for manufacture, costs associated with fuel economy (e.g. when used in a vehicle), and transportation and installation costs. Reduced weight gears are also more suitable for use in high speed transmission systems, particularly in the electric automotive or aerospace industries where weight reduction can significantly improve performance.

Although the gear has a significantly reduced weight, the configuration of the gear, particularly the side walls, helps the gear to retain or have even greater stiffness than a solid gear about important axes. The increased stiffness of the gear resists loading forces in use, so that radial rotation of the gear is reduced or minimised. The reduction in radial rotation of the gear rim has significant advantages in that misalignment of the gear teeth with teeth of adjacent gears is reduced. This prevents or significantly reduces non-conjugate tooth impacts as the teeth of adjacent gears mesh with one another. As such, local contact stress, transmission error and noise and wear of the gears are all reduced. The positioning of the first and second side walls of the gear helps to resist gear loading forces, particularly in the axial direction, which helps to resist radial rotation of the gear rim. This helps to ensure that the teeth of the gear do not become misaligned with respect to adjacent gears, thereby reducing gear teeth contacts, reducing gear local contact stress, transmission error and noise, thus mitigating premature wear of the gears.

Claim 1:
A gear (<NUM>) for use in a transmission system, the gear (<NUM>) comprising:
an outer annular element (<NUM>) having gear teeth (<NUM>) arranged helically on a radially outer surface thereof;
an inner support element (<NUM>) arranged coaxially with the outer annular element (<NUM>); and
first and second opposing side walls (<NUM>, <NUM>), each side wall extending from the outer annular element (<NUM>) to the inner support element (<NUM>) to form an annular space (<NUM>),
wherein both of the first and second side walls (<NUM>, <NUM>) extend from the outer annular element (<NUM>) at an angle greater than <NUM> degrees with respect to a direction perpendicular to the rotational axis of the gear (<NUM>),
wherein an angle of inclination of each of the first and second side walls (<NUM>, <NUM>) is constant along their respective lengths from the outer annular element (<NUM>) to the inner support element (<NUM>), and each of the first and second side walls (<NUM>, <NUM>) are rigidly connected to or integral with each of the outer annular element (<NUM>) and the inner support element (<NUM>) such that, in use, the first and second side walls resist gear loading forces in the axial direction,
wherein the inner support element (<NUM>) is an axially extending shaft extending from the first side wall to the second side wall,
wherein the outer annular element (<NUM>) is cylindrical and extends axially, and
wherein an axial length of the inner support element (<NUM>) is greater than an axial length of the outer annular element (<NUM>),
characterised in that the first and second opposing side walls (<NUM>, <NUM>) extend from respective axial ends of the outer element (<NUM>) towards the respective axial ends of the inner support element (<NUM>).