GB2471512A

GB2471512A - Gear set with helical gears which move axially to reduce imbalanced loads

Info

Publication number: GB2471512A
Application number: GB0911506A
Authority: GB
Inventors: Changxlu Zhou
Original assignee: Smart Manufacturing Technology Ltd
Current assignee: Smart Manufacturing Technology Ltd
Priority date: 2009-07-03
Filing date: 2009-07-03
Publication date: 2011-01-05
Also published as: GB2485290A; WO2011001155A3; GB201121893D0; GB0911506D0; WO2011001155A2; GB2485290B; CN102667237A; CN102667237B; WO2011001155A8

Abstract

A gear set 200 comprises an input shaft 201, an output shaft 215 and two or more pairs of helical gears 202, 203, 213, 214 of opposite hand Each pair of gears 202, 203, 213, 214 are fixed relative to each other but axially moveable in tandem and mounted on the input shaft 201 and/or the output shaft 215. Three or more lay shafts 207, 208, 209 each having an input gear 204, 205, 206 engaged with one of the pairs of helical gears 202, 203 and an output gear 210, 211, 212 engaged with one of the other helical gears 213, 214 on the output shaft 215 for transmitting rotational movement from the input shaft 201 to the output shaft 215. In operation an imbalance in load sharing between the lay shafts 207,208,209 results in axial movement of the pairs of helical gears 202, 203, 213, 214 tending to reduce the imbalance.

Description

GEAR SET

The invention relates to gear sets for converting torque and speed of a source of rotational power, and in particular to gear sets having multiple lay shafts for load sharing.

To reduce the size and mass of a gear box it is known to transmit the load through multiple intermediate shafts (also known as lay-or countershafts) rather than one larger lay shaft. This allows for a reduction in the overall size of the gear box, because loads can be transmitted more effectively over two smaller shafts rather than one larger shaft.

Imperfections in the gears, however, can cause one gear to engage before the others, leading to a larger proportion of the load being taken on a single lay shaft. This results in the lay shafts having to be engineered with increased safety factors, reducing the benefit of using such multiple shafts.

With two lay shafts it is possible to ensure that the load is distributed equally between the lay shafts using a configuration such as that disclosed in US1759689, figure 1 of which is reproduced in the appended drawings as figure 1. The illustrated gear set 100 provides for a reduction in speed between a drive shaft 19 and a driven shaft 11, and a consequent increase in torque. A pair of helical gears 22, 23 having equal and opposite helix angles drive two lay shafts 26, 28 through helical gears 24, 25 mounted on the lay shafts 26, 28.

The lay shafts 26, 28 engage with the driven shaft 19 through helical gears 32, 34, both of which mesh with an output helical gear 33.

When using helical gears it is known that an axial force will be generated that is roughly proportional to the applied torque on the helical gear, with a constant of proportionality given by the tangent of the helix angle. In the arrangement shown in figure 1, an axial force parallel to the drive shaft 19 is generated on each of the helical gears 22, 23. As the gears 22, 23 are of opposite hand and able to move axially in tandem, the load is shared between the two lay shafts. This is because any load imbalance causes the gear having the higher loading to move axially, and this movement causes this loading to reduce and the loading on the other gear of the pair of gears 22, 23 to increase.

With axial forces on the input gears 22, 23 being balanced, the proportion of the load taken by each shaft 26, 28 can be controlled by altering the helix angles of each gear set 22, 24 and 23, 25. Equal load sharing is obtained by using equal helix angles of opposite hand.

Because the load between the drive shaft 19 and the driven shaft 11 is shared between the lay shafts 26, 28, the overall size of the gear set can be reduced compared with a comparably rated gear set having only one lay shaft. Although in principle further lay shafts could lead to further reductions in size, the above technique of load sharing using a pair of helical gears of opposite hand cannot be applied for more than two lay shafts, because adding a further lay shaft connected to one of the input helical gears 22, 23 will not allow the load between this shaft and the other shaft connected to the same gear to be balanced. Furthermore, adding a further lay shaft connected through a separate helical gear will not result in this shaft being balanced with either of the existing lay shafts 26, 28.

It is consequently an object of the invention to address the problem of load sharing for a gear set having more than two lay shafts.

In accordance with a first aspect of the invention there is provided a gear set comprising: an input shaft; an output shaft; two or more pairs of helical gears of opposite hand, each pair of gears being fixed relative to each other, axially moveable in tandem and mounted on the input or the output shaft; three or more lay shafts each having an input gear engaged with one of the pairs of helical gears and an output gear engaged with a gear on the output shaft for transmitting rotational movement from the input shaft to the output shaft, wherein the gear set is configured such that an imbalance in load sharing between the lay shafts results in axial movement of the pairs of helical gears tending to reduce the imbalance.

A first pair of the two or more pairs of helical gears may be mounted on the input shaft and a second pair mounted on the output shaft, each of the input gears of the three or more lay shafts being engaged with only one of the first pair of helical gears, each of the output gears of the three or more lay shafts being engaged with only one gear of the second pair of helical gears.

The three or more lay shafts may comprise: a first lay shaft having an input gear engaged with a first one of the first pair of helical gears and an output gear engaged with a first one of the second pair of helical gears; a second lay shaft having an input gear engaged with the first one of the first pair of helical gears and an output gear engaged with a second one of the second pair of helical gears; and a third lay shaft having an input gear engaged with a second one of the first pair of helical gears and an output gear engaged with the second one of the second pair of helical gears.

The gear set may comprise a third pair of helical gears, the third pair being mounted on the output shaft, wherein the three or more lay shafts comprise: a first lay shaft having an input gear engaged with a first one of the first pair of helical gears and an output gear engaged with a first one of the second pair of helical gears; a second lay shaft having an input gear engaged with a second one of the first pair of helical gears and an output gear engaged with a first one of the third pair of helical gears; a third lay shaft having an input gear engaged with the first one of the first pair of helical gears and an output gear engaged with a second one of the second pair of helical gears; and a fourth lay shaft having an input gear engaged with the second one of the first pair of helical gears and an output gear engaged with a second one of the third pair of helical gears.

The gear set may comprise two or more pairs of helical gears mounted on the output shaft, each gear of the two or more pairs of helical gears being engaged with an output gear of a different lay shaft. This arrangement is particularly suitable for gear sets having even numbers of lay shafts.

The gear set may also or alternatively comprise two or more pairs of helical gears mounted on the input shaft, an input gear of each lay shaft being engaged with one of the gears of the two or more pairs of helical gears. Each gear of the two or more pairs of helical input gears may be engaged with an input gear of a different lay shaft.

The gear set may be configured so as to share load substantially equally among the lay shafts. This is preferably achieved by the helix angles of the gears of the pairs of input and output helical gears being substantially equal. For some embodiments, for example when a first gear of a pair of helical gears engages with only one lay shaft gear while the second gear of the pair engages with two gears, to achieve equal load sharing a tangent of the helix angle of the first gear is preferably approximately twice a tangent of the helix angle of the second gear.

Optionally, a first pair and a second pair of the two or more pairs of helical gears are mounted on the input shaft, an input gear of a first one of the lay shafts comprising a third pair of helical gears of opposite hand fixed relative to each other and axially moveable in tandem on the first lay shaft, the third pair of helical gears being engaged with respective gears of the first and second pairs of helical gears. In such an arrangement, a second one of the lay shafts preferably has an input gear engaged with one of the gears of the second pair of helical gears, and a third one of the lay shafts has an input gear engaged with one of the gears of the first pair of helical gears.

A fourth pair of helical gears may be mounted on the input shaft, with a fourth lay shaft having an input gear engaged with one of the gears of the fourth pair of helical gears, the input gear of the third lay shaft comprising a fifth pair of helical gears of opposite hand fixed relative to each other and axially moveable in tandem, the fifth pair of helical gears being engaged with respective gears of the second and fourth pairs of helical gears.

For embodiments having pairs of helical gears on the input shaft, one or more of the lay shaft output gears may be axially adjustable so as to balance a load between the lay shafts. Such adjustments are preferably made during assembly prior to fixing the adjustable gear in place, for example by bolting or welding the gear to the lay shaft.

A particular advantage of the invention is that three or more lay shafts can be used in the gear set and the load between them can be shared according to how the pairs of helical input and output gears are arranged. Typically, the gear set will be configured such that load sharing is equal between the lay shafts, by configuring the helical angles of the pairs of input and output gears, although other arrangements where the load is required to be shared in a different way between the lay shafts are possible through choice of the helical angles of the gears.

An aim of the invention is to allow a gear set with any number of lay shafts to obtain equal load sharing (or any other desired load sharing). To obtain load sharing among the lay shafts at least one additional axially moveable gear pair is required. Load sharing is assured by ensuring that each lay shaft has a unique transmission path from the input shaft to the output shaft.

The invention is illustrated in further detail below by way of exemplary embodiments, with reference to the accompanying drawings in which: figure 1 is a cross-sectional drawing of a known gear set having two lay shafts; figure 2 is a schematic cross-sectional drawing of a gear set having three lay shafts; figure 3 is a schematic cross-sectional drawing of a gear set having four lay shafts; figure 4 is a schematic diagram illustrating the power flow for the known gear set of figure 1; figure 5 is a schematic power flow diagram illustrating an extension to the gear set of figure 4; figure 6 is a schematic power flow diagram illustrating the embodiment show in figure 2; figure 7 is a schematic power flow diagram illustrating an alternative embodiment having four lay shafts; figure 8 is a schematic power flow diagram illustrating an alternative embodiment having five lay shafts; figure 9 is a schematic power flow diagram illustrating a further alternative embodiment for three lay shafts; figure 10 is a schematic diagram of an arrangement of pairs of helical gears used in the embodiment of figure 9; figure 11 is a schematic power flow diagram of a further alternative embodiment for four lay shafts; figure 12 is a schematic power flow diagram of a further alternative embodiment for axial load cancellation and load sharing among four lay shafts; figure 13 is a schematic power flow diagram of a further alternative embodiment for axial load cancellation and load sharing with two lay shafts; and figure 14 is a schematic power flow diagram of a further alternative embodiment for load sharing between four lay shafts.

Figure 1, which illustrates a known gear set from US1759689, has already been described

as part of the background to the invention above.

Shown in figure 2 is a gear set 200 having three lay shafts 207, 208, 209. Each lay shaft has an input gear 204, 205, 206 engaged with one of a pair of input helical gears 202, 203 mounted on an input shaft 201. The pair of helical input gears 202, 203 are configured to rotate with the input shaft 201. The helical input gears 202, 203 are fixed relative to one another, for example by being formed as a single unit, but are mounted such that a degree of axial movement of the pair is allowed. This may be achieved by the helical input gears 202, 203 being slidable along the axis 221 of the input shaft 201, which is preferably coincident with the axis of the output shaft 215 of the gear set 200.

The lay shafts 207, 208, 209 each also have an output gear 210, 211, 212 engaged with one of a pair of helical output gears 213, 214. The pair of helical output gears 213, 214 are, similarly to the pair of helical input gears 202, 203, configured such that they are fixed relative to one another but are axially moveable in tandem.

The pair of helical input gears 202, 203 ensures that load is shared appropriately between lay shaft 209 and lay shafts 207, 208, while the helical output gear pair 213, 214 ensures that load is shared between lay shaft 207 and lay shafts 208, 209. The combination of the two axially moveable gear pairs 202, 203 and 213, 214 ensures the overall load is split between the three shafts 207, 208, 209. For equal load sharing the tangent of the helix angle of gear 202 should be twice the tangent of the helix angle of 203 and the tangent of the helix angle of 213 should be twice the tangent of the helix angle of 214. This is because each pair of helical gears will need to balance a load from two lay shafts with the load from one lay shaft. The helix angle of a helical gear may be defined as being the angle between a tangent to the teeth of the gear and an axis about which the gear rotates.

References herein to pairs of helical gears being of opposite hand mean that the helix angles of the gears are opposite in sign but not necessarily equal in magnitude.

An alternative embodiment having four lay shafts is illustrated in schematic cross-section in figure 3. The gear set 300 in figure 3 comprises lay shafts 302, 303, 304, 305 and three pairs of axially moveable helical gears 306, 307, 316, 317 and 318, 319. A first pair of helical gears 306, 307 is mounted on the input shaft 301. Each of the lay shafts 302, 303, 304, 305 have an input gear 308, 309, 310, 311 engaged with only one of the gears of the input helical gear pair 306, 307. Each gear of the pair of helical input gears 306, 307 is engaged with input gears of two of the lay shafts, a first one 306 of the pair of helical input gears engaged with input gears 309, 311 of lay shafts 303, 305 and a second one 307 engaged with input gears 308, 310 of lay shafts 302, 304. Each of the lay shafts 302, 303, 304, 305 also have an output gear 312, 313, 314, 315 engaged with only one gear of the two pairs of helical output gears 316, 317 & 318, 319 mounted on the output shaft 320. In the case of four lay shafts, each one of the pairs of helical output gears 316, 317, 318, 319 is engaged with only one of the output gears 312, 313, 314, 315 of the lay shafts 302, 303, 304, 305. As with the above embodiment having three lay shafts, the gears in each pair of helical gears are fixed relative to one another but are axially moveable in tandem.

In the case of four lay shafts, equal load sharing can be achieved by having an equal helical angle on each of the pairs of helical input and output gears. Load sharing between a first pair of lay shafts 302, 304 and a second pair of lay shafts 303, 305 is achieved by axial movement of the input pair of helical gears 306, 307. Load sharing between lay shaft 303 and lay shaft 305 is achieved by axial movement of a first pair of helical output gears 316, 317. Load sharing between lay shaft 302 and lay shaft 304 is achieved by axial movement of a second pair of helical output gears 318, 319. Overall, therefore, the input load on the input shaft 301 is shared equally between the four lay shafts 302, 303, 304, 305.

As mentioned above, for equal load sharing an equal helix angle should preferably be chosen. However, other choices of helix angle can be chosen if a different degree of load sharing is required, for example to apply more load to one pair of lay shafts compared to the other by altering the helical angles of the input pair of gears 306, 307.

The use of the terms input' and output' to denote the relative positions of various gears in the gear sets described herein does not necessarily indicate that the arrangements described can only be used in the direction thereby implied. The gear sets may alternatively be used in the reverse direction, i.e. where an input gear becomes an output gear and vice versa. As an example, the use of two pairs of output gears, such as illustrated in the four lay shaft embodiment in figure 3, could be described instead as having two pairs of input gears if used in reverse.

Figures 4 to 7 are schematic power flow diagrams illustrating various possible arrangements using multiple lay shafts and different load sharing arrangements. In each of these diagrams, as well as in the later diagrams described below, straight horizontal lines represent shafts, filled ellipses represent pairs of helical gears of opposite hand axially moveable in tandem on the relevant shaft, and restricted to rotate with the shaft on which they are mounted. Filled rectangles represent normal gears, which for the purposes of load sharing can be considered to be constrained to rotate with the shafts they are mounted upon, as well as being axially fixed to that shaft. In practice, these normal gears may be selectively engaged with the shafts around which they would otherwise rotate. Vertical lines on the diagrams connecting ellipses and rectangles represent meshing of these gears. Intersections of lines without an ellipse or rectangle are an artefact of the diagram, and do not indicate any technical features. Vertical lines which do not start and end in rectangles and ellipses are also an artefact of the diagram, and indicate only that the horizontal lines at either end are in fact continuous, and would in three dimensions be a single straight shaft, parallel to the other lay shafts.

The orientation of a vertical mesh line of one gear towards another gear indicates the hand of the relevant helix of that gear. For example, figure 4 illustrates the gearing arrangement as disclosed in US1759689 and described above with reference to figure 1.

Vertical lines 401, 402, extending between a pair of helical gears 403 on an input shaft 404 and corresponding input gears 405, 406 on lay shafts 407, 408, represent meshing of either gears 22, 24 or gears 23, 25 of figure 1. Vertical lines 409, 410, extending between output gears 411, 412 on lay shafts 407, 408 and an output gear 413 on output shaft 414, represent meshing of gears 32, 34 on lay shafts 26, 28 with output gear 33 on the output shaft 11. The handedness of the gears 22, 23, 24, 25 is indicated according to whether a vertical line extends upwards or downwards from a gear, i.e. a right-handed gear is represented by a line travelling downwards from an ellipse and a left-handed gear by a line travelling upwards from an ellipse. The reverse of this would, of course, also apply.

Since it is well known that two normal external gears in mesh always have opposite hand (except, of course, for spur gears), this is not generally indicated in the diagrams by the direction of the lines joining them. Also, throughout the diagrams input and output shafts are notional and do not specify which one is the actual input (or driving) and output (or driven) shaft. Any mention of input and output in relation to these shafts is for notation only, and should not be construed as being a limitation on the use of a gear set.

From the diagrams it is easy to check the rotational sense of the system, i.e. to confirm that the gearing arrangement will work in practice and not seize. Each shaft, with each gear mounted on it, has a direction of rotation, being either positive or negative (clockwise or anti-clockwise). As the diagrams only represent external gears, each mesh reverses the direction of rotation. Therefore, assigning a positive rotation to an input shaft leads to all lay shafts connected to the input shaft having a negative rotation, and the output shaft having a positive rotation. In more complicated diagrams, all loops can be checked for consistency in this way.

The diagram of figure 5 illustrates how the arrangement of US1759689 can be extended to load sharing using four lay shafts 501, 502, 503, 504, through the use of intermediate shafts 505, 506. In principle this can be extended to any even number of lay shafts, i.e. by adding further pairs of lay shafts to connect with one or more of the outermost lay shafts 501, 504 indicated in the diagram. The four lay shaft output gears 507, 508, 509, 510 all mesh with a single gear 511 mounted on the output shaft 512. A disadvantage with this approach, however, is the need for additional intermediate lay shafts and gearing, which increases the size and cost of the gear set.

Figure 6 shows a gear set 600 having a three lay shaft load sharing arrangement corresponding to the embodiment illustrated in figure 2. The input shaft 601 has a first pair of helical gears 602 mounted thereon, a first one of the first pair of helical gears 602 engaged with input gears 603, 604 of a first lay shaft 605 and a second lay shaft 606, while a second one of the first pair of helical gears 602 is engaged with an input gear of a third lay shaft 607. An output gear 608 of the first lay shaft 605 is engaged with a first one of a second pair of helical gears 609 mounted on the output shaft 610, while output gears 611, 612 of the second and third lay shafts 606, 607 are engaged with a second one of the second pair of helical gears 609. It can be seen that the representation of the gear set 600 in figure 6 corresponds exactly with the arrangement shown in figure 2, described above.

Figure 7 shows a gear set 700 having a four lay shaft load sharing arrangement similar to the embodiment illustrated in figure 3. The arrangement illustrated in figure 7 differs from that shown in figure 3, in that the third lay shaft 703 is engaged with the same gear of the first pair of helical gears 706 as the first and second lay shafts 701, 702, while only the fourth lay shaft 704 is engaged with the other gear of the first pair of helical gears 706.

This is shown to demonstrate that the principle can be readily extended further, which the gear set 800 shown in figure 8 illustrates.

Figure 8 illustrates a gear set 800 in which load sharing is achieved among five lay shafts by adding a fifth lay shaft 805 to the four lay shafts 801, 802, 803, 804 arranged similarly to those in figure 7, and a further pair of helical gears 807 on the input shaft 808. The first lay shaft 801, instead of engaging with the pair of helical gears 806, engages with a first one of the further pair 807, while the fifth lay shaft 805 engages with a second one of the further pair 807. This principle of adding further lay shafts and further pairs of helical gears can, in theory, be extended to any number of lay shafts. In practice, however, the number of lay shafts possible using this technique will be limited by the distance the lay shafts are offset from the input and output shafts, and by the greatest diameter of the tips of the gear teeth of the largest lay shafts. Beyond 32 parallel lay shafts the benefits are expected to be limited, as the input and output gears would have to be rather large relative to the lay shaft gears in order to achieve a useful ratio between the input and output shafts. A practical upper limit on the number of lay shafts is therefore 32, with a typical arrangement having between 3 and 8 lay shafts.

It should be noted that, since the lay shafts have no allowed axial movement, the output gear sets on the right hand side of each of the diagrams have a handedness which is independent to the left hand side. The handedness of the output gears can therefore be inverted, and load sharing among the lay shafts will be unaffected. The relative location of gears on the input shaft and also on the output shaft is also not relevant, and will not affect the load sharing.

For applications such as automotive gearboxes, where gears mounted on the output shaft need to be selectively engaged to the output shaft to provide a choice of ratios, for example by means of one or more synchronisers, it is desirable to have all the load sharing paired helical gears at the input shaft end, to avoid having them on the output shaft as this would make the gearbox excessively long. The gear set 900 illustrated in figure 9 shows how this may be achieved for 3 lay shafts. A first pair 901 and a second pair 902 of helical gears are mounted on the input shaft 903, while normal gearing 904 is mounted on the output shaft 905. A third pair 906 of helical gears is mounted on the first lay shaft 907, the third pair 906 being engaged with one gear of both the first and second pairs 901, 902. Input gears 910, 911 of the second and third lay shafts 908, 909 are engaged with gears of the second and first pairs 902, 901 of helical gears with which the pair of helical gears 906 is not engaged. Each of the pairs of helical gears 901, 902, 906 are axially moveable on their respective shafts, allowing load sharing among the three lay shafts 907, 908, 909.

The arrangement of axially moveable pairs of helical gears shown in figure 9, and extended further in the gear set shown in figure 11 (described below), is characterised by the way in which only one gear on each axially moveable pair of helical gears is in mesh with a gear on another axially moveable gear pair. This is illustrated in figure 10, in which a first, second and third pair of helical gears 1001, 1002, 1003 are shown in mesh with each other. The first and second pairs 1001, 1002 are mounted, and axially moveable, on a first shaft 1004, while the third pair 1003 is mounted, and axially moveable, on a second shaft 1005. Axial movement of each of the pairs 1001, 1002, 1003 of helical gears allows for load sharing between the second shaft 1005 and two other shafts having gears meshing with the first and second pairs 1001, 1002. The third pair 1003 of helical gears and the second shaft 1005 in figure 10 correspond with the third pair 906 of helical gears and the first lay shaft 907 in figure 9, while the first and second pairs 1001, 1002 of helical gears in figure 10 correspond with the first and second pairs 901, 902 of helical gears in figure 9.

The gear set 1100 shown in figure 11 shows that, by adding another two axially moveable pairs of helical gears 1111, 1112, load can be shared over a fourth lay shaft 1110 in addition to the first, second and third lay shafts 1107, 1108, 1109. This change from 3 to 4 lay shafts can be iterated further when adding more lay shafts. In both of the gear sets 900, 1100 shown in figures 9 and 11, all of the axially moveable pairs of gears 901, 902, 1101, 1102, 1112 on the input shaft 903, 1103 are driven directly by the input shaft 903, 1103. This means that, for equal load sharing on all the lay shafts, the gears on the input shaft which mesh with just one gear on a lay shaft (i.e. the gears of pairs 901, 902 which mesh with gears on pair 906, and the gears of pairs 1101, 1102, 1112 which mesh with gears on pairs 1106 and 1111) will preferably need approximately twice the face width to have an equivalent strength, and will preferably have a helix angle with a tangent about double that of all the other driving gears on the input shaft to allow for equal load sharing.

From the diagrams, it can be seen that using the fewest number of axial moveable paired helical gears will always result in axial loads on the input and output shafts as well as at least one of the lay shafts. For certain heavy duty applications, such as for use in wind turbines, marine and other industrial gearboxes, such axial loading may be unacceptable.

To address this problem, for an even number of shafts it is possible to add an additional pair of gears of opposite hand on the input or output shaft. These gears need not be axially moveable, or even near each other. An exemplary embodiment of such a gear set 1200 is shown in figure 12, with additional gears 1201, 1202 provided on the input shaft 1203. Because the direction of axial force on a helical gear of a given hand is determined by whether it is driven or driving, for zero axial force on a lay shaft the driven and driving gears must be of the same hand. For the embodiment shown in figure 12, this means that the input gear of the second lay shaft 1205 engages with a gear of the first pair of helical gears 1208 having the same hand as the gear of the second pair of helical gears 1209 on the output shaft 1212. Additionally, the input gear of the third lay shaft 1207 engages with a gear of the first pair of helical gears 1208 having the same hand as the gear of the third pair of helical gears on the output shaft 1212. In a general aspect therefore, any lay shaft having input and output gears engaged with respective gears of pairs of helical gears is engaged with gears having the same hand.

An alternative embodiment of a gear set in which axial loading is minimised or cancelled is shown in figure 13. In this gear set 1300, two non-axially moveable gears 1301, 1302 mounted on the same lay shaft 1303 are used, instead of on two different lay shafts. As with the embodiments of figures 9 and 11, this can be extended to include further lay shafts, but in this case maintaining cancellation of axial loading. This alternative embodiment is able to achieve both equal load sharing and zero axial loading on all shafts for an odd number of lay shafts. For equal load sharing on the lay shafts in this case, all the helix angles of the input gears should be the same.

To ensure that no axial force is developed on the lay shafts by the output gear set(s) it is known that the output gears can either have zero helix angle (i.e. spur gears) or that they are of a herringbone nature (pairs of rigidly connected helix gears of opposite hand) and those gears mounted on the lay shafts are axially moveable to ensure that both flanks of opposite hand on the herringbone gear share the load equally.

For both the second method as well as the shaft axial load cancelling instance of it, the benefit of more than 32 parallel shafts is expected to become less effective, for the same ratio limiting reasons described above in relation to the other embodiments.

Shown in figure 14 is a schematic power flow diagram for a further alternative embodiment of a gear set 1400 having four lay shafts 1404, 1405, 1406, 1407, in which load sharing between a first pair of lay shafts 1404, 1407 is achieved by means of a first pair of helical gears 1401 on the input shaft 1403 and load sharing between a second pair of lay shafts 1405, 1406 is achieved by means of a second pair of helical gears 1402 on the input shaft 1403. This particular embodiment, in which the output gear 1408 is axially fixed on the output shaft 1409, is particularly suitable where the output gear is required to be large, for example in wind turbine applications, or where multiple gearing ratios are to be selected, such as in automotive applications. In both of the situations, axially movable pairs of output gears are either less practical or infeasible.

In use, all the gears on the lay shafts and the gear 1408 on the output shaft, with which all of the output gears 1410, 1411, 1412, 1413 engage, are axially fixed. The proportion of the load taken by the first pair of lay shafts 1404, 1407 compared with the load taken by the second pair of lay shafts 1405, 1406 is therefore fixed when the gear set 1400 is operational. During assembly, this proportion can be set by allowing one of the output gears 1410, 1411, 1412, 1413 to be adjusted, so asto allow for minor differences in gear meshing that can cause load to be taken preferentially by one pair of lay shafts. Such an adjustment can be done for example by measuring a torque on the lay shafts and adjusting the axial position of one of the gears 1410, 1411, 1412, 1413 until the torque is equal to the desired proportion of the input torque (which, in this case, will preferably be one quarter of the input torque). The gear can then be fixed in position, for example by bolting, welding or otherwise permanently fixing the gear in place on the lay shaft.

The principle of allowing one or more of the output gears on the lay shafts to be axially adjustable can be extended to embodiments where more than four lay shafts are used and where the output gearing 1408 is fixed. For higher numbers of lay shafts, more than one of the output gears on the lay shafts can be made adjustable axially on assembly, to allow for the torque taken by each lay shaft to be balanced during an initial set up of the gear set. The general principle to be applied is that the number of adjustable output gears will need to be equal or greater than the number of pairs of axially movable helical gears minus one.

The above principle can also be applied for applications where more than one gearing ratio can be selected on the output shaft. In this case, an output gear on each lay shaft will be engaged with two or more gears on the output shaft, each output gear being selectably engaged with the output shaft, for example by means of a synchroniser. The adjustment step for the one or more lay shaft output gears can be done for each output gear, the set up procedure involving an adjustment being made, followed by permanent fixing of the output gear, for each available gearing ratio.

Other embodiments are intentionally within the scope of the invention as defined by the appended claims.

Claims

CLAIMS1. A gear set comprising: an input shaft; an output shaft; two or more pairs of helical gears of opposite hand, each pair of gears being fixed relative to each other, axially moveable in tandem and mounted on the input or the output shaft; three or more lay shafts each having an input gear engaged with one of the pairs of helical gears and an output gear engaged with a gear on the output shaft for transmitting rotational movement from the input shaft to the output shaft, wherein the gear set is configured such that an imbalance in load sharing between the lay shafts results in axial movement of the pairs of helical gears tending to reduce the imbalance.
2. The gear set of claim 1, wherein a first pair of the two or more pairs of helical gears is mounted on the input shaft and a second pair is mounted on the output shaft, each of the input gears of the three or more lay shafts being engaged with only one of the first pair of helical gears, each of the output gears of the three or more lay shafts being engaged with only one gear of the second pair of helical gears.
3. The gear set of claim 2 wherein the three or more lay shafts comprise: a first lay shaft having an input gear engaged with a first one of the first pair of helical gears and an output gear engaged with a first one of the second pair of helical gears; a second lay shaft having an input gear engaged with the first one of the first pair of helical gears and an output gear engaged with a second one of the second pair of helical gears; and a third lay shaft having an input gear engaged with a second one of the first pair of helical gears and an output gear engaged with the second one of the second pair of helical gears.
4. The gear set of claim 1 comprising a third pair of helical gears, the third pair being mounted on the output shaft, wherein the three or more lay shafts comprise: a first lay shaft having an input gear engaged with a first one of the first pair of helical gears and an output gear engaged with a first one of the second pair of helical gears; a second lay shaft having an input gear engaged with a second one of the first pair of helical gears and an output gear engaged with a first one of the third pair of helical gears; a third lay shaft having an input gear engaged with the first one of the first pair of helical gears and an output gear engaged with a second one of the second pair of helical gears; and a fourth lay shaft having an input gear engaged with the second one of the first pair of helical gears and an output gear engaged with a second one of the third pair of helical gears.
5. The gear set of claim 1 wherein two or more pairs of helical gears are mounted on the output shaft, each gear of the two or more pairs of helical gears being engaged with an output gear of a different lay shaft.
6. The gear set of claim 1 or claim 5 wherein two or more pairs of helical gears are mounted on the input shaft, an input gear of each lay shaft being engaged with one of the gears of the two or more pairs of helical input gears.
7. The gear set of claim 6 wherein each gear of the two or more pairs of helical gears mounted on the input shaft is engaged with an input gear of a different lay shaft.
8. The gear set of any preceding claim wherein the gear set is configured so as to share load substantially equally among the lay shafts.
9. The gear set of claim 8 wherein the helix angles of the gears of the pairs of input and output helical gears are substantially equal.
10. The gear set of claim 3 wherein a tangent of the helix angle of the first one of the first pair of helical gears is approximately twice a tangent of the helix angle of the second one of the first pair of helical gears.
11. The gear set of claim 1 wherein a first pair and a second pair of the two or more pairs of helical gears are mounted on the input shaft, an input gear of a first one of the lay shafts comprising a third pair of helical gears of opposite hand fixed relative to each other and axially moveable in tandem on the first lay shaft, the third pair of helical gears being engaged with respective gears of the first and second pairs of helical gears.
12. The gear set of claim 11 wherein a second one of the lay shafts has an input gear engaged with one of the gears of the second pair of helical gears, and a third one of the lay shafts has an input gear engaged with one of the gears of the first pair of helical gears.
13. The gear set of claim 12 comprising a fourth pair of helical gears mounted on the input shaft, a fourth lay shaft having an input gear engaged with one of the gears of the fourth pair of helical gears, the input gear of the third lay shaft comprising a fifth pair of helical gears of opposite hand fixed relative to each other and axially moveable in tandem, the fifth pair of helical gears being engaged with respective gears of the second and fourth pairs of helical gears.
14. The gear set of claim 6 wherein an output gear of one or more of the lay shaft output gears is axially adjustable so as to balance a load between the lay shafts.
15. A gear set substantially as described herein, with reference to the drawings in figures 2, 3, and 5 to 14.