GB2573482A

GB2573482A - Traction drive arrangement

Info

Publication number: GB2573482A
Application number: GB1704682.2A
Authority: GB
Inventors: John George Patterson Mervyn; Phillip Jordan Matthew
Original assignee: Allison Transmission Inc
Current assignee: Allison Transmission Inc
Priority date: 2017-03-24
Filing date: 2017-03-24
Publication date: 2019-11-13
Anticipated expiration: 2037-03-24
Also published as: GB201704682D0; GB2573482B

Abstract

A traction drive arrangement comprising an inner convex rolling surface 14, an outer concave rolling surface 15 mounted for rotation about a common epicyclic axis 16, and at least one roller 13 comprising a rolling surface in rolling driving engagement with the inner and outer surfaces at inner and outer contact regions respectively. Each contact region has a respective contact width, wherein the outer contact region is radially further from the epicyclic axis than the inner contact region. The width of the outer contact region is less than the width of the inner contact region. The outer contact region may comprise one contact portion (fig. 3) or two contact portions (fig. 4) and the outer rolling surface may be an annulus or ring gear, the inner rolling surface may be a sun gear and the rollers may be planetary rollers. The reduced contact area between the outer surface and roller may increase the contact stress at the outer contact without increasing the contact stress at the inner contact.

Description

This invention relates to a traction drive arrangement, and has application to a traction drive epicyclic.

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Traction drives offer an efficient means of transferring power, comprising two surfaces in mutual driving rolling engagement and separated by a thin film of traction fluid which is drawn into the region (the contact) where the two surfaces are engaged experiences a high normal loads or stress since the surfaces are clamped together. The viscosity of the traction fluid in the contact, also subjected to these high stresses, rises dramatically to the point where it becomes 'glassy', thereby enabling shear forces to be transferred from one surface to the other with the minimum of slip. The stress in the fluid (and at each contact) is the key determining factor for both the transmission of torque or power, and for the durability of the rolling surfaces, as is known in the field of tribology. Spin in traction drives can also arise due to a difference in local velocities of at least one of the rolling surfaces across the contact; the resulting variation in slip across the contact causes a loss in efficiency compared with a pure rolling contact with zero spin. A traction epicyclic device (when configured in a similar fashion to a conventional simple epicyclic) typically has nominally zero 'contact spin' which creates a high efficiency drive mechanism. Because drive is transmitted without the need for drive teeth, the drive outputs minimal noise and is not subject to the usual ratio constraints of conventional geared epicyclics, allowing exceptionally high drive ratios of up to approximately 14:1. Another advantage of this device is the ability to accommodate high speeds and, for a given level of torque, high power capacity.

Though not limited to such a configuration, the traction epicyclic typically comprises an array of planets circumferentially spaced and arranged for rotation in a planetary carrier assembly which is disposed between a radially inner sun, and a radially outer annulus. The sun, planets and annulus are in driving engagement with one another as described above. Thus there are two sets of rolling contacts in series with one another: the sun-planet(s) contact(s), and the planet(s)-annulus contact(s). Load may be transmitted between the sun and annulus using a loading arrangement which may comprise an annulus which is deformed into a trilobular shape and slid over the sun and planets before being allowed to relax, thereby causing the annulus to bear inwards radially clamping the annulus against the planets and the planets against the sun. In this configuration it may be observed that the normal forces in the annulus-planet contacts (the 'outer contacts') are nominally equal to the forces in the planet-sun contacts (the 'inner contacts').

Problem to be solved

Challenges in configuring a traction epicyclic may best be explained with reference to the configuration which mimics a conventional epicyclic. However, it should be noted that the outer contact geometry is defined by the planet (a convex outer surface) which is engagement with the annulus inner surface (concave inner surface). The inner contact geometry is defined by the planet (a convex outer surface) which is engagement with the sun outer surface (convex outer surface). The outer contact is therefore convex-concave whilst the inner contact is convex-convex in nature. This means that the area over which the normal contact forces are spread at the inner contacts is less than the area over which the normal contact forces are spread at the outer contacts. The inner contact stresses are therefore higher than those at the outer contact. This means that the device cannot be optimised for durability since life (characterised typically by the number of cycles to failure) is a high power function of contact stress. Further, it has been found that certain traction fluids are less effective at transmitting shear force and power below a threshold contact stress. It may be desirable to increase the radial (normal contact) force in order to increase the contact stress at the outer contacts so that the requisite torque or power can be transmitted. However, this creates a compromise in that the stress and therefore durability of the inner contacts is compromised, a situation which is exacerbated by the high rate at which stress cycles are accumulated by the relatively high rotational speed of the sun. This can lead to the requirement to over-engineer the design (either in size, or through the use of costly superior materials) in order to alleviate the unfavourable conditions (high stress or insufficient power capacity) created by the aforementioned compromise.

An aim of this invention is to improve the power and/or torque transmission capacity of a traction epicyclic. A further aim is to improve the durability of a traction epicyclic. A further aim to enable both of the foregoing to be achieved.

invention

In a first aspect this invention provides a traction drive arrangement comprising an inner convex rolling surface, an outer concave rolling surface mounted for rotation about a common epicyclic axis, and at least one roller comprising a rolling surface in rolling driving engagement with the said inner and outer surfaces at inner and outer contact regions respectively, wherein the outer contact region is radially further from the epicyclic axis than the inner contact region, and wherein a mean normal contact stress of the inner contact region is less than twice the mean normal contact stress of the outer contact region.

The mean normal contact stress of the inner contact region may be less than one and a half times the mean normal contact stress of the outer contact region.

The mean normal contact stress of the inner contact region may be substantially the same as the mean normal contact stress of the outer contact region.

Whilst the outer concave surface or surfaces may be substantially or wholly parallel to the epicycle axis, it is understood that one or both of these surfaces may be inclined to the epicyclic axis. Likewise, the inner convex surface or surfaces may be substantially or wholly parallel to the epicycle axis, but it is understood that one or both of these surfaces may be inclined to the epicyclic axis.

In a second aspect this invention provides a traction drive arrangement comprising an inner convex rolling surface, an outer concave rolling surface mounted for rotation about a common epicyclic axis, and at least one roller comprising a rolling surface in rolling driving engagement with the said inner and outer surfaces at inner and outer contact regions respectively, wherein the outer contact region is radially further from the epicyclic axis than the inner contact region, and wherein a peak normal contact stress of the inner contact region is less than twice the peak normal contact stress of the outer contact region.

The peak normal contact stress of the inner contact region may be less than one and a half times the peak normal contact stress of the outer contact region.

The peak normal contact stress of the outer contact region may be substantially the same as the peak normal contact stress of the inner contact region.

The normal stress may be arrived at through application of Hertzian formulae. Alternatively, and perhaps preferably, the stresses may be determined through the use of Finite Element Analysis (FEA) in which the geometry of the part (which may be complex, which subtle micro-geometric features for the alleviation and tailoring of local stresses) are modelled in 2D or 3D. By creating such a geometric model, the loading conditions may be applied and an FEA mathematical simulation performed. The normal contact stresses (sometimes referred to as contact pressure) may be determined from output of such a mathematical procedure. The mean normal stress may be defined as the total force, determined from the integral of the stress over the total contact region or regions of interest, divided by the total area of the applicable contact regions. In this case, the 'total area' may be approximated as the total area over which the engaged surfaces make substantial contact (e.g. have a contact stress greater than 10% of the maximum stress in the contact region). The peak contact stress may be determined directly from the simulation output.

In a third aspect this invention provides a traction drive arrangement comprising an inner convex rolling surface, an outer concave rolling surface mounted for rotation about a common epicyclic axis, and at least one roller comprising a rolling surface in rolling driving engagement with the said inner and outer surfaces at inner and outer contact regions respectively, each contact region having a respective contact width, wherein the outer contact region is radially further from the epicyclic axis than the inner contact region, and wherein the width of the outer contact region is less than the width of the inner contact region.

The width of the outer contact region may be less than 75% of the width of the inner contact region.

The width of the outer contact region may be less than 50% of the width of the inner contact region.

The respective contact widths may be arrived at through application of Hertzian formulae. Alternatively, and perhaps preferably, the contact widths may be determined through the use of Finite Element Analysis (FEA) in which the geometry of the part may be defined by a 2D or 3D model, as described earlier. The width of the respective contact may be defined as the total width of contact (in the direction of the epicyclic axis) over which the local contact stress is greater than 10% of the maximum contact stress of the applicable contact region.

The contact regions and/or contact widths may comprise more than one portion, in which case the individual portions of the applicable contact regions or widths may be summed when determining the contact width or mean contact stress.

In any of the aforementioned aspects, the following optional features may be incorporated.

The outer rolling contact may comprise a plurality of contact portions. The outer rolling contact may comprise two contact portions. The outer concave rolling surfaces may define the said two outer contact portions. The two contact portions may be spaced apart in the direction of the epicyclic axis.

The inner convex rolling surface may be an outer periphery of a sun, the outer concave rolling surface may be an inner periphery of an annulus. The roller or rollers may be planets.

The roller or rollers may be clamped between the inner convex rolling surface and the outer concave surface.

The roller is preferably mounted for rotation about its own axis in a planetary carrier.

The drive ratio of the traction epicyclic, defined as the ratio of rotational speeds of the outer rolling surface about the epicyclic axis to the inner rolling surface about the epicyclic axis (with the carrier held stationary) may be in excess of 4, or in excess of 6, in excess of 10, or in excess of 12.

The traction epicyclic arrangement preferably comprises three rollers, and preferably comprises three and only three rollers. That is, the inner convex rolling surface is in driving contact with three rollers, though in some embodiments there may be further rollers between the said three rollers and the outer concave rolling surface. This makes for a statically determinate arrangement. Assembly of the device is also facilitated if a deformable annulus is installed over the planets during assembly.

The outer concave surface may be substantially parallel to the epicycle axis. The outer concave surface may alternatively be inclined to the epicycle axis.

The inner convex surface or surfaces may be substantially parallel to the epicycle axis. The inner convex surface or surfaces may alternatively be inclined to the epicycle axis.

Preferably a clamp load provides the inner and outer contact stresses. The outer rolling surface may be an interference fit on the rollers and inner rolling surface for the provision of the said clamp load. The outer rolling surface may be mechanically deformed prior to installing over the roller or rollers prior to being released to provide the said clamp load.

Figure 1 shows a conventional geared simple epicyclic.

Figure 2 shows a schematic of a traction epicyclic

Figure 3 shows the side elevation of a traction epicyclic according to an embodiment of the present invention;

Figure 4 shows the side elevation of a traction epicyclic according to a second embodiment of the present invention;

Figure 5 shows the side elevation of a traction epicyclic according to a third embodiment of the present invention;

With reference to Figure 1, a conventional epicyclic comprises a sun 2, a ring gear (or annulus) 1 which engage (mesh) with an array of planet gears 3. The sun 2 and annulus 1 are mounted for rotation about the epicyclic axis 6. The planets 3 are mounted for rotation about their own respective axes on a carrier, but the carrier itself rotates about the epicyclic axis 6. The planets 3 are arranged at equal circumferential intervals on the carrier. A typical maximum achievable ratio (defined as the speed of the sun divided by the speed) is 4.

With reference to Figure 2, one example of a traction epicyclic comprises a sun 12, a ring (or annulus) 11 which are in driving engagement with an array of planet s or rollers 13. The regions of contact between the sun 12 and planets 13 are termed the inner rolling contact 14. The regions of contact between the planets 13 and the annulus 11 are termed the outer rolling contact or contacts 15. The rolling contacts may be lubricated with traction oil which may be either fed directly or indirectly on to the running (rolling) surfaces. The sun 12 and annulus 11 are mounted for rotation about the traction epicyclic axis 16. The planets or rollers 13 are mounted for rotation about their own respective axes on a carrier, but the carrier itself rotates about the traction epicyclic axis 16. The planets or rollers 13 are arranged at equal circumferential intervals on the carrier. A typical maximum achievable ratio (defined as the speed of the sun divided by the speed) is 14 which is far greater than that which may be achieved using a conventional geared epicyclic.

One embodiment of the invention is shown in Figure 3. In this example, there is one outer contact 15 only, and one inner contact 14 only. There may be multiple contact regions which form the outer contact, and there may be multiple contact regions which form the inner contact. However, this embodiment shows that the total width of the outer contact 15 is less than the total width of the inner contact 14, which allows the peak and/or mean contact stress at the outer contact to be increased relative to the stress at the inner contact; another way of describing this is that the peak and/or mean contact stress at the inner contact to be decreased relative to the outer contact. In either case, the stress at the outer contact can be made to be high enough to increase the torque capacity of the traction epicyclic since a minimum contact stress is desirable in order to transmit the shear forces necessary for the transmission of the required torque. This increase in outer contact 15 stress may be achieved without the need to increase the radial clamping forces on the planets 13 (for example, by increasing the interference fit of the annulus 11), which would have a detrimental impact on durability of the sun 12 which, as described earlier, experiences relatively high contact stresses due to the adverse convex-convex curvatures of the planets 13 and sun 12 elements. Thus the device may be smaller and lighter than it would otherwise need to be to provide the same torque / power capacity or durability performance.

Figure 4 shows an alternative embodiment to that shown in Figure 3. The outer contact region is in two portions [15], thereby reducing the width and/or contact area of the outer contact 15 relative to the inner contact 14. This provides the annulus 11 with effective rocking or swashing stability.

Figure 5 shows a similar embodiment to that shown in Figure 4. The outer contact region is in two portions [15], thereby reducing the width and/or contact area of the outer contact 15 relative to the inner contact 14. This provides the annulus 11 with effective rocking or swashing stability. The planets or rollers 13 are mounted for rotation in bearings (shown).

Claims

1) A traction drive arrangement comprising an inner convex rolling surface, an outer concave rolling surface mounted for rotation about a common epicyclic axis, and at least one roller comprising a rolling surface in rolling driving engagement with the said inner and outer surfaces at inner and outer contact regions respectively, each contact region having a respective contact width, wherein the outer contact region is radially further from the epicyclic axis than the inner contact region, and wherein the width of the outer contact region is less than the width of the inner contact region.

2) A traction drive arrangement according to claim 1 wherein the outer contact comprises one contact portion that is narrower than any of the contact portion of the inner rolling contact portion.

3) A traction drive arrangement according to either claim 1 or claim 2 wherein the outer rolling contact comprise two contact portions.

4) A traction drive arrangement according to claim 3 wherein the outer concave rolling surfaces defines the said two outer contact portions.

5) A traction drive arrangement according to either claim 3 or claim 4 wherein the two contact portions are spaced apart in the direction of the epicyclic axis.

6) A traction drive arrangement according to any preceding claim in which the inner convex rolling surface is an outer periphery of a sun, the outer concave rolling surface is an inner periphery of an annulus, and wherein the roller is a planet.

7) A traction drive arrangement according to any preceding claim in which the roller is clamped between the inner convex rolling surface and the outer concave surface.

8) A traction drive arrangement according to any preceding claim in which the roller is mounted for rotation about its own axis in a planetary carrier.

9) A traction drive arrangement according to any preceding claim in which the drive ratio defined as the ratio of rotational speeds of the outer rolling surface about the epicyclic axis to the inner rolling surface about the epicyclic axis is in excess of 4.

10) A traction drive arrangement according to claim 9 in which drive ratio defined as the ratio of rotational speeds of the outer rolling surface about the epicyclic axis to the inner rolling surface about the epicyclic axis is in excess of 6.

11) A traction drive arrangement according to claim 10 in which drive ratio defined as the ratio of rotational speeds of the outer rolling surface about the epicyclic axis to the inner rolling surface about the epicyclic axis (with the carrier which holds the roller or rollers being stationary) is in excess of 10.

12) A traction drive arrangement according to claim 11 in which drive ratio defined as the ratio of rotational speeds of the outer rolling surface about the epicyclic axis (with the carrier which holds the roller or rollers being stationary) to the inner rolling surface about the epicyclic axis is in excess of 12.

13) A traction drive arrangement according to any preceding claim in which the traction epicyclic arrangement comprises three rollers.

14) A traction drive arrangement according to any preceding claim in which the traction epicyclic arrangement comprises three and only three rollers.

15) A traction epicyclic arrangement according to any preceding claim in which the outer concave surface is substantially parallel to the epicycle axis.

16) A traction epicyclic arrangement according to any preceding claim in which the outer concave surface is inclined to the epicycle axis.

17) A traction epicyclic arrangement according to any preceding claim in which the inner convex surface or surfaces are substantially parallel to the epicycle axis.

18) A traction epicyclic arrangement according to any preceding claim in which the inner convex surface or surfaces are inclined to the epicycle axis.

19) A traction epicyclic arrangement according to any preceding claim in which a clamp load provides the inner and outer contact stresses.

20) A traction epicyclic arrangement according to claim 19 in which the outer rolling surface is an interference fit on the rollers and inner rolling surface for the provision of the said clamp load.

21) A traction epicyclic arrangement according to claim 20 in which the outer rolling surface is mechanically deformed prior to installing over the roller or rollers prior to being released to provide the said clamp load.

22) A traction epicyclic arrangement according to any preceding claim in which the mean value of contact stress in the outer contact region is greater than 0.5 GPa and less than 2.5 GPa.

23) A traction epicyclic arrangement according to any preceding claim in which the mean value of contact stress in the outer contact region is greater than 1 GPa and less than 2 GPa.

24) A traction epicyclic arrangement according to any preceding claim in which the mean value of contact stress in the inner contact region is greater than 0.5 GPa and less than 2.5 GPa.

25) A traction epicyclic arrangement according to any preceding claim in which the mean value of contact stress in the inner contact region is greater than 1 GPa and less than 2 GPa.

26) A traction epicyclic arrangement according to any preceding claim in which the carrier is stationary.