Individually suspended and driven axle shaft assembly, drive system configured therewith and method for reducing traction force vibrations in such an axle shaft assembly

Abstract

Damped, individually suspended axle shaft assembly, comprising: a first axle shaft (100a) having a first torsional spring stiffness; and a second axle shaft (100b) having a second torsional spring stiffness; wherein a torsional spring stiffness difference between the first torsional spring stiffness and the second torsional spring stiffness is defined by a ratio between the first torsional spring stiffness and the second torsional spring stiffness, and wherein the ratio is greater than 1.4 to 1; wherein the first axle shaft (100a) has a first set of physical properties and the second axle shaft (100b) has a second set of physical properties, wherein a difference between the physical properties creates the torsional spring stiffness difference; characterized in that the difference between the physical properties is a relative difference between the first and second axle shafts (100a, 100b) in at least one of the properties cross-sectional diameter, strength and composition.

Description

TECHNICAL FIELD

[0001] The present invention relates generally to driven axle shafts of motor vehicles and more particularly to a damped axle shaft assembly according to the preamble of claim 1, as known for example from US 6 398 686 B1 or US 4 413 701 A.

[0002] With regard to the further prior art, reference is made to the documents US 4 240 517 A, US 4 771 842 A, US 6 681 913 B2 and US 4 699 235 A. BACKGROUND OF THE INVENTION

[0003] Motor vehicles with independent driven axle suspensions include a pair of axle shafts (also called split axles or half shafts), one for each wheel, as described for illustrative purposes only in U.S. Patent No. 4,699,235, issued October 13, 1987 to Anderson and assigned to the assignee of the present application.

[0004] It will now be discussed with reference to Fig. 1 briefly describes the split axle drive system of patent US 4 699 235 A as a reference, it being understood that the present invention is applicable to two-wheel or four-wheel drive systems.

[0005] Shown is a schematic plan view of a part-time four-wheel drive vehicle including an internal combustion engine 10, a transmission 12, and a transfer case 14 mounted on a vehicle chassis (not shown). The engine 10 and transmission 12 are well-known components, as is the transfer case 14, which typically includes an input shaft (not shown), a main output shaft 16, and a secondary output shaft 18. The main output shaft 16 is drivingly connected to, and usually aligned with, the input shaft in the transfer case 14. The secondary output shaft 18 is drivingly connected to, and usually offset from, the input shaft in the transfer case 14 by a clutch or the like.The transfer case clutch is operated by a suitable selector mechanism (not shown) which is generally controlled remotely by the vehicle operator.

[0006] The main output shaft 16 is drivingly connected to a rear drive shaft 20, which in turn is drivingly connected to a rear differential 22. The rear differential 22 drives the rear wheels 24 through split axles in a well-known manner. The auxiliary output shaft 18 is drivingly connected to a front drive shaft 26, which in turn is drivingly connected to a split axle drive mechanism 28 for selectively driving the front wheels 30 through split axles. The split axle drive mechanism 28 is attached to the vehicle chassis by means including a bracket 71 on an extension tube 66.

[0007] Suitable split axle members, commonly referred to as halfshafts, are well known from front-wheel drive motor vehicles. These can be used to connect the drive mechanism 28 to the front wheels 30. The drawings schematically illustrate a halfshaft of a conventional type for a drive connection with individually suspended, steerable vehicle wheels, comprising an axle shaft 76 having a universal plunger joint 78 at its inboard end adapted to connect to an output, such as flange 72 or 74, and the well-known Rzeppa universal joint 80 at its outboard end adapted to connect to the vehicle wheel 30.

[0008] Problematically, axle shafts often experience "traction vibration" when large torque is applied to them. Traction vibration typically occurs when tire friction relative to a road surface is periodically exceeded by low-frequency vibrations (i.e., below approximately 20 Hz) from the torsional twist of the axle shafts. Traction vibration represents a vibration reflected back to the suspension and driveline components and is perceptible to vehicle occupants, who describe the sensation as "bumping," "hitting," "bumping," or "bouncing."

[0009] Axle shafts are typically made of tubular steel and, as such, act as very efficient torsion springs. To reduce unwanted vibrations in the axle shafts, standard practice has been to adjust the size of the axle shafts (i.e., increase their diameter) to adjust the resonant vibrations in such a way that the negative influence of the vibrations is minimized by increasing the overall torsion spring stiffness of the axle shafts and thereby reducing traction vibration.However, increasing the diameter of the axle shafts leads to additional problems in terms of packaging, mass and associated costs, without addressing the central problem of directly damping traction-induced vibrations, namely: the lack of damping to absorb energy introduced into the powertrain by the negative damping characteristics of the tires during longitudinal acceleration or braking.

[0010] Accordingly, there is a clearly recognizable need in technology for axle shafts that are specifically damped in order to reduce traction vibration and the associated disturbances in the power transmission system, such as axle vibration.

[0011] The invention is therefore based on the task of meeting this need. SUMMARY OF THE INVENTION

[0012] This object is achieved with an axle shaft assembly having the features of claim 1, with a drive system having the features of claim 5 and with a method having the features of claim 9.

[0013] The present invention consists in an individually suspended, driven axle shaft assembly in which the axle shafts are asymmetrical with respect to one another, the asymmetry providing a reduction in tractive force vibration and the associated disturbances in the power transmission system, such as axle jitter.

[0014] According to a preferred embodiment of the present invention, the asymmetric axle shafts are adjusted asymmetrically so that the relative torsional stiffness between them differs by a ratio of substantially between about 1.4 to 1 and about 2.0 to 1. The asymmetry can be achieved by any known means that changes the torsional stiffness and is consistent with the operating load requirements of the axle shaft, such as by the axle shafts having the same length but different cross-sectional diameters; by the axle shafts having the same cross-sectional diameters but different lengths; by the axle shafts having different strengths (e.g., solid shafts versus hollow shafts); by the axle shafts having different material compositions; or by a combination of these.

[0015] The asymmetric axle shafts are operatively connected to a limited-slip differential to create an axle-to-axle friction torque coupling, which dampens out-of-phase torque oscillations between the asymmetric axle shafts. According to a preferred embodiment of the present invention, the asymmetric axle shafts are suspended in a fork, which in turn is connected to the vehicle frame or body either directly or via a plurality of resilient fork mounts. The fork mounts have a stiffness adjusted by a specific application to maximize the reduction of traction oscillation associated with the asymmetry of the axle shafts.

[0016] Accordingly, it is an object of the present invention to provide an individually suspended, driven axle shaft assembly in which the axle shafts are asymmetrical with respect to one another, the asymmetry providing a reduction in tractive force vibration and the associated disturbances of the powertrain, such as axle jitter.

[0017] This object, together with other objects, features and advantages of the present invention, will become clear from the following description of a preferred embodiment. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic plan view of a prior art switchable four-wheel drive vehicle. Fig. Figure 2 is a view of a rear suspension of a vehicle with asymmetric axle shafts according to the invention. Fig. 3 is a side view of an example of a first asymmetric axle shaft according to the invention. Fig. 3A is a cross-sectional view taken along line 3A-3A of Fig. 3. Fig. 4 is a first example of a second asymmetric axle shaft, which is arranged with respect to Fig. 3 is asymmetric. Fig. Figure 4A is a cross-sectional view taken along line 4A-4A of Fig. 4. Fig. 5 is a second example of a second asymmetric axle shaft, which is arranged with respect to Fig. 3 is asymmetric. Fig. Figure 5A is a cross-sectional view taken along line 5A-5A of Fig. 5. Fig. 5B is a cross-sectional view of a third example of a second asymmetric axle shaft shown in relation to Fig. 3 is asymmetric. Fig. Figure 6 is a graph of axle shaft torque versus time for a state-of-the-art symmetrical axle shaft assembly. Fig. Figure 7 is a graph of torque versus time for an asymmetric axle shaft assembly according to the invention. Fig. Figure 8 is a graph of drive shaft torque versus time comparing a symmetric and an asymmetric axle shaft assembly. Fig. Figure 9 is a graph of drive shaft torque versus time comparing a symmetric axle shaft assembly with highly damped fork mounts, an asymmetric axle shaft assembly with minimally damped fork mounts, and an asymmetric axle shaft assembly with highly damped fork mounts. Fig. Figure 10 is a graph of torque versus time for an asymmetric axle assembly at different limited-slip differential friction torque values. Fig. 11A is a cross-sectional view of a rear fork bracket. Fig. 11B is a cross-sectional view of a front fork bracket. Fig. 12 is a schematic view of a front wheel drive system wherein one of the asymmetric axle shafts includes an intermediate shaft. DESCRIPTION OF THE PREFERRED EMBODIMENT

[0018] In the set of drawings to which reference is now made, Fig. 2 to 12 show various aspects of individually suspended, driven, asymmetric axle shafts 100, 100' according to the present invention.

[0019] In Fig. Figure 2 depicts an example of a rear suspension 102 of a motor vehicle drive system incorporating asymmetric axle shafts 100. The asymmetric axle shafts 100 are configured as a set of two mutually asymmetric axle shafts: a first axle shaft 100a and a second axle shaft 100b, the asymmetry between which is such that each has a different torsional spring stiffness relative to the other. The rear suspension 102 includes a fork 104, which, in this application, is attached to a frame (not shown) of the motor vehicle by resilient fork mounts 106. A rear differential module 108 is connected to the fork 104 by means of resilient differential module mounts 110 and is further connected to the first and second axle shafts 100a, 100b of the asymmetric axle shafts 100 via constant velocity joints 112a, 112b, respectively.The first and second axle shafts 100a, 100b are each individually suspended via constant velocity joints 112a, 112b, allowing them to move independently of each other along the arrows 114a, 114b. A drive shaft 116 is connected at one end to a transmission (not shown) and at its other end to the rear differential module via a constant velocity (or other type) joint 118.

[0020] In Fig. 3 to 12, to which reference is now made additionally, structural and functional aspects of the asymmetric axle shafts 100, 100' are discussed in detail.

[0021] Fig. 3 and Fig. 3A illustrates a first axle shaft 100a, 100a', for which a length L1 is preselected and a cross-sectional diameter D1 is also preselected. The selection criteria used are those generally considered to be standard in the art with regard to durability and torque load behavior. In this regard, the first axle shaft has a selected torsional stiffness T1. For example, the first axle shaft 100a' is constructed of solid or hollow steel in a cylindrical configuration with teeth 122a, 122b at each end to engage constant velocity joints of the independent suspension.

[0022] In contrast, the second axle shaft 100b is asymmetric with respect to the first axle shaft 100a, so that its physical properties provide a different torsional stiffness T2, which may be greater or less than T1, with the torsional stiffness ratio being between approximately 1.4 to 1 and approximately 2.0 to 1. For example, the second axle shaft 100b is made of solid or hollow steel in a cylindrical configuration, also with teeth 122a, 122b at each end to engage constant velocity joints of the independent suspension.

[0023] Fig. 4 through 5B, which are discussed next, are examples of how physical differences between the first and second axle shafts 100a, 100b can provide the desired torsional spring stiffness difference.

[0024] Fig. 4 and Fig. 4A show a first example of a second axle shaft 100b, 100b', in which the length L2 is equal to the length L1; however, the cross-sectional diameter D2 is different from D1 (the teeth 122a, 122b are identical to Fig. 3). In the example shown, D2 > D1, it is of course also possible to make D2 < D1, whereby it is only necessary that D1 is not equal to D2, in such a way that the desired torsional spring stiffness difference is achieved, for which the ratio lies between approximately 1.4 to 1 and 2.0 to 1.

[0025] Fig. 5 and Fig. 5A show a second example of a second axle shaft 100b, 100b'', in which the cross-sectional diameter D2' is equal to D1; however, the length L2' is different from L1 (the teeth 122a, 122b are identical to those of Fig. 3). In the example shown, L2' < L1, it is of course also possible to make L2' > L1, whereby it is only necessary that L1 is not equal to L2, in such a way that the desired torsional spring stiffness difference is achieved, for which the ratio lies between approximately 1.4 to 1 and 2.0 to 1.

[0026] Of course, it is possible to change the physical properties in other ways to achieve the torsional stiffness difference between the first and second axle shafts 100a, 100b, for example, through a selected combination of cross-sectional diameter difference, length difference, strength difference (i.e., solid construction as opposed to hollow construction), or material composition difference (however, since different steel grades tend to all have approximately the same torsional stiffness for a given geometry, a sufficient difference is unlikely to be achieved by steel material substitution alone). An example of torsional stiffness asymmetry due to a strength difference can be seen by comparing Fig. 3 and Fig. 5B, in which a third example of a second axle shaft 100b, 100b'' is hollow and may have a larger or smaller cross-sectional diameter than D1 and a longer or shorter length than L1, thereby varying the torsional spring stiffness between them. As mentioned, either one or the other of the first axle shaft 100a and the second axle shaft 100b, or both, may be solid or hollow.

[0027] The asymmetric axle shafts 100, 100' are available with a limited-slip differential, either electrical or mechanical (such as 108 from Fig. 2 or Fig. 306 from Fig. 12), are operatively connected to create a mechanical axle-to-axle coupling, which damps out-of-phase torque oscillations between the asymmetric axle shafts. The mechanical coupling in a limited-slip differential provides a friction torque coupling between the asymmetric axle shafts, whereby, for example, through empirical testing or mathematical modeling, an optimal friction torque is provided that is optimal for a given torsional stiffness difference between the asymmetric axle shafts in a specific application.If there is no frictional torque coupling between the asymmetric axle shafts, the asymmetry between the axle shafts is not able to provide axle-to-axle damping through phase-shifted torque oscillations; on the other hand, if an open differential is used instead of a limited-slip differential, or if the coupling does not have slip between the asymmetric axle shafts, the torque oscillations between them tend to be in phase, and the damping is mitigated, i.e. reduced.

[0028] Fig. Figure 6 is a graph 200 of axle shaft torque versus time for conventional, symmetrical axle shafts, wherein the graphs 202, 204 each refer to a single axle shaft and wherein each axle shaft has a torsional stiffness of 525 Nm / degree (i.e., Newton meters per angular degree). It can be seen that the torque oscillations are in phase, thus not reducing the conditions for tractive force oscillation insofar as the torque oscillations of each axle shaft are structurally related to the other.

[0029] Fig. Fig. 7 is a graph 210 of axle shaft torque versus time for asymmetric axle shafts 100 according to the invention, wherein the graph 212 refers to the first axle shaft 100a, which has a torsional stiffness of 270 Nm / degree, and wherein the graph 214 refers to the second axle shaft 100b, which has a torsional stiffness of 525 Nm / degree. It can be seen that, in contrast to Fig. 6 the torque oscillations are out of phase, thereby mitigating the conditions for traction oscillation in that the torque oscillations of each axle shaft are destructive with respect to the other (with the out-of-phase torque oscillations being most pronounced during an initial portion of a traction oscillation event when the traction oscillation is most likely to be perceived by the vehicle occupants).

[0030] Fig. 8 is a graph 220 of the torque of the driving shaft (see 116 from Fig. 2) in comparison to the time for conventional, symmetrical axle shafts in the diagram 222, where each axle shaft has a torsional stiffness of 525 Nm / degree, where the driving shaft has a torsional stiffness of 138 Nm / degree, and where the damping of the fork mount (see 106 from Fig. 2) is 2 Ns / mm; and for asymmetric axle shafts 100 according to the invention in the graph 224, wherein the first axle shaft 100a has a torsional stiffness of 270 Nm / degree, and the second axle shaft 100b has a torsional stiffness of 525 Nm / degree, the drive shaft has a torsional stiffness of 138 Nm / degree, and with an electronic limited-slip differential with a friction torque of 400 Nm, the fork mount damping is 2 Ns / mm. It can be seen that the amplitudes of the torque oscillations in the graph initial section 222a are high, indicating that the traction oscillation has a sufficient amplitude to be perceived by occupants.On the other hand, the initial section 224a of the graph shows torque oscillations of a lower amplitude than the initial section 222a of the graph, indicating that the traction oscillation does not have sufficient amplitude to be perceived by the occupants. The fact that the subsequent section 224b of the graph 224 shows a larger residual amplitude than the subsequent section 222b of the graph.

[0031] diagram 222 is of negligible importance, since the amplitudes of these torque oscillations are not perceived by the vehicle occupants.

[0032] Fig. 9 is a graph 240 of drive shaft torque versus time for conventional symmetrical axle shafts at plot 242, wherein each axle shaft has a torsional stiffness of 525 Nm / deg, the drive shaft has a torsional stiffness of 138 Nm / deg, and fork mount damping is as high as approximately 2 Ns / mm; for inventive asymmetrical axle shafts 100 at plot 244, wherein the first axle shaft 100a has a torsional stiffness of 270 Nm / deg, and the second axle shaft 100b has a torsional stiffness of 525 Nm / deg, the drive shaft has a torsional stiffness of 138 Nm / deg, and fork mount damping is as low as approximately 0.2 Ns / mm at approximately 10 Hz; and for asymmetric axle shafts 100 according to the invention in the diagram 246, wherein the first axle shaft 100a has a torsional spring stiffness of 270 Nm / degree,and the second axle shaft 100b has a torsional stiffness of 525 Nm / degree, the drive shaft has a torsional stiffness of 138 Nm / degree, and the fork mount damping is high at approximately 2 Ns / mm. It can be seen that the amplitudes of the torque oscillations in the graph section 242a of the graph 242 are high, indicating that the traction oscillation has a sufficient amplitude to be perceived by the occupants, while those of the initial graph section 244a of the graph 244 and the initial graph section 246a of the graph 246 have amplitudes of the respective torque oscillations that are sufficiently low that the occupants would not perceive any traction oscillation. Furthermore, however, it can be seen that although the initial diagram section 244a has a torque oscillation with a relatively low amplitude,However, for the graph's subsequent section 244b, the amplitude of the torque oscillation increases to a level that can be perceived by the occupants. On the other hand, the graph 246 exhibits a torque oscillation with low amplitudes throughout, indicating that no traction oscillation would be perceived by the occupants. Accordingly, depending on the application, it may be desirable to provide highly damped fork mounts with the asymmetric axle shafts 100; however, it should also be noted that there are applications where fork mounts are not used, but where damping of the asymmetric axle shafts is still provided.

[0033] An illustration of the effect of the limited-slip differential friction torque is shown in Fig. 10 is shown, which illustrates a graph 250 of axle shaft torque versus time for asymmetric axle shafts 100 according to the present invention. In this illustration, the first axle shaft 100a has a torsional stiffness of 270 Nm / degree and the second axle shaft 100b has a torsional stiffness of 525 Nm / degree, the drive shaft has a torsional stiffness of 138 Nm / degree, and the fork mount damping is 2 Ns / mm. It can be seen that a friction torque of 100 Nm, according to graph 252, may be too low, a friction torque of 400 Nm, according to graph 254, may be optimal, and a friction torque of 2,000 Nm, according to graph 256, may be too high.

[0034] In the case of spring-loaded fork mounts 106 being used, the stiffness of the fork mounts is adjusted by the rubber configuration and selection. For illustration, Fig. 11A and Fig. 11B spring-loaded fork mounts shown, where Fig. 11A depicts a rear fork bracket 106' and Fig. 11B depicts a front fork bracket 106''. Each fork bracket 106', 106'' consists of an upper metal washer 106a, 106a', a lower metal washer 106b, 106b', a rubber core 106c, 106c', and an outer sleeve 106d, 106d'.

[0035] Fig. 12 is a schematic illustration of a front-wheel drive system 300 including an engine 302, a transmission 304, a limited-slip differential 306, and asymmetric axle shafts 100'. The first axle shaft 100a, 100a'' is, for example, as shown in Fig.3. The second axle shaft 100b, 100b''' is a combination of a second axle shaft component 100c and an intermediate shaft component 100d, which is drivingly connected thereto, for example at a fork bracket 106'''. It is understood that the asymmetry between the first and second axle shafts includes the physical properties (i.e., length, cross-sectional diameter, strength, composition, etc.) of the first axle shaft 100a, 100a'' with respect to the second axle shaft 100b, 100b'''' for the second axle shaft component 100c and the intermediate shaft component 100d, respectively, or for both of them together.

[0036] The following example is provided for illustrative purposes only, is not to be construed as a limitation, and is provided here for reference purposes only. EXAMPLE 1

[0037] For the asymmetric axle shafts, the first axle shaft 100a has a torsional stiffness of 270 Nm / degree (right axle shaft with a diameter of 35 mm between the teeth, a length of 0.6 meters, and a composition of 300M solid steel) and the second axle shaft 100b has a torsional stiffness of 525 Nm / degree (left axle shaft with a diameter of 55 mm between the teeth, a length of 0.52 meters, and a composition of 300M hollow steel with a wall thickness of 8 mm); the drive shaft has a torsional stiffness of 138 Nm / degree; the friction torque of the limited-slip differential is 400 Nm; and the fork bearings have a vertical damping of 2 Ns / mm.

Claims

[1] Damped, individually suspended axle shaft assembly, comprising: a first axle shaft (100a) having a first torsional spring stiffness; and a second axle shaft (100b) having a second torsional spring stiffness; wherein a torsional spring stiffness difference between the first torsional spring stiffness and the second torsional spring stiffness is defined by a ratio between the first torsional spring stiffness and the second torsional spring stiffness, and wherein the ratio is greater than 1.4 to 1; wherein the first axle shaft (100a) has a first set of physical properties and the second axle shaft (100b) has a second set of physical properties, wherein a difference between the physical properties creates the torsional spring stiffness difference; characterized by , that the difference between the physical properties is a relative difference between the first and second axle shafts (100a, 100b) in at least one of the cross-sectional diameter, strength and composition. [2] Damped axle shaft assembly according to claim 1, wherein the ratio is between 1.4 to 1 and 2.0 to 1. [3] A damped axle shaft assembly according to claim 1, wherein the second axle shaft (100b) comprises: a second axle shaft component (100c); and an intermediate shaft component (100a) which is drivingly connected to the second axle shaft component (100c); wherein the difference is created by selected physical properties of the second axle shaft component (100c) and the intermediate shaft component (100d). [4] Damped axle shaft assembly according to claim 3, wherein the ratio is between 1.4 to 1 and 2.0 to 1. [5] Drive system comprising: a limited-slip differential; a first axle shaft (100a) which is in driving connection with the differential, the first axle shaft (100a) having a first torsional spring stiffness; and a second axle shaft (100b) which is in driving connection with the differential, the second axle shaft (100b) having a second torsional spring stiffness; wherein a torsional spring stiffness difference between the first torsional spring stiffness and the second torsional spring stiffness is defined by a ratio between the first torsional spring stiffness and the second torsional spring stiffness, and wherein the ratio is greater than 1.4 to 1; wherein the limited-slip differential provides a predetermined friction torque with respect to the first and second axle shafts (100a, 100b); wherein the first axle shaft (100a) has a first set of physical properties and the second axle shaft (100b) has a second set of physical properties, wherein a difference between the physical properties creates the torsional spring stiffness difference; characterized by , that the difference between the physical properties is a relative difference between the first and second axle shafts (100a, 100b) in at least one of the cross-sectional diameter, strength and composition. [6] A drive system according to claim 5, wherein the ratio is between 1.4 to 1 and 2.0 to 1. [7] A drive system according to claim 5, wherein the second axle shaft (100b) comprises: a second axle shaft component (100c); and an intermediate shaft component (100d) which is drivingly connected to the second axle shaft component (100c); wherein the difference is created by selected physical properties of the second axle shaft component (100c) and the intermediate shaft component (100d). [8] A drive system according to claim 7, wherein the ratio is between 1.4 to 1 and 2.0 to 1. [9] A method for reducing traction vibration in an individually suspended, driven axle shaft assembly, comprising the steps of: a torsional spring stiffness difference is selected between a first torsional spring stiffness of a first axle shaft (100a) of the axle shaft assembly and a second torsional spring stiffness of a second axle shaft (100b) of the axle shaft assembly, wherein the torsional spring stiffness difference is defined by a ratio between the first torsional spring stiffness and the second torsional spring stiffness, and wherein the ratio is greater than 1.4; the first axle shaft (100a) is provided with the determined first torsional spring stiffness; and the second axle shaft (100b) is provided with the determined second torsional spring stiffness.

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