This invention relates to a transmission shaft for use as the longitudinal shaft (usually, and herein, termed the propeller shaft) of a motor vehicle. Such a shaft is used for power transmission between a forwardly mounted engine unit and a rearwardly mounted differential gear unit, or gearbox and differential, in a vehicle.

It is usual to provide for a vehicle's propeller shaft to be capable of accommodating a relatively small change in the length of the shaft. Such a change in length is necessary to facilitate installation of the shaft and to accommodate geometrical changes with suspension movement and movement of the engine unit in service and may be provided by a sliding splined joint forming part of the shaft. If, however, the vehicle is involved in an accident involving front and/or rear impact, the shortening of the shaft length allowed for in its design may be exceeded as the body structure of the vehicle collapses longitudinally. It will be appreciated that it is usual to design the body structure of some vehicles, particular modern passenger cars, to absorb impact energy by progressive collapse of the front and rear parts thereof. Under these circumstances, the propeller shaft will act as a rigid strut, which can buckle with possible dangerous consequences. For example, it may puncture the vehicle's fuel tank, cause damage to fuel lines, possibly enter the passenger compartment of the vehicle, or cause the engine unit to be displaced in an undesirable manner such as to enter the passenger compartment of the vehicle.

Accordingly, it would be desirable if the propeller shaft were capable of axial collapse under such accident conditions, to avoid the above mentioned dangerous effects. The shaft should, of course, perform as an ordinary shaft in normal service. Further, if the shaft were to be capable of collapsing progressively and absorbing energy in so doing, it could assist the energy absorbing design of the structure of the vehicle and increase overall vehicle safety. It is the object of the present invention to provide a propeller shaft which meets these requirements.

According to the invention, we provide a propeller shaft including two tubular shaft elements having portions interfitting axially with one another and of a cross-sectional shape providing for transmission of torque therebetween and to prevent relative axial displacement between said elements under axial loads encountered in normal service, and wherein the configuration of said interfitting portions is such that, during shortening of the shaft by relative axial displacement between said shaft elements under excess axial load, at least one of said shaft elements is subject to progressive deformation thereby to absorb energy.

By arranging at least one of the shaft elements progressively to be deformed by the other of the shaft elements as the shaft collapses axially, the requirement for energy absorbtion to enable the propeller shaft to partake in the energy absorbing deformation of the rest of the vehicle structure is met. The simplest safety requirement would, of course, be met if the shaft collapsed without further resistance after the initial resistance to collapse is overcome, but then the

shaft would take no further part in controlling the progressive deformation of the vehicle's structure.

In use, a motor vehicle propeller shaft may rotate at a very high speed, e.g. several thousands of revolution per minute. It is important that the bending stiffness of such a shaft should be sufficient to ensure that the shaft remains straight and in balance at such speeds. Therefore the axially interfitting torque transmitting connection between the two tubular shaft elements must be of such a character as ^" to provide sufficient bending rigidity in the shaft.

Preferably said shaft elements adjacent said interfitting portions are of circular cross-sectional shape, and said interfitting portions thereof are deformed together to a splined configuration, i.e. one having circumferentially spaced axially extending projections and recesses.

By deforming said portions of the shaft elements together, a backlash free connection is established therebetween, which is rigid with respect to bending of the shaft.

The shaft element whose interfitting portion is innermost may be arranged to be progressively deformed during collapse of the shaft, by having its undeformed outside diameter greater than the minor diameter of the deformed portion of the outer shaft element. As will be described hereafter, the difference between such dimensions determines the force required to be exerted during shortening of the shaft.

The invention also provides a method of manufacturing the propeller shaft as above described, comprising assembling portions of two tubular shaft elements together in interfitting relation and deforming said portions together to provide a cross-sectional shape for torque transmission therebetween and a connection of a tightness sufficient to resist relative axial displacement between said elements under axial loads encountered in normal service, at least one of said deformed portions having a dimension arranged to cause progressive deformation of the other sha ^' ft element during shortening of the shaft by relative axial displacement between said shaft elements under excess axial load.

The invention will now be described by way of example with reference to the accompanying drawings, of which:-

Figure 1 is an elevation, partly in section, of part of a propeller shaft according to the invention;

Figure 2 is a longitudinal cross-section, in two planes of Figure 3, of part of the shaft assembly illustrating manufacture thereof;

Figure 3 is a transverse cross-section through part of the shaft assembly.

Referring firstly to Figure 1 of the drawings, there is shown part of a propeller shaft according to the invention. At one end of the shaft, there is a Hookes joint 10 of which one yoke is connected to a drive flange 11 and the other yoke is welded to a tubular shaft element 12. The tubular shaft element 12 is connected at 13, by a torque transmitting connection as described

hereafter, to a further tubular shaft element 14 which is itself welded to the outer element 15 of a slip spline joint whose inner element is indicated at 16. The remainder of the shaft is not shown. The spline joint 15, 16 provides for length adjustment of the shaft to facilitate installation and to accommodate relatively small shaft length changes in use.

Referring now to Figures 2 and 3 of the drawing, the torque transmitting connection 13 and the method of manufacture thereof are described in or ^' e detail.

Initially, tubular element 12 is of appreciably greater diameter than tubular element 14 and a portion 17 thereof is swaged down to a smaller diameter. A portion 18 of tubular element 14 is expanded to a diameter such that portions 17, 18 closely telescopically interfit axially with one another. Such interfitting portions are then deformed simultaneously with one another to a splined torque transmitting configuration having interfitting circumferentially spaced axially extending projections and recesses. This deformation is effected by inserting a mandrel 19 into the portion 18 of shaft element 14, the mandrel having an external surface of the required configuration, and inwardly deforming circumferentially spaced parts 17a, 18a of the portions 17, 18 together. This is effected by axial movement of a plurality of circumferentially spaced rollers as 20, in the direction of arrow 21, in engagement with the exterior of portion 17. The extreme end 22 of portion 18 remains at its expanded dimension.

The dimensions of the shaft element 14, and roller 20 and mandrel 19, are selected so that the minor diameter of the deformed portion 17 of the outer tubular shaft element 12, indicated as A in Figure 3, is less

than the outside diameter of the undeformed inner shaft element 14, the latter dimension being indicated as B in Figure 2.

The length of the interfitting and deformed portions 17, 18 must be sufficient to ensure torsional rigidity and bending stiffness in the complete shaft.

In use of the shaft, frictional forces between portions 17, 18 as a result of the deformation thereof, together with the axial interfitting pro ^' vided by the minor diameter of portion 17 being less than dimension B and less than the diameter of the extreme end 22 of portion 18, are sufficient to prevent any relative movement between the shaft elements 12, 14 under axial forces which normally arise. In the event of an accident sufficiently severe to attempt to cause shortening of the propeller shaft beyond that able to be accommodated by the splined joint 15, 16, this is likely to apply a sufficient axial force to the shaft to overcome friction between portions 17, 18 and cause the shaft to collapse axially. During such axial collapse, the fact that dimension A is less than dimension B means that the shaft element 14 will be deformed by portions 17 of shaft element 12 as the shaft shortens. The shaft thus progressively absorbs energy during its axial collapse to enhance safety as above described. The difference between dimensions A and B determines the magnitude of such energy absorbtion.

Although one torque transmitting configuration to which the shaft elements 12, 14 are deformed has been described, it will be appreciated that other torque transmitting configurations could be adopted. Further, although as above described it is the shaft element 14

which is progressively deformed to absorb energy during axial collapse of the shaft, in a shaft of suitable configuration the other, or possibly both, of the shaft elements may provide for such energy absorbtion.