Driveshaft With Driving Elements Attached to It in Groups

The invention relates to a driveshaft. The driving elements referred to are meant to be those elements which are suitable for cooperating with other machinery parts such as gears, cams, but also parts of friction and roller bearings. Below, the invention will be described in connection with a camshaft intended to be incorporated in an internal combustion engine, but it is not limited to this application. In its EP-A-0 213 529, one of the applicants has proposed a process for attaching driving elements to a hollow shaft in the case of which the latter consists of commercial tube material, with the elements being slid on and subsequently fixed to it by expanding the tube portions underneath. The material of the tube itself is plastically deformed whereas that of the cam is only elastically deformed, with the spring-back of the latter causing the pressure resulting in the force-locking connection between the two parts. To enable the connection to withstand the loads occurring in operation, the product of the pressure force and the pressurised face must achieve a certain value. If, as has been the case so far, the width of the cam as well as the outer diameter of the hollow shaft are not increased beyond the dimensions applied for other design reasons, a relatively high expansion pressure (in the region of 2500 bars) is required for the shaft to undergo the necessary plastic deformation and the cam the necessary elastic deformation. This deformation is not limited to an inner region adjoining the shaft, but extends across the entire cross-section of the cam, and because the cam is not rotation-symmetrical, the extent of the deformation is azimuthally variable. It has been found that in consequence, one of the advantages aimed at with the above-mentioned process has been lost, i.e. to dimensionally finish-machine the cam prior to assembly and, possibly, specifically influence the material structure (for instance by surface hardening). Since expansion of cams hardened in such way may cause surface cracks, the applicants, in their German application P 37 17 534.3 which does not form part of the prior art, proposed to divide the driving elements into axially separate regions whose materials have different properties and especially different expanding coefficients.

It is the object of the present invention to provide a drive-shaft having driving elements attached to it in a force-locking way in the case of which the degree of elastic expansion is so slight that it does not extend as far as their boundary regions, thereby not putting the dimensional accuracy of the finish-machined driving elements at risk. Furthermore, the object is to reduce the necessary expansion pressure to approximately half the value given above, a measure which increases the service life of the seals which, on the probe used for expanding purposes, limits the regions to be expanded below the driving elements.

If, as proposed, the driving elements are at least partially connected to each other by thin-walled sleeves (which in consequence have a greater expansion ability), e.g. in the form of double cams such as they have recently been used for multi-valve internal combustion engines, on the one hand, the area available for producing the force-locking connection is increased, as a result of which the degree of elastic deformation may be reduced, and because the sleeves are more easily expandable due to their walls being thinner, the expansion pressure itself may be reduced. The elastic deformation in the driving elements may be limited to a radially inner region. It is also possible to combine driving elements of different kinds into groups, such a cams and bearing bushes, or the connecting sleeves themselves, by selecting a suitable type of material and surface condition, may be used for the latter purpose.

In a further embodiment of the invention, the driving elements combined in a group and the associated connecting sleeves have been produced in one single part; axially different material properties are possible but not necessary.

In accordance with an alternative embodiment, the driving elements may be fixed on the one hand through internal expansion of the shaft which is hollow for this purpose or, on the other hand, by the equivalent process of shrinking on. As with the latter process, in view of the stability of the material structure, the driving elements cannot be heated to any temperature, the area pressures achieved in the course of the subsequent cooling process are relatively low, so that an increase in the pressurised area is particularly significant.

Especially for driving elements which, like cams, do not have a rotation-symmetrical cross-section, it is proposed to assist the reduction in elastic deformation between the inner face and the outer face of the element by reducing the material thickness of the latter along a certain region looking at its longitudinal section so that the outside of the element is no longer subject to any resulting deformation.

It is proposed to limit the force-locking effect to the region between the sleeves and the shaft, but to increase slightly the inner diameter of the driving elements so that the expansion of the shaft only leads to the gap between the two parts to be closed without noticeably expanding the element.

The requirements in respect of the expanding ability of the material of the driving elements may be kept relatively low so that these may be produced from malleable cast iron for example, a material which has other advantageous properties which make it particularly suitable for this application.

The FIGURE is a sectional view of a driveshaft according to the present invention.

Embodiments of the invention are illustrated in the drawing in the form of axial longitudinal sections. Double cams 2 have been slid on to the hollow shaft 1, with each double cam 2 consisting of two portions 21 forming the actual cam and a sleeve 22 connecting them. A hydraulic pressure probe which is not illustrated here but which is known to the person skilled in the art, is used to expand regions 11 of the hollow shaft which are plastically deformed in the process. The deformation is passed on to the connecting sleeves 22, but only within the elastic range; their spring-back achieved the force-locking connection connecting the shaft and the driving elements. The expanded range 11 is limited (lefthand half of the Figure) to that part of the hollow shaft 1 positioned opposite the connecting sleeve 22 so that the cam portions 21 are not or only slightly affected by elastic deformation. The gap remaining between the hollow shaft 1 and the cams has been given exaggeratedly large dimensions in the Figure. In a further embodiment (righthand half of the Figure), the expanded region 11 extends over the entire double cam 2. In order to prevent the deformation of the cam 21 from extending over an unacceptably long distance, especially as far as its surface 25, either the cams 21 may have a slightly larger internal diameter than the connecting sleeves 22 from the start so that in this case, too, elastic deformation is practically passed on to the latter only, or it is possible to provide recesses 27 as a result of which webs 24 are formed between a radially inner region 23 which is also elastically expanded and the outer region 25, with the elastic deformation being eliminated completely in the webs 24.