GB2070194A - Resilient shaft couplings - Google Patents

Abstract

A vehicle drive shaft with high flexibility includes two relatively rotatable tubes (12, 14) connected by elastomeric bushes (15) arranged to be stressed in shear when the shaft carries torque. The shaft further includes elastomeric elements (21) arranged to be compressed between radially extending portions (20, 22) of the tubes only after a pre-determined relative rotation of the tubes has been effected. Thus the drive shaft stiffness increases for loadings in excess of a pre-determined value to give a low rate at low loadings but restricted deflections at high loadings. <IMAGE>

Description

SPECIFICATION Vehicle drive shafts This invention relates to vehicle drive shafts, and particularly but not exclusively to vehicle propshafts.

Propshafts are known which are provided with high torsional flexibility to help alleviate hetrodyne problems and over-run boom. Typically such shafts consist of a pair of coaxial steel tubes each connected to a respective end coupling and located one within the other, the tubes being connected and located radially relative to one another by annular rubber bushes bonded to the tubes. When the propshaft is subjected to torque, shear stresses are induced in the bushes which distort to permit relative angular movement of the tubes.

Viewed from one aspect, the invention provides a vehicle drive shaft having flexible elements arranged so as to provide a given torsional stiffness for initial torsional deflections and increased stiffness at greater torsional deflections.

Preferably the stiffness is increased smoothly for any deflection greater than sixty per cent of the maximum designed working deflection of the drive shaft.

Viewed from another aspect, the present invention provides a vehicle drive shaft having a pair of co-axially arranged tubes coupled by an elastomeric element arranged so as to be stressed when the drive shaft is subjected to torque, and a radially extending portion on one of said tubes co-operable with a portion of the other of said tubes to compress a further elastomeric element only after a predetermined relative angular movement of the tubes from their unloaded position, there being not more than four said radially extending portions on a tube at any one axial location.

Thus after a predetermined relative rotation of the tubes the stiffness of the drive shaft is increased due to compression of the further elastomeric element(s). Hence it is possible to have a relatively low stiffness drive shaft at low loads and deflections such as is desirable to reduce transmission of unwanted vibrations, but to maintain an acceptably low deflection at high loads.

Preferably the further elastomeric element is compressed for any deflection greater than sixty per cent of the maximum designed working deflection of the tubes, to ensure a progressive increase in stiffness.

Embodiments of the invention will now be described by way of example and with reference to the accompanying drawings in which Figure 1 is a longitudinal cross section through part of a propshaft according to the invention; Figure 2 is a transverse cross section taken on the line A-A of Fig. 1; Figure 3 is- a longitudinal cross section through a further embodiment of the invention; and Figure 4 is a typical graph of load versus torsional deflection between the ends of a propshaft in accordance with the invention.

As shown in Fig. 1, a propshaft 511 for a motor vehicle comprises an outer closed section steel tube 1 2 of circular cross section, a coaxial steel sleeve 1 3 bonded with tube 1 2 and a coaxial inner closed section steel tube 14 located within sleeve 1 3 by means of annular elastomeric bushes 1 5. Bushes 1 5 are similar to those used in prior art propshafts, but are not bonded in place for reasons ex piained below.

Inner tube 14 comprises two sections 1 6 and 17, one end of section 1 6 being a close fit within the section 1 7 where they are bonded together. Section 1 6 is splined internally to receive a splined shaft 1 8 of a Hooke's joint yoke 1 9 to allow variations in length by relative sliding of the splines. Section 1 7 has three equi-spaced radially and axially extending portions 20; each carrying elastomeric elements 21 bonded on its radially extending faces. Inter-spaced between the portions 20 are three inwardly directed radially add axially extending portions of the sleeve 13, equal clearances 23 being left between the elastomeric elements 21 and the portions 22 when the propshaft is under zero load.

When the propshaft transmits torque, the elastomeric bushes 1 5 are stressed in shear as the tubes move relative to one another, but for the initial deflection the elements 21 are not compressed and thus the propshaft stiffness is relatively low. At greater deflections under higher torque however, clearances 23, between pairs of portions 20 and 22 are closed and stiffness of the propshaft is a function of both shear in bushes 1 5 and compression of elements 21 to produce relatively higher stiffness. As torque and deflection increase still further the elements 21 are deformed and the stiffnes increases further.

Fig. 4 shows a typical graph of torque versus torsional deflection between the ends of a propshaft according to the invention. It will be seen that for deflections up to 10 degrees the characteristics of load versus deflection is linear as the bushes are stressed in shear. After 10 degrees, the clearances 23 have been closed up and the stiffness progressively increases until at 20 degrees the maximum designed working deflection, the torque loading is double that which bushes alone would support. Thus in the present case any deflection of greater than 50% of the maximum designed working deflection results in an increase in stiffness compared to the stiffness for initial deflections.

Because the elements 21 act over a substantial proportion of the maximum designed deflection they can be made relatively large and soft so that there is not sharp rise in stiffness as they began to act, but instead there is a smooth transition. An even smoother transition and a different shape of graph could be achieved by using still larger elements of softer material which act at smaller deflections, possibly having an additional set of smaller elastomeric elements which act only after a larger deflection to further increase stiffness at high deflections. It is envisaged that at least one set of elastomeric elements will undergo compression within the first 60% of deflection up to the maximum designed working deflection if a smooth versus deflection curve is desired.

Although it is within the scope of the invention to bond the bushes 1 5 in place as in the prior art, it is not necessary in the case of the present embodiment. This is because the stiffness of the bushes is substantially less than in prior art propshafts for the same application and at maximum deflection, which can generally be less than in prior art examples, the shear loading on the bush is less and the need for bonding to prevent slip is obviated.

Moreover, because the shear loading carried by the bushes is reduced, a smaller volume of rubber can be used.

Thus the above described embodiment provides a propshaft which has a low stiffness at small deflections such as to occur when the vehicle is cruising, and is thus well suited to avoid transmitting vibrations under these conditions. At high torques such as when the vehicle is accelerating, the increased stiffness of the propshaft limits deflection to an acceptable level.

Fig. 3 illustrates an alternative embodiment in accordance with the invention, for applications wherein the propshaft can be of fixed length. A one piece inner tube 1 4a is used supported by annular elastomeric bushes 1 5a of different diameters, shown in Fig. 2. Elastomeric elements in compression are incorporated similar to those shown in Fig. 2.

It will be apparent that numerous variations from the illustrated embodiments could be effected without going beyond the scope of the invention.

For example, a different number of annular bushes could be incorporated, and a different number of portions 20, corresponding portions 22 and elastomeric elements 21 could be incorporated. For propshaft applications however, it is not- envisaged that more than four elements 20, 22 would be employed at any given axial location. To use more elements of sufficient strength to carry typical torque loadings it would either be necessary to reduce the volume of the elastomeric elements and make their operation too abrupt or else the structure for supporting elements 20, 22 of reduced thickness would need to be unduly massive and complex to manufacture.

The elements could of course be staggered axially if a greater number were required for some reason.

In a broader aspect of the invention, other means could be employed to achieve a variable stiffness propshaft. For example steel springs could be employed, some of which only act after a predetermined deflection of the propshaft. Alternatively, foamed elastomeric materials could be employed in compression alone, since these materials exhibit a variable stiffness in compression. Again, some form of Neihart coupling could be used, as again this gives a variable torsional stiffness.

In most embodiments however, it is envisaged that the torsional stiffness would be increased for at least forty per cent of the range of possible deflections up to the maximum designed deflection.

Claims (5)

1. A vehicle drive shaft having flexible elements arranged so as to provide a given torsional stiffness for initial torsional deflections and increased stiffness at greater torsional deflections.

2. A drive shaft as claimed in claim 1, wherein the stiffness is increased for any deflection greater than sixty per cent of the maximum designed working deflection of the drive shaft.

3. A drive shaft as claimed in claim 1 or 2, wherein at least part of the initial stiffness is provided by an elastomeric element arranged so as to be stressed in shear when the drive shaft is subjected to torque.

4. A drive shaft as claimed in claim 1, 2 or 3, comprising two coaxial tubes wherein a radially extending portion on one of said tubes co-operates with a portion of the other tube to compress an elastomeric element only after a pre-determined relative angular movement of said tubes from their unloaded position, to provide the increase in torsional stiffness after the initial deflection.

5. A vehicle incorporating a vehicle drive shaft as claimed in any preceding claim.

5. A drive shaft as claimed in claim 4, wherein there are no more than four said radially extending portions on a tube at any one axial location.

6. A drive shaft as claimed in claim 5, wherein three radially extending portions are provided on each tube.

7. A drive shaft as claimed in claim 3 including two coaxial tubes, wherein said element stressed in shear is a bush which serves to locate one tube coaxially within the other.

8. A drive shaft as claimed in claim 4 or any claim dependent thereon, wherein said radially extending portion has an elastomeric element for compression bonded to each of two substantially radially and axially extending faces thereof, whereby respective elements are compressed dependent upon the direction of torque transmission through the drive shaft.

9. A vehicle drive shaft substantially as hereinbefore described with reference to Figs.

1, 2 and 4, or Fig. 3 of the accompanying drawings.

10. A vehicle incorporating a vehicle drive shaft as claimed in any preceding claim.

CLAIMS (23 Feb 1981)

1. A vehicle drive shaft comprising two tubes located coaxially one within the other by an elastomeric bush arranged to be stresed in shear to transmit torque from one shaft to the other, a radially extending portion on one tube co-operating with a portion of the other tube to compress an elastomeric element only after a pre-determined relative angular movement of said tubes from their unloaded position of not more than sixty per cent of the maximum designed working deflection of the tubes, there being not more than four said radially extending portions on a tube at any one axial location, whereby said shaft is arranged to permit a substantial working deflection and to exhibit increased torsional stiffness after an initial deflection.

2. A drive shaft as claimed in claim 1, wherein there are three radially extending portions on said one tube.

3. A drive shaft as claimed in claim 1 or 2, wherein said radially extending portion has an elastomeric element for compression bonded to each of two substantially radially and axially extending faces thereof, whereby respective elements are compressed dependent upon the direction of torque transmission through the drive shaft.

4. A vehicle drive shaft substantially as hereinbefore described with reference to Figs.

1, 2 and 4, or Fig. 3 of the accompanying drawings.