Constant velocity gets consistent

May 2002 cover feature storey:Constant velocity gets consistent An Australian inventor claims to have solved a puzzle that has vexed engineers since the invention of the Cardan joint some 450 years ago. The Thompson Coupling allows two rotating shafts to be joined without the mechanical stresses, wear and vibration prevalent in most existing designs. It does this by eliminating load-bearing sliding surfaces and by tolerating axial and radial loads without any loss of efficiency. It combines the structural strength of a Cardan joint with the function of a constant velocity joint. The implications of this design could be huge. The most obvious industry to benefit is automotive, where Cardan joints have formed the mainstay of drive transfer since the year dot. Power transmissions across general industry could also see the benefits on offer as axial alignment in power transmission systems is not always possible, either due to design or operating conditions. The elimination of harmful factors such as wear, stress and vibration could prolong significantly the life of many transmission systems. Thompson also envisages their use in marine and aeronautical applications. The design has been vouched for by two of Australia's leading experts on machine theory: Professor Jack Phillips, author of a best-selling book on machine theory published by the Cambridge University Press, and Dr John Gal, a senior lecturer in the School of Engineering and Industrial Design at the University of Western Sydney. They have described the Thompson Coupling as "spectacular". This particular Eureka moment came when Glenn Thompson, who is also a keen pilot, was considering how you navigate when flying - especially the fact that the shortest route between two points on the globe is a 'great circle' and not a straight line. "With the curvature of the Earth, I imagined how lines drawn through points on the Earth's surface could intersect at the centre of the Earth. I have a lot of other interests and I had also been thinking about how to improve the design of CV joints. Suddenly, last June, the ideas came together and I realised that it should be possible to have a spherical mechanism at the coupling's centre." The result was a device, not too dissimilar in appearance from existing couplings, which uses a clever centring mechanism to restrain the input and output shaft - keeping the assembly constantly in the most efficient orientation. Cardan joints have been used in vehicles for many years, helping to counter instances of axial misalignment. However, when the shafts are out of alignment, vibration and stresses are induced which diminish both the quality of the ride and overall life of the coupling. The problem is that, when at an angle, the two shafts do not rotate in a uniform manner. Both complete a full revolution in the same time, but the relative angular velocities vary. During one revolution one shaft goes faster than the other twice and slower than the other twice. In instances where high speed and high load are encountered, such as automotive applications, this can create problems over the life of the joint. A strict relationship The problem was countered, to a certain degree, by Robert Hooke, who, in the mid 1600s, placed an intermediate shaft between two Cardan joints - a configuration which is used to this very day on many vehicles. The main drawback, however, is the strict geometric relationship which must be maintained between the two joints. The Thompson coupling is essentially two Cardan joints within the same envelope. They are joined co-axially by trunnions which are constrained to remain within the most efficient (homokinetic) plane of the joint. A two-segment, spherical four-bar linkage or spherical draglink forms the crux of the joint. One end of the drag link is connected to a pin that extends out from the input shaft, the other end of the link is attached to a pin located on the inside of the yoke of the output shaft The central axis of the drag link is located onto a pin which projects from the centre of an additional C-shaped member which sits (at 90°) within the 'arms' of the output yoke. In operation, the centre axis of the draglink (and therefore the centre of the C-shaped member) continually bisects the included acute angle between the extended axis of the input shaft and the output shaft. It therefore lies continuously on the axis of the homokinetic plane. This 'restraint' forces the trunnions connecting the inner and outer Cardan joints to lie in the homokinetic plain of the joint which is the most efficient position to be in. "The coupling is a CV joint which has no load bearing sliding surfaces," adds Thompson, "and combines the structural strength of a Cardan Joint with the function of a constant velocity joint. "It can be used in truck drivelines and heavy industrial applications, and is suitable for front-wheel drive and four-wheel drive vehicles. Commenting on the construction, Professor Phillips says: "The great thing you notice about the coupling is that all of the joints within the device are ordinary hinge joints which can be made with roller or needle bearings." Thompson has also created the Thompson Double Coupling which, as the name suggests, is a double Cardan joint but with a centring mechanism which ensures that both the input and output shafts each from an identical angle with a short, tubular, intermediate shaft. The mechanism is a variation on that used in the single coupling but uses two spherical four-bar linkages arranged to move in co-operation with each other, with the bars sharing a common axis located in the centre of the intermediate shaft. As both couplings share similar operating principles with the Cardan joints they are designed to replace, the learning curve for designers and engineers is substantially reduced. The major difference is that the major components are not subject to the same wear and tear - due to the self-inflicted dynamic loads - with only the easily replaceable bearing suffering any ill affects, if any.