Variators improved with power
Tom Shelley reports on the latest development in stepless ratio power transmission
A redesigned mechanical variator has been developed that saves space by using very wide angle cones, saves cost by running on ordinary power transmission fluid, and has an improved ratio changing mechanism.
Designed for industrial use, the device provided continuous variation of output speed over a 5:1 range from electric motors rated at up to 1.85kW. Possible automotive use of almost flat cone variators continues to be researched.
The V series of units from Bonfiglioli work rather like planetary gearboxes, except that they use planetary discs instead of planetary gears. Power transmission ratio is changed by turning a hand wheel, which brings the driving cones closer together or allows them to be further apart. Bringing them closer together forces the planetary discs between them outwards while allowing them to be further apart allows the discs to come inwards. The discs are held in slots in a spider that allows them to move in and out. The outer raceway in which the peripheries of the discs run is in the form of a slot. Since the whole arrangement is full of oil, viscous drag between the edges of the discs and the sides of the slots makes the discs rotate in a similar manner to planetary discs in a planetary gearbox. Power output is from a shaft connected to the disc spider.
Mechanical speed variators have been known since the nineteenth century. Probably the best known is the Kopp Variator which uses large balls and varies its ratio by changing the ball rotation axes angles, relative to those of the input and output shafts. Changing ball axis direction changes the circumferences of the contact paths between balls and input and output cones. Various companies have tried to develop both ball and cone variators for automotive use, but so far, none have succeeded commercially. The best known automotive ball variator is the Torotrak development and Professor N Gulia and other researchers in the Machine Design Department of Moscow State Industrial University, working in conjunction with the AMO ZIL plant, have patented automotive cone variators not dissimilar to the Bonfiglioli design, with 5 degree cones. All the automotive designs so far, seem to have come unstuck on their very demanding load requirements and consequent dependence on special lubricants such as Findett's 'Santotrack-50'. The Bonfiglioli design is covered by patent EP 1201967, which was published in May last year, but whose details are, "Not yet available." Ratio variation can be servo motor controlled if desired.
* Variators are suitable for motor ratings up to 1.85kW
* Speed ratio is about 5:1
* A new speed regulation mechanism, with proportional control, confers greater progressiveness and sensitivity in the fine tuning of speed ratio, with reduced consequent stresses.
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