When starting any mechanical assembly, friction and inertia are making this phase the more consumming one in terms of efforts, power consumption and part dimensions. A biker suffers during the first meters, and if a cardan joint must break, it will break when the vehicle starts.

Varistart, in its simpliest version, is a starting assembly based on a variable reducer adaptable to all turning machines. Varistart is operating on the first turn only and becomes transparent after that.

Now a days , the start phase is managed by different systems dedicated mainly to speed regulation, mostly attached to clutch systems and gear boxes. Varistart can complete these systems on the cinematic chain or totally replaces them.

Technical description

 

 

 

 

 

 

 

 

 

 

 

 

 

VARISTART is a coaxial variator whose reduction ratio finishes to a 1:1 direct drive after a single turn of the output shaft. It uses 4 circular and non-circular gears and has very little friction loss.

The heart is a couple of identical non-circular gears, with an open primitive curb which has the shape of a spiral. This is the most basic variator on earth. It gives one turn out for one turn in, but the instantaneous speeds are different. The reduction ratio goes from R/r to r/R, with R and r being the extremes radius of the spiral. The variation of speed is (R/r)²  which can easily reach 25 ! The obvious drawback is the fact that only one turn can be achieved, before reaching the discontinuity point where the system should stop before getting destroyed !

The VARISTART concept manages that transition by using a second couple of conventional gears, a small input wheel having a radius equal to r, driving a large wheel of radius R which is linked to the leading spiral wheel by a planetary shaft able to turn around the common input-output shaft.

Let’s consider this planetary shaft blocked to start with and let’s position the spiral wheels at the discontinuity point (large driver, small driven). We have a coaxial reductor with a ratio of (R/r)². This ratio starts decreasing and after R/r turns of the input shaft, we arrive again at the discontinuity point (small driver, large driven) and a ratio of R/r x r/R = 1 and an output shaft having made one full turn.

We have blocked the planetary shaft, but effort reactions show it would have been sufficient to block it in one direction, i-e the reverse rotation of both input and output shafts. If we do that, an automatic transition takes place just before reaching the blocking point. The mechanism has the choice of having a 1:1 transmission through 4 meshing gears or a direct drive, with the whole system turning around the common axis and no wheel in relative motion. The difference of friction between those 2 ways of transmission makes the system passing smoothly from the first state to the second one. At that point, the planetary mounting starts to turn while the 4 gears stop their relative rotations.

To distribute the effort and to have a better balance, it is possible to have 2 or more planetary shafts with a same number of planetary wheels meshing with a single input wheel and a pile of output spiral wheels.

The shape of the spiral primitive curve is dictated by the desired ratio variation and can be also designed to take into account angular relative variation of input torque as PMH.

For higher ratios, 2 or more VARISTART can be mounted in series.

The positioning of the spiral wheels before and after the discontinuity point can be achieve with different methods according to the destination of the mechanism as well as the command of the different phases.