PatTra: Extending the transmission range of belt CVT
In a belt CVT for a car, scooter, ATV etc, an even lower gear ratio at low-gear improves the acceleration, and an even higher gear ratio at high-gear improves the mileage and reduces the noise. This is why the Transmission Gear Ratio Range (TGRR), defined as the ratio of the higher gear ratio to the lower gear ratio, is so important.
The new generation of steel belt CVTs for cars expands the Transmission Gear Ratio Range from the previous 6.0 to the current global top level of 7.0
The allowed axial displacement Lmax of the one conical-pulley-half relative to the other conical-pulley-half is limited by the effective width W of the belt. If an axial displacement L wider than W is applied to the one conical-pulley-half towards the other conical-pulley-half, it will cause collision of the working conical surfaces of the conical pulley.
The need for a higher "Transmission Gear Ratio Range" is more crucial in trucks, in agricultural tractors, in machinery equipment etc.
In the following image the PatTra mechanism (shown at "slow motion" at right):
allows the belt to run at bigger diameters, enabling a larger Transmission Gear Ratio Range.
A single-step PatTra (as in the animation above), using the same belt profile and the same minimum actual-diameter in the conical pulleys, doubles the "Transmission Gear Ratio Range".
The bigger the "Transmission Gear Ratio Range", the faster the accelerations of the vehicle, the steeper the uphills it can climb, the lower the revs at cruising and the better the mileage.
A two-step PatTra:
enables a four times larger TGRR (the TGRR=7.0, which is the current world top, goes to TGRR=28.0).
Given the angle F of the conical pulleys, given the working width W of the belt, and given the minimum effective radius Ri wherein the belt is allowed to run on the pulley, the transmission gear ratio range (TGRR) is defined (and limited).
The effective radius R wherein the belt runs is calculated as:
wherein L is the axial displacement (i.e. the displacement along the rotation axis of the conical pulley shaft) of the one conical-pulley-half relative to the other (with L=0 when the belt runs at the minimum effective radius Ri).
The tan(F) is the tangent of the angle F.
For specific Ri and F, the R gets maximized when the L is maximized, so the maximum possible effective radius Ro at which the belt can run on the conical pulley is:
The maximum allowed radial displacement Ro-Ri of the belt as it cooperates with the conical pulley is:
For instance, with a minimum radius Ri=50mm, a conical pulley angle F=79degrees (i.e. 90degrees-F=11degrees) and a belt width W=32mm:
In the image above, the belt runs on the inner conical surface S1 of the left pulley half.
In the image below, the belt runs on the outer conical surface S2 (the inner section of the left pulley half, that with the conical surface S1 on it, is displaced to the left by the right half of the pulley).
In the following image the control is mechanical (a variator in the drive pulley, at top, a spring with an actuation linkage in the driven pulley, at bottom) :
A different, and simple, approach, preferable for some applications, is the "comb" conical pulley wherein the two pulleys can get closer to each other without collision:
Another different, and simple, approach, preferable for some applications, is the "radial comb" conical pulley wherein the two pulleys can approach each other without collision (stereoscopic image):
The grooves on the one conical pulley half allow the conical surface of the other pulley half to enter without collision, and vice versa.
This type fits with continues belts (like the elastic belts used in the scooters).