Continuously variable transmission having a variable...

Interrelated power delivery controls – including engine control – Transmission control – Continuously variable friction transmission

Reexamination Certificate

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Details

C477S046000

Reexamination Certificate

active

06524220

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a continuously variable transmission, comprising:
a primary pulley for rotating at a rotational speed;
a secondary pulley, whereby a drive belt is wrapped around said pulleys at variable radial positions; and
a transmission control means for controlling the transmission ratio between a maximum and a minimum value, whose quotient defines a transmission ratio coverage at least in relation to a rotational speed of the primary pulley.
The present invention also relates to a method for controlling the transmission ratio accordingly.
DESCRIPTION OF THE RELATED ART
Such a continuously variable transmission is disclosed in EP-A-0 767 324. In particular it is known therefrom, to have transmission control means influence the actual transmission ratio within a ratio control area. The transmission speed ratio is known to be defined as the quotient between the bending radius of the drive belt when running around said secondary pulley and the bending radius of the drive belt running around said primary pulley. The ratio control area is defined in a so called variogram, which is a graphical representation of the primary pulley rotational, or input speed, which is usually equal to the engine speed, and the secondary pulley rotational, or output speed, which apart from the presence of reduction means may be considered to represent the vehicle speed. The currently generally adopted ratio control area in the variogram (see
FIG. 2
) is a limited area surrounded by a maximum input speed Nin, a maximum output speed Nout, a straight line indicating the maximum transmission torque ratio Rmax (=the lowest speed transmission ratio, i.e. Low) and a straight line indicating the minimum transmission torque ratio Rmin (=the highest speed transmission gear, i.e. overdrive Od). The actual transmission ratio can be controlled to lie within the ratio control area, whereby Rmax and Rmin, respectively Low and OD are extreme transmission ratios, whose quotient in the known continuously variable transmission defines a fixed transmission ratio coverage value. At realising a transmission design these extreme transmission ratios usually defined by determining the smallest radial positions of the drive belt attainable for both the primary and the secondary pulley such that during operation a load on the drive belt does not exceed an empirically determined maximum load value, which positions together with a desired distance between the rotational centres of the pulleys, i.e. the transmission size, determine the above-mentioned extreme transmission ratios and thus its transmission ratio coverage value. Hereby, the smallest radial position of the drive belt for the primary pulley is determined by the maximum attainable primary pulley input speed that is usually related to a maximum speed of an engine driving a load through the transmission, whereas the smallest radial position of the drive belt for the secondary pulley is determined by the maximum attainable secondary pulley output speed that is usually related to a maximum rotational speed of a load, e.g. passenger vehicle, driven by the engine through the transmission. The maximum rotational speed of the load thereby being determined by an equilibrium between a mechanical power generated by the engine and a mechanical power needed to sustain the rotational speed of the load, e.g. to overcome resistance factors such as a wind and a rolling resistance.
Within the nowadays available electronically controlled continuously variable transmissions full use of the transmission possibilities is made, whereby its application parameters however lie within the strictly bounded above-mentioned ratio control area.
SUMMARY OF THE INVENTION
It is an aim of the present invention to further extend the possibilities of a continuously variable transmission i.e. to break through the above-mentioned boundaries, in particular to extend the application and control possibilities of the prior art transmissions.
Thereto the continuously variable transmission according to the present invention is characterised in that the transmission control means are arranged for controlling said transmission ratio such that the transmission ratio coverage varies in relation to the rotational speed of the primary pulley.
Surprisingly it has been found that there is no absolute need to confine the transmission ratio control in relation to the rotational speed of the primary pulley to a transmission ratio control coverage having a fixed value. It has been found that a variable transmission ratio coverage from a point of view of the drive belt may vary and need not have a fixed value. This per se is a break through opening new and further application possibilities for continuously variable transmissions, in particular at the now extended outer limits of the control at minimum (high gear, i.e. overdrive Od) transmission speed ratio and/or maximum (low gear, i.e. Low) transmission speed ratio. This increases the area of the known ratio control area, hence extending the application possibilities and control limits of the transmission according to the invention.
It is an important aspect of the present invention that the total mechanical tension Ttotal in the drive belt, apart from other less significant tension components, is at least determined by a torque tension Tbelt related to the torque transmitted by the drive belt, a bending tension Tbend related to the amount of bending imposed on the drive belt when it runs around the primary and secondary pulleys, e.g. as quantified by the respective running radius, and a centrifugal tension Tcentr. related to centrifugal forces acting on the belt as a result of the drive belt speed and bending. It holds roughly that:
T
total=
T
belt+
T
bend+
T
centr.
It may be noted that the mechanical tension Ttotal in the drive belt during operation must be limited to a boundary value to prevent premature failure of the belt, for example due to fatiguing in case of a metallic drive belt, such boundary value conforming to the said maximum load value.
According to an important notion of the present invention Tbend may vary, and in particular be made dependent on the combined influence of Tbelt and Tcentr. without exceeding said above mentioned boundary value of Ttotal.
As for the drive belt speed, the centrifugal tension Tcentr. depends on the cubic of the drive belt speed. Since the belt speed depends on the rotational speed of the primary pulley and the transmission ratio through the belt's radial position at, at such lowered speeds the cubic thereof is lowered even more, and therefore that allowable amount of bending and the bending tension Tbend resulting therefrom may be increased without Ttotal exceeding said boundary value. Hence, for a relatively low rotational speed of the primary pulley, the maximum transmission speed ratio (low gear) may be increased and/or the minimum transmission speed ratio (high gear) may be decreased, by reducing the smallest radial position of the drive belt for the primary pulley and/or the secondary pulley respectively, thus widening the allowable ratio control area with respect to that at a relatively high rotational speed of the primary pulley.
As for the torque tension Tbelt, it is usually true that said torque tension increases with increasing speed of the primary pulley at least up to a certain speed of said pulley, as a result of the engine speed/torque characteristic of an engine driving said primary pulley. Because of this the allowable amount of bending of the drive belt and the bending tension Tbend resulting therefrom may be increased even further above said certain speed without the total mechanical tension Ttotal exceeding said boundary value, at least for relatively low rotational speed of the primary pulley. It is however noted that the torque transmitted is only directly dependent on the speed of the primary pulley, if there is a fixed connection between the engine and the primary pulley. The fixed connection usually exists when the transmi

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