Belt type continuously variable transmission and control method thereof

1. A belt type continuously variable transmission comprising:

a continuously variable shift mechanism having a primary pulley and a secondary pulley, respective pulley widths of which can be modified, and a belt wound around the respective pulleys, which varies a shift ratio by varying the pulley width in order to vary a contact radius between the respective pulleys and the belt;

a pulley thrust calculation unit that calculates a pulley thrust, which is used to bias the respective pulleys in a direction for reducing the pulley width, so as to include a predetermined margin;

a hydraulic control unit that controls an oil pressure supplied to each of the pulleys on the basis of the calculated pulley thrust; and

an upshift determination unit that determines whether or not an upshift, during which the speed ratio is reduced, is underway,

wherein the pulley thrust calculation unit sets the predetermined margin to be smaller when an upshift is determined to be underway than when an upshift is determined not to be underway.

2. The belt type continuously variable transmission as defined in claim 1 , wherein the pulley thrust calculation unit increases an amount by which the predetermined margin is reduced steadily as the shift ratio following determination of an upshift increases.

3. The belt type continuously variable transmission as defined in claim 1 , wherein the pulley thrust calculation unit reduces the predetermined margin after a predetermined time delay has elapsed following determination of an upshift.

4. The belt type continuously variable transmission as defined in claim 1 , wherein a torque from an engine is input into the primary pulley,

a reduction rate determination unit is provided to determine whether or not a reduction rate of a rotation speed of the engine is lower than a predetermined reduction rate, and

when the reduction rate of the rotation speed of the engine is determined to be lower than the predetermined reduction rate, the pulley thrust calculation unit sets the predetermined margin to be larger than a current margin.

5. A control method for a belt type continuously variable transmission which includes a continuously variable shift mechanism having a primary pulley and a secondary pulley, respective pulley widths of which can be modified, and a belt wound around the respective pulleys, which varies a shift ratio by varying the pulley width in order to vary a contact radius between the respective pulleys and the belt, comprising:

calculating a pulley thrust which is used to bias the respective pulleys in a direction for reducing the pulley width so as to include a predetermined margin;

controlling an oil pressure supplied to each of the pulleys on the basis of the calculated pulley thrust; and

determining whether or not an upshift, during which the speed ratio is reduced, is underway,

wherein the predetermined margin is set to be smaller when an upshift is determined to be underway than when an upshift is determined not to be underway.

6. The control method for the belt type continuously variable transmission as defined in claim 5 , wherein an amount by which the predetermined margin is reduced steadily increases as the shift ratio following determination of an upshift increases.

7. The control method for the belt type continuously variable transmission as defined in claim 5 , wherein the predetermined margin reduces after a predetermined time delay has elapsed following determination of an upshift.

8. The control method for the belt type continuously variable transmission as defined in claim 5 , wherein determining whether or not a reduction rate of a rotation speed of a engine is lower than a predetermined reduction rate, and

when the reduction rate of the rotation speed of the engine is determined to be lower than the predetermined reduction rate, the predetermined margin is set to be larger than a current margin.

9. A belt type continuously variable transmission comprising:

a continuously variable shift mechanism having a primary pulley and a secondary pulley, respective pulley widths of which can be modified, and a belt wound around the respective pulleys, which varies a shift ratio by varying the pulley width in order to vary a contact radius between the respective pulleys and the belt;

pulley thrust calculation means for calculating a pulley thrust, which is used to bias the respective pulleys in a direction for reducing the pulley width, so as to include a predetermined margin;

hydraulic control means for controlling an oil pressure supplied to each of the pulleys on the basis of the calculated pulley thrust; and

upshift determination means for determining whether or not an upshift, during which the speed ratio is reduced, is underway,

wherein the pulley thrust calculation means sets the predetermined margin to be smaller when an upshift is determined to be underway than when an upshift is determined not to be underway.

10. The belt type continuously variable transmission as defined in claim 9 , wherein the pulley thrust calculation unit increases an amount by which the predetermined margin is reduced steadily as the shift ratio following determination of an upshift increases.

11. The belt type continuously variable transmission as defined in claim 9 , wherein the pulley thrust calculation means reduces the predetermined margin after a predetermined time delay has elapsed following determination of an upshift.

12. The belt type continuously variable transmission as defined in claim 9 , wherein a torque from an engine is input into the primary pulley,

reduction rate determination means is provided to determine whether or not a reduction rate of a rotation speed of the engine is lower than a predetermined reduction rate, and

when the reduction rate of the rotation speed of the engine is determined to be lower than the predetermined reduction rate, the pulley thrust calculation means sets the predetermined margin to be larger than a current margin.