There is a huge difference between the CVT in an ATV or UTV which is simply a couple of variable sheaves and a belt.
Then the CVT in a combine or tractor which uses multiple power shifts and a varilbe sheave with a chain and some even have a limited range hydrostatic drive incorporated with the power shifts.
How does a CVT work?
A car jacked up on one side provides a useful analogy to explain the basic principle of a CVT gearbox.
If you were to put the car in first gear and release the clutch, the differential in the axle would transfer the drive through the path of least resistance, causing the wheel up in the air to spin quickly, while the one on the ground stays completely still.
However, if you apply resistance to the wheel in the air, energy will be transferred to the other side and the car will fall off the jack.
A CVT essentially recreates this process using a combination of hydraulic and mechanical drives linked via a planetary gear system, comprising a sun gear, planet gears and a ring gear around the outside of the two.
The central sun gear is likened to the wheel on the ground, as it is mechanically geared to the engine and rear and front axles, so is initially the path of most resistance.
A ZF cPower CVT transmission in a loading shovel has been shown to offer considerable fuel savings over a conventional powershift transmission
The ring gear is likened to the wheel in the air, as it has the least resistance. It drives a hydraulic pump, which in turn feeds a hydraulic motor that is also directly geared to the front and rear axles.
When the engine is turned on, power is transferred through the path of least resistance, turning the hydraulic pump. But when the operator chooses to accelerate, the hydrostatic pump swings out and gradually increases oil flow to the motor, which in turn drives the tractor almost entirely hydraulically.
As the tractor continues to accelerate, the pump steadily reaches its maximum swing angle until its output is at maximum capacity.
The hydraulic motor then reverses the oil flow back towards the pump, decelerating the ring gear and steadily transferring the path of least resistance to the sun gear, so the tractor is driven 100% mechanically as it hits maximum speed.
To go into reverse, the hydrostatic pump is swung in the opposite direction, so the suction side is switched and the tractor is driven in reverse hydraulically. A shuttle clutch aids this transition.
The whole process is completely stepless from a standing start to top road speed, with no need for the operator to clutch or shift between forward and reverse.
Some manufacturers, such as John Deere, New Holland and Case IH, now have CVT transmissions that use electronic clutches to move seamlessly between a number of ranges.
