How CVTs work

A stepless gearbox lets a machine run from a standstill to maximum speed with no distinct ratio changes or ladder-rung gears.

Two sorts of CVT have been around long enough to grow whiskers. The simplest uses a pair of variable-diameter pulleys or sheaves connected by a belt; you’ll know them from combines, some ATVs and mopeds.

Hydrostatic drives uses oil lines to connect a variable-displacement pump to a fixed- or variable-displacement motor. Moving a control lever sends more or less oil to the motor, changing its speed.

Both systems deliver stepless speed change. Both are limited either by torque capacity (belt drive) or relatively low efficiency (hydrostatics).

Modern tractor CVTs blend the high efficiency of a mechanical drive with the speed flexibility of hydrostatics. Two basic designs currently rule:

Fendt’s Vario sends torque down mechanical and hydrostatic paths, with the hydrostat section controlling travel speed. At a standstill, no oil flows and the tractor can’t roll forward or back; this is active standstill. The driver calls for tractor movement using a joystick or foot pedal. Drive is initially 100% hydrostatic, then moves to 100% mechanical as speed picks up. By using a variable-displacement pump and two motors, all with exceptionally wide operating angles, Fendt’s CVT delivers a broad speed range – although for better efficiency, separate ranges for field and 50k roadwork are needed.

The Case-Steyr and John Deere transmissions are more complex. They too move from 100% hydrostatic to 100% mechanical drive as speed picks up, but because they use a fixed-displacement motor and a pump with a narrower operating angle than Fendt’s, the core set-up can’t deliver the same speed range. Consequently both add a completely automatic four-step mechanical section for 0-50kph travel.

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