BRANDON – There must be a myriad of high tech secrets inside a continuously variable transmission that can move a tractor from 0.018 mph without a creeper gear, all the way to 31 mph without a road gear, and never needs to shift.
“No. The Fendt transmission is remarkably simple,” said Ross Tufford, owner of Ag West Equipment in Portage la Prairie, Man.
“It’s continuously variable. There’s no changing from one gear to another because there are no gears. No shifts. No jerking. No clutching. No shock load.
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“You can keep your engine speed right at 1,700 rpm and go from a very slow crawl all the way to 31 mph without shifting.”
Tufford said the output shaft from the engine runs directly to the input side of the transmission. There is a vibration dampener on this shaft, but no clutch anywhere in the drive train.
“Whatever speed the engine is turning, that’s the exact speed the input shaft into the transmission is turning. The input shaft drives a planet carrier which consists of three spider gears with a ring gear on the outside,” Tufford said.
“There’s a sun gear at the centre and it’s geared directly to the transmission output shaft. As the back axle of the tractor turns, so does the sun gear turn.
“When the tractor stops, the sun gear stops. When the tractor goes fast, the sun gear goes fast.”
Tufford said the only task of the ring gear is to drive the transmission’s hydraulic pump.
“There are only two places power can go. Power can go to the sun gear to drive the tractor or power can go to the ring gear to drive the hydraulic pump. Power cannot go anywhere else.
“So, if the tractor is sitting in the yard idling, there’s no resistance on the ring gear because the pump isn’t pumping.
“Now, when I get up in the cab and decide to drive, I grab the stick and push it forward a little and we start to move. That’s because the stick is connected to the pump so now the pump swings into action and pumps oil.”
The pump literally does swing, up to 45 degrees, depending on how far the operator pushes the stick. More stick gives the ring gear-driven swash plate more angle and thus more oil and more power. The angle of the pump is controlled by two servo motors activated by the stick.
The pump delivers oil directly to the hydraulic motor, which also changes angle along with the pump. As the hydraulic motor begins to turn, it forces the swash plate to turn.
The rotating output of the swash plate connects to a collecting shaft, which feeds the sun gear and the tractor starts to move.
“So now we have two different forces exerting torque on the output shaft. We have hydrostatic torque that originated with the pump and transfers through the hydraulic motor. And we have mechanical torque from the sun gear.
“What comes out the final drive of the transmission is the sum of the two torque sources. It’s power splitting.
“The ratio of mechanical torque to hydrostatic torque changes as your speed changes. The only time it goes into straight mechanical is at 33 mph with the engine throttled back to 1,500 rpm. That’s almost never.”
When the operator wants to go into reverse, it’s a matter of stopping the tractor and putting the stick into reverse. No clutch work is involved because there is no clutch pedal. The pump and motor first swing back to zero angle and the tractor stops because the oil flow stops.
The pump then swings back into the pumping mode, but the hydraulic motor swings the opposite direction and goes over centre. The motor is now rotating the collecting shaft in the opposite direction, so the tractor is running in reverse.
For more information, visit www.fendt.com.
