Tractor- Drive Train

A Few Drive Train Details

Click for larger image: 41 KB This is a simple LDRAW model of the 2-speed transmission. It is built around the center differential (the one between the front and rear differentials).
This assembly is very simple and does not perform too well. I would have liked to have used the transmission parts out of the recent Lego supercars, but I didn't have any at the time of building my tractor. I think they would have helped to make a better transmission that was easier to operate.

Click for larger image: 38 KB The shifting takes place when the middle axle is moved forward or back. The 1x5 liftarm on the middle axle is pushed or pulled to engage the gears.
Low gear is when the 8-toothed gear is pushed forward to engage the 24-toothed gear on the differential body. High gear is when the toothed half-bushing is pushed back to engage the dark gray 16-toothed gear. This gear (the "clutch" gear) then meshes with the 16-toothed gear on the differential body. Neutral can be obtained by carefully pushing the 1x5 liftarm to where nothing is engaged (a middle position).
Problems arise when the middle axle is moved back or forth. If it doesn't slide freely, then the 1x5 liftarm will slide the 8-toothed gear, or half-bushing, along the stuck middle axle and out of alignment. Then the transmission will not work properly.
One note: there is further gear reduction between the center differential and the wheel differentials, I just didn't include them in the CAD model.

Click for larger image: 54 KB The other problem is actuation of the transmission. My goal was to motorize the shifting so that it could be done by remote control.
My solution was to use 2 10L threaded axles to hold on to the 1x5 liftarm and then move the axles back and forth. I placed worm gears on either axle and used them as rack gears. The 8-toothed gears ran the worm gears back and forth, therefore moving the 1x5 liftarm back and forth.
The threaded axles had to be restrained from rotation, so I used a triangle which has axle holes (cross-shaped holes) on either end (the yellow part in the picture to the left). The threaded axles run through the triange's end holes which restrain the axles from rotation and also help hold them in place.

My final conclusion is that although this transmission held up under the high torque, it basically sucked because of shifting problems. It also took up too much room. Not pictured in the CAD drawings is the motor used to move the shifter. It took up a lot of room too.

I will definitely use the new transmission parts from the supercars the next time I build a transmission.
Yes, I know these parts aren't "new". They've been around for many years now, but I haven't owned any until just a few weeks ago ;-)

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Click for larger image: 41 KB		This is a simple LDRAW model of the 2-speed transmission. It is built around the center differential (the one between the front and rear differentials). This assembly is very simple and does not perform too well. I would have liked to have used the transmission parts out of the recent Lego supercars, but I didn't have any at the time of building my tractor. I think they would have helped to make a better transmission that was easier to operate.



Click for larger image: 38 KB		The shifting takes place when the middle axle is moved forward or back. The 1x5 liftarm on the middle axle is pushed or pulled to engage the gears. Low gear is when the 8-toothed gear is pushed forward to engage the 24-toothed gear on the differential body. High gear is when the toothed half-bushing is pushed back to engage the dark gray 16-toothed gear. This gear (the "clutch" gear) then meshes with the 16-toothed gear on the differential body. Neutral can be obtained by carefully pushing the 1x5 liftarm to where nothing is engaged (a middle position). Problems arise when the middle axle is moved back or forth. If it doesn't slide freely, then the 1x5 liftarm will slide the 8-toothed gear, or half-bushing, along the stuck middle axle and out of alignment. Then the transmission will not work properly. One note: there is further gear reduction between the center differential and the wheel differentials, I just didn't include them in the CAD model.



Click for larger image: 54 KB		The other problem is actuation of the transmission. My goal was to motorize the shifting so that it could be done by remote control. My solution was to use 2 10L threaded axles to hold on to the 1x5 liftarm and then move the axles back and forth. I placed worm gears on either axle and used them as rack gears. The 8-toothed gears ran the worm gears back and forth, therefore moving the 1x5 liftarm back and forth. The threaded axles had to be restrained from rotation, so I used a triangle which has axle holes (cross-shaped holes) on either end (the yellow part in the picture to the left). The threaded axles run through the triange's end holes which restrain the axles from rotation and also help hold them in place.