Automatic 2 speed Gearbox (Unimog RC edition)

This is the second stage gearbox I'm designing for my Unimog RC project. The second photo shows my UNIMOG RC assembly in progress…

While the first stage (to come) is a classic (fixed ratio) gearbox with a ratio of 5.5:1 (meaning that if the motor goes at 550 Rpm the output would be 100 Rpm), this gearbox has a 1:2.6 ratio by default (so that's basically overdrive) leading to an output of 260 Rpm in my example.

BUT under load the gearbox should reduce speed and increase torque by becoming a pass through gearbox (1:1 ratio) so that the output is 100 Rpm again !

The way this works is by using two things:

• a slipping gear that will “sense” the load and … slip when sufficient load is detected
• an open differential that is used like an adding device

There are two paths for power transmission:

• through the differential - that's -100 Rpm(rotates in the opposite direction)
• through the “adder axles” that move the differential cage, powered by the slipping gear: 
  1. 20 teeth to 10  teeth: that's x2 or 200 Rpm
  2. 8 teeth to 10teeth: that's x0.8 or 160 Rpm
  3. 10 teeth to 20 teeth: that's x.5 or 80 Rpm (again -80 Rpm with rotation direction)

So the adder axles will move the diff cage at -80 Rpm so that the output axle of the differential will rotate at -2×80-100 = -260 Rpm while the input axle is at 100 Rpm …

Now, under load the slipping gear slips and the diff cage does not move anymore and the output speed is just -100 Rpm (as 2×0 -100 = -100 !!!).

I've provided 2 housing tops. The one named “test” allows to … make tests.

The diff cage is from my bigger differential designs with ball bearings on the spider gears. Yes, this time we need them because the normal operation will have the spider gears rotate all the time while, when it was used as a differential, they would only rotate when the car was turning.

Mounting tips:

All the screw and nuts are M3.

For the diff cage you can look at my differentials #1 or #2 ; you will need 4 ball bearings  

2 MR117-2RS (drive gears) and 2 MR115-2RS (spider gears)

The 2 ball bearing holders hold quite a lot of BBs:

3 MR117-2RS and 1 6700ZZ , each !

So that's 8 MR117 , 2 MR115 and 2 6700zz

Some parts are called “in” while others are called “out” and if you are wondering, “in” is where the slipping gear is. There are spacers (SPx) that can go in only one place. For the slipping gear I've provided 2 spacers. But I used only one because of print overall tolerances. If you need to chose put at least one between the housing and the slipping gear. 

The crown gear (output side) is not glued (in case dismounting is needed). It is held in place by the spacer that looks like a big washer.

The “main axles” are glued onto the diff cage parts and sacrificial support layers need to be removed (some screw holes + drive axles). You need to clean the supports very well from one of the ball bearing holders otherwise the BBs may not sit deep enough and you would not be able to fit everything in the housing.

Print the clutch in PETG (it needs to flex and accept some heat - so PETG is better than PLA).

As usual my naming convention for parts is:

XXXnn-kk-name

XXX = sub assembly

nn part number within this assembly

kk number of instances needed in one single instance of the sub assembly