RC CAR Differential #4 with limited slip

This works like an open differential but unlike an open diff it will send some torque to the non slipping  wheel.

It is compatible with my RC car V2 https://www.printables.com/model/281110-new-design-rc-car-v2-almost-fully-printed; I've replaced the rear differential with this one and it works fine. Reduction ratio is still 1:2  and I've kept the crown gear and the pinion from my differentials #1 and #2. 

My differential #3 is to small to fit in a limited slip function so it is is just a smaller open diff.

How does this work ? 

The drive axles have a worm gear instead of the bevel gears in an open diff. The bevel spider gears are replaced by a set of 4 spur gears that connect the left and the right axle (upper and lower set of 2 spur gears for symmetry ).  When the car is going straight the spur gears do not turn on their axles and the crown gear axle and the drive axles turn in sync (same as an open diff). 

When the car turns, the outer wheel axle will go slightly faster than the inner one. Relative to the diff cup the wheel axles are turning in opposite directions driving the meshing spur gears (no lock provided the speed of the axles is exactly the same while in opposite directions)

To better understand this, let's say the the gearbox delivers 200 rpm. Then the crown gear runs at 100 rpm.  With the 19° angle limiter on my RC CAR V2, the sharpest cornering will lead to a 18% speed difference vs. the crown gear; so the inner wheel will run at 82 rmp while the outer will run at 118 rpm. So relative to the diff cup, one axle will rotate 18 rpm in one direction while the other one rotates in the opposite direction at 18 rpm (no lock).

When one wheel slips, it starts going much faster than the other one like in the open diff. The slipping worm will drive the spur gears on its side which will drive the meshing spur gears of the other axle. These spur gears cannot drive the corresponding worm gear. So they will lock on the slower worm gear after a short while; this basically locks the differential that now acts like a closed diff and gives torque to the good wheel that starts turning so that the car is not stuck. So basically the good wheel gets 50% of the torque which should be enough to break free of the slipping condition.

A simple way to test it: On my RC CAR V2 mount an open diff on the front wheels and this limited slip diff on the rear wheels:

• Put the car on top of a filament spool (all 4 wheels in the air)
• Run the car. All wheels are spinning.
• Block one front wheel with your hand. The other wheel runs faster and you feel no torque on the blocked wheel
• Try slowing one rear wheel; you feel it has some torque. Do not block it because you will break the spur gears (as the gearbox + diff reduction is  1:32, there is a lot of torque coming from the engine)

Mounting instructions: 

The pictures should help; if you need to change the rotation of the wheels so that they go in the same direction as the other differential of a 4WD simply flip the diff over before mounting the supports. If you still cannot figure everything look at the pictures in my other differentials designs (#1 and #2 mainly).

Ball bearings: You will need 

6700 ZZ QTY 2 (main axle)

MR117-2RS QTY 5 (drive axles - 2 ea and pinion 1)

Printing:

The casing is printed floating in the air so that the inside is as accurate as possible (we do not care so much for the outside). I used variable layer height.

The worm gears are positioned at 45 degrees (best printing results).

I retain my naming convention for parts:

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