RCCAR Differential #7 with limited slip using a clutch pack

This differential is a clutch pack 2 way limited slip differential. It has the exact same dimensions as my diff #6. (so it mounts seamlessly in my new RC car replacing diff #6)

Why did I design it ? Well my previous limited slip differentials were based on a simplified torsen mechanism where a spur gear (it should be a worm wheel but a spur gear works the same) locks onto a worm gear. When this happens the differential locks and you get torque on the non-slipping wheel.

BUT and that may be a big but, because everything is plastic, if you are stuck, you end up breaking the weakest part and in this case it is the spur gear. So even if diff#6 was stronger than my other LS diffs, it ended up breaking at some point in time (after ca 20 mn of rough handling in the garden).

Now I hope this new design will achieve a longer life span (!) 

If you are not familiar with limited slip differentials here is a short explanation on how this works, differences with my previous designs, and how I have chosen to implement the clutch pack diff in 3D print / plastic. Now if you are not into too much explanation simply go to printing !!!

---

My very simplified previous LS diffs designs are not torque multipliers (as real torsen diffs). What they do is that they lock because after a while the slipping worm gear rotates its fellow spur gear enough so that the spur gear of the other worm reaches a locking position. But if for some reason the car cannot move, you end up with messy plastic shavings mixed with grease inside the diff cup...

So, as everything is plastic, If we do not want to brake gears the non slipping function needs to be progressive. And well, that's what a clutch is about, isn't it' ?

A clutch pack diff is preloaded with some initial torque (with a spring) ; the preload comes in competition with the primary function of a differential which is to allow speed differences between wheels when cornering. Because of the preloaded torque it is a bit more difficult to get a speed difference when turning.

That's why you don't want too much preload, otherwise you end up with a closed/welded differential ! 

The clutch pack consists of 2 groups of friction plates on both sides. The first group is connected to the wheel axle and the second is connected to the diff cup which is rotating from the input of the motor through the pinion. Now if these plates are loose, there is no friction ; that's why we have a preload spring on each side to get some initial friction that locks the diff cup and the wheel axles.

There is an additional locking mechanism that comes from the spider gears  pressing the parts called pressure rings; when pushed by the spider gears axle, the pressure rings add pressure on the friction plates from the other side. This is achieved when accelerating. Strong acceleration will give more torque on the non slipping wheel. This happens both in drive and reverse (on this design) ; that's why it is a "2 way". You could have a 1 way (drive only) or a 1.5 way (mainly drive and some in reverse).

Now because my clutch pack module needed to fit in the space I inherited from diff #6,  I only have one carrier plate and one gear/wheel plate on each side.  In order to preload the plates with some friction, I'm using a TPU pad that's slightly bigger than the space available.

As a round shape slides better, the spider gear axle (acceleration locking) really needed to be round and If you've been building some of my designs, you know the shafts or axles are never round because they don't print (at least in the direction we want them)! So I'm using the heads of 2 M3 screws to achieve this; you will need cylindrical socket heads for this (M3 Philips screws won't work).

This is work in progress and as you can imagine there is still some tuning to do (!)

Some part numbers/names are the same as the ones of Diff #6 but they might be different... Use the stl / print file of this model. However if you have printed Diff#6 you can reuse the housing and the pinion+ pinion shaft. We won't need the MR63ZZ ball bearings anymore. However we still need the following ball bearings 6702-2RS QTY 4;  6700ZZ QTY 3(for one differential).

Mounting tips:

The spider gears need to rotate freely on the cross shaft. If needed sand the axle a little bit.

The screws need to allow this rotation so don't tighten them too much.

As for grease, we want a little on the gears but not to much because we don't want grease in the clutch !(and by the way the gears will only rotate when the car turns so wear is not really an issue).

I've slightly modified the pressure rings and the plates. For the plates, the teeth are usually perimeters but the top inner layer is usually a diagonal fill which increases friction (good!).

Mounting is a little bit finicky because everything moves as you try to assemble it; check that the spider axle screws are well positioned into the grooves of the pressure rings.

Also as the TPU pad is too thick, it can rotate before the screws of the crown gear or the diff cup axle tighten it in place. I might need to add 2 screws on each side to even the pressure.

I've not yet tested it inside the car but when holding the pinion, if I exert enough force and rotate one drive axle, the other rotates in the other direction (yay !) So compared to a open diff it is more difficult to turn which was the goal (!) BUT you can still have different wheel speeds.