RCCAR Differential #6 with limited slip

This is my 3rd limited slip Diff...

So what's new ?

This time I've tried to be as careful as possible with tolerances and play, with the idea to make not only a working RC car differential, but also one that could be more durable (as much as this is possible with PLA).

But bear in mind that lubrication is the most important thing when you want a printed gearbox to work as long as possible. Whatever the filament a well lubricated gearbox will last a long time with very little wear.

I've also decided to put more plastic so tear resistance could be increased.

In previous designs I was constrained by the fact that I was doing scale models (Unimog, Peugeot 3008...) Well not this time ! Stay tuned ... A new RC car should be released a few months from now...

As for looks, even-tough my previous diff housings looked nicer, because this part is usually buried inside the car, I've decided to choose a boxy design, easier to print.

Main modifications:

The diameter of the drive axles is increased from 7 to 10 mm (more plastic).

I'm using a new ball bearing size (6702) 21x15x4 for the diff cup axle while the cup itself now looks more like a ... box !

I've increased the module of some gears (the worm gears , the spur gears) i.e. more plastic.

I've tried to improve my use of the ball bearings in general; this is not as easy as it seems: to best use a BB you need a very tight fit on the inside or the outside of the BB and tight fits otherwise.  (meaning one of the 2 sides is very tight , the other is tight...) With 3D printing you always compromise and tight is ... relative !  So sometimes the fit is not as tight as it should be. Also you need to avoid a fixed part to be in contact with a moving ring of the BB.  So the spacers, blockers need to be in contact with only one ring of the BB  - not with both.

Mounting tips:

Needed ball bearings (for one differential) : 6702-2RS QTY 4;  6700ZZ QTY 3; MR63ZZ QTY 4

Look at the pictures ; usually I've embossed the length of the M3 screws to use (if not, try to measure first, so you don't break anything).

Limited slip function: the "magic" happens between the worm gears and the 4 spur gears ; I've explained that here.

The spur gear "axles" are a tight fit so once in place they will not move. The spur gears real axle is the MR63 BB. You need to mount the BB flush inside the spur gear; do it on a hard flat surface. The gear should be aligned with the BB; do not lose that alignment when mounting on the axle (the metal to plastic friction keeps the gear centered, which should be enough) . The chamfered part of the spacer should NOT face the gear. Put a lot of grease on all of the gears before you close the cup/box.

Once you've finished mounting the Diff cup the worm gears axle is still not rigid enough; this will be achieved with the external 6702 BB and the outer diameter of the drive shaft. This way the worm gear axle will be held by two BBs (one 6702 indirectly and one 6700 directly).

When rotating the worm / drive axles please keep in mind that these axles can either rotate in the same direction at the same speed or in opposite directions at the same speed.  When holding one axle the other one will be blocked roughly after one half turn (locked diff).

The crown gear mounts on the diff cup with 2 screws. Assemble the 4 walls of the main housing with screws after putting the pinion and the crown gear inside. 

The bottom lid is necessary to mount properly the main housing because it holds the the 4 walls from the other side. The other (bigger)  lid can just be mounted with 2x  6 mm screws to keep the grease tidy while you wait to mount it on the chassis (the 4 mounting screws will be then screwed from the other side).

Printing:

Most of the parts are easy to print; the only tricky ones are the worm gears. I've personally tried many different orientations and the one that has given the best result is the horizontal one with supports and brim. I used organic and 0.15 layer height +  4 perimeters. Cleaning the supports is a bit tedious but gives a strong nice part and yes some sanding helps. Please be aware that the hex needs to be clean for mounting the drive shaft and there is a cylindrical shape just below the worm that will fit tightly into the BB. So these 2 places need to be clean. I've provided 2 MK3S print files. If you own a different printer try to mimic my choices (orientation and supports).

PS: as usual the first 2 digits number is the part number and second one tells you the number of parts you need to print for one assembly (one differential)