R/C Ball Differential Cutaway (OpenSCAD Animation)

Pushing the limits of what OpenSCAD can do. Sharing this video as a curiosity. More information about this part can be found here: • Narrow 2WD Differentia...
In case you are wondering how this animation was created, here are some details. A top-level UNIX shell script computes time stamps and camera positions, and for each it calls OpenSCAD from the command-line, with instructions to save to disc a PNG image file. The PNG images are then stitched together into video using FFmpeg.
There are some intricate details esp. concerning the cutaway, because with OpenSCAD there are "color bleeding issues" when cutting at two objects of different color at the same plane. To solve this, the cutaway plane is budged very slightly for each object being cut away. This describes the process, but only in a very general terms.

[@GHOSTGXZ]

how does this work without some slippage of plate against balls? there aren't any teethed beveled gears?

[@NeriEngineering-oq6vh]

Yes, good question. These are small parts intended for a 1/10th scale R/C car. The steel plates are pushed up against the balls with a great amount of force, it's upwards of 20 lbs. There is potentially some slippage, but another device called a "slipper plate" which is part of the top shaft is designed to slip before the balls slip. If the balls do slip, it's not a big deal, only that a lot of slipping will cause a groove to wear in the rings, which isn't desired. In most cases, the R/C car industry uses tungsten carbide balls for some reason, while they tend to make the plates out of some sort of stamped shiny steel. The balls are very round and they resist compression because of the material they're made from, so they stay round even under compression. The coefficient of static friction between steel and tungsten carbide isn't ideal; I did a little bit of research and found that steel/steel would give a significantly higher coefficient of friction. (Still, for some reason, the industry chooses tungsten carbide balls and after doing extensive analysis I found that there does not seem to be good reason for that.) Furthermore, the gear with its holes is made of some sort of nylon plastic or similar (stronger) material, and the balls need to slip inside the holes, w.r.t. the plastic material. They use silicon lube for that, which supposedly decreases the coef. of friction between balls and plastic significantly while keeping the coef. of friction between plates and balls relatively unchanged. The size of these balls is approximately 1/8 inch diameter, to give you an idea of the scale of this object. In the genre of 2WD 1/10th scale off road electric cars especially, this sort of design has been proved to be extremely effective and cars equipped with this sort of ball differential continue to win important competitions, even to this day (while bevel gear, and other designs are an alternative to this system). I should also mention that the ball differential offers a high degree of structural integrity between the two diff halves, whereas the bevel gear differential relies on auxiliary structural support for its integrity. The balls are in essence a thrust bearing which keeps the two diff halves aligned on the same [parallel] axis.

[@GHOSTGXZ]

​@@NeriEngineering-oq6vh I realize the size and scale is small, I didn't realize the plates were compressed against the balls. I appreciate the thorough answer, why isn't a normal differential with bevel gears used instead though?

[@NeriEngineering-oq6vh]

@@GHOSTGXZ I would suspect that the reasons why a ball differential is preferred in this case (for a 2WD car that already does not use bevel gears to begin with, typically) is for several reasons. The cost of manufacture is much less. The structural integrity with ball diff is superior. The weight is less. I would have to do a further analysis, or recall my analysis which I did years ago, to give you a better answer. There are three yellow/gold-colored Belleville washers in the interior of the part, right next to the M3 threaded nut. That is part # P6432033 at www.springmasters.com, and it has an advertised working load of 36 lbs. The three are stacked alternating in my depiction, which would mean that the load remains the same but the distance of compression to achieve that load is increased. The exact same differential is featured in the following assembly animation: kzbin.info/www/bejne/ZobMmJ2bhM9-lbs . You may find some analysis of bevel gear vs. ball diff for 2WD cars (where the design inherently calls for only straight-toothed spur gears and not bevel gears) online, but claims such as "smooth operation" seem to be prevalent. In the world of 4WD cars, the typical design nowadays is one that employs a center shaft to connect front wheels to rear wheels (with a center diff in some cases), and for that, bevel gears are used between center shaft and diff gears, b/c they meet at a 90 degree angle. Ball differentials and bevel gear transfers do not play well together because bevel gears exert huge lateral forces on the "diff gear", whereas the ball differential relies on there being relatively little lateral force on the gear. So in other words, bevel gear differentials are very common with 4WD cars. I disagree with the current 4WD designs; I have come up with an efficient belt system that is far superior (which you can see here: instagram.com/p/C5VFDvRLkuO/ ), and it avoids bevel gears altogether.

[@paradiselost9946]

im sure you can play with that a bit more to actually SHOW how they work... the cutaways, etc... whats transparent... like, i know, its just balls carried by the crown wheel, squeezed between two plates, but seriously, this was confusing... and i know exactly how they work!

[@NeriEngineering-oq6vh]

Thanks for stopping by.