So as a fun cad challenge to myself I decided to create a differential gear box, modeled after the Lego version, using FRC parts. (I’m not that good at cad currently, but practicing to get better).
Obviously this isn’t something that feasible to use on an FRC robot though.
However I had an issue and I have no idea what the proper mesh distances are for bevel gears. Any help with that would be appreciated.
Bevel gear mesh distances are specific to the gear set, and bevel gears can typically only be mated with one other specific gear. The bevel gear you’re using can only be mated with andymark’s 20 tooth bevel gear (am-2621) (or another bevel of same module and tooth count) – it cannot mate with itself, as you have. To see the mounting distances for this bevel gear set, look at the layout prints for both gears in the set.
Huh. I did not know that. I guess I’ll be modifying the design slightly, hopefully it should be an easy swap out, and only have to change a few dimensions. Thanks!
Additionally, the bevel angles* of the two gears must add up to the angle between the shafts. Using the gears you have, with approximately 60 degree bevels, the shafts would be about 120 degrees apart to get the teeth to mesh. If you want equal gears to turn 90 degrees, you need to find gears with a 45 degree bevel angle.
* The angle the tooth ridges and valleys make with the axis of rotation.
One easier trick for bevel gears: If you extend a line from the crowns (or roots) of the gear teeth, they’ll all meet in a single point called the “Apex”. The Apexes of two meshing bevel gears must be coincident. See below:
So in your 3D cad program, you can draw a sketch on the midplane of each of your gears to find the apex point. Then in the assembly, mate the two apex points together. Then measure to see how far off the rest of the assembly is
Good luck with the project! There isn’t a lot of need for differentials in FRC, but they do come up once in a while:
Just to note: your differential has a 2:1 increase, assuming you’re driving it from the 40 tooth gear and running wheels off the 20 tooth bevel gears. This is usually counterproductive, as motors usually spin faster and with lower torque than the application requires (long range flywheel shooters being a notable exception). You may want to consider at least an option to drive from the small crown gear, which doesn’t look too difficult given your general layout…
Oh, yeah. Reading this (and it seems pretty obvious now, otherwise teeth would interfere on the inner edge and gap on the outer edge), that means that the angle along the crowns will be slightly larger than that along the roots. The real bevel angle is the angle of the pitch cone, roughly half way in between.
Actually not. The entire housing is rotating with the outer gear. If the two output shafts are running at the same speed (i.e. no differentializing) then the bevel gears do not turn relative to each other and the two output shafts are running at the same speed as the large gear.
If one of the output shafts is running slower than the housing, the other will run faster such that the average of the two is equal to the housing rotation speed. The Gear ratio in the bevels only affects how fast the pinion gear rotates relative to the differential speed of the two side gears. But ultimately, there is not gear reduction from the main gear to the average speed of the two output shafts in any differential regardless of the gear ratio of the bevel gears that form the pinion gears and side gears.
Have you ever looked at how they build them for cars? The small gears are used as idlers, the large gears are used as the “side gears” and are splined to the axle shafts. The small gears just have a round hole in the center, and ride on a steel shaft, which is anchored to the housing by a roll pin or bolt. Do some googling, for ideas. “Differential Carrier” might be good search term.
Fun fact (if you have played with the Lego differential, you probably already know this): Once the side gears and pinion gears are installed, they are self retained and don’t require any sort of retainer on the shaft to hold them in place.
On automotive axle differentials, once the gearset is slid into the housing a single cross pin is inserted to hold the pinion gears and then the axle shafts themselves hold the side gears. Many designs utilize a spherical back face and spherical washer so that the pinion gears can be installed into mesh with the side gears in the “window” of the carrier and then the whole assembly is rotated into position around the axle shaft axis until the holes in the pinions line up with the holes in the carrier at which point the retaining pin is inserted. but those designs use 2 pinion gears to carry the drive torque over more teeth. With this design having only a single pinion gear, each side gear can be installed once the pinion gear is in place by just rotating it into position with the meshing of the teeth.
