Here is the CAD for a design I made to replace the COTS shifting gearboxes we have on our practice robot. The housing will be 3D printed using ABS, and the plates will be machined from 6061 1/4" aluminum plate. It also utilizes a VEX Mag encoder hex bore housing held inside the 2 by 1 frame rail. All shafting will be steel.
Full disclosure: I am a programmer by trade, not a CADer. - GrabCAD link -
Specs:
5.45:1 reduction for a theoretical free speed of 17.98 ft/s
10t keyed pinion, 44t steel driven gear, 50t final gear on the output shaft.
CAD looks great - love it when people enclose their gearboxes.
17.98 fps seems awfully high for a single-speed drive - you might have difficulties accelerating. Most teams aim somewhere around 12fps for a SS drive, but your mileage may vary.
As always, the speed you should aim for is always dependent upon the game and how your team is going to play it. 18 fps could be a totally reasonable gearing. For example, I know that 1747 was geared at around 17.5 fps in 2019 and it worked out really well for them.
That all being said, 18 fps is probably too high. For more data and discussion on drivetrain speed, there’s always this thread.
All I know is that our lead mechanical has spent a lot of time in the ILITE simulator and the number he gave me was ~18 fps… It is before-losses anyway, and seeing as it’s for a practice robot I’m not too worried.
It’s only supported by the bearing in the aluminum plate. I did this after I saw a large number of posts in this thread talking about how it was okay to cantilever that specific shaft as I have it. It shouldn’t undergo much stress, so I figured it would be okay. Feel free to let me know your thoughts, though.
Are you using the correct gear teeth? It’s 10:44:50. It very well could be me calculating it wrong, however. Let me know how you got that number, if I’m wrong I’ll update my numbers for sure.
Just to make a note, it worked very well for their driver because he was extremely skilled. He took a lot advantage of being able to drift it. The robot was design for him to drive, and him alone. I’ve seen many students struggle to use it at it’s full potential.
If you have a driver up for the task, go for it, but I would definitely say that the driver for 1747 from 2017-2019 was a rare one indeed.
I think you’re calculating it wrong. When you have idler gears in the geartrain, they get ignored for the purpose of calculating the reduction. So your final reduction would be 50/10 = 5:1. You can think about it as (44/10)*(50/44) and it’s clear that the idler gear cancels out and leaves you with just the first and last gears in the train.
Edit to add:
Cantilevered means the load is on the outside of the two bearings supporting the shaft, as opposed to between them. Think of a cantilevered WCD wheel; there are bearings in both sides of the tube and the wheel sits off to the side of both of them. It doesn’t mean that you can use a single bearing to support the shaft. The bearing will not be able to take up non-axial moments in the shaft and you will have serious problems. With very few exceptions, all shafts should be supported by exactly two bearings; no more no less.
Thanks for the insight. That was actually something that I was planning to work on this year after seeing some teams like 3538 at MSC doing some really cool “powerslides” around areas like the rocket. This gearbox was designed with something like 2017 in mind, however, where you are making long runs with small wheels, rather than short sprints as in 2019.
Ok, that was where my error was. I was under the impression that it would be (44/10)+(50/44). Seeing as that’s the case, I’ll update my numbers later today when I have the time.
What would those exceptions be? I was thinking that this would be one of them, seeing as the shaft is only ~.821 inches long and it’s an idler, but I guess not. I was planning on holding in the rear end of that shaft with a bolt and nut but I could add a bearing into that 3D printed housing cover I guess.
176’s 2018 intake had a very neat example of a shaft supported by a “single” bearing. Any guesses as to why this works as an exception to the rule?
The cantilever only works because the shaft is already supported at 2 points, and the gear is close enough to the plate that shaft deflection isn’t really an issue.
For the benefit of other readers who may come later...
What do bearings do? They constrain motion. We would like to assume the shaft is a rigid body [1] and only allow it to rotate about its axis, which we’ll arbitrarily label Z.
Objects in our 3 dimensional cartesian space have 6 degrees of freedom:
Translation along the X, Y, and Z axes. (The notation for these dofs will have the form “X”.)
Rotation about X, Y, and Z axes. (The notation for these dofs will have the form “XX”.)
We would like to fix the 3 translational dofs and 2 of the rotational dofs. We choose to leave the one remaining dof (ZZ) free so the shaft is able to spin.
Radial bearings provide constraint normal to the axis of rotation, as the balls bear against the inner and outer races. This constrains the two translational dofs in the radial directions, X and Y. When you support a rigid shaft by exactly 2 radial bearings, as is typical practice, then the constraint provided by the pair of bearings locks rotation in the XX and YY directions.
Thrust bearings, on the other hand, constrain the third translational dof in the axial direction Z as well as the rotational dofs XX and YY.
It is possible for a single bearing to act like both a radial bearing and a thrust bearing at the same time. A deep-groove ball bearing is one example of a bearing that will work this way. The radial aspects of the bearing prevent motion in X and Y, and the thrust aspects of the bearing prevent motion in Z, XX, and YY. All together, we have locked motion in every direction except the one direction we have chosen to keep free: the rotation ZZ of the shaft about its axis.
What we have in 176’s case is a single deep-groove ball bearing. Here’s what’s happening:
The radial qualities of the bearing constrain the shaft’s motion in the X and Y directions. Loads in these 2 directions are passed into the next part (the plate) by the cylindrical outer surface of the bearing’s outer race.
The thrust qualities of the bearing constrain the shaft’s motion in the Z, XX, and YY directions. Loads in these 3 directions are passed into the next part (the plate) primarily by the bearing’s flange… but possibly also by binding as the bearing slightly rotates within the bore in the plate.
[1] this is not always the case and may not be an appropriate assumption!
Not to get too off topic, but one way to practice would be to add additional omni’s onto already existing drivetrains. You could then see some really nice slides in action.
The output shaft of the Versa transmissions actually engages with the last carrier plate then goes thru the output bearings (2) that are slightly seperated forming a cantilever. It looks like the bevel gear is ou tight to the output bearing, minimizing overhang. In 2018 we ran cantilevered wheels( same 4" green wheels) spaced out about 1" from the output bearing on Versaplanetary Transmissions with little trouble.