[QUERY] AWD-RWD, 2-speed gearbox

[Krxlion]

Hi everyone!

I need help regarding designing a gearbox as mentioned in the topic. During my work on a new project, I created a gearbox, which works very nice, but I wanted to enhance it with additional feature - selecting RWD in faster output and AWD in slower output. I was struggling with the idea for couple of days now, and I am slowly giving up on it, but maybe you can help me. First, let me show you how it works right now:

The red 2L axle connector is an input of high rpm motor, then it follows to 46490 steering hub, which acts as a transmission (5.4:1). Then, the power output splits in two by using one 20-tooth and two 16-tooth gears (this helps a lot with skipping gears). After that we get to the simple, yet robust gearbox which choose between 24-tooth and 16-tooth gears and then of course following gears below. Finally, we are back on one axle and below are the ratios if someone is interested. :D
Slow output: 5.4:1
Fast output: 2.16:1

Now that you understand the idea of my gearbox, I would like it to modify it to act as I mentioned in the first paragraph. I would of course want to maintain as much of my design as well, but heavier modifies are also welcome. Switching gearing is done by a motor connected to a clutch (46834+46835) and then by 6641 changeover catch. I also want to use only one motor for changing gears and powering axle if possible.

Thanks for help in advance :)

[damian_kane_iv]

If I were you, I would look into nico71's use of two differentials for a CVT. A single motor powers the whole driveline and the torque applied to the wheels affect the net gearing from motor to wheels pretty nicely: https://www.nico71.fr/trophy-truck-with-continuously-variable-transmission/

[Stereo]

If you add another 20t gear meshing with one or both of the red 16t gears, for your front wheel output, that would do it, right?   So the bottom shaft goes to the rear in both gears, and then another one only connects the fronts in low gear.

 

If you could replace the 20t with a differential, that seems like it should also work - the higher ratio would be feeding into one side of the diff (and thus not power the thing sticking out the other end) and the lower ratio would drive the diff directly, and thus do both outputs.  I'm not sure what gear spacing would make a diff work there though.  Maybe use 20t clutch to the 16 end of an old style diff?  Or use a clutch piece to connect the pinhole 20t gear to one side of a diff, with the two pieces pushed together so it's just always engaged.

Edited April 2 by Stereo

[Krxlion]

11 hours ago, damian_kane_iv said:

If I were you, I would look into nico71's use of two differentials for a CVT. A single motor powers the whole driveline and the torque applied to the wheels affect the net gearing from motor to wheels pretty nicely: https://www.nico71.fr/trophy-truck-with-continuously-variable-transmission/

Thanks for your suggestion. The idea of CVT is very nice, I might do something like this in a different MOC. But in this one I want to be in control of shifting. Slower gear helps a lot in maneuver over various obstacles, and this is something I am aiming into. The RWD-2nd gear will just be used in flatter and easier terrains.

7 hours ago, Stereo said:

If you add another 20t gear meshing with one or both of the red 16t gears, for your front wheel output, that would do it, right?   So the bottom shaft goes to the rear in both gears, and then another one only connects the fronts in low gear.

 

If you could replace the 20t with a differential, that seems like it should also work - the higher ratio would be feeding into one side of the diff (and thus not power the thing sticking out the other end) and the lower ratio would drive the diff directly, and thus do both outputs.  I'm not sure what gear spacing would make a diff work there though.  Maybe use 20t clutch to the 16 end of an old style diff?  Or use a clutch piece to connect the pinhole 20t gear to one side of a diff, with the two pieces pushed together so it's just always engaged.

I will look into this idea once I return home. Right now during working day it is hard for me to imagine everything. :D If I will struggle with your idea, I will let you know, thanks.

[gyenesvi]

This Jeep Wrangler MOC has exactly such gearbox, maybe you can get some inspiration from it:

The key idea is this: it uses two switches, controlled together; one side switching the gears, the other side engaging/disengaging the FWD.

One note though; used with planetary wheel hubs (as in this model), RWD high gear is not so effective, because then the wheels are back-spinning the front of the driveshaft at a pretty high speed, causing excess friction. Could work better with plain hubs though.

Edited April 3 by gyenesvi

[Krxlion]

On 4/3/2024 at 10:45 AM, gyenesvi said:

This Jeep Wrangler MOC has exactly such gearbox, maybe you can get some inspiration from it:

The key idea is this: it uses two switches, controlled together; one side switching the gears, the other side engaging/disengaging the FWD.

One note though; used with planetary wheel hubs (as in this model), RWD high gear is not so effective, because then the wheels are back-spinning the front of the driveshaft at a pretty high speed, causing excess friction. Could work better with plain hubs though.

Thanks for the idea @gyenesvi. I've tried to replicate your gearbox just to get a picture of how it works, but I cannot see bottom gears, haha. Anyway, I am looking forward to this topic as I am struggling with this gearbox idea for over a week now.

PS Am I seeing it clear, or this drivetrain is not symmetrical, and one axle is off one stud to the symmetrical line of a car

Edit: I can see that it is. Pretty unusual solution, but I don't know if it would work in my case. Although it looks like for this gearbox it is necessary.
Edit 2(a lot of edits :D): I figure out how you design that gearbox. I must have been tired lately and couldn't understand all the design. It is very clever, but I am wondering about the robustness of it. It gives me some hope back, I will get back to the drawing board with this knowledge. Thank you once again!

Edited April 4 by Krxlion

[gyenesvi]

Yes, the gearbox is asymmetrical in the sense that the actual 2-speed switcher is on one side and the FWD/RWD switcher is on the other side, and hence the front driveshaft is offset 2 studs from the center. I also think that this asymmetry is required if you want to operate it with one switcher.

Here is a picture of the bottom, hope it helps:

[Krxlion]

I actually did something that make your gearbox symmetrical and axles turn in one direction. Blue 12t gear do not connect with the 16t gear next to it. :)

Adding one gear solve the issue, but on my picture you can of course see that it lost a lot of robustness. Anyway, it was just a concept. I will try to design a gearbox around moving axles that has gears and depending on the position of the axle one or two axles are rotating, and also it would involve switching gears. Might be a dream, but worth a try. I once tried it and I got a lot of skipping gears, but I have an idea to improve it.

[Divitis]

On 4/2/2024 at 8:38 PM, Krxlion said:

Switching gearing is done by a motor connected to a clutch (46834+46835) and then by 6641 changeover catch. I also want to use only one motor for changing gears and powering axle if possible.

Have you built your design yet? I would be curious to know if the new clutch gear isn't too hard for this purpose.

I just got my first copy and it feels really hard, so that the two catch pieces will have snapped out of place before it starts skipping.

[Krxlion]

14 minutes ago, Divitis said:

Have you built your design yet? I would be curious to know if the new clutch gear isn't too hard for this purpose.

I just got my first copy and it feels really hard, so that the two catch pieces will have snapped out of place before it starts skipping.

I have built it, and it is a working design. The clutch isn't so hard for this job, as its only purpose is to limit movement to not be over +/- 45 degree.

On the back of the white clutch part, there is of course gray one and geek-servo motor connected to it. But to be honest, any Lego motor can do the job if you could limit rotations (to 45 +/- degrees) using some Technic bricks around.

Edited April 4 by Krxlion

[Divitis]

That makes sense. I thought you were using the catch  driving ring interaction as the limiter.

[Stereo]

On 4/3/2024 at 2:33 AM, Krxlion said:

I will look into this idea once I return home. Right now during working day it is hard for me to imagine everything. :D If I will struggle with your idea, I will let you know, thanks.

Here's one option, makes it a bit taller physically, I built it in 7x11 frames to make it easier for me to locate stuff. (20:16/16:28 = 0.71 in low gear, 20:16/24:20 = 1.5 in high gear)

The red output is only active in low gear, blue one in both gears.

[Krxlion]

3 hours ago, Stereo said:

Here's one option, makes it a bit taller physically, I built it in 7x11 frames to make it easier for me to locate stuff. (20:16/16:28 = 0.71 in low gear, 20:16/24:20 = 1.5 in high gear)

The red output is only active in low gear, blue one in both gears.

Cool idea, but not 100% what I was looking for. When we engage both red 16t clutch gears (low gear), we can force only one of two axles to work. In a scenario when one axle is hanging loose, it will transfer all energy to that axle, which is not great. But in the high gear the rear axle (blue one) will be engaged 100% sure, but it might as well use some of the power on the front, but that solution works great for me. I really like the fact that this gearbox is so simple, and I think robust as well.