2-Speed Planetary Automatic Gearbox

[2GodBDGlory]

Here is a new automatic gearbox design I developed last winter, after I got my first planetary hubs.

It is a very unusual gearbox for Technic, without any traditional gears, but offers high strength and efficiency, and can shift for itself.

Its operating principle is as follows:

The input shaft goes to the input side of the planetary hub, and the output is hooked up to the output of the hub. However, with only this setup, any load will cause the hub assembly itself to turn, rather than the output, so a small tire is added to create friction between the hub body and the input. With this setup, the gearbox will tend to stay at a locked 1:1 ratio, but it is still capable of slipping and sending all its power into rotating the hub body against the friction. To prevent this, a pair of ratchet-type things are added, which only allow the hub body to rotate one direction. Thus, under light load, the gearbox will again spin at 1:1, but at higher loads, the hub body will slip against the rubber in the opposite direction UNTIL it strikes the ratchet. At this point, the body is stopped, so the gearbox works at the standard planetary hub reduction of 1:5.5.

The main disadvantages of this gearbox are that it is difficult to adjust the friction, and that it only really works going forwards. If the motors are reversed, it causes the gearbox to automatically shift down into first (good), but it also is able to slip indefinitely, so it is not good for off-roaders that will need to reverse in challenging terrain.

Additionally, multiple copies of this gearbox can be attached in series to add more speeds, with one speed being added for each additional assembly. However, this increases in volume rapidly, and the only possible gears are one to powers of five, so 1:1, 1:5.5, 1:30.25, 1:166.375, 1:915.0625, and so on. As you can see, the ratios quickly become far too low for any practical use.

I tested this gearbox in my Toyota Land Cruiser 70 model, and it met durability requirements. I used two gearboxes side-by side, essentially doubling the resistance needed for a downshift. Unfortunately, it would very rarely get into the high gear, due to the high weight of the model. Also, the rubber tires I was using got somewhat worn down.

I feel that it could be valuable for fast, lightweight off-road buggies that need a little extra torque once in a while to get over obstacles.

Let me know what you think!

 

[JLiu15]

1 hour ago, 2GodBDGlory said:

it only really works going forwards

I’m guessing you could incorporate something like a drive/reverse selector and only have the drive motor run in one direction or something like that. But anyways, nice gearbox! 

[AMX]

Cool gearbox you have there!

i wanted to make this kind of gearbox when I first saw these planetary hubs, but unfortunately I haven’t got my hands on them so far

[BusterHaus]

The small size and simplicity of this gearbox are great. I'm wondering if the friction wheel couldn't somehow be replaced by a flyweight mechanism like ones found on governors. 

I can't imagine it being as small as your current solution. 

[2GodBDGlory]

19 hours ago, JLiu15 said:

I’m guessing you could incorporate something like a drive/reverse selector and only have the drive motor run in one direction or something like that. But anyways, nice gearbox! 

Yeah, I'm sure that could be done. It would make it larger, less reliable, and less efficient, but it might be worth it.

[JLiu15]

1 hour ago, 2GodBDGlory said:

Yeah, I'm sure that could be done. It would make it larger, less reliable, and less efficient, but it might be worth it.

You could use this piece and put 3 12T bevel gears in there, the ones on the sides connected to 16T gears going to the 16T gears w/clutch that'll be engaged by the driving ring. The axle with the driving ring and the middle 12T bevel gear will be the input/output, and you could easily motorize the shifting. Totally worth it imo  

[2GodBDGlory]

That sounds like it is probably the best solution for models that require lots of torque in reversing. It is able to reverse on flat ground (with a light enough model), but when the going gets tough, it will give up, so a reverse gear like your suggestion would be good for heavy models or off-roaders.

[Bluehose]

make the ratchet controled by the motor's way of rotation

[2GodBDGlory]

I can see that working, perhaps with an auxiliary Servo motor hooked up to the driving channel. It could be hooked up to the ratchet using a rubber axle joiner, allowing it to rotate the ratchet 180 degrees while adding a spring-loaded effect. Maybe when I get some time I can build that...

[Bluehose]

You can just put a pulley on the input on wich you put a rubber band to a second pulley wich command the ratchet selector

Good luck

Edited June 22, 2020 by Bluehose
Good luck

[2GodBDGlory]

10 hours ago, Bluehose said:

You can just put a pulley on the input on wich you put a rubber band to a second pulley wich command the ratchet selector

Good luck

Good idea. It would be a bit less energy-efficient (always slipping the rubber band), but a whole lot more space-efficient.

[Bluehose]

If you don't want slipping rubber you can put the input and the output on the sides of a differentiel and the ratchet selector on the body

[2GodBDGlory]

Last night/this morning I took some of these suggestions and built a compact, reversible gearbox, using the rubber band idea. I should get it posted here soon. I am pleased with it!

[Bluehose]

To my mind this design is the best trialtruck gearbox so far. It have the same range than the nico71's cvt but more efficient.

[Bluehose]

About your problem with heavy moc : you can speed up gears before the gearbox and speed down after, this way the gearbox will handle less torque and will be more often in high gear.

[2GodBDGlory]

On 6/24/2020 at 2:21 AM, Bluehose said:

To my mind this design is the best trialtruck gearbox so far. It have the same range than the nico71's cvt but more efficient.

Thanks!

On 6/24/2020 at 9:58 AM, Bluehose said:

About your problem with heavy moc : you can speed up gears before the gearbox and speed down after, this way the gearbox will handle less torque and will be more often in high gear.

That is a good point. It could sometimes be a packaging problem, but it is probably the best way to adjust shift points.

Here is V2!

It uses the rubber-band ratchet idea, and has an updated friction mechanism using rubber track inserts. It, like the previous design, will wear down the rubber friction elements eventually.

https://youtu.be/DY6t_UeW71U

Edited June 27, 2020 by 2GodBDGlory

[Bluehose]

Indeed the wearing is the main issue.

Does the output lose power between the time of the slipping and the time of the ratchet blocking ?

Edited June 25, 2020 by Bluehose

[2GodBDGlory]

12 hours ago, Bluehose said:

Indeed the wearing is the main issue.

Does the output lose power between the time of the slipping and the time of the ratchet blocking ?

Yes, I think it does. This could be solved by essentially putting a gear ring around the planetary gearset for the ratchet to mesh with, but I think it would be fairly hard to do in Lego. A large turntable might work, but that would just be... large.