[MOC] 3 Speed automatic transmission gearbox

[TechnicBrickPower]

Here's my 3 speed automatic gearbox MOC. The gearbox automatically switches between one of 3 gear ratios, namely a ratio of 1, 1/3rd and 1/9th depending on the load on the output. It uses two 2 speed automatic transmissions in series to create the three speed automatic gearbox. It works on the principle of 2 degrees of freedom within a pair of differentials. This creates two gear paths for the transmission of force and the path of least resistance is taken.

The loading points at which this lego gearbox switches gears is adjustable via two linear actuators at the bottom of the gearbox, which adjust the amount of artificially introduced friction to control the switch over points.  Hope you like it.

 

 

[Bluehose]

They are two thing wrong about this gearbox :

1) it's not two 2-speed gearbox, it's two cvt. When the two sides of the differenials works together, the ratio is set somewhere between the min and the max so there are too much differentials : you could just set 1:1 and 1:9 to the sides of the two first differentials.

2) if the load is to big, all the power goes in the friction axles and there is no output.

 

The friction axles are a good innovation IMO.

Edited July 1, 2020 by Bluehose

[TeamThrifty]

2 hours ago, Bluehose said:

They are two thing wrong about this gearbox :

Everyone's a critic!

I've seen a lot of amazing stuff from @TechnicBrickPower over the last few months, credit where its due... he's winning the gearbox section by a country mile!

[Bluehose]

37 minutes ago, TeamThrifty said:

amazing stuff 

Yes, it's true his other gearboxes are impressives, innovatives and bring a lot to the commmunity ! But it must not impeach you to look how they are working and exchange with him on technical points, to my mind it's the kind of debat wich should interrest him.

Edited July 1, 2020 by Bluehose

[TeamThrifty]

You are correct about debate, its very valuable... i think i perhaps misjudged your first comment, i apologise for that 

[TechnicBrickPower]

21 hours ago, Bluehose said:

They are two thing wrong about this gearbox :

1) it's not two 2-speed gearbox, it's two cvt. When the two sides of the differenials works together, the ratio is set somewhere between the min and the max so there are too much differentials : you could just set 1:1 and 1:9 to the sides of the two first differentials.

2) if the load is to big, all the power goes in the friction axles and there is no output.

 

The friction axles are a good innovation IMO.

Hi Bluehose Yes good feedback. I expected someone would raise those points. 1) You are right it is a cvt but when there is enough load all the power does go through the 1:3 ratio (I have seen the 1:1 path stop in practice), but in between it travels through both paths and the ratio is anywhere between 1 and 1/3 - hard to tell what the exact relationship is between the load is and the ratio.  Good point about just using one 1/9 so maybe just one pair of diffs is enough - I thought I had a moment of inspiration when I put two in series.

Point 2) In practice I haven't seen that happen. The blue pin (or the linear actuators) add a constant amount of load "c" to the output (for a constant speed) so the overall load is L = c + O ( O = load on output axle). Of course if L is large enough the whole thing stops (like any gearbox) and in fact there is no power transfer at all as nothing is moving so it can't be going into the pins at that point. If the load O is just below the this threshold then I am not sure what % of the work is lost in the blue pins vs the gear friction. An open question lol!

Thanks!

 

18 hours ago, TeamThrifty said:

You are correct about debate, its very valuable... i think i perhaps misjudged your first comment, i apologise for that 

 

19 hours ago, TeamThrifty said:

Everyone's a critic!

I've seen a lot of amazing stuff from @TechnicBrickPower over the last few months, credit where its due... he's winning the gearbox section by a country mile!

Thanks @TeamThrify & @Bluehose. All points raised are good ones. Thanks for your support - much appreciated.

 

[Pas_de_nice]

Nice gearbox, I like it in it's simplest form with only one stage (2 speedish). The 3 speed is definitely too bulky to be incorporated in a vehicle.
However I kind of agree with @Bluehose: you loose a lot of power in the friction pin and you can easily quantify it:

You have one input (from the motor), and two outputs (the friction pin and the actual output), kind of like a simple differential. The power is distributed among the multiple outputs.

If we call Ωi and Ti the input rotation speed and torque, Wo and To the output rotation and torque, and finally Ωf and Tf the ones for the friction output. You have then, neglecting all the friction losses except the one in the friction pin: total input power P=Ωi x Ti =Ωo x To + Ωf x Tf.
Then when you are in the 1:1 ratio, you get Ωf=0, so no loss in power, but as soon as Ωf is non zero, the amount of dissipated power is Ωf x Tf.
You may get a higher torque To when using the 1:3 ratio, but at the cost of a serious loss in power...

Edited July 2, 2020 by Pas_de_nice
typo

[Ivorrr]

Amazing idea!

I feel recently you guys are stepping into another level of engineering with all the CVT and automatic gearbox. 

I am wondering if we will ever see an automatic in a LEGO set

[2GodBDGlory]

15 hours ago, TechnicBrickPower said:

 

Point 2) In practice I haven't seen that happen. The blue pin (or the linear actuators) add a constant amount of load "c" to the output (for a constant speed) so the overall load is L = c + O ( O = load on output axle). Of course if L is large enough the whole thing stops (like any gearbox) and in fact there is no power transfer at all as nothing is moving so it can't be going into the pins at that point. If the load O is just below the this threshold then I am not sure what % of the work is lost in the blue pins vs the gear friction. An open question lol!

I can't say whether or not it is possible for this gearbox to slip entirely, but I once built Nico71's similar CVT transmission and tested it in a 4x4. I found that in extremely difficult terrain it could slip, but it sure took a lot to get it to that point. Still, though, I am not sure how that happened.

It looks like an interesting and original gearbox!

[TechnicBrickPower]

On 7/3/2020 at 2:04 AM, Pas_de_nice said:

Nice gearbox, I like it in it's simplest form with only one stage (2 speedish). The 3 speed is definitely too bulky to be incorporated in a vehicle.
However I kind of agree with @Bluehose: you loose a lot of power in the friction pin and you can easily quantify it:

You have one input (from the motor), and two outputs (the friction pin and the actual output), kind of like a simple differential. The power is distributed among the multiple outputs.

If we call Ωi and Ti the input rotation speed and torque, Wo and To the output rotation and torque, and finally Ωf and Tf the ones for the friction output. You have then, neglecting all the friction losses except the one in the friction pin: total input power P=Ωi x Ti =Ωo x To + Ωf x Tf.
Then when you are in the 1:1 ratio, you get Ωf=0, so no loss in power, but as soon as Ωf is non zero, the amount of dissipated power is Ωf x Tf.
You may get a higher torque To when using the 1:3 ratio, but at the cost of a serious loss in power...

Hi @Pas_de_nice Thanks for that calculation of power loss - that will help me with future designs. I will try to estimate these numbers and see what % of power is actually lost - will be interesting to see what that is.