Understanding LEGO 4-speed sequential gearboxes

[Didumos69]

26 minutes ago, Filthy Fox said:

Could I also post a video of the problem I'm having?

Sure.

Edited June 22, 2020 by Didumos69

[Filthy Fox]

@Didumos69 Here is a quick and crappy video demonstrating the "only works in one direction in high gears, problem." This is just one example. It's very similar to the 5-speed + reverse @allanpshared a pic of, but with a high/low selector. (This was from someone's moc). There's also my Sian problem at the end. Also, please ease excuse my baby hands and my @ing, I am relatively new to Eurobricks 

 

Edited June 22, 2020 by Filthy Fox

[allanp]

8 hours ago, Didumos69 said:

@Filthy Fox, @allanp, the stock Lamborghini gearbox has 4 idlers transmitting torque, of which one is very suspicious. I really don't understand why they made it so complex, because the same ratios can be obtained with much less gear meshes.

Without having the gearbox in my possession it's hard to be sure but I think you are right, and @Filthy Fox it makes sense now for 7th (which I think is actually second gear) to be the gear in which it locks up. In (what I assume is) 2nd gear the upper blue 20t gear is engaged with the drive ring. This tries to turn its mated 16t grey gear slightly faster which then sends it's torque through a winding path down through the idler gear arrowed by @Didumos69, which is a gear that is also transmitting torque. In 4th gear (which also follows a similar torque path through that gear) the ratio is slower, requiring less torque to turn, so it doesn't quite lock up. In 1st and 3rd the power is not being transmitted by that gear and in gear 5-8 the torque requirement is less due to the lower ratio of the second 2 speed/4 position gearbox. A good way to check would be to turn the car on it's side and lock the gearbox in the trouble gear by turning the wheels. Then while locked try to fiddle with that one gear side to side and see in it moves a little.

Many people might not have this problem, but we are so close to the edge of being barely functional due to these over-complex designs that any barely noticeable manufacturing flaw can cause this to happen. I noticed when a lot of people were having problems with the 42009 outriggers barely moving that some axles straight out of the box were ever so slightly bent. So I built an axle tester by placing a bunch of 1x2 Technic bricks next to each other on a 2x8 brick. Half the axles fell straight through but many of then fell slowly and many didn't fall through at all. After building 42009 using my tester on every axle the outriggers worked as well as I've seen in any video (not great, but they worked). Tiny imperfections like this caused some people to have the issue, and some people not to.

@Filthy Fox As for gears only locking up in one direction of rotation, I'm pretty sure it's to do with the way gears react to each other when torque is transmitted through them. Remember every action has an equal and opposite reaction? Well take another look at the example gearbox:

On gear 2, which is the problem gear, there are 2 forces that we need to consider (there are other forces n other directions but fully drawing up the vector diagrams is not important to this). The first force is the force that tries to push gears 2 and 3 apart whenever two gears are transmitting power. It's the force that requires us to brace gears to prevent them coming apart under high loads. In this example this force between gear 2 and 3 is always pushing gear 2 to the upper right of the screen, and this never changes. The second force being applied to gear 2 that is relevant to this issue is that which is applied by gear 1. Not only is gear 1 trying to push gear 2 upwards, but it will also be exerting an even greater force left or right (this is what causes gear 2 to turn after all), and the direction in which that happens will depend on which way gear one is turning. So in one direction of rotation the two forces somewhat work against each other, so gear 2 doesn't want to move anywhere. In fact, if you was to remove the axle on which gear 2 is riding on it probably would stay there transmitting power for a second or two. However, if you then turn gear one in the opposite direction the two forces are now acting in the same direction and are therefore added together, in this example trying to push gear 2 to the upper right of the screen. This increases the friction on the axle on which it rotates. This is why lockups are often dependant on direction of rotation, because in one direction the forces kinda cancel out, when in the other direction the forces add together. Without owning the Sian I can't say for sure but I highly suspect the same is happening in there.

Spoiler

 

Wouldn't it be nice.....

 

 

[Filthy Fox]

@allanp Thanks once again for explaining this! I guess I've just been unlucky with production quality. 

And now that most of my Technic parts are very used through years of play, I won't be able to expect much. 

Why doesn't Lego make such practical gearboxes like the one you've shown above? Are the parts too large?

[Didumos69]

10 hours ago, Filthy Fox said:

@Didumos69 Here is a quick and crappy video demonstrating the "only works in one direction in high gears, problem." This is just one example. It's very similar to the 5-speed + reverse @allanpshared a pic of, but with a high/low selector. (This was from someone's moc). There's also my Sian problem at the end. Also, please ease excuse my baby hands and my @ing, I am relatively new to Eurobricks 

It's hard to tell, but I suppose this is a real life example of what @allanp pointed out.

5 hours ago, Filthy Fox said:

@allanp Thanks once again for explaining this! I guess I've just been unlucky with production quality. 

And now that most of my Technic parts are very used through years of play, I won't be able to expect much. 

Why doesn't Lego make such practical gearboxes like the one you've shown above? Are the parts too large?

Perhaps the upcoming pimped Sian by @jb70 will bring salvation.

7 hours ago, allanp said:

Many people might not have this problem, but we are so close to the edge of being barely functional due to these over-complex designs that any barely noticeable manufacturing flaw can cause this to happen. I noticed when a lot of people were having problems with the 42009 outriggers barely moving that some axles straight out of the box were ever so slightly bent. So I built an axle tester by placing a bunch of 1x2 Technic bricks next to each other on a 2x8 brick. Half the axles fell straight through but many of then fell slowly and many didn't fall through at all. After building 42009 using my tester on every axle the outriggers worked as well as I've seen in any video (not great, but they worked). Tiny imperfections like this caused some people to have the issue, and some people not to.

I also noticed that many axles are slightly bent out of the box. This is killing for drivetrain that need to run smoothly. I also always check my axles before I use them in a gearbox. I use the bent ones for construction only.

[allanp]

7 hours ago, Filthy Fox said:

@allanp Thanks once again for explaining this! I guess I've just been unlucky with production quality. 

And now that most of my Technic parts are very used through years of play, I won't be able to expect much. 

Why doesn't Lego make such practical gearboxes like the one you've shown above? Are the parts too large?

The practical gearbox could be much smaller, as in shorter, but we can't make it anyway because the parts don't exist. That was a non functional mock up to show the idea. 

As for your Sian, it would be a shame if it didn't work the way you want it. If you don't fancy rebuilding the whole thing again while checking all the axles for straightness (which may or may not work) why not try a drop of lubrication? Just remember to clean your parts once you disassemble it before putting the parts away as a tiny amount of lube will cover everything! And use a plastic/rubber tyre safe lubricant like silicone (NOT WD40!)

[Filthy Fox]

6 hours ago, allanp said:

The practical gearbox could be much smaller, as in shorter, but we can't make it anyway because the parts don't exist. That was a non functional mock up to show the idea. 

As for your Sian, it would be a shame if it didn't work the way you want it. If you don't fancy rebuilding the whole thing again while checking all the axles for straightness (which may or may not work) why not try a drop of lubrication? Just remember to clean your parts once you disassemble it before putting the parts away as a tiny amount of lube will cover everything! And use a plastic/rubber tyre safe lubricant like silicone (NOT WD40!)

I don't like to use non-Lego things with Lego. I'm a bit of a purist, except I'm ok with illegal techniques.

[Voldemort87]

Nice explanation, thanks for sharing!