(Help) 4 speed gearbox

[Oliver 79]

Hello fellow brickers.

I'm attempting my first "Supercar" MOC and I'm having a little trouble with the gears.

I've based the four speed gearbox on Sariel's from his Unofficial Technic guide.

My problem is the third and fourth gears have a great deal of friction. I've geared it down a fair bit to try and compensate, but in fourth gear the car doesn't "roll" at all.

Do any of you car builders have any suggestions?

here are some pictures and the LDD file is here

Thanks for looking!

[JJ2]

Have you tried rebuilding it? I have built the same gearbox and it had very little friction, also when building it keep the parts a little to far apart and loose and when finished push them together slightly and test friction.

[Lipko]

There are way too many 8-teeth gears. You seem to have sufficient longitudinal space to use axles with universal joints to connect the gearbox with the differential. Also, if you still have to have 8-teeth gears, make sure to secure them well, and from both sides.

The fake engine is placed too high, you could also save some gears with better placing the engine.

Aaand another thing:

take the fake engine input from the shaft with the 32-teeth gear instead of the shaft with the 8-teeth gear. I hope that's understandable.

Edited April 28, 2016 by Lipko

[deehtha]

Do you need all the 12/20 gear combinations right before the diff? Why not take the axle with the 8 tooth and extend it all the way back, cutting out 2 pairs of gears and the friction from them. You could even extend the black axle all he way to the last 12/20 pair and mesh to the 20 with a 16 tooth I believe. These solutions will of course change the final ratios.

Edited April 28, 2016 by deehtha

[DrJB]

If you're going to 'gear down', it is best to do it with a single stage reduction (large ratio) as opposed to many of small ratio. This way, you minimize the friction forces (parasitic losses).

Some basics about transmissions:

When a transmission has 5 speeds, typically the ratios of output to input rpms are around

Gear 1: 0.6 - output (drive shaft) runs slower than engine

Gear 2: 0.8

Gear 3: 1.0 - output runs at about same speed as engine crank

Gear 4: 1.26

Gear 5: 1.6 - output runs FASTER than engine

Now, typically the drive axle/differential has a reduction around 3 to 1.

The above numbers should give you an idea on what gears to use, to have a 'realistic' vehicle.

In your attempt/LDD, you're using a very high (staggered) reduction, and ... maybe that is why the vehicle does not move at all

Edited April 28, 2016 by DrJB

[Oliver 79]

Thanks for the relies.

Have you tried rebuilding it?

Ha... Many, many times!

There are way too many 8-teeth gears. You seem to have sufficient longitudinal space to use axles with universal joints to connect the gearbox with the differential. Also, if you still have to have 8-teeth gears, make sure to secure them well, and from both sides.

The fake engine is placed too high, you could also save some gears with better placing the engine.

Aaand another thing:

take the fake engine input from the shaft with the 32-teeth gear instead of the shaft with the 8-teeth gear. I hope that's understandable.

I first tried universal joints and it was much worse. That's why I geared it down after the differential. I don't think the engine is too high. It's as low as I could get it. Here is what it looks like with everything else,

If you're going to 'gear down', it is best to do it with a single stage reduction (large ratio) as opposed to many of small ratio.

I will try this to see what happens. I'm starting to think that lego is just not up to the job for gearboxes.

Thanks All.

[Lipko]

Oh, I see your gearbox is unusually low-placed.

Anyway, the very first thing you should try is to connect the engine to the shaft with the 24-teeth gear, not the 8-teeth gear (I accidentally wrote 32-teeth in my previous post).

That last gearing-up can be the cause of the jamming. And I still stand for my opinion that there is way too much 8-teeth gears there. Only one is absolutely needed.

EDIT: if you place the gearbox one stud higher, than with 16-20 gear couples you could get rid of all the unnecessary 8-teeth gears. It may be worth to consider this trade-off (I guess you have a good reason why the gearbox is so low placed).

Edited April 28, 2016 by Lipko

[Oliver 79]

OK, maybe I'll try to move the gearbox up (I'm working to a blueprint of a real car), get rid of the 8 tooth gears and only gear down once after the differential.

When it wasn't geared down the pistons went crazy on forth gear and just kind of got stuck....

Edited April 28, 2016 by Oliver 79

[Lipko]

With the gearbox moved one stud up, the setup could be something like this:

The most important thing is to put the engine input to the 24-teeth shaft. You can use opposite gearing at the differential (an save another gear with it), or opposite in before the engine. Whatever works better. With a well braced gearbox, even gearing-up at both points should work.

Edited April 28, 2016 by Lipko

[Oliver 79]

With the gearbox moved one stud up, the setup could be something like this...

Awesome. I'll give it a go. Many thanks!

[Didumos69]

If I understand correctly, you geared down your build because of the bad performing gearbox. My impression is that this should not be necessary.

When I study your build I see two more possible issues, besides the ones that have already been mentioned:

1.) The gearbox casing - Make sure all clutch gears have substantial play and all individual axles run smootly. Also make sure the casing does not squeeze under the weight of the car

2.) Axles not secured - The endpoints of the axles inserted into the driving ring connectors are free to move beyond their ideal position. This might cause the engaged clutch gear to get clamped between the driver ring and the perpendicular axle-pin connector directly next to the clutch gear. I had this experience myself with another gearbox. I resolved this by making sure the axle is confined by adjacent parts, so it only gets the minimal required play and can not move out of its ideal position.

Good luck!

EDIT: I used axle pin connectors to keep the gearbox axles in place::

Edited April 28, 2016 by Didumos69

[Oliver 79]

If I understand correctly, you geared down your build because of the bad performing gearbox.

I geared it down because I didn't want the pistons to be going so wild in top gear. I might play around with gearing down after the gearbox but for now I'm trying LIpko's suggestions. I'm pretty much starting the chassis again!

I think you are right with the bracing too. It's back to the drawing board I go.

Many thanks.

[Sariel]

My gearbox uses 10 gear wheels. If I'm correct, you've added extra 15. That's where friction comes from. Also, if you want pistons in the engine moving slowly, then gear down between the gearbox and the pistons. Or connect the pistons to motors, not the gearbox - then you'll have them moving at constant speed at all times.

[DrJB]

1.) The gearbox casing - Make sure all clutch gears have substantial play and all individual axles run smootly. Also make sure the casing does not squeeze under the weight of the car

2.) Axles not secured - The endpoints of the axles inserted into the driving ring connectors are free to move beyond their ideal position. This might cause the engaged clutch gear to get clamped between the driver ring and the perpendicular axle-pin connector directly next to the clutch gear. I had this experience myself with another gearbox. I resolved this by making sure the axle is confined by adjacent parts, so it only gets the minimal required play and can not move out of its ideal position.

+1

Most of us assumed it was more 'technical' when in fact, your answer emphasizes that one should always go back and try the basic (more common) issues first.

[Oliver 79]

Also, if you want pistons in the engine moving slowly, then gear down between the gearbox and the pistons.

Yeah. That's what I think I'll try. Thanks. I've taken out all the gears before the gearbox, so I'll have to test it out with "real" bricks soon.