Hello,

Hopefully I'm not breaking any forum rules but posing the following questions (In my defense, I'm double checked where this should be posted).

Long story short:

I'm rebuilding my MOC (yellow) tank, this time with 4 x XL engines, as the previous setup (2 x XL) was too slow to power this beast (I want it to be *big*), with each two XL engines connected to a differential (see picture(s) below) and to one edge of a PF V2 receiver.

Currently I'm facing two issues:

1. The 90 degree non-beveled gear (cog) between the differential and the external gears seems to slip under load. Most likely I'm using the wrong part. Any idea which part I should use? AFAIR the differential P/N is 73071.

2. As noted above, I've got 2 x 2 x XL engines connected to the PF V2 receiver. Under load, it seems to cut off. Am I tripping the over-heating and/or over-current protection? Any possible solution, beyond buying SBrick / BuWizz?

2a. Assume the answer above is a resounding *Yes*. Any suggestions? Will both will be powerful enough to power 4 x XL engines + ~2-3 XL/L engines (for turret, gun elevation, etc)?

 

Thanks!

 

4 x XL power pack: https://imgur.com/a/EiVx6MB

Edited April 18, 20204 yr by gilboa

What is the point of putting a diff between the motors? I would recommend to just dive the front and rear sprocket wheel with 1 Motor each

Every motor has slightly different rpms with the same voltage. It's better to put diff between motors, but I don't do it too)

Just now, Gray Gear said:

What is the point of putting a diff between the motors? I would recommend to just dive the front and rear sprocket wheel with 1 Motor each

The previous tank iteration had one XL engine on each side, and it simply didn't have sufficient power to move it fast enough (to my liking).
As for the differential, as romashkaman said, as far as I know, not all XL engine are created equal, minute RPM differences can shorten their life considerably when combined in a direct drive.

I can't really say anything about lifetime, did not have problems so far. Also the rpm differs, when the engines are under load so it should even out. A simpler solution would be the following:

I would suggest making a drive train where all four motors are direct coupled and delivering drive through a adder/subtracter gearbox. motors being direct coupled is not a problem. It's only at no load situations they have a significant speed difference, and under high load they will all be using their power to move the thank rather than fighting each other. No motor moving a heavy model like this will ever reach it's top RPM, so the extra heat generated by direct coupling them will be negligible. Take this from a electronics engineer...

I've done this for tank iterations I've made, and it removes any problem with motors running the belts at different speed, as well as giving much better control. It does however require a fifth motor for steering.

You will quickly find that a traditional diff can't handle the load of two or in my case four motors. I designed a diff around the large turntable to solve this problem. See pictures below.

I'm currently using this design with two buggy motors on a ripsaw based moc I'm working on, so it should be more than strong enough. Just be very careful with the gear placement as it is easily possible to get one or more of the planetary gears out of alignment when you close up the diff. It should have no undue resistance when you test it.

 

I know this solution requires a lot of space, but on a tank of this scale it should be no problem. Design for placement of the diffs can be a bit hard, but I suggest placing them vertical to the tracks so that you can have a up gearing after them to the drive sprockets. This solution can be a bit hard when it comes to delivering power into the diffs as right angles in a drivetrain with 4 XL motors is always going to be a weak point. But if you can solve that you should have a very powerful drivetrain that gives you a very gradual control and a lot of power to the tracks.

 

-ED-

 

Edit: Sorry. The pictures above is a old design with a fatal flaw. It's almost impossible to dismantle... The picture below shows the evolution I've had on this design, and the leftmost design is one of the newest and solves this problem as well as using fewer parts. the internals are still the same though...

And the following picture shoves the same design in real life. I've made one more iteration on who the turntable is connected but have no pictures available.

 

Edited April 18, 20204 yr by Nazgarot
Wrong pictures...

10 hours ago, Jundis said:

I can't really say anything about lifetime, did not have problems so far. Also the rpm differs, when the engines are under load so it should even out. A simpler solution would be the following:

I considered using a direct drive like this, but decided against it (due to the reasons above). I may reverse the call if there's no solution to item A (cog slipping).

Follow up question: What type of receiver are you using? V1/V2 or SBrick/BuWizz?

9 hours ago, Nazgarot said:

I would suggest making a drive train where all four motors are direct coupled and delivering drive through a adder/subtracter gearbox. motors being direct coupled is not a problem. It's only at no load situations they have a significant speed difference, and under high load they will all be using their power to move the thank rather than fighting each other. No motor moving a heavy model like this will ever reach it's top RPM, so the extra heat generated by direct coupling them will be negligible. Take this from a electronics engineer...

I've done this for tank iterations I've made, and it removes any problem with motors running the belts at different speed, as well as giving much better control. It does however require a fifth motor for steering.

You will quickly find that a traditional diff can't handle the load of two or in my case four motors. I designed a diff around the large turntable to solve this problem. See pictures below.

I'm currently using this design with two buggy motors on a ripsaw based moc I'm working on, so it should be more than strong enough. Just be very careful with the gear placement as it is easily possible to get one or more of the planetary gears out of alignment when you close up the diff. It should have no undue resistance when you test it.

I know this solution requires a lot of space, but on a tank of this scale it should be no problem. Design for placement of the diffs can be a bit hard, but I suggest placing them vertical to the tracks so that you can have a up gearing after them to the drive sprockets. This solution can be a bit hard when it comes to delivering power into the diffs as right angles in a drivetrain with 4 XL motors is always going to be a weak point. But if you can solve that you should have a very powerful drivetrain that gives you a very gradual control and a lot of power to the tracks.

-ED-

Edit: Sorry. The pictures above is a old design with a fatal flaw. It's almost impossible to dismantle... The picture below shows the evolution I've had on this design, and the leftmost design is one of the newest and solves this problem as well as using fewer parts. the internals are still the same though...

And the following picture shoves the same design in real life. I've made one more iteration on who the turntable is connected but have no pictures available.

 

Before I begin, thanks for the long and very detailed answer.

Just to make sure I understand your point, there's not much use in using the "normal" differential as there were never designed to handle 2+ XL engines and this is the reason the 90 degree gear (cog) is slipping?

I understand the general design of your turntable based differential, not sure I get the the minor details but I guess I'll be able to build one on my own. Never the less, a couple of questions on your design:

1. Did you connect four engines to this differential? Two on each side? Didn't you have issues with the output (I assume you used the turntable cog as "output") simply melting / breaking teeth of the cog you connected to?

2. How did you steer? IRL tank drives usually use a clutch mechanism to engage / disengage the driving wheels.

3. Did you connect the 4 x XL engines directly to the battery box? (And by that, avoided tripping the over current / thermal protection on the PF receiver?)

 

Thanks again.

A simpler  Heavy Duty Differential avoiding use of fragile 12t bevel gears - Original version used for my TC12 Boydell Stomper entry.

Red #2 axle connectors kept in place with 4 x 2L axles - which can be extracted OK using a needle point.

Improved Heavy Duty Differential version allowing for easy disassemble using knob gears.

r

Edited April 19, 20204 yr by Doug72

5 hours ago, gilboa said:

Before I begin, thanks for the long and very detailed answer.

Just to make sure I understand your point, there's not much use in using the "normal" differential as there were never designed to handle 2+ XL engines and this is the reason the 90 degree gear (cog) is slipping?

I understand the general design of your turntable based differential, not sure I get the the minor details but I guess I'll be able to build one on my own. Never the less, a couple of questions on your design:

1. Did you connect four engines to this differential? Two on each side? Didn't you have issues with the output (I assume you used the turntable cog as "output") simply melting / breaking teeth of the cog you connected to?

2. How did you steer? IRL tank drives usually use a clutch mechanism to engage / disengage the driving wheels.

3. Did you connect the 4 x XL engines directly to the battery box? (And by that, avoided tripping the over current / thermal protection on the PF receiver?)

 

Thanks again.

1: I did in deed connect 4 x XL motors to one diff to test what it could handle, and it has no problem handling the torque. I did however in the end decide to use two buggy motors instead as they have a lot higher RPMs. That way I avoid a lot of gearing to get the speed I wanted for my model. I did however make many iterations with 4 XL motors with a common input to both diffs.

2: This is the point I was making about easier steering. I've made several designs of high speed tanks with one set of motors pr belt, and do to difference in speed of the motors it is very hard to steer them. That is why I started designing this diff. I use them in an adder/subtractor gearbox where a extra motor is aaplied for steering. This extra motor will add speed to one output while reducing it on the other, while the main drive setup gives a common input to both diffs. Sariel explains it very well here: http://sariel.pl/2009/01/subtractor/

3: I have tried using v2 Recievers, and it you use one V2 pr 2 XL motors it works if they have individual battery boxes, but it makes for a very heavy model. You could also direct connect them to two battery box with a switch controled by a smal motor or similar to power on or off, but you loose all speed control this way. After some testing I decided to go for a BuWizz in my high speed builds. For a more normal tank 4 XL with two V2 receivers and 2x traditional battery boxes should do the trick. use on output on both recievers for 2x XL and on of the other for a M that you gear down for steering. Then you have an additional output for other functions, and you can add another reciever to the other battery box for turret control if you need it. In this case the weight might even bee a positive in making the tank behave realistically. Steering, turrets and other functions normally requires very little int the way of power and can easily be combined with 4 XL motors in a 2x battery box setup. 

-ED-

Edit: If you really want to use "clutches" and brake the tracks for steering there is options using the diffs as clutches as well, but it will not be as reliable as a adder/subtractor... You could for example design a mechanism that physically brakes the diffs, and them release the brake on the side you want to reduce power on. this would be very similar to an adder/subtractor gearbox, but would seem more realistic while still not damaging any parts, or applying undue force to the motors. I used a similar mechanism in a mechanical two function sequencer, though that was automated... see this thread for ideas: 

 

Edited April 19, 20204 yr by Nazgarot
Additional info

On 4/18/2020 at 11:43 AM, gilboa said:

Currently I'm facing two issues:

1. The 90 degree non-beveled gear (cog) between the differential and the external gears seems to slip under load. Most likely I'm using the wrong part. Any idea which part I should use? AFAIR the differential P/N is 73071.

2. As noted above, I've got 2 x 2 x XL engines connected to the PF V2 receiver. Under load, it seems to cut off. Am I tripping the over-heating and/or over-current protection? Any possible solution, beyond buying SBrick / BuWizz?

2a. Assume the answer above is a resounding *Yes*. Any suggestions? Will both will be powerful enough to power 4 x XL engines + ~2-3 XL/L engines (for turret, gun elevation, etc)?

 

 

1. Old 28z differentials are not meant to be used with 16z gears, so slipping is to be expected. I'd eliminate this entirely.

2. Yes, you are. My solution would be to use either separate receivers and battery boxes, or servo controlled switches. Buwizz or Sbrick would be helpful, but are not requirements.

2b. Since the tank is so large, I'd use a separate battery box for the turret. When I built a large tank, I need a battery box to counter balance the main gun.

Now, for a large model like this, I would not use differentials in an adder configuration at all. This does not really solve any sort of real problems, and indeed introduces many new ones. I'd also do some gearing down to the XL motors. I think one problem you might be having is the fact that you don't have enough torque, and that is one reason your model was slow, the motors were overworked. Also, I'd be wary of doing anything with clutches and brakes. That, again just introduces complexity for no real gain. I'd just steer by stopping one or the other motors, like 8275 does. Now, if you were feeling fancy, you could go for a subtractor, but I have found that sometimes in a subtractor setup, you lose a lot of power because the differentials backdrive the steering motor instead of driving the vehicle forwards.

Also, these notes can be helpful:http://www.gizmology.net/tracked.htm

I built a tank which weighs 6 kilograms and is only driven by two XL motors. Not sure if your vehicle will be smaller or larger but using differentials, especially old 28-tooth differentials is a no-no for heavy Lego tanks because they slip under high torque. The effect of a subtractor can be achieved by using a Lego Train remote control or an SBrick/BuWizz.

 

Edited April 22, 20204 yr by Tarix819

On 4/18/2020 at 8:10 PM, romashkaman said:

Every motor has slightly different rpms with the same voltage. It's better to put diff between motors, but I don't do it too)

The difference is so small, if you are using the same type of motor. It's not worth it, since you add more friction with additional gears and axles for the diff. Did you test it before? did you connect two of the xl motors to a diff? Did diff turn a lot in one direction?

In my tests it never did. So I discarded my plans to couple motors with diffs.

There is no advantage of using diffs to couple motors. Technically speaking 9398, 41999 and 42099 have hard coupled drive motors too, with each being on their axles, yet coupled via ground and wheels. With such big and heavy projects keep it as simple as possible. Have every motor drive the tracks in front and rear with minimum amount of gears as possible.

On 4/19/2020 at 11:17 AM, Doug72 said:

A simpler  Heavy Duty Differential avoiding use of fragile 12t bevel gears - Original version used for my TC12 Boydell Stomper entry.

Red #2 axle connectors kept in place with 4 x 2L axles - which can be extracted OK using a needle point.

 

Given the fact that the turntable was never designed to handle fast rotation, did you have any long term issues using it as a differential axis?

 

On 4/19/2020 at 2:03 PM, Nazgarot said:

1: I did in deed connect 4 x XL motors to one diff to test what it could handle, and it has no problem handling the torque. I did however in the end decide to use two buggy motors instead as they have a lot higher RPMs. That way I avoid a lot of gearing to get the speed I wanted for my model. I did however make many iterations with 4 XL motors with a common input to both diffs.

2: This is the point I was making about easier steering. I've made several designs of high speed tanks with one set of motors pr belt, and do to difference in speed of the motors it is very hard to steer them. That is why I started designing this diff. I use them in an adder/subtractor gearbox where a extra motor is aaplied for steering. This extra motor will add speed to one output while reducing it on the other, while the main drive setup gives a common input to both diffs. Sariel explains it very well here: http://sariel.pl/2009/01/subtractor/

3: I have tried using v2 Recievers, and it you use one V2 pr 2 XL motors it works if they have individual battery boxes, but it makes for a very heavy model. You could also direct connect them to two battery box with a switch controled by a smal motor or similar to power on or off, but you loose all speed control this way. After some testing I decided to go for a BuWizz in my high speed builds. For a more normal tank 4 XL with two V2 receivers and 2x traditional battery boxes should do the trick. use on output on both recievers for 2x XL and on of the other for a M that you gear down for steering. Then you have an additional output for other functions, and you can add another reciever to the other battery box for turret control if you need it. In this case the weight might even bee a positive in making the tank behave realistically. Steering, turrets and other functions normally requires very little int the way of power and can easily be combined with 4 XL motors in a 2x battery box setup. 

-ED-

Edit: If you really want to use "clutches" and brake the tracks for steering there is options using the diffs as clutches as well, but it will not be as reliable as a adder/subtractor... You could for example design a mechanism that physically brakes the diffs, and them release the brake on the side you want to reduce power on. this would be very similar to an adder/subtractor gearbox, but would seem more realistic while still not damaging any parts, or applying undue force to the motors. I used a similar mechanism in a mechanical two function sequencer, though that was automated... see this thread for ideas: 

 

1. OK thanks.

2. Sounds very interesting. I'll check it out.

3. I was planning to have two battery box, one for the drive-train (4 x XL) and one inside the turret to drive the turret and the gun elevation. I'll have to see which is cheaper, getting an additional remote + v2 + battery box or biting the bullet (price wise) and moving to BuWizz.

4. I was thinking about a add a third small motor that will "break" the output side of differential, but this will require a second differential on each side, which in turn will add additional failure points and take huge amount of space.

 

On 4/22/2020 at 7:26 AM, Saberwing40k said:

1. Old 28z differentials are not meant to be used with 16z gears, so slipping is to be expected. I'd eliminate this entirely.

2. Yes, you are. My solution would be to use either separate receivers and battery boxes, or servo controlled switches. Buwizz or Sbrick would be helpful, but are not requirements.

2b. Since the tank is so large, I'd use a separate battery box for the turret. When I built a large tank, I need a battery box to counter balance the main gun.

Now, for a large model like this, I would not use differentials in an adder configuration at all. This does not really solve any sort of real problems, and indeed introduces many new ones. I'd also do some gearing down to the XL motors. I think one problem you might be having is the fact that you don't have enough torque, and that is one reason your model was slow, the motors were overworked. Also, I'd be wary of doing anything with clutches and brakes. That, again just introduces complexity for no real gain. I'd just steer by stopping one or the other motors, like 8275 does. Now, if you were feeling fancy, you could go for a subtractor, but I have found that sometimes in a subtractor setup, you lose a lot of power because the differentials backdrive the steering motor instead of driving the vehicle forwards.

Also, these notes can be helpful:http://www.gizmology.net/tracked.htm

1, What type of gear should be used with 28z? (I couldn't find a definite answer)

2a. See item 2 above.

2b. That was my original plan. (58119 for the power-train, 64228 inside the turret to drive the gun / turret).

3. I plan to test a direct drive setup, and check how it goes.

 

On 4/22/2020 at 10:49 PM, Tarix819 said:

I built a tank which weighs 6 kilograms and is only driven by two XL motors. Not sure if your vehicle will be smaller or larger but using differentials, especially old 28-tooth differentials is a no-no for heavy Lego tanks because they slip under high torque. The effect of a subtractor can be achieved by using a Lego Train remote control or an SBrick/BuWizz.

 

First, amazing work! It'll take me a couple of iteration (and years) to come anywhere close to your MOC.

The reason I'm overshooting for 4 x XL engines drive train is simple (and kinda stupid): All the tank MOCs I've seen are fairly slow. Having served in a tank battalion IRL, I really want to recreate the sheer awe generated by ~60 ton beast charging headlong at ~70km/h

 

On 4/22/2020 at 11:48 PM, Andman said:

The difference is so small, if you are using the same type of motor. It's not worth it, since you add more friction with additional gears and axles for the diff. Did you test it before? did you connect two of the xl motors to a diff? Did diff turn a lot in one direction?

In my tests it never did. So I discarded my plans to couple motors with diffs.

Two sets of XL engines.

In each set, each XL engine is connected to one end of the differential.

As stated above, I plan to test a direct drive setup and see how it goes.

 

Many thanks to you all for the answers.

- Gilboa

Edited April 24, 20204 yr by gilboa

6 hours ago, gilboa said:

Given the fact that the turntable was never designed to handle fast rotation, did you have any long term issues using it as a differential axis?

 

 

No problems using it as a differential axle, had a very good speed when on full throttle.
Used a 60T turntable witha 12T bevel gear driven by an L motor. 

Edited April 24, 20204 yr by Doug72

23 hours ago, gilboa said:

<snip>

1, What type of gear should be used with 28z? (I couldn't find a definite answer)

<snip>

The old 28-toothed differential uses 14 tooth bevel gears. They are very weak and where phased out in the mid-90's in favor of the reinforced 12-tooth bevel gears.

https://www.bricklink.com/v2/catalog/catalogitem.page?P=4143

Hello all,

Tried going with the far simpler setup - read: I dropped the differential and and switched to two XL engines (connected directly) / v1 receiver / battery box on each side, and at least for now, it seems to work just fine.

I'll update if the XL engines go up in flames... :/

Thanks all.

Edited May 3, 20204 yr by gilboa