Drive Train for Heavy Treaded Vehicle

[mxp]

Hi Everyone,

I have a pretty heavy model that I would like to put on treads. The final model will probably weigh something around 12-15kg. The model is also supposed to move at a certain speed (around 10cm per second). As a result I think I will need to combine several PF XL motors to get the necessary torque and speed.

In my opinion I have 2 options:

Option 1: Adders + Subtractor

I would connect several PF XL motors via adders for torque and then a subtractor for steering. I think I would need to heavily (9:1) gear up each motor before the differentials because the small 12 tooth bevel gears in the differentials (for adders and subtractor) don't handle torque very well. So I would first gear up and then, behind the subtractor gear down, so that I get more torque again.

Advantage: Model can go straight (thanks to the subtractor); No hard-coupling (motors won't suffer)

Disadvantage: A lot of power from the motors gets lost because there are so many gears. I'm not sure how fast lego gears and axles can spin for a long time without breaking (PF XL motor 200rpm with 9:1 gear up = 1800rpm)

 

Option 2: Several PF XL motors on each tread

I would have multiple sprocket wheals for each tread that is driven by a PF XL motor.

Advantage: Simpler construction; less power loss

Disadvantage: Hard-coupled motors (coupled through the tread links); model never goes straight because all motors won't turn at exactly the same speed.

 

My question is, do any of you have experience with very heavy treaded models?

What's the best way to drive them? (so that they can go straight)

How many PF XL motors would I roughly need for a 12-15kg model?

 

Thanks for your help and let me know if my description of the problem doesn't make sense :)

 

 

[BrickbyBrickTechnic]

I'm not sure how you plan to gear up to a subtractor with 9:1 ratio and then drive 12-15 kg. Did you mean 1:9 gearing? Because you would need alot of XL's and batteries. 1 battery box stalls 2 XL motors at 5:1 gearing with a light, tiny vehicle (300-400g).

[mxp]

Just now, BrickbyBrickTechnic said:

I'm not sure how you plan to gear up to a subtractor with 9:1 ratio and then drive 12-15 kg. Did you mean 1:9 gearing? Because you would need alot of XL's and batteries. 1 battery box stalls 2 XL motors at 5:1 gearing with a light, tiny vehicle (300-400g).

I would use a combination of an 8 tooth gear and a 24 tooth gear twice, giving me a ratio of 1:9. Do you think I would need even more gearing up? The batteries shouldn't be an issue, I'm not using the lego battery box but will build my own, using AA or LiPo batteries. The lego battery box is just too limited.

[suffocation]

You could try two hard-coupled XLs geared down 1:3 on each sprocket for a total of 8 XLs. A BuWizz unit should be able to handle all the motors on its own, reducing the model's weight. Not too sure about the 10 cm/s but with the BuWizz set on fast it shouldn't be too far off.

Edit: just tried 2 hard-coupled XLs geared down 1:7 and powered by a LiPo on a 1.5 kg model - speed is more or less 10 cm/s. Some more numbers for you to crunch.

Edited January 2, 2018 by suffocation

[Saberwing40k]

I'm sorry, but I see several critical flaws in your plan. I don't mean to be mean, or come across as harsh, but you asked for advice, because you're at the stage where you don't have a lot of experience with Technic, and want advice. We've all been there. But, that does not mean I'll sugercoat it, that much.

Firstly, what are you building that would weigh 12-15 kilograms? For comparison, 42055 weighs 3.5 kilos, and @Sariel's Maus tank weighs 5.3 kilos. Those are really quite big. I mean, this isn't an inherent problem, if you're building a scale model of something large. But, are you building the model large for the sake of being large, or are you building it as big as it needs to be? It's fine to build a large model, but building as big as possible is not a good design practice. What you are wanting to build also dictates the track and motor configuration. A bucket wheel excavator, for instance, has many small tracks, whereas a crawler crane has two or four large ones. I initially though what you were building would be near the top end of what is possible, but @Dawid_Szmandra recently posted a crawler crane that weighs in at a whopping 27 kilos. I'd maybe ask him how he drives the tracks on that thing.

Secondly, you have some very bad ideas regarding adders and subtractors. Using adders is more or less unneeded, as Lego motors don't differ that much, and do not suffer very much from hard coupling. In addition, using an adder with more than two motors is a nightmare of complexity. Plus, you're adding gearing that accomplishes nothing. Same with gearing up and then gearing down. I've seen some people suggest using it, but it is a terrible idea, and horribly inefficient. It also does not remove any stress from the drivetrain. Also, bevel gears are going to be the LEAST of your concerns at the scale you're talking about. At your scale, all manner of components, including axles, can be broken by poor design. Don't ask how I know this. In addition, subtractors, at least in my experience, actually cause you to lose a lot of power in the drivetrain, as the drive motors backdrive the steering motor, instead of driving the vehicle forward.

Thirdly, in my experience, using two Lego motors to drive a tank does not result in a vehicle that can't travel straight. Lego motors don't differ that much, and the weight of the vehicle actually forces the motors to move at more or less the same speed. Plus, any differences in motors can be accounted for by the operator with relative ease, the differing motors speeds only really matter with a self guided robot. Also, as I alluded to earlier, hard coupling motors will not damage them any more than normal use. Lego motors are quite good for what they are, and the other system parts actually equalize things fairly well, at least from my experience.

Basically, you should use hard coupled motors, and keep the drivetrain, if any, as short as possible. With XL motors, you should be able to gear them down 3:1 and maintain the speed you want.

Best of luck, I hope you come back with some actual WIP pictures.

[Sariel]

I concur with everything said by Saberwing, and you should forget about putting any differentials, whether for adders or subtractors, into anything heavier than 3 kg. The bevel gears inside them will be shredded at the torque needed to move something that heavy at the speed you're aiming for. My Maus tank at 5.7 kg was driven by simply sticking a PF XL motor into each of four sprockets. There were 8 gear wheels in the entire drivetrain, none of them a bevel gear, and the only thing connecting the motors were the tracks. That's how you want to build. Keep it dead simple or you'll be busy replacing destroyed gears all the time. Don't worry about driving a 12kg model straight, worry about driving it at all.

Edited January 2, 2018 by Sariel

[doug72]

2 hours ago, Sariel said:

I concur with everything said by Saberwing, and you should forget about putting any differentials, whether for adders or subtractors, into anything heavier than 3 kg. The bevel gears inside them will be shredded at the torque needed to move something that heavy at the speed you're aiming for. My Maus tank at 5.7 kg was driven by simply sticking a PF XL motor into each of four sprockets. There were 8 gear wheels in the entire drivetrain, none of them a bevel gear, and the only thing connecting the motors were the tracks. That's how you want to build. Keep it dead simple or you'll be busy replacing destroyed gears all the time. Don't worry about driving a 12kg model straight, worry about driving it at all.

The heavy duty differentials I developed for my Boydell Stommper TC12 entry using a Z60 turntable and knob gears could be a solution.
I used an L motor but an XL motor would also be OK.
Various gear ratios are possible to drive the turntable either by spur gears or for a high ratio by worm gears.

You could have two per track i.e. one at each end.

 

Edited January 2, 2018 by Doug72
Added image

[mxp]

Thank you everybody for your feedback, I really appreciate it. There seems to be a consensus here what I should do: So I will go with hard coupled motors that are directly connected to the sprockets.

@suffocation Thank you for your advice and even trying it out. That's seems to be what I will be needing (also from what others said)

@Saberwing40k Why would I be offended? Thank you so much for your feedback and advice. So unfortunately, I can't reduce the model's size. It's probably not what you were thinking of and in most people's opinion here probably not a great build, but since you asked, here is a video. What I'm trying to do is to put the following on to wheels/treads:

It's WIP and I'm still working on some details.

It weighs (including the base that will be removed later) 8.8kg. I assume that with a structure that can drive it, it will come out somewhere around 12-15kg.

@Sariel Wow, thanks for your answer. I actually read your book and watch your workshop reports on youtube every month. So thanks for taking the time and helping me. I was actually afraid of the bevel gears breaking, that's why I wanted to gear up a lot. But from what you and other people are saying here that doesn't seem to work. So I will go with connecting the motors directly to the sprockets and hard coupling multiple of them.

@Doug72 Thanks for your suggestion Doug. I think your idea would work. The only problem is, that because I need to couple so many motors to get enough torque to move 12kg that I would need a lot of those differentials. Since it is quiet big, the model would become even heavier and bigger. It's a good idea but I think in my specific case, I will go with the simpler design.

 

Again, thank you everybody for your feedback. It was very helpful and I really appreciate it.

[suffocation]

I love it - works and looks great! I wouldn't even know where to start with a build like that. I think you're a lot more talented than you're willing to admit

If you can get your hands on a BuWizz or are able to build your own comparable unit then go for the 1:9 reduction (you want to gear down, not up). In the end you'll be closer to 12 kg than 15 - even with eight motors, a decent battery pack, a ton of 5x7 frames, the tracks and all the other structural elements, I doubt the base will exceed 3 kg - so that should take some stress off the motors.

Really looking forward to seeing the final build

[mxp]

Thanks @suffocation I will try 1:9 and see how that works.

I could post some pictures once I'm done, but that might take a while

[Carsten Svendsen]

Wow is that an old school RCX unit I see there? Cool, I like it already!

[Aleh]

4 XL motors geared DOWN 1:9 without any differentials and having minimum geas meybe could move your creation at all, but nor fast.

[TheMindGarage]

I don't think the motors will be the problem - it'll be the complexity. Your gears will grind and possibly break, especially if you use adders and subtractors. It probably depends on other factors like the power sources, but I reckon 4 XL motors might be enough (2 on each tread) to get it to crawl along, given that one is enough to drive the BWE (admittedly extremely slowly). You might need 6 or 8 if you want it to achieve the speed you want. What's probably more important is having extremely robust gearing. Ideally each motor should have its own gear reduction - you might be able to group them in pairs if you sandwich the gears between two well-braced beams. However, I think with this much torque going through the final shaft leading to the sprockets, twisting them might be a possibility. If you are using the larger sprocket wheels (those with an axle hole in the middle, six pin holes and room for 10 track links around the circumference), you should definitely consider attaching them to the LEGO portal axle hub (they fit!) which also provides a sturdy casing for a 1:3 gear reduction.

I wish you luck with your project, but 12-15kg is no laughing matter. Above about 5kg, bending becomes a serious issue. For a tracked vehicle, it'll probably be a case of the middle sagging as the only things supporting the vehicle's mass are the two tracks at the far ends - how much of a problem this will be depends on your chassis and how wide it will be.