Maximum safe torque for single bevel gears in 3L differentials

[Hrafn]

I managed to break two 12t single-bevel gears in 3L differentials before concluding that trying to use them with an ungeared XL motor really was a bad idea. My question is, how much torque can those gears take (in the 3L differential) before breaking? It's clearly somewhat less than the maximum torque produced by the XL motor, since my gears broke when I was powering the motor with the 7V rechargeable battery and the dial on the battery was (I think) turned to just below the maximum setting. Will two L motors break the bevel gears? Both the official stats and Philo's site show the L motors as having just under half the torque of the XL (though they list different absolute values).

On a related note, will the gears survive more torque in an old-style 4L differential when two half-bushes and a fourth bevel gear are added to reinforce them?

Thanks!

[Boxerlego]

Question: Did you have the differential axle go straight to the tire?

[Hrafn]

Question: Did you have the differential axle go straight to the tire?

Yes, I did - it was a small vehicle with no room for gearing after the diff.

More generally, though, I'm trying to figure out how much force the 12t gears can take; if they can survive 2 L motors ungeared then they can survive 4 L motors geared 2:1, and so on, right?

Edited January 3, 2014 by Hrafn

[Ape Fight]

I managed to break two 12t single-bevel gears in 3L differentials before concluding that trying to use them with an ungeared XL motor really was a bad idea. My question is, how much torque can those gears take (in the 3L differential) before breaking? It's clearly somewhat less than the maximum torque produced by the XL motor, since my gears broke when I was powering the motor with the 7V rechargeable battery and the dial on the battery was (I think) turned to just below the maximum setting. Will two L motors break the bevel gears? Both the official stats and Philo's site show the L motors as having just under half the torque of the XL (though they list different absolute values).

On a related note, will the gears survive more torque in an old-style 4L differential when two half-bushes and a fourth bevel gear are added to reinforce them?

Thanks!

I have a truck powered by four XL motors (geared down once that further increases torque) and the power goes through one 3L diff. The 12t gears in the diff are fine.

I broke the middle gear when the truck was geared down twice (and about six other gears), but with it geared down once it's been fine. It seems you either had a weak gear or that something isn't quite right with your drive train.

Edited January 3, 2014 by Ape Fight

[Junpei]

Yes, I did - it was a small vehicle with no room for gearing after the diff.

More generally, though, I'm trying to figure out how much force the 12t gears can take; if they can survive 2 L motors ungeared then they can survive 4 L motors geared 2:1, and so on, right?

I'm pretty sure you're thinking about this the wrong way; you're thinking of speed, not torque. 4 L motors geared 2:1 would increase the torque on the diffs at least 6 times. Correct me if I'm wrong though.

Or are you talking about gearing after the differential?

[Hrafn]

I'm pretty sure you're thinking about this the wrong way; you're thinking of speed, not torque. 4 L motors geared 2:1 would increase the torque on the diffs at least 6 times. Correct me if I'm wrong though.

Or are you talking about gearing after the differential?

This is why I shouldn't post on the net before having my morning coffee.

I should have said geared 1:2 - geared so the axle spins faster but carries less torque to the differentials. I totally understand that you can gear up after motors, then down before the wheels, to spare the gear train (including diffs) from enduring too much torque. What I'm trying to figure out is *how much* I have to gear down to avoid fatal levels of torque; and right now I don't know what the fatal level of torque is for 12t single bevels.

I have a truck powered by four XL motors (geared down once that further increases torque) and the power goes through one 3L diff. The 12t gears in the diff are fine.

I broke the middle gear when the truck was geared down twice (and about six other gears), but with it geared down once it's been fine. It seems you either had a weak gear or that something isn't quite right with your drive train.

Hm, maybe some of my gears are older and weaker. It's hard to know since some came off of BrickLink. The drive train was very simple: motor to 20t to 3L differential, with the 20t and diff held in a 5x7 frame. The diff outputs went directly to wheels, with no suspension.

[Zerobricks]

The setup you described should nate have failed...Proboably some bad gears

[Hrafn]

The setup you described should nate have failed...Proboably some bad gears

Thanks, that's good to hear. Are there any visible tell-tale signs of a weak gear? Examining the rest of my 12t gears I don't see any obvious cracks or chips or anything. Are certain colors weaker? Both of the broken ones were light bluish gray.

[allanp]

If they slide onto the axle way too easily that's a sign that they are bad. But I would not be surprised if a new gear in great condition broke with an XL motor driving it. The official Lego crawler has a diff driven by an L motor at a 12:20 ratio so I guess that is safe. Two ungeared L motors would produce about the same torque in most cases. I have found buggy motors to be much more useful. Twice as powerful as XL motor without breaking gears!

[Hrafn]

If they slide onto the axle way too easily that's a sign that they are bad. But I would not be surprised if a new gear in great condition broke with an XL motor driving it. The official Lego crawler has a diff driven by an L motor at a 12:20 ratio so I guess that is safe. Two ungeared L motors would produce about the same torque in most cases. I have found buggy motors to be much more useful. Twice as powerful as XL motor without breaking gears!

Thanks! All my remaining ones fit tightly on axles so they should be fine. And I should have two buggy motors arriving soon, so I can try those out - though they'll likely need to be geared down significantly for an indoor rally car.

[Tamas Juhasz]

I totally understand that you can gear up after motors, then down before the wheels, to spare the gear train (including diffs) from enduring too much torque. What I'm trying to figure out is *how much* I have to gear down to avoid fatal levels of torque; and right now I don't know what the fatal level of torque is for 12t single bevels.

One very important thing: if you gear up before the diff, then down, then the efficency of the drivetrain will be very bad. You should use this as the last opportunity. lego motors have relatively less power, so every percent of power which can go to the driven wheels are important.

Anyway, measuring how much torque can handle a 12t bevel gear, is very easy: just fix the gear in a vise, then use a big lever arm with a weighing scale. From the displyed weight (in the moment when the gear breaks) you can count the force, and you know the arm's length --> M = F*l.

Then you can count with the motor's maximum torque and the drivetrain's gearing until the 12t bevel gears, how much torque go to them. If it's higher than the measured one, you should modify the construction.

Are there any visible tell-tale signs of a weak gear? Examining the rest of my 12t gears I don't see any obvious cracks or chips or anything. Are certain colors weaker? Both of the broken ones were light bluish gray.

The old 14t gears are the strongest. The tan, new 12t gears are a little bit stronger (not prooved fact, only experiments), than the grey ones. The 90's years light grey 12t bevel gears are the weakest, imho.

And I should have two buggy motors arriving soon, so I can try those out - though they'll likely need to be geared down significantly for an indoor rally car.

You can't say this if you don't say which wheel do you want to use. If you use a small wheel, then you need no or less gearing. With a 94 mm wheel you need more. RC buggy motors are enough strong to drive with no gearing a car with 62 mm wheels. It all depends on, what is your goal with the vehicle (going uphill, or only driving in a flat surface in the room, etc.).

[allanp]

Buggy motors are the best, but you might not need as much gearing down as you think. They spin very fast, the slower of the two axles spinning three times as fast as an L motor but they still have almost the same torque as an L motor.

[Hrafn]

One very important thing: if you gear up before the diff, then down, then the efficency of the drivetrain will be very bad. You should use this as the last opportunity. lego motors have relatively less power, so every percent of power which can go to the driven wheels are important.

Yes, that's one of the reasons I've been avoiding doing this.

Anyway, measuring how much torque can handle a 12t bevel gear, is very easy: just fix the gear in a vise, then use a big lever arm with a weighing scale. From the displyed weight (in the moment when the gear breaks) you can count the force, and you know the arm's length --> M = F*l.

Then you can count with the motor's maximum torque and the drivetrain's gearing until the 12t bevel gears, how much torque go to them. If it's higher than the measured one, you should modify the construction.

I was hoping someone would have a clearcut answer from their own experiences, so I wouldn't have to go the route of breaking more gears in the name of science.

The old 14t gears are the strongest. The tan, new 12t gears are a little bit stronger (not prooved fact, only experiments), than the grey ones. The 90's years light grey 12t bevel gears are the weakest, imho.

Very interesting! I'm surprised you say the 14t gears are strong, I thought they were phased out partly because they were weak. Or was it just because they didn't mesh well with anything else?

You can't say this if you don't say which wheel do you want to use. If you use a small wheel, then you need no or less gearing. With a 94 mm wheel you need more. RC buggy motors are enough strong to drive with no gearing a car with 62 mm wheels. It all depends on, what is your goal with the vehicle (going uphill, or only driving in a flat surface in the room, etc.).

I currently plan to use 56x28 wheels. The issue is speed and the lack of a straightaway in our apartment. It's a decent size (~1000 sq ft = 93 m^2) but broken up into a number of small rooms and strangely shaped hallways. I suspect a vehicle moving at any decent speed will quickly get out of IR range. Outdoors is not really an option right now - we live in New England and it's 12 F (-11 C) (in the middle of the day) and snowy! I haven't used the buggy motors yet, so I was making a very rough guess based on the published specs of their RPMs.

[Tamas Juhasz]

Very interesting! I'm surprised you say the 14t gears are strong, I thought they were phased out partly because they were weak. Or was it just because they didn't mesh well with anything else?

The strength of 14t gears is also an experienced "fact". Me and some technic clubmates in Malug (hungarian Lego club) also found that they are the strongest, during the last 10 years. They are strong because of the center part, because of the amount of material in the center (the bevel 12t gears brake mostly in center). 12t gears has a weak cross-section in the center, 14t don't. 14t gears were phased out, because they can be used with less gears properly, they only fit well (seen with TLC Lego technic model builder eye) with the 28t old style differential, and each other. They aren't exact stud wide (less than a half), and have 14 teeth, you can make with them "strange" ratios. But for these properties, many of us like them. Edited January 3, 2014 by Mbmc

[Hrafn]

The strength of 14t gears is also an experienced "fact". Me and some technic clubmates in Malug (hungarian Lego club) also found that they are the strongest, during the last 10 years. They are strong because of the center part, because of the amount of material in the center (the bevel 12t gears brake mostly in center). 12t gears has a weak cross-section in the center, 14t don't. 14t gears were phased out, because they can be used with less gears properly, they only fit well (seen with TLC Lego technic model builder eye) with the 28t old style differential, and each other. They aren't exact stud wide (less than a half), and have 14 teeth, you can make with them "strange" ratios. But for these properties, many of us like them.

Good to know, thanks!