# What are the torque limits of gears?

I am using Lego in the classroom to introduce students to geared machines.

As part of this I am asking them to design a winch, using a small 1.5V DC motor and a collection of the common Lego gears, i.e. the 8, 16, 24 and 40-tooth spur gears, and the special worm gear.

Based on rough back-of-the-envelope calculations, I will require about 5 N.cm of torque at the winch drum.

• Do I have to worry about breaking any gears due to excessive torque?

• Do I have to worry about gears slipping (skipping teeth) due to high torque?

• Has anyone performed experimental (destructive) tests to quantify the maximum torque these gears can withstand? Order of magnitude is OK - will the gears break at 1 N.cm, 10 N.cm, 100 N.cm, ... ?

• One thing to bare in mind is that some LEGO gears exist in several variants, each of which may have different torque limits. The following tutorial cites some of these differences: sariel.pl/2009/09/gears-tutorial Oct 14, 2013 at 11:18

Do I have to worry about breaking any gears due to excessive torque?

LEGO Gears are one of the strongest LEGO components. When you apply great amounts of torque, they may slip or grind, but never break.

Do I have to worry about gears slipping (skipping teeth) due to high torque?

If you apply high amounts of torque, your gears will slip. This happens even with moderately high amounts. They may also slide down the axle and grind the side of the other gear, or the axle.

Has anyone performed experimental (destructive) tests to quantify the maximum torque these gears can withstand? Order of magnitude is OK - will the gears break at 1 N.cm, 10 N.cm, 100 N.cm, ... ?

Your gears will withstand the torque quite well - it's probably other components, like the axles and beams, that will give way first. Don't worry about breaking the gears - the worst situation might be that an axle or two might bend.

• Actually it is quite possible to break gears when using very high gearing ratios. See firstroboticscanada.org/main/wp-content/uploads/LEGO-Gears.pdf . It gives some circumstances in which the author has broken gears, though it doesn't include any actual torque measurements. Oct 16, 2013 at 5:52
• There are more elements to breaking gears than torque, like pressure and speed. Oct 16, 2013 at 11:17

Simply put, they won't break. I've put LEGO gears through everything I could and they were fine. That includes spinning an 8-tooth gear at around 80,000 RPM (sustained for only a second or two) via a flywheel-based stored-energy device. I've driven a 9lb vehicle with about 40 gears in the drivetrain, and none broke.

Slipping teeth is rather common. As a guide, if you sandwich spur gears between two beams or bricks with holes, you're not going to grind them. I would trust that for 100Ncm or more. If you are going to put the structural beams further away from the gears, the torque limit reduces. You can increase it by adding bushes (to stop gears from sliding along axles) or by adding short beams that span two axles - this stops the axles from bending and disengaging the gears.

### If anything breaks, the gears won't primarily be the problem.

Do I have to worry about breaking any gears due to excessive torque?

Do I have to worry about gears slipping (skipping teeth) due to high torque?

I have seen gears get to nearly twenty N.cm without issue, other than slipping along the axle, (easily fixed with bushings), or skipping, (easily fixed by super-gluing your chassis pieces together.)

However! Not all gears are made equal, and I have broken a couple of (gear) teeth in my day. Your 40-tooth gears might be in for a rude awakening as to their structural integrity under high torque.

This can be easily solved, I think, by changing the gear ratios, which will be a great lesson for your students, how big gears driving small ones can make things more efficient.

The "real problem" will most likely be the bricks. Bricks don't stand up to torque as well. If you compress a brick too far, it will snap, despite how tough they normally are, or at least chip.

This may be a hard issue to solve, not to mention the other "real problem" of the binding power of bricks to each other. Fortunately, there is a solution! You just may not like the fix.

Superglue is tough. It stand up to a lot of pressure. As I said in the first bit, super-gluing your chassis-pieces together may be the only way to handle a challenge like this, if your students have trouble with structure.

So say you don't like ruining your pieces' chance of use ever again, like me. I suggest that you look up good ways to structure a Technic chassis, and if a build is iffy, tell them what needs to be fixed.

But for argument's sake, I have tried many things like this, and everything was fine. Just make sure that somehow, every piece stays in place.

But only if you're using LEGOs for the chassis at all!