Hi, my team uses belt for our drive train, and we noticed when we shift between low and high during cycle runs, that the thread on the belts starts ripping, causing that side of the drive train not work properly. It will be helpful for some advice of the issue or some knowledge of why this happens.

We had an issue similar to this last year (2019). Our biggest issue was the size of the pulley, if your pulleys are 36t or smaller you may not be getting enough teeth in the mesh to support the belt. Another issue was that the axles were not supported on both sides, so the belt tension pulled the axle out of line and loosened the belts. If neither of these are the problem, maybe send some pictures of the drive train so the problem is easier to see.

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you can kinda see the belt on the left pulley is missing its thread in the second pic and the first pic is basically how its set up in our drive rail. We are running a custom gear box

This looks like the result of belt slippage and skipped teeth. My guess is that you are sending enough torque through the belt that it either shears the teeth, or the belt slips and the belt teeth rub on the pulley teeth, wearing them down. Is the belt loose or tight? If it’s loose, you can prevent skipping by tightening the belt

An alternate solution is to decrease the amount of torque on your pulleys by reducing the power in your drive gearbox. The easiest way to do this is by reducing the output voltage from the motor controllers.


What is the tooth count of the pulleys? They look awfully small.


This. For comparison’s sake, the kitbot has used 15mm wide, 5mm pitch, 42tooth pulleys on single-loaded belts for the past six years. (The kit used chain previously.) If your robot is going to be significantly under weight, dropping to 36 or even 30 tooth pulleys might work, but those look even smaller.

What were your values for:

  • Number and type of motors on each side
  • Gear ratios, especially slowest gear ratio
  • Wheel Diameter
  • Pulley tooth count, belt width, pitch, and profile
  • Robot Weight, INCLUDING battery and bumpers
  • Did any of your belts drive two or more wheels*?

* In your first photo, it looks like your gearbox shaft is driving a belt to a near wheel, then there is a belt from that wheel driving a far wheel. If this is what we’re looking at, then that near belt is driving both visible wheels.

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We’ve used 24t pulleys on 4" wheels for 125lbs robots before, it worked, but I would still go with at least 30t. Those pulleys in the picture look like 18t to me, which is way too small

As others have said - you’re definitely exceeding the allowable working load of the belt. You can find in this document from SDP-SI on pg T-15 that the recommended working tension on 1" width of 5mm pitch HTD belt is 102lbf or 454N. Again note this number is for 1" width and you’re only using 15mm (0.59"). You can do some math to determine what your belt working tension is using the pitch diameter of the belt, wheel diameter and traction limited point of your wheel (based on CoF of wheels and weight on the wheel). Of course this load is meant if you want your belts to last a lot longer than they need to in FRC, and there’s some safety factor built in.

Some rough math:

Robot Weight: 150 lb
Wheel CoF: 1
Wheel Diameter: 4"
Number of wheels bearing weight: 4
Max torque / wheel before breaking traction = 150 lb / 4 wheels * 2 in = 75 in lb
Pitch diameter of pulley = Tooth Count * (5 mm pitch / (25.4 mm/in)) / pi
An 18T pulley has a PD of 1.127" or a radius of 0.564"

Max working load on belt = Max Torque / pitch radius
For an 18T pulley this means you’re loading the belt with (75 in lb / 0.534 in) which is 133 lbf of tension.

Additionally you’ll notice the belt that failed transmits all the torque to two wheels. The more weight you have on wheels drive off a single belt the more load you can apply to the belt before your wheels slip. If your weight is shifted onto the two wheels driven off that middle belt you may be putting half the weight of the robot on the wheels driven from a single belt allowing for 266 lb of tension!

You can help reduce this load if you run a belt from the gearbox output to the far wheel instead of using the middle belt to transfer the torque to the outer wheel. But really you want to run at least 24T pulleys if not larger. We typically run 24T pulleys on the gearbox output and 42T pulleys at the wheel.


One word: Chain.
Preferably 25H.
We had this problem in 2017 and 2018 and after trying multiple solutions came to the conclusion that belts just weren’t gonna cut it for the powerful drivetrains that my team likes to design.

This isn’t really accurate considering there are a variety of factors that come into play.

Plenty posts above mention this already, but all belt profiles, pulley diameters, and belt types have their own performance ratings you need to design around. You can also use a combination of belt and chain, or belt and gear drive, etc…

However, I also prefer chain b/c I’m a lazy designer who changes stuff a lot and leaves details to the last minute.

I was just saying that my team tried multiple different belt setups, thicknesses, pulley sizes. and managed to strip or break belts in all situations and we’ve had no trouble with chains. They’re also much easier to repair.

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While true, proper design using belts (and making sure they avoid metal in the system) shouldn’t need repair.


Akash already pointed out that this conclusion is not accurate but I want to reiterate - this statement is explicitly false. Not nuanced false, not false for some interpretations.


My brother walked through the math above, I recommend evaluating what you are doing based off of physics rather than anecdotal data.

paper: Belt vs. Chain Drive Evaluation - Team 234 is an old, but interesting study by FRC234 on the topic of chain vs belt. There’ve likely been some changes since then however they outline a fairly thorough test methodology that may be useful to emulate in a new revision of this document.

(Who can tell I’m sick of “well 254 does it so I have to” logic?)


I can find the pictures of the stripped and broken belts if you want… Not saying belts can’t be used in drivetrains I’m just saying that’s my teams 2017 and 2018 robots stripped and broke belts with multiple combinations of belts and pulleys so we decided to switch to chain which can hand much higher loads.

I can confirm that cody is telling the truth, 1073 broke belts on many occasions with many different variations in center to center and pulley size. We have found through our testing that chain can handle a higher load than belts.

Do I need to quote you saying that belts can’t handle the powerful drivetrains you like to build… again?

I’m not taking issue with your use of chain, I’m taking issue with blanket statements that chain can handle higher loads than belts. It’s factually incorrect. Is #35 chain going to handle more load than a 9mm wide 5mm HTD belt? Yeah, obviously.

But there’s a huge range of belts, a lot of variables, and to make a blanket statement like I keep seeing is wrong.

I’ve run belts without issue for years in many high power drivetrains as well as other mechanisms. Much larger, higher power systems function fine with belts.


Maybe you could start by being a little more GP this is the FIRST community not the bar down the street…

The reason chains work better with shifting transmissions is since there is a small amount of slack in the chain due to it being a lot of different pieces rather than a single entity it is able to handle the sudden jerk much better.
A 25H chain also has a higher tensile strength than a 15mm htd timing belt which is the type of belt most commonly used in FRC drivetrains.
I’m not saying belts don’t work I’m simply saying they are not physically as string as belts.

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I’m sorry, but you’d greatly help your case by including chain and belt strength data. Andrew is simply making the point that you cannot make a blanket statement like that. It’s going to be dependent on the belt and the chain. You’re arguing based on your experience that all belt is weaker than all chain.

By the way, I have a belt running the equivalent of 5 CIMs of power. Can your chain do that? Conversely, I’ve seen #35 chain fail in FRC drivetrain applications. Would a belt do the same? It depends on the unit… and I deliberately held back the size of belt I’m using.


I’ve definitely used chains in a situation where belts would have been a better choice, as well as the other way around. You need to consider the amount of torque transfer, pulley/sprocket pitch diameter, belt/chain speed, your tolerance for “stretching” vs capability for tensioning, and how much width you’ve got to work with. Different answers to these key numbers can result in significantly different answers for your best solution.

Along the lines of what @EricH was saying, pop the hood of your car. Are you certain that a single chain strand could drive your air conditioner and alternator as well as all of the other stuff on that serpentine belt - for about 120,000 miles according to my experience, with little if any re-tensioning?