Original post here
I’ve been doing some calcs. A 28T to 60T GT3 3mm pitch belt is mechanically capable of giving 46.5 lbs of forward thrust using a 90mm wheel. And that’s while staying in spec.
the 28T HTD pulley can handle 7.0 inlbs of torque. The GT3 pulley can handle 17.9 inlbs. These are all ballpark numbers but it’s a general trend, over twice the load capacity…
So that brings up my question. As the DIY community has so many people pushing the boundaries of what is possible and putting our components past their limits, why does nobody use GT3 belts?
That 46.5 lbs of thrust is the max while in spec… We obviously are exceeding the load capacity of our belts hence why people tend to shred them so often or have issues with skipping. So if we push GT3 to the limit we could surely get much better force out of it.
This may not apply to acceleration since we are usually motor limited, but i think it’s important for braking. If you can’t push your brakes to the max without skipping belt then you’re not at peak performance.
And in addition to better wear resistance and load rating, they also cite quieter operation.
EDIT: After further research, I have come to these conclusions.
HTD VS GT2
Here are some direct comparisons followed by interpretations of the data.
General Lifetime:
This is the general lifetime testing of the different belts. Each test was conducted with the same pitch, pulley sizes, speeds, and torque. It concluded upon belt replacement was necessary, or until enough data was collected (no need to go further, in the case of GT). The baseline for these tests was the HTD performance, set at 100%. These tests were conducted at speeds and pulley sizes that result in accelerated belt wear, which will be explained further down this post. GT 5M has nearly 190% the lifespan of HTD 5M.
Tooth Jump:
These results are not particularly favorable for GT 5mm pitch, at least upon first glance. HTD and GT seem to be very similar in the 5mm pitch, however it is important to see that this test was only tested up to 2300 RPM, while we typically run our motors around 6000 rpm. This results in much worse tooth skip, as it is far beyond the specifications of the tooth profiles. This higher rpm with higher torques is where GT shines greatest, and as you can see GT and HTD profiles begin to diverge as installation tension decreases. We run our tension somewhere in the 2-3 lb range. Some with idlers as low as 0 or 1 lb of tension.
Operating Belt Tension:
Here is the exact maximum belt specification for the tension on the taut side. For explanation’s sake, I’m going to divide numbers by 2, so the belt width is equal to 12.7mm, which is close to our use case. HTD 5M is rated at 51 lbs of tension. GT is is rated at 80 lbs. Please note that this is independant of ratcheting or pulley size, this is only the maximum allowable tension on the belt.
Minimum Pulley Size Per RPM Unit:
This is fairly straightforward. A 22 tooth HTD 5M pulley is rated to operate at 1160 RPM. This is over 5 times less than the 6000 rpm we operate at. A 22 tooth GT 5M pulley is rated to operate at 2000 RPM. Again, we are far out of spec, but by much less. This is 3 times less than the 6000 rpm we operate at. Unfortunately, both will experience accelerated wear, but HTD will suffer much worse.
Exact RPM And Torque Ratings:
These graphs show the operating RPM and torque ratings. HTD 5M is rated to 75 in lbs of torque at 6000 RPM. GT 5M is rated to 250 in lbs of torque. This means that GT 5M can run at 3 times the torque of HTD 5M. While we are not typically exceeding these, it still means that GT will suffer much less wear over time. Please note that this data is unrelated to pulley size, that data will come next and will show further disparity between HTD and GT.
Exact Torque Ratings Per Pulley Tooth Count:
I’m going to use the math for 5000 RPM, as it is the closest common value to 6000 rpm between the two graphs. HTD 5M is rated to 11.6 in lbs of torque with an 18T pulley. GT 5M is rated to 25.2 in lbs of torque with the same rpm and pulley.
What is more interesting is the low RPM values. At 100 rpm, HTD is rated to 25.7 in lbs of torqe, while GT is acceptable up to 58.5 in lbs with an 18T pulley. Our intense startup torques far exceed HTD specifications.
However, HTD is already in the “accelerated wear” category all the way up to 28T pulleys at 5000 rpm. HTD suffers from excessive wear from an 18T pulley at any rpm of 1160 and up. GT is acceptable wear at 5000 rpm at any pulley size 18T and up. GT is functional from 200 rpm to 14000 rpm with an 18T pulley. It only suffers from excessive wear from 100 rpm and lower, which we are rarely in when riding. For a visualization, this is when the motor makes a full turn every .6 seconds, which is a very slow creep for us.
We will be exceeding the torque ratings of these, so GT will also suffer accelerated wear, but not nearly to the extent of HTD since it is already going to wear excessively at any torque.
Other Details, Unrelated to Performance:
Ignore this if all you care about is raw performance, only the above information will be relevant to you.
The sound levels are not easily interpreted from this. Decibels are determined on a logarithmic scale, not linear. So 100 dBA is much more than 2x louder than 50 dBA. GT is significantly quieter than HTD.
We use backside idlers. GT and HTD perform the same here, but the data shows that we are seriously exceeding the spec here lol. Reversed bend radius very negatively affects belt life (given that all other wear factors are already in spec. We use idlers so other factors like belt slippage don’t occur, which is objectively a greater concern for wear).
Totally random thing I found about belt movement that I thought was kinda neat:
I found this very interesting. In a tuned pulley setup where they are in line and not twisted of offset at all, the belt will always ride to one side. If you have a precision CNC’d motor mount setup that ensures your pulleys are properly lined up, then you will still suffer from belts moving sideways. It is dependant on the direction the fibers internal to the belt are twisted. S twist is the opposite of Z twist. So if you’re using two of the same belts, they will both move in the same direction, ie: one will move away from your truck and the other towards it. Just thought this was interesting to read about.
TLDR:
All Gt profiles are far superior to HTD, particularly in lifespan, torque ratings, and rpm. If you want a quick interpretation of the data, here is a comparison of many profiles:
That’s why - HTD is much rounder (the 20mm at the bottom is HTD and is basically circular) while GT3 are “squarish”