My team lost our programmer this year and now I have to learn more programming. I’m horrible with technology and this is probably a stupid question, but if we are using an 84-tooth gear then a 60-tooth gear and another 84-tooth gear on the drive train, what are the gear ratios and rpm? I looked it up several times, but I’m having a lot of trouble with this. Can someone help me? Thank you!
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This is from PLTW: gear ratio = teeth in driven gear /teeth in driver gear .
So it would be 84 teeth/84 teeth =1:1 gear ratio
This formula is how you find all gear ratios. Just divide the teeth of the outer most gear (the one farthest from the driven gear) and divide it by the teeth of the driven gear (the one being powered by a motor)
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Well lucky for you, gear ratios don’t require any programming knowledge
A picture would be helpful if you want help with your specific situation.
In general, you can use fractions to determine the output rpm of a system.
EX: The driven gear is a 60T on a 200 RPM motor, and the wheel is on a 36T gear. The RPM of the wheel would be: 200 * (60/36) = 333.33 RPM
Another thing to note is that all VEX gears are multiples of 12, so the 60/36 in the example could be simplified to 5/3
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Ok, thank you! When I used 84/60, I got 1.4. However, I can’t use decimals in VEXcode V5 Blocks. I hope this picture helps.
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Yeah that’s 1:1 the middle gear is just really a place holder to make sure that the 84 tooth gears go in the same direction. The front wheel will spin at the same speed as the back wheel.
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That ratio ends up being 1:1 (the wheels spin at the same rate) because the ratio cancels out on either side
I don’t know what you mean about programming decimals, you can just tell the motors to spin
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Oh my goodness. Thank you so much. I just told my partner this and she had the same reaction as I did. I feel so stupid right now.
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Something to note- this fraction formula is past its limits once you transition from a simple gear train to a compound gear train. Simple gear trains are where all the gears are on separate axles, but in a compound gear train the gears (or sprockets, or pulleys, or whatever you’re using to transfer the power) the way you go about calculating the ratio is slightly different. Say you have an 84t driving a 12t. The 12t is sharing the shaft with a second 84t, and that second 84t is driving a second 12t. This creates a 1:49 ratio because with compound gear trains you multiply the ratio of the first set of meshing gears by the ratio of the second set of meshing gears, as seen in the example with (12/84)x(12/84)=(1/49) which simplifies down to
(1/7)x(1/7)=1/49. This means that the final 12t rotates 49 times faster than the first 84t, but also means that it has 1/49th the torque (angular velocity and torque are inverses here).
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The gear in the middle is refered to as an idler gear. It in no way changes the gear ratio, just is used to connect the others while changing the direction of the next gear in line.
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Oh that makes sense. Thx for the note
Thx so much. You would not believe how stupid I feel rn
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Nope, don’t feel stupid right now. You reached out, asked a very good question and got some good reference points. Lots of other out don’t ask, end up direct driving things with poor results. (Bent shafts, burned out motors, unhappy robot movements, etc.)
You should take a look at the VEX teacher material. https://education.vex.com/parent-wrapper.php?id=mechanical-advantage is a good section on how to really leverage gear trains.
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Yes you can! Can you elaborate on the error?
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Well thank you very much. I know this will definitely help me figure some of this out.
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On the drivetrain settings, I was trying to enter 1.4 as the input for gear ratios, and it kept on going back to what I had previously entered.
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I think for drivetrain settings, it wants you to enter the ratio of the gear cartridges
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This here is another knowledge on gear ratios.
When you have gears setup like this where 2 gears go on the same shaft, it is called compound gears. To calculate its gear ratio, you need to multiply the first gear ratio by the second gear ratio. For example if you have 12t to 36t on first stage and 12t to 60t on the second state, the gear ratio will be (1:3)*(1:5) = 1:15
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Wow, that’s really interesting. It’s like a whole new world I never knew anything about. Thanks so much for all the help y’all!
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This is unrelated to gear ratios for the most part, but based on your picture, I would suggest using bearing flats on all your axles. They will greatly reduce friction on the spinning axles and are deemed necessary for all moving axles.
https://www.vexrobotics.com/shafts-and-hardware.html
They are part 276-1209 on this page.
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As said above, the gear in the middle is known as an idler gear, so for example, just think if you take the first gear and put it into a gear ratio, then the middle gear spins fastest, right? Then you take that faster central gear and deconvert it back to normal, so it turns in the right direction and right speed(ratio) to match both wheels. So it is still 1:1. The formula is gear ratio = teeth in driven gear/teeth in driver gear. The ratio is the same and is 1:1.
Short Story
The ratio is the same and is 1:1, so the wheels spin the same way and speed! 
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