# How do you calculate rpm?

I have no prior experience in dealing with rpms, and I would appreciate it if y’all could provide a easy, concise, explanation of how rpm works in mechanisms and how you calculate rpms. Also, how much rpms is considered “slow” and how much is considered “fast”?

Thanks for the help y’all! Vex community is amazing.

To find the RPM of an output shaft/mechanism, take the input RPM (i.e. the motor speed), and multiply it by the gear ratio (the ratio of the number of teeth on the input gear to the number of teeth on the output gear).

Example: A flywheel with a 600 RPM motor powering a 60 tooth gear which, in turn, spins a 12 tooth gear, which is on the same axle as the flywheel. Its output RPM will be

600 RPM * (60/12) = 3000 RPM.

The consideration of a “slow” or “fast” RPM depends on the mechanism. For a drivetrain, for example, 100 RPM would be considered slow, 300+ RPM would be considered fast.

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aight thanks for the quick reply

A more general formula would be something like `rpm = (input speed)*(driven gear/output gear)`

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oh yeah just one more thing, whats the best rpm for an x-drive and for a dr4b?

Of course, it depends on what kind of tasks you want to complete with either, but these are pretty general.

X-Drive: Generally 200 RPM motors are used, creating an effective drive speed of about 200*1.414 = 283 RPM.

DR4B: Usually something like 100 RPM on a 1:5 gear ratio for an effective speed of 20 RPM. Of course, your optimal output speed depends on the weight you are carrying as well as the length of the four-bar arms.

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FYI. Incase you were not aware RPM stands for Rotations Per Minute. It is important to note that a 4” wheel drivetrain on 200rpm will be faster than a 3.25” drivetrain on 200rpm because every rotation on a 3.25” wheel is less than that same rotation on a 4” wheel. Also on the forums people tend to go ahead and account for the differences in speeds based on wheel sizes and it is reasonable to assume that when someone says x rpm and does not include a wheel size, then that rpm has already been calculated or measured to be on a 4” wheel

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How do you put that in account into the equation for rpm though?

And by the way I put up a discussion already and answers the question. I made it a couple of days ago. I think it literally had the same exact title

For example, for the drive, you find the output RPM (say 200), then to find the speed of the drive, you multiply by the circumference of the wheel to get distance per minute. For a 200 RPM drive on 4" wheels in tank drive (not x-drive), the robot travels at

200 RPM * 1.05 ft. = 210 ft/m = 3.5 ft/s

So the speed of the base will be 3.5 feet per second.

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No I am saying like lets say you have 5/3 gear ratio with a 200 rpm motor. And for your wheels you have 4" wheels. So if you were not to account for the wheels rpm the 36 tooth gear is going 334 rpm. But what acctually matters is the wheel. So how do you account for than in the equation or am I looking at it from the wrong perspective.

every time the 4 inch wheel rotates, it travels the distance of it’s circumference. you can plug that into rpm, personally I convert it to feet per second

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wait would a 5/3 gear ratio with a 200 rpm motor be too fast for a drive train with 3.25" omni wheels or would it be ok. Because that gear ratio translates to 330 rpm. Or would it be ok because that is what I have on my cad.

Ok, here we go:

200 RPM * (5/3) * 0.822 = 274 ft/m = 4.56 ft/s

So your drive speed will be 4.56 feet per second.

No what I am asking is will my drive have too high of a ratio and burn out because there will be like no torque.

that’s a good speed as long as your robot doesn’t weigh a ton. I’m using that exact ratio for my hoodbot.

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Ok that is what i’m doing for mine too. Thank you. If you want to see the cad of my robot I have planned it’s this

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