7/27/2019 3000 to 150 rpm

1/1

Note: Reduction annotation is usually written with how many input rotations ittakes to get one output rotation. Ex. 3.5 (input) : 1 (output)

There are also multistaged reductions which involve many gears which can reductions of over 1000:1! It just depends upon the required output speed and torque (we'll get to that later). So, how can you determine what the gearing is of aset of gears/sprockets/pulleys? Well, gears, timing belts, and sprockets are easy because they are labeled according to how many teeth they have. If you have aninput gear with 10 teeth on it and an output gear with 40 teeth then the 10 tooth gear will have to rotate 4 times (40/10) to get the 40 tooth gear to spin once. Therefore we have a 4:1 single stage reduction. V-Belt pulleys, on the otherhand are based upon pulley diameter.

Well, how do we determine the final reduction of a multistaged gearbox? It's really pretty easy. Multiply the reduction of the first set of gear times thereduction of the next set times the reduction of the next set and so on until you have included them all. That will give you the total gear reduction. So, if wehad a three stage gearbox where the first gear set was reduced 4:1, the secondset reduced 5:1, and the third set 6:1 then we would multiply 4 x 5 x 6 to get 120:1. (Or you could just look at the motor specs :-p) Now, let's use the motor that we talked about at the beginning and put this gearbox on it and then attacha wheel to the output shaft. Input rpm is 3000. With a 120:1 reduction we divide3000 by 120 to get 125 rpm. If we attach a 6 inch wheel to that then our bot would move at 2.32 miles per hour. Hmmmmm.... That's a little slow for our taste s

o we'll have to come up with a gear box that gives us what we are looking for. So, let's determine what type of reduction we would need to achieve a target speed of 15 miles per hour for our bot (that is a quick bot!) First, we know that weare using 6 inch wheels and our motor spins at 3000 rpm and are target speed is15 mph and our constant is 336. Plug them into this formula ((wheel size) x (motor rpm))/((target speed) x 336). If we plug in our numbers we would get (6 x 3000)/(15 x 336) = 3.57:1. It would be pretty hard to get that exact reduction butwe can get close using a 10 tooth input sprocket or gear and a 35 or 36 tooth output sprocket or gear. But, also remember that the 3000 rpm is for an unloadedmotor. Loaded motors will spin at a slower speed but determining that speed is beyond the scope of this help section.

Now, there are those in the builders community who say that your reduction

should be the same as your wheel size. i.e. If you are using a 6 inch wheel thenyou need 6:1 reduction or if you are using a 10 inch wheel then you need 10:1 reduction. I personally think that is over generalizing since different motor spin at different rates.

What are the advantages and disadvantages of gear reduction?

Well, the two main disadvantages are #1 you lose speed and #2 you have added weight for the gear box. But, on the other hand, there are some great advantages to using gear reduction. First, you bring the bot down to a manageable speed.Second, the motor doesn't have to work as hard to spin the wheel which means itwon't draw as much current from your batteries. And third, along those lines, the torque produced by the output is inversely proportional to the amount of redu

ction in the gear box. Say what? Basically, if you have a 4:1 gear box then thebot moves 1/4 as fast but has 4 times the torque! So you can have a 120 pound robot with the right gearing that will push a 400 pound load across the floor!

The optimum configuration will give you greatest speed but still have enough torque to cause the wheels to break traction (peel out) before the motor stalls. That optimum configuration varies from bot to bot and is up to you to figureout how to best implement it with your own robot.