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### Author Topic: Electric Auto Gear Ratio  (Read 3926 times)

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#### garriwilson

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• Posts: 345
##### Electric Auto Gear Ratio
« on: January 18, 2008, 11:00:48 PM »
Hi everybody! In my engineering club/class, we are making an electric auto. Here are the detailed instructions:

http://www.fsea.org/pdf/EA1%20Electric%20Autopdf.PDF

We have 5 gears: 10, 20, 30, 40, and 50 tooth. It says to put gears on both sides of the jackshaft. How do I calculate the best gear ratio first for greater speed AND acceleration, and then the maximum torque?

I just found these instructions, and I never thought of putting gears on both sides of the jackshaft:

Quote
Correct utilization of the jackshaft gear would be to put one size gear on
one side to mesh with the motor, and one gear on the other side of
the jackshaft to drive the axle gear.

If say on the right side I have the motor with the 40 tooth gear on it and I mesh it with the 30 tooth still on the right side of the jackshaft. On the left, I put the 10 tooth, and on the axle itself the 20 tooth gear. I want to put 10 tooth gear on the left side so that I can keep some of the power by then meshing it with the final, 20 tooth gear to get acceleration. Am I thinking right?

-Garri

#### SmAsH

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##### Re: Electric Auto Gear Ratio
« Reply #1 on: January 18, 2008, 11:20:28 PM »
i dont quite know what you mean. but the car looks quite kool.
for the first Q the greatest speed would probibly be 1o tooth on wheel and 50 on motor but you would need quite a bit of torque...what are the specs of the motors you will use?

~smash
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#### ed1380

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##### Re: Electric Auto Gear Ratio
« Reply #2 on: January 19, 2008, 12:23:04 AM »
mostly alot of math or just trial and error.

also like smash said what motor are you using? a powerful motor will be able to use a taller gear ratio and still accel good
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#### SmAsH

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##### Re: Electric Auto Gear Ratio
« Reply #3 on: January 19, 2008, 12:48:03 AM »
one thing i forgot to say way it also depends on how fast you want it to go.  but as ed1380 said google it look on different sites that will help you. or just keep trying until you find what you want.

~smash
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#### garriwilson

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• Posts: 345
##### Re: Electric Auto Gear Ratio
« Reply #4 on: January 19, 2008, 01:43:25 PM »
I've been trying to search for the specs of the motor, but unsuccessfully. There is no label on the motor that has its specs. My teacher says I need to experiment. I don't like to experiment, I like to calculate. So I want to verify if this is right. Imagine this ratio 50-20-40-10, 10 being on the drive axle and 50 on the motor:

V1 = 5/2 * V1
V3 = 2 * V2
V4 = 4 * V3
And by substitution property: V4 = 20 * V1

That is the ratio of the the speed of the first gear and the speed of the last gear, as you can see it is dramatic.

But I figured since A = F/m then I could find the mass of each gear and if it is greater, then the acceleration is lower.
So:

m2 = 2/5 * m1
m3 = 2 * m2
m4 = 1/4 m3
And again by substitution property: m4 = 1/5 * m1

Phew, that was kinda hard to write. You get what I'm saying? The speed gets 20 times greater, and since the mass is 1/5 at the end, it accelerates faster. Am I right?

-Garri

#### ed1380

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##### Re: Electric Auto Gear Ratio
« Reply #5 on: January 19, 2008, 02:18:51 PM »
wait so you have to use all the gears?

if so then you use the first and last one in the math

if not then it's just a simple ratio.

and since you dont have teh specs of teh motor, you cant really use math to figure it out
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#### garriwilson

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##### Re: Electric Auto Gear Ratio
« Reply #6 on: January 19, 2008, 02:32:14 PM »
No, you can use any gear ratio with any number of gears.

Quote
and since you dont have teh specs of teh motor, you cant really use math to figure it out
so the whole thingamabab I did on the last post is wrong?

#### ed1380

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##### Re: Electric Auto Gear Ratio
« Reply #7 on: January 19, 2008, 02:45:25 PM »
Quote
V1 = 5/2 * V1

you mean v2=5/2*v1 ?

i dont really get what you're saying

all that math makes my head hurt.

if motor 10 tooth and axle 10 tooth, then wheels spin at same speed as motor because of 1:1 ratio

motor 10. axle 50. you get 5x less speed and 5x more torque.

motor 50. axle 10. you get 5x more speed 5x less torue

i dont see a need for all the v1, v2, etc stuff.

but methinks there is a 25x diference from lowest to highest possible speed with given gears

as with teh mass of the gear and how it would effect accelaration. it's not great enough diference to really matter

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#### garriwilson

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##### Re: Electric Auto Gear Ratio
« Reply #8 on: January 19, 2008, 02:52:20 PM »
Is there like an actual equation for calculating torque? I found that the diameter of the largest gear is 15.92 mm. I don't know the exact speed of the motor, but you could assume any number if you just want to see the relationship between two gears right? I tried searching the tutorials on the website but I didn't find an equation for  a gear's torque. Is trial and error the only way to find the best ratio? I just don't have that kind of time. The next time we meet is the race. And I can't take the car home.

Quote
you mean v2=5/2*v1 ?
Oops, yeah I mean that.

Quote
if motor 10 tooth and axle 10 tooth, then wheels spin at same speed as motor because of 1:1 ratio

motor 10. axle 50. you get 5x less speed and 5x more torque.

motor 50. axle 10. you get 5x more speed 5x less torue
so you're saying that the gears in between don't matter?

#### SmAsH

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##### Re: Electric Auto Gear Ratio
« Reply #9 on: January 19, 2008, 06:24:57 PM »

so you're saying that the gears in between don't matter?
[/quote]
it still does doesnt it? will this work http://www.societyofrobots.com/RMF_calculator.shtml
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#### ed1380

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##### Re: Electric Auto Gear Ratio
« Reply #10 on: January 19, 2008, 07:04:43 PM »
you use the diameter of pulleys and the teeth count on gears to calculate speed/torque transfer

I'll try to make an equation(even though i suck at it)

. imput gear teeth #
------------------------ = transfer ratio
output gear teeth #

imput speed/torque * transfer ratio = output speed/torque
« Last Edit: January 19, 2008, 10:48:53 PM by ed1380 »
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#### SmAsH

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##### Re: Electric Auto Gear Ratio
« Reply #11 on: January 19, 2008, 10:26:25 PM »
teath? is that something else or is it teeth spelt wrong?

~confuzzled smash
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#### ed1380

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##### Re: Electric Auto Gear Ratio
« Reply #12 on: January 19, 2008, 10:49:08 PM »
nice catch.

fixd
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#### SmAsH

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##### Re: Electric Auto Gear Ratio
« Reply #13 on: January 20, 2008, 02:19:54 AM »
lol my mum used to be an english teacher
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##### Re: Electric Auto Gear Ratio
« Reply #14 on: January 26, 2008, 11:35:10 AM »
Quote
How do I calculate the best gear ratio first for greater speed AND acceleration, and then the maximum torque?
maximum speed ~= 1/maximum acceleration

If you increase one, you decrease the other. Its a balance.

For example, lets take a car. You need some force to push the car, from the motor. Increase gearing to raise velocity at the cost of torque. Torque = force * distance . . . reducing the torque means you also reduce the force (to balance the equation), where distance is defined/fixed by the shape of the car parts. Reducing the force in F=m*a, given a fixed mass, reduces acceleration (to balance the equation).

btw, what you want is angular acceleration:
http://en.wikipedia.org/wiki/Rotation_around_a_fixed_axis