Gear Ratio (Understanding)

Take a look at this: CLICK HERE FOR PRINTABLE WORKSHEET

GA again, Lyn, great worksheet. @ Samantha, yes, you are correct.

Thanks. It beats trying to put it into words.

OP, get your hands on a copy of Machinery's Handbook, any edition.

Thank you Lyn for that sheet, here are my results are they correct?

1a: ---> 0.5:1 - RPM will increase

2a: ---> 3:1 - RPM will decrease

3a: ---> 4:1 - RPM will decrease

That is how I would of write the gear ratios but according to the sheet, the ratios would have be written as 2:1; 1:3; 1:4; Is this correct???

correct.

Thanks jpool, but just wanted to clarify your response, am I correct with 0.5:1 and the sheet 2:1 is incorrect? Referring to the first question on the sheet?

You're correct with the first part, but back-to-front with the second. The worksheet explains that the ratio is Driver:Driven, (distance moved by driver:distance moved by driven). So in 1), distance moved by driver = 1 turn, distance moved by driven = 2 turns; ratio is 1:2.

it depends upon which one is the driver .

for your example if the small gear is the driver one then RPM will decrease by the same ratio ie 5oo*.39/1 = 159 rpm

if the big gear is the driver one then output speed for the small gear will be

500*1/.39 = 1282 rpm and So on.

I hope it's clear

The generally-accepted convention is that the first number is the input. 1:4 would produce an increase in speed and 4:1 a decrease.

A motor fitted with a 20:1 gearbox would give 1 turn of the output shaft for 20 turns of the motor.

Thank you JohnDG,

so can I say 1:4 is the same as 0.25:1? That should be the same correct?

Same ratio, yes. In gearbox datasheets it's usually converted to whole numbers either side of the colon. The ratio is just the number of turns of the input shaft:number of turns of the output shaft.

Sometimes the numbers get silly, like 7:43 - this would be 1:6.1428571428571428571428571428571 or 0.16279069767441860465116279069767:1, so it makes sense to stick with whole numbers.

Thank you JohnDG for that explanation. I understand now and it does make sense to stick with whole numbers

Your understanding is correct. However, I'm not sure that everyone knows of or follows this convention, so it might be good to spell out both input and output rpm. (Or at least say speed increaser/reducer rather than just x:1 gearbox.)

The purpose of gears and transmissions is almost always to increase torque and/or to adjust torque to load thereby increasing efficiency....The first number being the drive and the second the load....In a typical automobile transmission you may start out at 3 to 1 ratio in first, and end at .5 to 1 in high gear....

Wiki has great info...

http://en.wikipedia.org/wiki/Gear_ratio

Thank you Lyn for that sheet, here are my results hopefully they correct because that is how I would of write the gear ratios.

1a: ---> 0.5:1 - RPM will increase

2a: ---> 3:1 - RPM will decrease

3a: ---> 4:1 - RPM will decrease

Now according to the sheet, the ratios would have be written as 2:1; 1:3; 1:4; Is this correct or am I missing something???

Great explanation guys and thanks for the help in understanding gear ratios