Quick Gear Ratio Question

**phlegma** · 09-18-07, 08:35 AM #1

Hey Everyone,
I am an extreme novice in understanding gear ratios, so if anyone could be of a little help, that would be much appreciated.

I'm going to be turning two gears with a belt, with a motor attached to one of the gears. Simple setup. I'm going to be attaching a prop of about 6 or so hanging from the belt, that will go around and around in in circles.

Basically, I want to know a rough estimate what rpm / torque motor I should buy to get this prop to move at approximately 10-15mph.

Nothing too precise, but don't want the prop flying! It's for a theatre setup.

Thanks for any advice.

**bontai Joe** · 09-18-07, 10:13 AM #2

Is this a 6" prop or a 6 foot prop? What is the weight of the prop? if 6 foot. what safe guards will there be to keep people from getting whacked by it?

**bontai Joe** · 09-18-07, 12:34 PM #3

I had one more question, Did you want this to go 15 MPH or 15 revs. per minute? Rotationg things are usually measured in RPMs.

**puey61** · 09-18-07, 02:05 PM #4

I think I can picture what you're trying to accomplish. You're looking for a prop prop. Ah, just my attempt at comedy. Anyhow, 120 volt motors generally run anywhere from 1200 to 3600 revolutions per minute (RPM) and you want a 6 foot diameter propeller that turns at or about probably 2 to 3-hundred RPM. Which equates to a need to reduce the motor RPM - assuming an 1200 RPM motor is used - by four to achieve 300 RPM at the propeller. In other words, the motor pulley must be one-fourth the diameter of the propeller pulley. So, with this scenario, you'll want to use either a 2 inch motor pulley coupled with a 8 inch propeller pulley OR a 3 inch motor pulley coupled with a 12 inch propeller pulley. Now you have to consider the horsepower rating of the motor you will purchase based on the net mass (weight) of the propeller assembly, but figure that a 1/2 to 3/4 horsepower motor should take care of what I suspect you'll use for materials on your project. I recommend using a 1/2 inch V-belt arrangement and you'll need to make either the motor or the propeller adjustable so that you can obtain sufficient belt tension, unless you incorporate some sort of spring-loaded idler arm & backside pulley arrangement. Most flange motors will allow for minimal adjustment so you'll need a belt that is close to tight even with the motor positioned and full slack on the slots in the flange. See this website for ideas on motors http://www.electricmotors.com/56cflangemotor2.html

**phlegma** · 09-18-07, 05:42 PM #5

thanks puey for all the info. sorry if my description was so lousy, but let me try again.

the motor and pulley will be the same size, no larger than 8". They will be separated by about a 20 foot distance. The device ( a headlight ), will hang approxmiately 6 feet below the system, and weigh no more than 5 lbs.

It is essentially a mock of the ghost train at Mr. Toad's Wild ride, Disney World.

I would like this light rotate approxmiately 15 - 20 miles per hour. My question is essentially what speed RPM motor would be best for that job. Would need a small amount of torque to carry that weight too.

**cheese** · 09-18-07, 11:25 PM #6

If both pulleys are going to be the same size, then you're going to have a headlight flying off at breakneck speed. You'll need to change pulley sizes to reduce the ratio. I still don't have a good idea of what you're trying to accomplish. I'm sue I can help, along with the other folks here.

I have done some animatronics and museum sets in the past, and got pretty good at figuring out how to accomplish things like that.

**bontai Joe** · 09-21-07, 07:52 AM #7

I'm a little clearer than before as to what you want to do, but still don't understand the rotating light at 15 miles per hour. It would be MUCH easier to come up with gearing and ratios if we knew how many revolutions per minute you need for the light. Miles per hour doesn't translate well to revolutions of a stationary light of unknown diameter (not enough info).

**phlegma** · 09-21-07, 08:44 AM #8

Maybe this drawing will help.

http://theriddleremedy.com/light.gif

**puey61** · 09-21-07, 12:36 PM #9

You'll need a motor to run at 800 rpm for 19 mph and 650 rpm to run at 15.5 mph. You'll never find a standard motor to run at such low rpm's so you'll then have to change the ratio of the two pulleys by setting up a third pulley arrangement. See this site for help on rpm to mph conversion. http://wahiduddin.net/calc/calc_speed_rpm.htm

**cheese** · 09-21-07, 08:59 PM #10

Ok, I see by your illustration now what you have in mind. The light will be on the belt, and travelling the speed of the belt.

A 1750rpm motor is a common speed motor available most everywhere motors are sold (tractor supply has them). A 1750rpm motor turning an 8" pulley will create a belt speed of 13.257 miles per hour. Exactly what you're looking for. (8" of belt per rpm, so 14000 inches per minute=233.33 inches per second=13.257mph).

I have a question though...how are you going to power the light without the wires twisting/winding as the light goes around and around?

**phlegma** · 09-22-07, 12:31 AM #11

haha, great question. i don't quite know, some kind of gyro mechanism that drives 120volt AC???

**geogrubb** · 09-22-07, 08:29 AM #12

Get an old ceiling fan and attach the pully to it then you have 3 different speeds. However, I don't think the belt is going to stay on the pulleys at a distance of 15-20 feet with a 5lb light hanging on it. My $.02. Have a good one. Geo

**cheese** · 09-22-07, 06:56 PM #13

I'm wondering how you'll attatch the light to the belt so that it will stay on the belt and allow the belt to travel around the pullies. Are you just wanting a light to move back and forth?

**phlegma** · 09-22-07, 07:01 PM #14

yeah, basically just want the light to travel back and forth.

**cheese** · 09-22-07, 08:23 PM #15

Ok, here's a simple solution that wouldn't be an engineering feat to build, and should solve both problems mentioned.

How about run a cable for the light to run along. Hang the light from a pulley from this cable. Then use the belt and pulley system with a heavy piece of fishing line tied to the belt. Then put a swivel in the fishing line, so it doesn't wind up or twist, and then tie the other end of the line to the pulley. Now the belt will go around and around, pulling the light back and forth as it does. It won't have to support the weight of the light, and the light can be powered by a regular cord.

**phlegma** · 09-22-07, 08:30 PM #16

great, thanks cheese.

Quick Gear Ratio Question

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