Thread: RPMs - Can someone check my math please?

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This sounds too good to be true, so it probably is. I've been looking for a drive motor for my Spinning Tunnel, but have been discouraged by the hefty prices of geared-down electric motors. So I started looking around my garage to see if there was a motor I could "borrow" from one of my power tools. My first thought was my drill press (which has the 5-speed belt & pulley system under the hood), but it is only 1/3HP, and I suspect it wouldn't last long. Same thing with an old miter saw. Then I dug out my old 2-wheel bench-top grinder... 3/4HP at 2850 rpm with a 1/2" threaded shaft. Ok, back to McMaster-Carr to find a pulley to gear it down... but first I needed to calculate the pulley size. And that's when I got that "too good to be true" feeling.

By my calculations, to get a 9' 6" tunnel ring (358" circumferance) to turn at 5 rpm, a 2850 rpm motor would need a 5/8" drive wheel. That's not a drive wheel - that's a few wraps of duct tape! Or maybe a short piece of garden hose. Could this be right?

I was hoping to use at least 1HP (preferably 1.5), but I think 3/4 would work if I give the tunnel a start by hand.

Thoughts?

I use to know a site that would give you the gear ratio you would need but I can't find it. But I'm 99.9% positive that your thinking is off

Maybe you would be right if you wanted to drive the 9'6 ring with a belt and have that to drive your prop....but other wise I'm fairly sure it won't work the way you're thinking.

Let me throw a solution to your problem at you. If you have a thrift store in your area go and see if they have a treadmill that runs off a motor. I see them all the time at the thrift store near me for 50 dollars and under. Not only will you get the motor that runs the treadmill but also the controller to speed it up or slow it down.

Another solution would be to use two wiper motors to turn the tunnel and instead of one drive wheel use 2. ....or use 2 gears/sprocket/pulleys (depending on how you drive the tunnel) one on each side of the drive wheel and a wiper motor for each side.

Kill joy. But I like the treadmill idea.

Originally Posted by madmax

... I'm 99.9% positive that your thinking is off

Let me throw a solution to your problem at you. If you have a thrift store in your area go and see if they have a treadmill that runs off a motor. I see them all the time at the thrift store near me for 50 dollars and under. Not only will you get the motor that runs the treadmill but also the controller to speed it up or slow it down.

I come up with 15.6 rpm using the formula I got from Ugly's Electrical References


Driving pulley Diam. X Driving RPM
---------------------------------------------
Driven Pulley Diam.

Which using the measurements you posted work out to

.6250 X 2850
--------------------- = 15.625
114
Unless I'm making a mistake somewhere .

How long will this be? I would be looking at torque values to make surethe motor will be strongg enough to turn it. There have been other people that have made this . You could do a search and find a lot of information on the construction. Hope this helps.

Here's a treadmill I picked up for $10.00 at the thrift store. They had $50 on it but when I pointed out that when I plugged it up it wouldn't come on....the "key" was missing and I also pointed out that the top brace was also missing.

The manager told a worker to put in back to throw out. I told her I would still give $10 just for the parts on it and take a chance on getting it to work.


Got it home and put a small magnet (pictured) on the round key and plugged it and it was working. Not only does it have a large DC motor but also the controller and it also has a DC LINEAR ACTUATOR like the one pictured below

Originally Posted by bfjou812

I come up with 15.6 rpm using the formula I got from Ugly's Electrical References


Driving pulley Diam. X Driving RPM
---------------------------------------------
Driven Pulley Diam.

Which using the measurements you posted work out to

.6250 X 2850
--------------------- = 15.625
114
Unless I'm making a mistake somewhere .

How long will this be? I would be looking at torque values to make surethe motor will be strongg enough to turn it. There have been other people that have made this . You could do a search and find a lot of information on the construction. Hope this helps.

Maybe I'm mistaken but I believe the problem is that LV Scott is leaving a key part to the equation out. The 2850 rpm motor isn't driving the 9'6 loop it's driving smaller drive wheel and that's driving the 9'6 loop.

5/8 diameter drive wheel is just shy of 2" circumference. Multiply by 2850 and number comes out 5600. Divide by 358 and 15.6 RPM is what you get.

For a friction drive on such a large wheel, I think at least one gear or chain reduction would be in order to allow the actual friction drive roller to be much larger. I would think a 3 inch caster type wheel would be a good choice.

So a 3" caster is around 9.5 inches in diameter. You want 5 RPM. 358 times 5 is 1790 inches a minute. Divide by 9.5 and you get your target RPM for the caster roller. So that is 188 RPM. Divide 2850 by 188 and you get your needed reduction for your gear or chain drive. That would be. 15 to 1. So you need a 10 tooth sprocket for the motor and a 150 tooth pulley mounted to the caster shaft.

TBQH I think you would be better off using some motor that already has some gear reduction. Like a 1/2 inch drill motor.

Like this.

http://www.harborfreight.com/1-2-hal...ill-93632.html

This unit would have the kind of load capacity you are looking for. Plus you have variable speed to work with. At maximum speed the reduction is about 100 to 1. So a 1 " diameter drive roller might work.

No 15.6 is correct. I did the math in my previous post. To get only 5 RPM on the big wheel with a 2850 RPM motor the friction drive roller would have to be about 1/5 " in diameter.

358 " X 5 = 1790 " / 2850 = .62 This means we need a roller with a .62 " circumference. Divide PI by 5 3.1428 / 4 = just about .62 "

Seems impossible I know but inches are inches. A motor turning a shaft 1/5 inches in diameter at 2850 RPM has a resulting inches traveled in rotation of 1790 inches. At 5 RPM a huge 9 FT 6 " wheel will travel 1790 inches of rotation. Simple math.

One of us is missing something and maybe it's me because I'm not using math and just going by what I seen others use for the Vortex Tunnel.

To give a example here is someone that actually made this prop and as you can see the motor is 288 RPM and it's driving a wheel a lot larger than 2 inches....maybe a 20 inch rim and that rim drives the large loop that turns the vortex tunnel.





see this one in action at bottom of this page http://timepasses.com/vortex/vortex3.html


I understand the math of a small drive wheel using friction turning the large 9'6 loop to reduce the speed but I just don't think it's going to work very well for the vortex tunnel. Especially not a 1/2 drill motor. Like I said I'm probably wrong because I'm going by what I've seen other people use for the vortex. I think this set up would work a lot better.

Ok, I'm not sure what I did before, but now I am also getting 15.6RPM. Oh, and by the way MadMax, adding an additional wheel edge-to-edge makes no difference, since the surface transfers the Feet-Per-Minute equally. However, if I drive a 9" wheel attached to a 3" wheel which drives the large ring, THEN I get 5.2RPM, and THAT I can live with.