# Off-Topic >  Need help with sprocket ratios?

## Captainleeward

:Head Scratch: Need help with sprocket ratios?  :Head Scratch: 
I have a 12 volt dc motor that has a gear head on it and turns at 41 rpms . on the motor shaft i have put a 11 tooth gear and on the other shaft is a 60 tooth sprocket which gives me 5.5 rpm. I need 1 or 2 rpm at most what should I do?? can I put a 30 tooth sprocket on the motor will this give me the desired rpm or is the torque wrong.Should I get a gear head reducer and put it in line with the gears I have now. what is the best??
Thanks for looking. L@@KING. :Clapping:

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## GlennR

I've been using this for pulleys. Maybe it will help.

Pulley Calculator. RPM, Belt Length, Speed, Animated Diagrams

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Captainleeward (Mar 7, 2015),

PJs (Jan 25, 2016),

Toolmaker51 (May 24, 2016)

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## Captainleeward

I can't find a tooth reference to rpms :O( Nice of you to mention it thanks...:O)

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## GlennR

I reread your question. Since it's a DC motor, why not just add a speed control and adjust the speed by turning the knob down?

Variable speed is the beauty of DC motors, right?


Btw, here's a site for helping choose sprockets for motorcycles. I doubt it's useful for this, but might be handy sometime.

http://www.gearingcommander.com/

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Captainleeward (Mar 12, 2015),

Paul Jones (Oct 1, 2015)

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## Captainleeward

I will check the specks for the motor and see if that is possible THANKS for your input. :Bow: 
I will let you know the out come....:O)

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## Captainleeward

Well that was a good idea but only starts at 41 rpm won't go down in rpm.

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## DIYer

Good, informative sites, GlennR. Although it didn't help the captain much, I'm sure it's going to be a useful reference for someone in the future.

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Captainleeward (Mar 12, 2015),

Paul Jones (Oct 1, 2015)

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## GlennR

1 RPM is really slow. You didn't say the size of the motor, but I'm assuming it's not very large since it's 12V.

You might be better off shopping for a gear motor that's already mated & sized to work together. I used to shop at Surpluscenter.com a bit. Sometimes they'd have odd assemblies that sound like what you are looking for. Sometimes odd things like that are cheaper than trying to find good deals on the separate components. 

What's a "stepper motor"? That sounds like a slow rpm motor, but I can't remember what they are for. Might be something to look at.

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Captainleeward (Mar 12, 2015)

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## Doc

Stepper motors are often found in old printers or many of the cheap Chinese sites for a penny each. They are popular for use in DIY CNC machines, 3D printers or plotters and the like.

They might be perfect for the job.

Try here

Regards

Doc  :Hat Tip:

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Captainleeward (Mar 12, 2015),

Jon (Mar 10, 2015)

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## Captainleeward

The size of the motor is 1/4 hp and is as big as a car starter you can see one on my bead roller here.
the easiest solution is to use a larger gear on the motor side. but I don't know if that would do the trick. vurently there is a 60 tooth sprocket on the shaft and a 11 tooth sprocket on the motor.
this gearing is 5.5 rpms.
Next best one is to put a reducer gear box on the shaft to reduce the ratio further.
this motor is 41 rpms I need 1 or 2 rpms. The shaft I'm turning is balanced and does not require much torque.

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## Frank S

Actually your RPMs with the 11/60 sprocket set would be more like 7.5 RPMs 
take 41 multiply by 11 the number of teeth in the driving sprocket then divide by the number of teeth in the driven sprocket 60 and it would look like this
1x11=451 /60=7.5167 to put it another way 60/11=5.4545 to 1 or 41/5.4545=7.5167 RPMs
if you have room to install a jack shaft to mount the 60t sprocket on one end then mount a small sprocket on the other end then a large sprocket on the item that you need to turn at 1 to 2 RPM you can accomplish the reduction you need
say you start with your arrangement of 11 and 60 with the 60 mounted on the jack-shaft , put another 11 tooth sprocket on the other end of the jack shaft and a second 60 tooth sprocket on the work piece you final RPMs will be 1.378
one note; with each reduction the torque is increased. your motor is starting out with a 310 in.lb. output torque after the first reduction is approximately 1719 in.lb. after a second reduction to the 1.378 RPM will be 9533 in.lb. or 794 ft.lb. 
Sp particular attention must be taken into account for the design horsepower of the chain 
Very small motors can become scary strong when reductions are great enough

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Captainleeward (Mar 14, 2016)

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## Captainleeward

Well Thank you Frank for that wonderful dissertation on gear ratios I'm impressed.....:O)

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## Toolmaker51

Frank S nailed it with the jack shaft. Way easier than a lot of arrangements to build and service. I'd of liked to help spin the topic; you know, helping Cap'n crunch the numbers.

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PJs (Aug 25, 2016)

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## Captainleeward

Who said that Capn Crunch that is insubordination.... :ROFL: 

Any wa as it turned out the rpms for my machine worked out fine as they were... :Clapping:

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PJs (Aug 25, 2016)

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## hooby

I haven't used any.

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