# Pulley Size Calculations For Rpms



## calstar

Not electrical but motors drive pulleys and this shows the size pulleys needed to get a desired rpm. 
Don't get confused by the formulas(like me), watch the short vid a few times and its really simple.

If this is the wrong forum pleased relocate

Brian


----------



## John Hasler

calstar said:


> Not electrical but motors drive pulleys and this shows the size pulleys needed to get a desired rpm.
> Don't get confused by the formulas(like me), watch the short vid a few times and its really simple.
> 
> If this is the wrong forum pleased relocate
> 
> Brian


No mention of effective pitch diameter, which is the tricky part with V-belts.


----------



## calstar

John Hasler said:


> No mention of effective pitch diameter, which is the tricky part with V-belts.



You mean the depth/width of the pulley's v? Unless you need a very specific rpm(+-20) doesn't seem like that would make a whole lot of difference in the hobby machine shop(but what do I know). I did a _quick_ google search, just found screw pitch diameter info.

heres a link to pulley size and speed calculator:  
http://www.blocklayer.com/pulley-belteng.aspx


Brian


----------



## Charles Spencer

Great link!  Thanks, Brian.

Oh, I generally use an Excel spreadsheet to calculate pulley RPMs.


----------



## wrmiller

Guys unless I'm completely missing something here, it's not rocket science if as said above you are ok with a +/- of some amount. Take the stated motor rpm (or measured) and simply apply the appropriate ratio to the diameter of the motor pulley to get the speed you want. To make this easier:

Motor rpm = 2000 rpm
Motor drive pulley dia. = 2" (just for sake of argument)
2000 rpm, use a 2" driven pulley (1/1)
1000 rpm, use a 4" driven pulley (1/2)
4000 rpm use a 1" driven pulley (or increase your drive to 4" and your driven to 2", but it's still a 2/1)

Not that hard?


----------



## roadie33

Bill,
That's the way I do the math when I have a 3450 RPM motor and need to reduce it to 1725. 2/1
Very simple and doesn't take rocket science to figure it out.


----------



## JimDawson

This is the formula I use.


----------



## markba633csi

Yep blocklayer is good
MS


----------



## John Hasler

calstar said:


> You mean the depth/width of the pulley's v? Unless you need a very specific rpm(+-20) doesn't seem like that would make a whole lot of difference in the hobby machine shop(but what do I know). I did a _quick_ google search, just found screw pitch diameter info.
> 
> heres a link to pulley size and speed calculator:
> http://www.blocklayer.com/pulley-belteng.aspx
> 
> 
> Brian


The effective diameter of a v belt pulley is not the top of the groove but the point where the pitch line of the belt contacts it.  The pitch line of the belt is the line on the side that does not change length when you flex it.  It is somewhere between 1/3 and 1/2 the thickness of the belt down from the top depending on who you ask.  It varies a bit depending on operating conditions.  Thus the effective diameter of an A belt  pulley can be as much as 5/16" less than the outer diameter if the belt rides flush.  This can make a significant difference with small pulleys: a 2" o.d. and an 8" o.d. pulley might give a 4.6:1 speed ratio rather than 4:1.

If you are designing with new, standard belts and new, standard pulleys the manufacturer's tables can get you as close as makes sense with v belts (they slip anyway).  If you are using recycled stuff...

Then there is calculating belt length, especially for systems with multiple pulleys and idlers.

https://www.google.com/search?sclient=psy-ab&btnG=Search&q=v+belt+pitch+diameter

Note that it's now "datum diameter" rather than "pitch diameter".


----------



## Bill W.

JimDawson said:


> This is the formula I use.
> 
> View attachment 131588


Jim... About half way down... Capital P should be small p.
Throws off the whole equation.
Bill


----------



## Fabrickator

Here's the formula I use for calculating pulley size and to find feet per minute (FPM).   In this instance, I had to use a triple reduction to slow my band saw blade speed down.  I've checked it for accuracy with a tachometer.


----------



## wrmiller

Bill W. said:


> Jim... About half way down... Capital P should be small p.
> Throws off the whole equation.
> Bill


----------



## microshop dinker

JimDawson said:


> This is the formula I use.
> 
> 
> JimDawson said:
> 
> 
> 
> This is the formula I use.
> 
> View attachment 131588
> 
> 
> 
> 
> 
> 
> JimDawson said:
> 
> 
> 
> This is the formula I use.
> 
> View attachment 131588
> 
> Click to expand...
> 
> 
> 
> View attachment 131588
Click to expand...

Jim D---Do you use a computer or SLIDE RULE???     Sammy


----------



## JimDawson

microshop dinker said:


> Jim D---Do you use a computer or SLIDE RULE???     Sammy



I use an abacus


----------

