Calculating pulley size

[OllyO]

I can't seem to get my head around this so was wondering if someone knows a simple equation I could use?

I'm fitting an electric motor to my treadle lathe for wood turning.

The motor is 1460rpm and has no pulley at present but has a 1/2" keyed shaft.

The lathe has a 4 step pulley dropping from 170mm to 90mm.

Centre height is only 4" or maybe 4.5" at a push so max diameter of stock would be 9" which I believe equates to about 700rpm. Min say 1" which requires around 3000rpm.

Any thoughts on calculating the motor pulley size would be greatly appreciated

 

[bletchmonster]

This online calculator works well
http://www.blocklayer.com/pulley-belt.aspx

 

[eSCHEn]

Pulleys, like gears, work on ratios.

Motor runs at 1460.
You want 700.
Let's say that's half (it's a wee bit less but no problems) the speed.

So if the pulley is 90mm and you want the motor to spin at half speed then you'd need a pulley half the size. So 45mm

Repeating the calculation for the larger size (170mm) will require a pulley of 85mm.

 

[ronan]

The rule when working gear/pulley ratios is driver over driven. So the diameter or number of teeth of the driver divided by the diameter or number of teeth of the driven.

 

[roofman]

I always use this method...motor RPM times motor pully diameter divided by driven pully diameter gives the driven shaft RPM

 

[Pete.]

Are you sure that the treadle lathe spindle can suffer 3000rpm?

 

[OllyO]

Thanks all for your input. Plenty for me to work with here. Am going to make a split pattern and cast an ally one. Will let you know results

 

[OllyO]

One last question: is pulley size measured by the OD of the pulley outer edge or the OD of the groove that the v belt runs in?

 

[eSCHEn]

As long as you measure the same point on each pulley it doesn't matter. I use the seat or top edge of the vee profile; whichever's easiest to get at

 

[OllyO]

Have seen an off the shelf pulley that may be suitable so sounds like it's best to contact supplier to check where they measure from