# Stupid Newbie ?- How to Measure Pulley Diameter?



## HMF (Jul 4, 2011)

Hi guys,

I was asked to measure the 3 inside pulleys on my Heavy 10 underdrive. Being an idiot, I had no clue how to do it. I used a large outside caliper and a straightedge.

What is the right way to do it please?

Thanks,


Nelson


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## Neil74 (Jul 4, 2011)

I like a simple answer as I haven't the equiptment my former bosses supplied,circumference divided by Pi. Unless inspecting or working in tenths, simplest is always best.


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## pdentrem (Jul 4, 2011)

You can use a thread or fishing line and measure the length and divide by pi. Remember that those flat pulleys are not flat. They have a crown to keep the belt centered.


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## HMF (Jul 6, 2011)

pdentrem link=topic=2674.msg18137#msg18137 date=1309831240 said:
			
		

> You can use a thread or fishing line and measure the length and divide by pi. Remember that those flat pulleys are not flat. They have a crown to keep the belt centered.



Question- I am aware of the crown, and the measurement needs to be fairly acccurate-

Won't thread stretch a bit and ruin the accuracy?

Is there a way to use an outside calipers to do it? How would you find the center if you used them? (See the third picture in my original post).

Thanks,


Nelson


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## Tony Wells (Jul 6, 2011)

If you have a pair of good dial calipers, preferably 12", you can use the chord/segment height method. To find center, dye it up and scribe a line along the area you will use to take your measurement.

Just how accurately do you need to know?


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## pdentrem (Jul 6, 2011)

Does your wife have a flexible tape to measure your waistline? 


OR
The setup to it this way will be PITA but it will work. This is not the way I would do it.
I have had to do it this way on some of our machines to register the jackshafts etc.
Put a straight edge across the pulleys that is parallel with the shaft. Measure to distance to the shaft at the largest pulley and add half the shaft diameter. Now you know that pulley size. On the other steps measure the distance to the pulley from the straight edge and remove 2 x that reading from the largest pulley diameter.


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## HMF (Jul 7, 2011)

Tony Wells link=topic=2674.msg18235#msg18235 date=1309994102 said:
			
		

> If you have a pair of good dial calipers, preferably 12", you can use the chord/segment height method. To find center, dye it up and scribe a line along the area you will use to take your measurement.
> 
> Just how accurately do you need to know?



I need to be sure it's center when I measure- that is my problem, how do I determine that for sure with caliper dividers?

Thanks,


Nelson


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## HMF (Jul 7, 2011)

I will try each of these methods and see if they are accurate enough.

Thanks for helping me out on this.


Nelson


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## HMF (Jul 22, 2011)

I have one- haven't used it yet but it looks excellent.


Nelson


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## Frank Ford (Jul 22, 2011)

Or you can buy one of these, for about ten bucks:




It measures diameter directly:


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