Measuring new v belt length woes

[Sugarkryptonite]

Hi guys

I recently installed my servo motor on my machine, having trouble measuring the belt for it. I used a string, got it decently tight wrapped around both pulleys in their loosest position (as in no tension would be on the belt), measured the string on the ground and got 39 inches.

Later that day I went to buy a 39 inch belt, got home and it's not even close! A few inches short at least. It clearly says on it 39, and I measured with the string multiple times to be sure.

Now after that, I used a couple calculators online for v belt length where you put in both pulley diameters and the center to center spacing between the two, and I get roughly 41 inches, which sounds like it could work, but why such a big difference with the string? When I measured the pulley diameters I used the outside diameter, not the inner sheave portion, it is unclear on all these sites where you must measure exactly.

Any thoughts?

Edited January 29, 2019 by Sugarkryptonite

[brmax]

I also found measuring the out side of the pulleys are the best and closest. So I think your on the right track!

Although this is if were using a 3L or similar 3/8” width  pulleys and belt. I found measuring the outside of the belt is my best bet.  

 

Good day

Floyd

[Constabulary]

On my side of the pond you find 3 different measures for V-belts - abbreviation are:  LI, LW and LA

• LI = length inner side of the belt (German: Länge innnen)
• LW= datum length of the belt (not sure if this is the correct translation)  (German:  Wirklänge)
• LA= length outer side of the belt  (A = außen = outer suide) (German: Länge außen)

So there can be a difference, but not sure how the official measuring on your side of the pond is.

Just wanted to give this as a hint.

[trash treasure]

I use a tailor's cloth tape measure, and measure around the outside of the pulleys - Usually get it pretty close.

If you buy from a place like McMaster Carr, they list belts by the outer circumference, so that measuring method works well :

https://www.mcmaster.com/drive-belts

Also, there are on-line belt calculators : http://www.calculatoredge.com/mech/vbelt length.htm

Edited January 29, 2019 by trash treasure
more info

[lambchop]

Lenght of Belt =
Distance between pully centers X2
+ 1/2 Circumference of top pully
+ 1/2 Cirumference of bottom = Lenght of Belt
if using correct belt width for pulleys

Edited January 29, 2019 by lambchop

[Sugarkryptonite]

Thanks all, I got a 42" and it fits like a glove. Note for newbies, do NOT measure inside the sheaves like I did with your string if you do it that method. I suggest the calculators shown here and online for calculating V belt lengths (as well as lambchop's formula, calc is just easier).