Measuring For Belt (Pulley) Size - How To?

[Ian1783]

I did search for this ( http://leatherworker.net/forum/index.php?showtopic=45569&hl=%20belt%20%20sizes&st=0 ), and did get that formula, which was great, but I'm wondering how to measure pulley size.

Do I measure the inside diameter? That is, the diameter of the pulley at the bottom of the V? Or is pulley size determined by the outside diameter of the pulley?

Ian

SF Bay Area

http://imcinnis.blogspot.com/

[Northmount]

I did search for this ( http://leatherworker.net/forum/index.php?showtopic=45569&hl=%20belt%20%20sizes&st=0 ), and did get that formula, which was great, but I'm wondering how to measure pulley size.

Do I measure the inside diameter? That is, the diameter of the pulley at the bottom of the V? Or is pulley size determined by the outside diameter of the pulley?

Ian

SF Bay Area

http://imcinnis.blogspot.com/

Diameter of the top edge where the top of the belt rides, in other words outside diameter.

Tom

[Ian1783]

So, using the formula: distance between shaft centers (15.5") X 2= 31", + smaller driver diameter (2.125") X 1.57"= 3.3 + driven diameter (3.125") X 1.57= 4.9= 39.2". A 40" belt then. But my 41" belt is way too loose. It seems to me that a 40" belt won't take up that slack.

By the way, I have the motor's adjustor at the halfway point.

If I use the same formula using the outside diameter of the the larger of the two drivers that I have, I get: 31" + 4.9" (driven dia.) + 4.71" (driver dia.), I get 40.61". My 41" belt is far too loose for this result. Am I missing something here?

I realize I can eyeball it and pick up what I think I should use, and exchange it for another if that doesn't work. I'm just surprised that a pulley would be measured at the outside diameter. For one thing, the original pulley (smaller of the two) allows the belt to ride a bit lower in the groove, for an effective outside dia. of about 1 7/8".

Well, I don't want to beat this to death. I'll stop by the auto store and see what they have. I'll try a 38" belt and see if I can get that to work. Who knows, maybe the one belt can be used for the two driver pulleys.

Ian

SF Bay Area

http://imcinnis.blogspot.com/

[dirtclod]

I've always just took a string and pulled it around the pulleys marked the string where the ends meet and measured then got that length belt.

[Ian1783]

Dirt, that's what I did for my initial measurement. I just bought a few belts and will see which works and how that may or may not jibe with the formula. Thanks!

Ian

SF Bay Area

http://imcinnis.blogspot.com/

[Ian1783]

OK. I put on a 38" belt with the 3" driver (measuring the outside diameter of the pulley), which I had figured for about a 40" belt, but thought that 1" wouldn't be enough to do it. Way too tight with the 38".

So I put on the 2.125" driver (again, measuring the outside dia.) and that did the trick, though with the machine just nestling into place; tight enough to require a little loosening of the motor's adjuster.

I was leaning toward the smaller driver, as it was original to the machine's setup (off the original clutch motor), and shortening the adjuster should strengthen the motor-mount setup.

For belt tension, I loosed the adjustor until its nut was loose and let the weight of the motor determine the tension (belt tightness). I figured that was about right, unless I hear other wise.

With things setup as well as I could, I measured using the string trick: 36". So, I think Tom's outer diameter measurement is probably the right way to go, considering the way the belt nestles a bit deeper into the driver than it does on the machine's driven, or the driver that came with the new servo motor. A 40" belt with the larger driver might have worked. I think what is illustrated is that an inch either way can be a big difference.

I think I've got this down. Cool. Thanks all!

Ian

SF Bay Area

http://imcinnis.blogspot.com/