Where to measure the pulley for drill press speed

[john davey]

I am rebuilding a drill press and need to get the pulleys/sheaves sized. It came with 2, 3 step pulleys. One is 1/4 smaller across the board. meaning the OD of the bigger pulleys largest step is 3 1/4" and the smallest is 2" OD. They are 2 1/2 and 1 1/4 on the ID diameters. The smaller pulley is 3" and 2 1/4 on the OD small and large pulleys and 1 3/4 and 1" on the ID. Using the 1" ID measurement for the drive and 2 1/2" ID for the shaft of the press I get 690rpm on OWWM.com's calculator. If I use the OD of both 1 3/4 & 3 1/4 I get 939rpm. And obviously different speeds on the other 2 pulleys. I want to set this up to be at one speed that works the best with brad points. I am hoping to get a speed around 800 rpm which should manage brad points from 1/4 to 1/2 inch. So do I use the ID or OD of the pulley in the calculation? Thanks, John..

[Jeff Duncan]

I'm thinking it's in between.....I believe the speed is going to depend on where the belt actually contacts the pulley(s). So not the outside or inside diameter, but somewhere in the middle? In all honesty though it won't make that big a difference as the brad bits will work on either your high or low end. Only difference will be how fast you can drill! Personally I'd want a bit faster for that size bit, at least 1200 rpm or so.

good luck,
JeffD

[Ronald Blue]

Pulley's are sized on the outside diameter. I believe therefore that the outside diameter is used to calculate rpms. It would only add a layer of confusion to things if it was sized on the od and measured on the smaller inner diameter. It doesn't matter where the belt rides on the vee other then you never want it to bottom out. If a belt rode high in the vee you would gain a slight increase in rpms.

[Myk Rian]

+1. The OD is used.

[john davey]

OD it is. Thanks for the help....John

[Pat Barry]

OD may be where the pulley size measurement comes from but that makes no sense with regard to the question of sizing the pulley for the correct RPM when driven by a belt. For that you need the point at which the belt contacts the pulley. Since that likely varies from pulley to pulley it would make the most sense to target the midpoint of the belt as it rides on the pullley and where it is gripped - that is the nominal diameter - its value would approximately be the pulley OD minus the belt thickness. You may be close enough for government work just using the OD stamped on the pulley itself, but the error in this will be magnified for small pulleys and less of an issue for large pulleys.

[Michael W. Clark]

If you look in manufacturer's selection guides for pulleys, gears, sprockets, they usually use the PD or pitch diameter. This is similar to what Pat is referring to. However, since the pulleys are designed for the same belt, the cross-section of the pulley groove should be the same. If in the field trying to measure speeds (fan pulleys for example), I usually use the pulley ODs and motor RPM. This can introduce some error, because we would usually check the fan shaft with a tach as well + the motor RPM may not be exactly the name plate RPM. If you need to nail it precisely, get a tach, or measure the OD and go to the vendor's catalogue to get the PD or see if you can back into it by knowing the belt type. However, I think the OD's will get you close enough for what you are trying to do.

[John TenEyck]

Assuming all the pulleys are of the same design and you are using the same belt types, it doesn't matter where you measure the diameter as long as you measure all of them in the same way. You are calculating rpm, not surface velocity, so it's just the ratio of the pulley diameters x motor rpm.

John

[Steve Baumgartner]

Assuming all the pulleys are of the same design and you are using the same belt types, it doesn't matter where you measure the diameter as long as you measure all of them in the same way. You are calculating rpm, not surface velocity, so it's just the ratio of the pulley diameters x motor rpm.

John

Not true! For instance, consider two pulleys each with a 1/2 inch deep groove for the belt, one pulley with inside diameter 2" (hence OD 3") and the other with ID 3" (hence OD 4"). If you use the ID ratio, you get 2/3 = 0.666…. If you use the OD ratio, you get 3/4 = 0.75.

[John TenEyck]

Not true! For instance, consider two pulleys each with a 1/2 inch deep groove for the belt, one pulley with inside diameter 2" (hence OD 3") and the other with ID 3" (hence OD 4"). If you use the ID ratio, you get 2/3 = 0.666…. If you use the OD ratio, you get 3/4 = 0.75.

By golly, you're right. Sorry for the misleading info.

John