pulley problems

polingrran · Aug 18, 2017

I've built a band saw sawmill with just a few cinches to work out. One, which has been my biggest problem so far, is getting the correct size pulleys, or sheaves for the drive shaft and motor. It either kills the motor when putting it into drive, or it eventually burns up the belt. It has an 8 horse motor with smaller wheels from a manual cultivator, so I'm guessing the issue is needing larger pulleys. Anyone have any thoughts?

texasranger556 · Aug 18, 2017

A picture is worth 1000 words here. What size pulleys have you tried? What size belt? How are you tensioning the belt? How does your alignment look? There are a ton of variables that could affect belt life.

CincyFlyer · Aug 18, 2017

It shouldn't kill the motor simply engaging without a load, even if it's geared a bit on the high side! You can look up the linear speed recommended for the blade, and then work backward. But if you need more torque at the wheels, you'll need a smaller pulley on the engine. I always recommend a larger engine. For pretty much any situation, ever.

polingrran · Aug 18, 2017

I have a 5.25" on the motor with an inch dia. on the shaft and 4.00" on the driveshaft. The tension pulley is spring loaded to take it in and out of gear. That's been my issue: once getting it in to gear, the motor pulley is spinning too fast, so once I start a cut, the torque's not there and slows, or stops the blade after 4, or 5 seconds and starts burning up the belt. If I go to smaller pulleys, the speed increases with more torque. My wheels are 12" with somewhat heavy rims as they are from a 49' cultivator.

texasranger556 · Aug 18, 2017

Let's run the numbers. Say your blade is supposed to run at 3500 feet per minute and you have 12" wheels that the band rides on. The circumference of the wheel is 1 foot * pi (3.14) = 3.14 feet. So 3500 fpm/3.14 feet is 1114 RPM for the drive wheel. Then let's say you have a typical gas engine that is rated at 3600 RPM. 3600/1114 = 3.23 so you want roughly a 3:1 reduction between your crankshaft and drive wheel.

Using your 5.5" pulley on the engine, you would need between a 17-18" pulley on the drive wheel. Since that's bigger than your drive wheel, you'll probably want to downsize both pulleys but keep the same ratio. So for example run a 12" pulley on the drive wheel and a 4" pulley on the engine.

Then you run into practical limits such as minimum belt contact patch in the pulley so it doesn't slip. At 8hp this probably isn't a major problem if your pulleys and belts are in good shape.

I just picked a blade speed, it depends on your blade and what you're cutting so adjust accordingly. In any case I'd say your're running the blade way too fast.

polingrran · Aug 18, 2017

Thank you, that was what I thought the issue was. Your numbers are about right on the motor and the blade, so I'll give it a try and see what happens. I wasn't sure there was some kind of ratio I could go by, so great information!

jemclimber · Aug 18, 2017

Texasranger did all the math for you and he also picked a very good linear feet per minute for an 8hp engine. He must have a bandsaw. Recommended fpm ranges from about 3000-6000 depending on the hp of the engine. 12" diameter wheels are very small for a sawmill and will break blades prematurely because of the increased flexing they cause, unless you are refering to the rim size and not the outside diameter where the band rides. If so then all that previous math needs to be redone. Most bandsaws have a minimum of around 19" wheels for the blade to ride on.

texasranger556 · Aug 18, 2017

I don't have one, but have been doing some research to eventually build one.

This is what I'll probably use for the blade wheels, just have to figure out what length belt fits perfectly in the groove...or cut one to exact length and somehow glue or screw it in place.

18.75 O.D. H-BUSHING SINGLE GROOVE PULLEY | Split Taper Bushed Bore Pulleys | Pulleys | Power Transmission | www.surpluscenter.com

mapper · Aug 18, 2017

texasranger556 said:
I don't have one, but have been doing some research to eventually build one.

This is what I'll probably use for the blade wheels, just have to figure out what length belt fits perfectly in the groove...or cut one to exact length and somehow glue or screw it in place.

18.75 O.D. H-BUSHING SINGLE GROOVE PULLEY | Split Taper Bushed Bore Pulleys | Pulleys | Power Transmission | www.surpluscenter.com

Seems like those pulleys are really narrow for a band saw, vee-belts are flat on the back so getting the blade to track well would be very difficult. My little 16" grizzly has wheels that are 1.5" wide with a 1" crowned tire on it.

texasranger556 · Aug 18, 2017

I hear what you're saying and I only started considering them because there are several "kits" that include similar or perhaps identical setup.

An example: 19" BAND SAWMILL WHEELS,PORTABLE BAND SAWMILL,DIY BANDMILL,LUMBER,WOOD | eBay