Drive wheel size

[Brad Lilly]

My grinder has a 4" drive wheel. I'm using a direct drive 3 hp variable speed motor. I find right now that I can't get enough speed out of my set up. The obvious thing is to step up to a 6" drive wheel but I stumbled on an old flat belt pulley. The pulley is 2" wide with a nice crown however it has an 8" diameter instead of 6" like I want. So before I start making a bushing and wasting a lot of time dose anyone run an 8" drive wheel? Do you guys think I will loose too much power?

 

[EdCaffreyMS]

Are you running a KBAC 29 VFD? If so, do you have the internal jumper(s) set for "double speed"? I have 3hp/1740rpm motors on both the grinders in the finish shop, both with 4" drive wheels, running off KBAC 29D controller...... the only time I'm ever at 100% on the controller with with the 10" contact wheel when I'm profiling. With a flat platen, I rarely exceed 60% on the controller. If you're not sure about the "double speed", download the manual http://www.kbelectronics.com/manuals/kbac_manual.pdf and look on page 24. It's a jumper inside the controller. Just make sure you cut the power to the controller, let the capacitors drain (the lights on the front panel will go out)...... then set the jumper to "double speed".

The old Square wheel machine ran a 10" drive wheel from the factory......and the belt speed on those machines just SCREAM! Way too fast for just about anything.

I suspect the issue with that wheel would be the weight, and more so the BALANCE. Those old wheels (like the one you pictured) were designed to run at low rpm. Just an educated guess, but I suspect if you turn that wheel more then a few hundred rpm or so... it might just fly apart on you.....that is if the grinder and bench it's attached to don't literally walk out the door first. I'd be afraid to run it on a modern motor, just out of fear that the balance would be so far off that it would wreck the shaft bearings in the motor.

 

[KenH]

What rpm is your current 3hp motor? I'd expect perhaps 1725/1800 rpm? If so, with a 4" drive wheel and running at 100% you would only have 1800 to 1900 sfpm which is pretty slow. By changing to the 8" drive wheel your max speed at 1750 rpm would be 3600/3700 SFPM which is pretty good. Below is a table showing typical belt speeds with a direct drive motor with different size drive wheels at different motor RPM. The 3450 can be a 1725 rpm motor run with double speed jumper in, or 200% of base 60 hz.

Motor RPM	Drive Wheel Diameter	Belt SFM
1750	4	1833
1750	6	2749
1750	8	3665
3450	4	3613
3450	6	5419
3450	8	7226

 

[Brad Lilly]

My motor is 1750 rpm. I had planned on trying to balance the wheel, when I checked the runout it was around 10 thousandths of an inch. I suspect if I spent a little more time cleaning it up I could be better.

 

[timgunn]

As a fairly general rule, 4-pole 3-phase motors (plated for a little under 1800 RPM) can be run at higher frequencies when used with a VFD without exceeding their design speed.

For manufacturing reasons, it is much cheaper to build all motors of a particular design with identical components and then fit either 2-pole (just under 3600 RPM at 60 Hz), 4-pole (just under 1800 RPM at 60 Hz) or more-pole windings at assembly to suit the speed and power rating required.

This means that all the motors in most ranges will have bearings and rotating components that are good to 3600 RPM. Most VFD and motor manufacturers seem to recommend running 4-pole motors with VFDs in the speed range 10 Hz-100 Hz for a 10:1 speed turndown. There's usually a reduction in power and efficiency as frequencies increase above 100 Hz, but it is small enough that it doesn't seem to be a problem for most knifemakers and many run their "1750 RPM" motors up to 3500 RPM, just as they would a "3500 RPM" motor.

The KBAC drives have a double speed jumper which makes things easy. All other drives I've encountered in the last 25 years or so have also had the ability to run at higher frequencies than 60 Hz, though it can sometimes be a pain finding the correct parameters to change. What drive are you using?