Band Saw Speed - What is too fast?

[Richard Butler]

I have been doing a little research into band saw blade speeds and it seems that I am getting some conflicting numbers.

It seems that the old Delta (1943 vintage) 14" saw had a blade speed of 2200 SFPM. The new Delta saws are being shipped with the blade travelling at 3000 SFPM.

My saw is a 28-243 and that saw had two different sized pulley pairs. The old version was 2.75 drive pulley and 6" driven pulley. The newer version of that saw was shipped with a 2.25" drive pulley and 4.625" driven pulley set.

According to http://www.owwm.com/Math/SFPM.asp those pulley sets yield blade speeds of 2896 SFPM and 3074 SFPM respectively.

If you look at the same owwm.com page you will see the highest speed for a 14" saw is 3300 SFPM.

Do I dare bump up the blade speed of my bandsaw to 3300 SFPM? Iturra runs his saw at 4000 SFPM!

I was doing some other calculations trying to correlate RPM to line frequency (Hz) and came up with a factor of 28.75. 1725/60=28.75 which to me is off. I would have expected something like 30.00. How does one calculate this?

[Rod Sheridan]

Hi, synchronous speed for a 4 pole motor is indeed 1,800 RPM at 60 Hz, however you have an induction motor, not a synchronous motor.

The induction motor will run at less than synchronous speed to induce currents in the rotor bars. The difference between synchronous and actual speed is called slip, and 1,725 RPM is a standard speed for 4 pole induction motors at 60 Hz.

(a 2 pole motor would be 3,450 RPM)

Synchronous speed = 120 X frequency/# of poles

regards, Rod.

[Richard Butler]

Ahh..so that is what slip is. Where does that excess energy go? If it were converted into heat I would think that it would be a fair amount.

[Richard Keller]

Ahh..so that is what slip is. Where does that excess energy go? If it were converted into heat I would think that it would be a fair amount.

Actually slip is two things. In an induction motor (not sync) slip is necessary for the motor to work. The rotor is chasing the rotating stator field. (This is electro magnetic rotation, not mechanical) The stator field is going at 1800 rpm for a four pole motor, as previously stated here. The electro magnetic field of the stator induces current flow in the squirrel cage (attached to the rotor, and where the name squirrel cage motor comes from) which generates another magnetic field. This field is attracted to the rotating stator field and begins to chase it, but it will never catch up. Part two of slip is the cause - friction (bearings), windage in the motor, etc. Also, if the rotor ever did catch up (which I assure you will never happen) the inductive effect that the stator field has on it would stop (no longer relative motion, which is required to induce current flow) therfore, the magnetic field in the squirrel cage would stop, therfore the rotor would stop. Slip is about 2-3% for a standard motor. Not enough to worry about unless you need perfect constant speed, which is not really required in power tools. The heat generated in motors is more resultant from core losses than anything. The remainder will be from current flow through the windings. Unless the bearings are in really bad shape, they should not get very hot.

Richard.

[Richard Butler]

If I calculate the % of slip at 60 cycles, will that same percentage hold true at higher and lower frequencies?