2x72 Belt Grinder Belt/Motor Speed
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  1. #1
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    Default 2x72 Belt Grinder Belt/Motor Speed

    Hello all,

    I am build a 2x72 belt grinder using a DC treadmill motor. The 1.5 hp motor I have is rated at 90V on the tag. No RPM is given.

    I am driving it with an 120V SCR speed control fed through a bridge rectifier. I get a maximum of 108V at present. This gives me a motor speed of 6700 rpm. If i drop back to 90V, I get about 5600 rpm.

    My question is twofold, I suppose.
    1st, I need to determine if I want to run the motor above the rated voltage. 18V over is 20%. I guess I would be comfortable with 10%, but not sure about 20%. Now, this would be design maximum speed, and not necessarily my most common speed I suppose. But I would still want to have comfort running at maximum occasionally.

    2nd, would be the belt speed range I want. I need to fabricate my drive pulley still. I have seen 5,000 to 6,000 sfpm as an upper range limit for premium belts. Is this a good target to hit?

    I want to due most of my grinding in the meat of my torque curve on the motor, but can only make assumptions as to where that is.

    Thanks

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    It depends on your drive wheel size. On the grinders I make I use Leeson 3 phase 1800 rpm AC motors overclocked via the VFD to run at 2500 rpm. This drive's a 6" drive wheel and give me plenty of torque at 20 rpm to full speed and runs any ceramic belt very well.

    With a DC motor like your using you should have near full torque at all rpm's. That said your rpm's are super high unless your using 2-3" drive wheel.

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    A good belt speed might be 4500SFM so you might target that for mid range.

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    Say my operating range will be 1,000 - 5,800SFPM, which is pretty wide range I suppose

    If I where to use a 4" pulley, that will nearly match my motor RPMs at 1,000 to 5,600.

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    i ordered a 2x72 grinder few weeks ago. they wanted to run it around 7000 ft/min. 3600 motor 4hp
    7” drive wheel

    seen a few videos of it goin through some 1/4” plate and it cuts through it much faster than you would think possible. they were using a ceramic belt also

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    yeah ceramic belts area a must for cool cutting long life and fast removal. My grinder does darn slow to 3000 rpm but with a 8" drive wheel, could do with a bigger motor HP wise, but for most of the small stuff i do with it its great, more speed is always nice, generally abrasives cut faster and last longer the higher the surface speed as it makes the steel a lot softer to cut through do to the frictional heat generated in the cut zone, yet cut fast enough and you cut the heat off as it conducts in and the part stays pretty cool!

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    Quote Originally Posted by acrosteve View Post
    Say my operating range will be 1,000 - 5,800SFPM, which is pretty wide range I suppose

    If I where to use a 4" pulley, that will nearly match my motor RPMs at 1,000 to 5,600.
    6000 is a good grinding speed so your 5800 is good IMHO.. likely you may lose a little RPM with work load.. You could kick it up to something over 6000 with having the variable control and that would be good also....Standard pulley (s) is good so you can catalog one if needing another.

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    20171230_150253.jpg20171230_150300.jpg


    I ended up using a 4.125" pulley - which is just layered plywood. I had a hole saw that was close, and then trued up the blank on my lathe and bored the center. I have not done anything yet to limit my speed on the upper end. I am considering installing a tachometer so that I can see the upper end of my range and just leaving it at that.
    Last edited by acrosteve; 12-31-2017 at 08:41 AM.


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