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Can this machine handle increased torque?

  1. Aug 30, 2011 #1
    I have a big, old horizontal blade mixer. There is no indication as to who manufactured it. No clue what year it was made...somewhere between 1910 and 1952. It currently has a 7.5HP motor. I tried mixing my viscoelastic goo. The ammeter showed the motor drawing about 2.5x the maximum rated current (and whining like no tomorrow!) until I shut it down. Empty, the machine draws about 30% of the maximum current.

    Through various means, I have estimated that I need about 15-20HP to mix my goo. A modern 30HP motor is the same frame size as the old 7.5HP unit. So I plan to upgrade from 7.5HP to 30HP. That should give me some margin as well. I'll install a variable frequency drive to reduce the speed of the mixing blades from current 65RPM to about 20RPM. That should help reduce some of the viscoelastic pushback from the material and reduce forces on the mixing blades.

    I am trying to figure out if this is even feasible. I have had two process engineering firms look at it and walk...they don't want to take a chance on it...and the job is too small for them to be interested in chance. They'd much rather spec out a new $150,000 machine for me.

    So, it is left to me, the non-engineer, former computer science guy, to figure out if I am better off selling it and getting a different machine vs. giving it a try.

    The 7.5HP motor runs at 1800RPM and a gear/sprocket on the shaft drives a 3" wide steel chain. The chain delivers power to a gear on a shaft. Then power goes through a series of gears (my guess is a 3:1 and a 10:1) until the mixing arms rotate at ~60RPM. The drive shafts are about 2 1/2" diameter.

    I don't know exactly what type of metal/alloy anything is made of. The gears look pretty good once I removed 60 years of gummed up crud from them. They are kind of golden colored metal when clean. A screwdriver doesn't scratch them. They don't appear to be scored or worn down. An occassional nick or cracked tooth is all I found.

    I am willing to chance breaking gears and shafts. I just really don't want to break the sigma mixing blades since they cost more to replace than what I paid for the machine.

    BTW, someone suggested to me that the mixing blades and bowl were mild steel.

    How do I even start? It seems like step 1 is to figure if the shafts and gears can take the load. Step 2 is more complicated math to figure if the sigma blades can handle the forces applied to them.

    Your expertise is appreciated!! I love to learn and I love to tinker.

    I attached some photos to put things in perspective.
     

    Attached Files:

  2. jcsd
  3. Aug 30, 2011 #2
    To make a long story short, you will need to do some basic machine design calculations for the shaft, gears, and blades.

    A very good reference for this is Machine Design chapters in: "Mechanical Engineering Reference Manual for the PE Exam" by Lindeburg. Machinery's Handbook is also a good source.
     
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