While a mixer drive is also a gearbox, a gearbox is not necessarily a mixer drive – and the difference can be costly if you get the wrong one.

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Not all parts of a mixer wear out at the same time.  If your shaft and impeller have plenty of life left, but your mixer drive is worn out, retrofitting could be a money-saving option.  Although it’s not quite “plug and play” if you know what to expect it can be a painless experience.

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Pulp and paper mills use a lot of water, almost 17,000 gallons per ton of paper produced. Then they have to treat the water so that it can be re-introduced into our environment. Typically this is done utilizing large volume basins or lagoons. 

Mill management doesn’t like these lagoons, they smell bad, they can foam a lot, they can cost a million dollars or more a year in energy and the real gorilla in the room…sooner or later management is going to have to deal with the accumulated deposition and short circuiting caused from 20-30 years of running 30+ year old vertical aeration technology.

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I recently visited a wet limestone FGD system because the customer called complaining that there were solids building up in the bottom of their limestone slurry storage tank.  I arrived at the site and went to look at the agitator (not a Philly) to try to get a handle on the problem.  The agitator drive was running smoothly, quiet and cool.   There was no apparent mechanical problem on the top side of the system.

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A mixer is a complex mechanical system consisting of both rigid and flexible components. For the purposes of this vibration discussion, please consider a top-entry single-impeller mixer which consists of an A/C motor driving a mixer-duty gear reducer (one that can handle high bending moments) whose output shaft is rigidly coupled to an agitator shaft that extends unsupported into a mixing vessel and which drives a mixing impeller connected to the bottommost end.  The lower portion of the gear box is typically bolted to a tank flange, support bridge or concrete.  For this configuration, the shaft and impeller can be thought of as a large tuning fork.

Like a tuning fork, the agitator shaft has a fundamental vibration frequency. The type of shaft-impeller movement associated with the lowest vibration frequency is similar to that of a pendulum – all portions of the agitator-impeller assembly move back and forth in the same direction, more motion at the impeller and less as you approach the upper bearing. The agitator-impeller assembly also has higher vibration modes (harmonics). The rate of vibration for these modes is also fixed for a given shaft geometric configuration. The motion of the second mode has a characteristic motionless node, often just a few feet above the impeller. At a given instant in time, those portions of the shaft above and below the node move in opposite directions. An almost infinite number of harmonics exist; although the visible deflection associated with each higher harmonic drops off exponentially (first mode has the largest deflection).  Therefore, vibrations at the first and second modes are the most problematic and are the key modes to consider when designing an agitator shaft.

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