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.

Read the rest of this entry »

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.

Read the rest of this entry »

The life blood of a mixer drive is its lubrication system, which provides an oil film at the contacting surfaces of all the working components to reduce friction and wear, and dissipates heat to prevent temperatures from rising to excessive levels.  If the lubrication system fails in any of these functions, it could result in the premature breakdown of your mixer drive.

Read the rest of this entry »

Food grade lubricants are more frequently requested than ever before.  Used most often in food/beverage handling and water treatment environments, food grade lubes are expanding also into areas where environmental contamination is hard to control.  Food grade lubricants are safer because the quantities of harmful chemicals are strictly limited.

Read the rest of this entry »

Proper mounting of a top-entry mixer is a combination of fit, alignment, and rigidity.  While many customers and tank manufacturers are skilled in handling fit and alignment of a mixer, rigidity is sometimes overlooked.

Read the rest of this entry »