Dave Dickey Forum Moderator 298 Posts
Re: What is causing the mechanical problems with agitators?7 April 2011 at 1:29pm
Mechanical problems with agitators are rarely as simple as steady bearings or larger shafts. Either approach may solve the problem or make it worse. You have identified a couple of advantages and problems for each option, but others are possible. From the information you have provided, I can only guess at possible solutions.
However, from your description, the problem may be in the rigidity of the mounting. If both agitators are having similar problems, it is likely that the support structure is at least contributing to the problems. To make any recommendation for or against the options you mentioned would require a much more thorough analysis than a simple description of the agitators.
Detailed dimensions and drawings would be needed to completely evaluate the strength of the agitator impellers and shafts. The drives and supports are potential sources of problems. Even your process conditions need to be examined to establish the loads on the agitators. Other mechanical problems such as natural frequencies for both the agitators and the supports can cause motion problems.
For an independent start, an article entitled, "Consider Mechanical Design of Agitators," by Fasano, Miller, and Pasley, in Chemical Engineering Progress, August 1995. pp. 60-71, has some good information on mechanical design of agitators and supports.
The answers by this expert are based on the best available interpretation of the information provided. The consequences of the application of this information are the responsibility of the user. If clarification is needed, please submit a further question.
I am currently mixing 4 tons of product with a density of 8.4lb/gal that is very sensitive to over mixing. My current mixer design has reliability issues. There are two counter rotating agitators in the tank which are about 8' long by 5' wide. While running, the agitators sway due to either fabrication quality or design flaws. I currently have 2" and 4" shafts extending down from the gear box into the tank. I am being sold on two different design options: 1.) Add a steady bearing to bottom of tank to reduce movement and my issues with this are the following: a. Ability to CIP; b. Have not corrected the problem just hid they symptom; c. Failure of the tank due to shaft hitting steady bearing in the repeatedly in the same place. 2.) Increase the size of the shaft to 6" and 3" my issues are the following: a. Work required to perform repair (cut off top of tank and replace with new cover); b. Cost associated with new gear box installs. My question is for this application in a pharmaceutical environment, what would be the best course of action? Is there any published literature on the subject? Lastly are these design solution accurate or should I be looking at other alternatives?
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