Have you ever seen an industrial mixer rocking like a metronome on its mounting base? Ever noticed an industrial mixer shaft shake violently as it accelerates, but rotate smoothly when it comes up to full speed? Have you ever found you can’t run your variable speed laboratory mixer at certain speeds with certain impellers because it vibrates, but it runs fine at faster or slower speeds?

If you answered yes to any of these questions, you have witnessed how critical speed directly affects the performance of your industrial mixer. Critical speed is a very important Industrial mixer design consideration. Understanding how critical speed affects the performance of your industrial mixer will help you stop/prevent it from shaking.

The materials used in industrial mixer construction are elastic; that is, they are “springy.” If you fix one end of an elastic industrial mixer shaft and strike the other end with a mallet, the shaft rings like a bell. The frequency of the ring depends on the length of the mixer shaft and its elasticity. If you add weight (an impeller) on the end of the mixer shaft, the frequency gets lower. This natural frequency is called the critical speed of the shaft.

When the mixer shaft is pushed off center, it will attempt to spring back because it is elastic. The frequency of the spring back is the critical speed. If the next push has the same frequency as the critical speed, the deflection will be larger. This is analogous to a child on a swing. They throw their feet back and forth at each cycle in time with the natural frequency of the swing, and they go higher and higher.

The industrial mixer design problem arises because many of the radial forces on a mixer shaft are at or are harmonics of its operating speed. If the operating speed is close to the critical speed, these forces go in resonance with the natural frequency of the shaft. As the mixer shaft starts to move more, the off-center hydraulic forces also increases making the problem worse. Each deflection is larger than the last, and ultimately the mixer shaft bends or the mounting structure fails.

To learn more about how critical speed affects your industrial mixer, read the complete article.

If you are unable to stop your industrial mixer from shaking, contact ProQuip Industrial Mixers at 330-468-1850 or applications@proquipinc.com.

 

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