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The Effect of Critical Speed on Industrial Tank Agitator Performance

Updated October 4. 2018 (videos added)

The performance of an industrial tank agitator can be directly affected by the critical speed of the shaft and impeller assembly.  Operating an agitator at a rotational speed that is close to or at the critical speed for the shaft assembly can result in significant shaking.

This article provides a detailed overview of critical speed including three videos demonstrating the effect of operating your industrial tank agitator below, near and above critical speed.

How Critical Speed Affects the Performance of Your Industrial Mixer

Understanding how critical speed affects the performance of your industrial mixer will help you stop/prevent it from shaking.

Most top-entry industrial mixers are designed with a so-called free overhung shaft. The impeller shaft is supported by anti-friction bearings in the gearbox or pedestal of the tank. The mixer shaft extends down into the vessel where the impeller is mounted.

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.

No industrial mixer assembly is perfectly balanced. Even if this were possible, the forces on the mixer shaft generated by stirring the contents of a mixing vessel are never balanced. They are always being pushed back-and-forth by periodic mechanical and hydraulic forces when the industrial mixer is in operation.

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 increase making the problem worse. Each deflection is larger than the last, and ultimately the mixer shaft bends or the mounting structure fails.

Lab Scale Mixer Critical Speed Demonstration

The following videos were prepared to illustrate the effect of shaft critical speed in industrial mixer design.

The demonstration mixer uses a 3/8” impeller shaft with a four blade axial turbine mounted at the end of the shaft.  This was run in a baffled, 15” diameter tank filled with water.  The impeller diameter/tank diameter is 1/3.

 

Below Critical Speed

The mixer shaft speed is approximately 50% of the shaft/impeller first critical speed.  Under this condition, there is no noticeable vibration with the impeller assembly.

 

At or Near Critical Speed

The mixer shaft speed is increased slowly to where it is operating very close to the calculated critical speed.  Accordingly, significant vibration develops for the shaft due to unbalances and the hydraulic forces.

 

Above Critical Speed

The mixer shaft speed is now increased to a speed that is well above the calculated first critical speed.  Under these conditions, there is again no noticeable vibration.  Many small mixers are indeed designed to operate above first critical speed.

 

At What Percentage of Critical Speed Should I Operate My Industrial Mixer?

Critical speed can be calculated and is always taken into account in industrial mixer design. The following are operating limits for critical speed based on the type of industrial mixer:

• Heavy duty industrial mixers (high shock or unbalanced load mixing – e.g., gas dispersion): 65% of critical speed
• Standard industrial mixers (uniform load mixing): 70% of critical speed
• Small laboratory mixers: 80% of critical speed

Many small Industrial mixers (e.g., lab scale or portable type mixers) operate above critical speed. You often see these mixers shake when they go through critical speed as they build up to full speed.

What Design Considerations Affect the Critical Speed of My Industrial Mixer?

Even though you may not design industrial mixers, it is important to know several design considerations related to critical speed that could lead to safety issues or mechanical failure. These include:

• Mixer mounting stiffness
• Number of impellers
• Impeller location on shaft
• Shaft length
• Shaft material
• Drive type

The critical speed of your industrial mixer depends on the stiffness of the mixer mounting. If the mixer mounting is too elastic, the critical speed is lower. Your mixing vessel designer needs to take this into account in their calculations. Mixer design forces are always shown on the ProQuip Certified Drawing. Make sure your mixing vessel designer knows these values and designs the mixer support accordingly.

The manufacturer of your industrial mixer should also be consulted prior to making any modifications to your mixer design. Adding an impeller, changing the location(s) of an impeller, changing the length of the mixer shaft, or putting a variable speed drive on an industrial mixer that was not designed for one might create a real hazard. Any mechanical modification of an industrial mixer could put the operating speed too close to its critical speed and lead to mechanical failure of the system.

For More Information

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