How to Prevent Fatigue Failure in the Gearbox Shaft of Your Industrial Mixer


Industrial Mixer Shaft Design (2 of 4)

In the second of our four part series on industrial mixer shaft design, we’ll explore the factors that can lead to fatigue failure in the gearbox shaft. These factors include shaft size and the mounting arrangement.

A good industrial mixer design cannot use a gearbox that has an output shaft significantly smaller than the mixer in tank shaft, even if the overall critical speed of the system would be sufficient. A large change in stiffness close to the top of the shafting (where the small gearbox shaft is coupled to the large tank shaft) concentrates the shaft deflection on the smaller diameter.  The result is ultimate fatigue failure of the gearbox shaft.

The industrial mixer designer can address this problem in two ways.  First, they can choose a gearbox with an output shaft about the same diameter as the tank shaft.  When critical speed controls the diameter, this will require a gearbox capable of handling significantly more torque and bending than is actually required by the application.  The result is a gearbox with a very high service factor that is more costly than required by the industrial mixer motor size and less efficiency because it is under loaded.

The second way to avoid fatigue failure of the gearbox shaft is to mount the tank shaft in bearings in the gearbox support pedestal.  The gearbox can then be chosen based on the power required, and it can be flexibly coupled to the top of the tank shaft. However, this can result in a more complicated mounting arrangement and require additional maintenance and spare parts.  The gearbox cost reduction versus the increase in cost of the shaft mounting on larger pedestals is often “a wash.”

Other factors related to the shaft diameter of your industrial mixer will result in indirect costs including larger impeller hubs and shaft couplings. The in-tank assembly will also be heavier than required by the torque and bending loads if critical speed considerations had not controlled the minimum shaft diameter.

For More Information

Preventing fatigue failure in your gearbox shaft will help ensure reliable, cost-effective operation of your industrial mixer. For more information about gearbox considerations in industrial mixer shaft design, contact ProQuip at 330-468-1850 or

To learn more about industrial mixer shaft design, look for our next blog post, “When and When Not to Use a Steady Bearing in Your Industrial Mixer Shaft Design” or read the first post in this series,5  Things to Consider When Designing and Industrial Mixer Shaft.”

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