Mixer Basics- Step 7: The Shaft

When you are buying your mixer, you’ll most likely need to buy a shaft. The shaft is simply the piece of material that connects the motor to the impeller. The shaft can vary by diameter and length. The diameter of your shaft depends on the amount of torque required for mixing and the length depends on the size of the impeller.

The more viscous your mixture is, the more torque your impeller needs. The more torque your impeller needs, the thicker your shaft needs to be. As the motor twists the shaft, the motor’s torque is transferred into the shaft. The shaft is then supposed to transfer the torque to the impeller. However, if the blade uses more torque than the shaft can transfer, then the shaft will vibrate and might possibly break. It would be like trying to turn a plastic spoon stuck in a mound of frozen ice cream. If the ice cream doesn't give, the spoon will break.


 Plastic spoons and rock hard ice cream don't mix.

To get the right diameter for your shaft, just solve this equation for "d."


 D is the diameter. M is the bending moment of the shaft. Tq is the torqe, and sigma is the tensile or shear stress.


If that equation intimidates you like it does me, then you can call one of our engineers and they just tell you the right shaft size.

The length of your shaft depends on the size of the mixing blade. If the shaft is too short, you can cause vortexing. Vortexing is when the mixture starts spinning so fast that the surface of mixture gets below the impeller. It's like creating a liquid tornado in your mixing vessel. Once the tornado touches the blade, you've got problems. It would mean that you could see the blade while it is spinning in your mixture. For most mixtures, this creates problems because when you can see the blade it means you are mixing air into your mixture.

Causing a vortex while mixing will unfortunately blend air into your mixture.

In order to avoid vortexing you need to have good coverage. "Coverage" is how far the impeller is underneath the surface of the mixture. We usually recommend that the blade be underneath the surface of the mixture by two times the length of the diameter of the impeller. So if the impeller is one foot wide, then you want the blade to be two or more feet underneath the surface of the mixture. If you need your impeller to be at least two feet underneath the surface of the mixture, then your shaft will need to be longer than two feet in order to accommodate the motor, the blade, the coverage and the freeboard (freeboard is the distance from the surface of the mixture to the top of the container).

Once you've figured out what kind of shaft you need, you should consider what kind of mount or stand you need.