Mixer School: Agitator Shaft Design

Agitator shafts are required to transmit the power from the mixer drive to the impeller(s). The shaft must handle the loads occurring including the transmission of torque, overhung moment due to hydraulic impeller forces, and thrust. The shaft must also be stiff enough to limit vibration and deflection to acceptable levels. Improperly designed, a mixer shaft may fail and cause catastrophic damage. Agitator shafts are commonly built from solid round bar, hollow shafting, or a combination of both.

Shaft strength is determined by the section modulus of the shaft.

SOLID SHAFTING has been the most common choice among other mixer manufacturers for many years because of availability, ease of construction and the fact that many impeller hubs require a key to transmit torque. These hubs slide onto the shaft and are held in place with a key and set-screw. The disadvantage of keyed one-piece hubs is that they can be difficult to service, steady bearings must be disassembled to allow clearance for impeller removal, and the impeller will usually weigh more due to the required thickness at the bolted blade attachment area. This extra weight coupled to the heavier solid shaft equates to a greater thrust load on the mixer bearings, limiting bearing life.

HOLLOW SHAFTING is Sharpe Mixers' preferred choice due to the design advantages offered, increased availability, and innovative methods of attaching impellers to the shaft without the use of key-ways, like our split hub design. Comparing a solid shaft with a hollow shaft of equal section modulus, both will transmit torque with equal stress levels, but the hollow shaft will be stiffer, or rather will deflect less under the same overhung moment. This translates to longer allowable shaft lengths at a given stress level, and less weight at a given shaft length. Less weight means less thrust transmitted to the drive bearings, and a longer expected bearing life. Less weight and higher stiffness also helps avoid critical speed, frequently a limiting factor in mixer design.

Regardless of the style of shafting used on a mixer, the important concern is that it is designed correctly for the application. Many factors regarding the tank and process enter into the design of a mixer shaft. Let us determine the best design for you today!