Mixer Shafts by Sharpe Mixers

Mixer shafts are required to transmit the power from the mixer drive to the impeller(s). With the transmission of this power, the shaft must handle the loads occurring including the transmission of torque, overhung moment due to hydraulic forces, and thrust. The shaft must also be designed to be stiff enough to limit vibration and deflection to acceptable levels. Improperly designed, a mixer shaft may fail and cause catastrophic damage to equipment and to personnel. If a shaft is marginally undersized, the excessive run-out and shaft whip may cause premature wear to drive bearings and seals.

Mixer shafts are commonly built from solid round bar, hollow shafting, or a combination of both. The strength of a shaft is determined by the section modulus of the shaft. For a given section modulus, a hollow shaft will be larger in outside diameter, but will weigh less than the equivalent solid shaft. The advantages of each are shown below:

1. SOLID SHAFTING: Solid shafting has been the most common choice among mixer manufacturers for many years because of availability, ease of construction and the fact that many impeller hubs require a key to transmit torque. Many mixer manufacturers stock one-piece impeller hub castings to which they bolt blades. These hubs slide onto the shaft from one end and are held in place with a key and set-screw. Since the required mating keyway would cut through a hollow shaft, a solid shaft is required. This design and the required inventory is a commitment to solid shafting by default. The disadvantage of keyed one-piece hubs is that they can be difficult to remove from the shaft, 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.

2. HOLLOW SHAFTING: Hollow shafting has become a preferred choice in more recent years due to the design advantages hollow shafting offers, increased availability, and more innovative methods of attaching impellers to the shaft without the use of key- ways (see- impellers; split hubs). When comparing a solid shaft with a hollow shaft of equal section modulus, both will transmit the 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 most important concern is that it is sized correctly for the application. Many factors regarding the tank and process enter into the design of a mixer shaft, and must be known before a mixer can be properly sized, such as:

• Tank shape

• Low liquid level

• Baffle configuration (or lack thereof)

• Mixer positioning in the tank

• Cross-flow in the tank

• Presence of settling solids

• Operational liquid level

• Presence of debits in tank.

Shaft Couplings

Shaft couplings are used to couple two adjacent sections of shafting together. The most typical use for a shaft coupling is to couple a solid gearbox output shaft to the mixer shaft (Many smaller mixers use "hollow quill" gearboxes where the end of the mixer shaft is machined to a close tolerance to fit into the gearbox quill, and therefore do not require a shaft coupling). Couplings are also required when the overall shaft length is greater than can be safely transported and/or handled during installation. The two most common types of shaft couplings are the rigid flanged coupling and the rigid split coupling (Flexible couplings are rarely used on mixer output shafts, and are more often referred to regarding high speed motor couplings). These coupling designs are described as follows:

1.RIGID FLANGED COUPLINGS - Flanged couplings are essentially a thick disk shaped flange welded or bolted to the end of a shaft and machined square and concentric to the axis of the shaft. A male register, or pilot, is machined into the face of the coupling, and a female register is machined into the mating shaft coupling to guarantee alignment. A series of bolts extend through both halves of the couplings for assembly. Removable flanged couplings require a close tolerance fit with the shaft to which it is installed, and a key to transmit torque. The coupling is held in place axially by the use of a retaining bolt and washer on the end of the shaft.

2.RIGID SPLIT COUPLINGS - Split couplings are used when ease of disassembly is required. A common example is a mixer with a mechanical seal which may need to be serviced. To service the seal, it must be removed from the end of the shaft, and therefore the coupling must be removed. A split coupling allows easy removal and reinstallation.