# Topic: Re: How do I calculate agitator shaft size?

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Calculating a shaft size uses basic mechanical design equations for a beam with different supports.&nbsp;The calculation for a bottom support or steady bearing is calculated as a beam with a fixed support at the top and a simple support at the bottom.&nbsp; The impeller imposes a lateral or sideways load at a point between the two ends.&nbsp; The load imposed by the impeller is a function of the torque on the impeller and the impeller diameter, along with a hydraulic force factor for different applications.&nbsp; The hydraulic force factor can be 1.0 for constantly submerged applications and 3.0 for periodic operation at the liquid surface, such as during filling or emptying of the vessel.&nbsp; The equations for this calculation can be found in the Shaft-Design-Hydrofoil-Steady-Bearing.pdf.&nbsp; The first calculation is for the torque on the shaft.&nbsp; The motor power is used in the torque and bending calculations to design for maximum load.&nbsp; The second calculation is for the lateral or sideways load imposed by the impeller.&nbsp; The torque and impeller diameter are used to estimate this load, along with the hydraulic force factor.&nbsp; The impeller force is applied to the shaft at the location calculated from the top of the mixer shaft.&nbsp; The overall shaft length is the location of the bottom bearing.&nbsp;The shaft torque and bending loads are combined to calculate a shaft diameter based on the shear stresses in the shaft.&nbsp; An allowable stress limit, based on metal strength and fatigue limits, is used in the calculation.&nbsp; A similar calculation for shaft diameter is done based on the tensile strength of the shaft.&nbsp; The two calculated diameters are compared and the larger of the two diameters is the minimum diameter for the shaft.&nbsp; The next larger standard bar stock size is normally used for the actual shaft.&nbsp;A similar series of calculations is done for the case of a middle shaft support, Shaft-Design-Hydrofoil-Middle-Bearing.pdf.&nbsp; The difference is in the lengths used for the bending moment calculations.&nbsp; The length from the mixer drive to the middle support and length to the impeller are used to calculate the maximum bending moment on the mixer shaft.&nbsp;For either calculation, the location of the shaft supports must be precisely aligned with the location where the shaft hangs naturally from the mixer drive.&nbsp; This location can only be determined after the mixer is mounted on the vessel.&nbsp; Any secondary shaft support is equivalent to trying to align three bearings on the same shaft.&nbsp; The alignment is impossible to achieve perfectly, but with care the alignment can be done within acceptable limits.&nbsp; Any misalignment will result in premature wear of one or more bearings.&nbsp; The bottom or intermediate bearings are usually the ones to wear quickest.
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