## Design Criteria

torsional stress The torsional stress in the shafting may be calculated by the following equations:

Ss = for solid shafting pD0

Ss =-16T- for tubular shafting

where SS = torsional shear stress, lb/in2 (N/m2) T = transmitted torque, in • lb (N • m)

FIGURE 22 Sections of flexible shafts and intermediate guide bearings used to transmit torque from motor located above flood elevation to pump located below ground (Flowserve Corporation)

FIGURE 21 Tubular intermediate shafting and universal units (H. S. Watson)

FIGURE 22 Sections of flexible shafts and intermediate guide bearings used to transmit torque from motor located above flood elevation to pump located below ground (Flowserve Corporation)

Do = shaft outer diameter, in (m) D = shaft inner diameter (tubular shaft only), in (m)

The allowable shear stress depends upon the material being used and whether it is subjected to other loads, such as bending or compression. The design safety factor on the shafting should be equal to or greater than those of the other components in the drive train.

critical speed The critical speed of a drive shaft is determined by the deflection, or "sag," of the shaft in a horizontal position under its own weight. The less the sag, the higher the critical speed. In practical terms, a long, slender shaft will have a low critical speed and a short, large-diameter shaft will have a very high critical speed. The deflection of a simply supported shaft is calculated as follows:

5wL4 384EI

noting that shaft deflection, in (m)

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