Page 378 - Centrifugal Pumps 2E

Shaft Design and Axial Thrust 343

Key section:

3
Using diameter divided by 4 as a rule of thumb, a /4-in. square key
will be used.

Shear stress:

• 316 stainless steel.
• Yield Strength = 30,000 psi.
« For shear use .5 x 30,000 = 15,000 psi.
« With a safety factor of 1.5, the design stress should be 10,000 psi.
« Key shear area = Width x Length.

Shear stress is:

Hence

Shaft Design, and Axial Thrust

Use 2%

See Figure 16-4 for a graphic illustration of the solution to this ex-
ample.

Axial Thrust

In a centrifugal pump, axial thrust loads result from internal pressures
acting on the exposed areas of the rotating element. These pressures can
be calculated; however, such values should only be considered approxi-
mate, as they are affected by many variables. These include location of
the impeller relative to the stationary walls, impeller shroud symmetry,
surface roughness of the walls, wear ring clearance, and balance hole ge-
ometry. Calculated axial thrust is therefore based on a number of as-
sumptions and should only be considered an approximation.