LESSON 3 SHAFT DESIGN

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LESSION 3 SHAFT DESIGN

Shafts are usually of circular cross section; either solid or hollow sections can be used. A hollow shaft weighs considerably less than a solid shaft of comparable strength, but is somewhat more expensive. Shafts are subjected to torsion, bending, or a combination of these two; in unusual cases, other stresses might also become involved. Careful location of bearings can do much to control the size of shafts, as the loading is affected by the position of mountings. After a shaft size is computed, its diameter is often modified (upward only) to fit a standard bearing. Calculations merely indicate the minimum size.

Common Shaft Sizes

Table 3-1 lists some of the common available sizes for steel (round, solid) shafting. These are nominal sizes only. A designer must accurately compute the exact size so that it will fit properly into bearings. Since any machining is costly, a minimal amount of metal should be removed from stock sizes. Any metal removed in certain locations changes the shaft diameter in various axial positions. Therefore, proper radii must be provided to minimize stress concentrations. Abrupt changes in diameter without sufficient radii produce so-called stress raisers. Thus it is desirable——from the standpoint of stress... to provide large radii. However, large radii also make it difficult to mount such other components as pulleys, cams, gears, and so on because of radius

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