Machine Design
In the calculation of induced shear stress in helical springs, the Wahl's stress factor is used to take care of

Combined effect of transverse shear stress and bending stress in the wire
Combined effect of transverse shear stress and curvature of wire
Combined effect of torsional shear stress and transverse shear stress in the wire
Combined effect of bending stress and curvature of the wire

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Machine Design
A hollow saddle key is

Provided in pairs at right angles and each key is to withstand torsion in one direction only
A taper key which fits in a key way of the hub and is flat on the shaft
A taper key which fits in a key way of the hub and the bottom of the key is shaped to fit the curved surface of the shaft
A taper key which fits half in the key way of hub and half in the key way of shaft

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Machine Design
Set screws are

Used to prevent relative motion between parts
Slotted for a screw driver and generally used with a nut
Similar to stud
Similar to small size tap bolts except that a greater variety of shapes of heads are available

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