Machine Design
When an open coiled helical compression spring is subjected to an axial compressive load, the maximum shear stress induced in the wire is (where D = Mean diameter of the spring coil, d = Diameter of the spring wire, K = Wahl's stress factor, and W = Axial compressive load on the spring)

(8WD/ πd3) × K
(4WD/ πd3) × K
(WD/ πd3) × K
(2WD/ πd3) × K

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Machine Design
A woodruff key is

Capable of tilting in a recess milled out in the shaft
Fitted in pairs at right angles
A tapered key with head at one end
All of these

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