Machine Design
The cracks in helical springs used in railway carriages usually start on the inner side of the coil because of the fact that

It is subjected to a higher stress than the outer side
It has a lower curvature than the outer side
It is subjected to a higher cyclic loading than the outer side
It is more stretched than the outer side during the manufacturing process

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Machine Design
Cap screws are

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

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