Machine Design
The contact ratio is the ratio of

Length of arc of recess to the circular pitch
Length of path of contact to the circular pitch
Length of arc of contact to the circular pitch
Length of arc of approach to the circular pitch

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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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