Theory of Machine
The advantage of the piston valve over the slide valve is that in the former case

Both wear and power absorbed are low
Power absorbed is less
The pressure developed being high provides tight sealing
Wear is less

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Theory of Machine
Angle of dwell of cam is defined as the angle

Through which the cam rotates during the period in which the follower remains in the highest position
Moved by the cam from the instant the follower begins to rise, till it reaches its highest position
Of rotation of the cam for definite displacement of the follower
During which the follower returns to its initial position

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