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diameter '' d + δ. ''The instant the green arrow is pointing downwards, the blue circle is pressed against the red circle at the bottom (point ''A ''on the blue circle). The force rotating counterclockwise causes the blue circle to roll around the red circle clockwise. When it has rolled a distance '' πd'', the circumference of the blue circle, point '' A'' again touches the red circle. Since the circumference of the red circle is π(''d'' + δ), point '' A'' touches the red circle a distance πδ clockwise from the bottom.]] | diameter '' d + δ. ''The instant the green arrow is pointing downwards, the blue circle is pressed against the red circle at the bottom (point ''A ''on the blue circle). The force rotating counterclockwise causes the blue circle to roll around the red circle clockwise. When it has rolled a distance '' πd'', the circumference of the blue circle, point '' A'' again touches the red circle. Since the circumference of the red circle is π(''d'' + δ), point '' A'' touches the red circle a distance πδ clockwise from the bottom.]] | ||
'''Precession''', also called '''epicyclic fretting precession''', (or more accurately '''hypocyclic fretting precession''' since "epicyclic" applies to a round part spinning outside a circle and "hypocyclic" applies to a round part spinning inside a circle) is the process of a round part in a round hole rotating with respect to that hole because of [[Engineering tolerance|clearance]] between them and a radial force on the part that changes direction. The direction of rotation of the inner part is opposite to the direction of rotation of the radial force. [[Fretting]] between the part and the hole is often a result of this motion. "In machinery, fretting is the micro-motion of tightly fitting parts that superficially appear immobile with respect to each other." | '''Precession''', also called '''epicyclic fretting precession''', (or more accurately '''hypocyclic fretting precession''' since "epicyclic" applies to a round part spinning outside a circle and "hypocyclic" applies to a round part spinning inside a circle) is the process of a round part in a round hole rotating with respect to that hole because of [[Engineering tolerance|clearance]] between them and a radial force on the part that changes direction. The direction of rotation of the inner part is opposite to the direction of rotation of the radial force. [[Fretting]] between the part and the hole is often a result of this motion. "In machinery, fretting is the micro-motion of tightly fitting parts that superficially appear immobile with respect to each other." | ||
"For a pedal, a rotating load arises from downward pedaling force on a spindle rotating with its crank making the predominantly downward force effectively rotate about the pedal spindle. What may be less evident is that even tightly fitting parts have relative clearance due to their elasticity, metals not being rigid materials as is evident from steel springs. Under load, micro deformations, enough to cause motion, occur in such joints. This can be seen from wear marks where pedal spindles seat on crank faces." | "For a pedal, a rotating load arises from downward pedaling force on a spindle rotating with its crank making the predominantly downward force effectively rotate about the pedal spindle. What may be less evident is that even tightly fitting parts have relative clearance due to their elasticity, metals not being rigid materials as is evident from steel springs. Under load, micro deformations, enough to cause motion, occur in such joints. This can be seen from wear marks where pedal spindles seat on crank faces." | ||