Have a look at this animation; it may help put your question into perspective. It visualises what airmon said. Notice how the speed of the crank (measured in radians) stays the same. However the liner velocity of the shaft is always changing. The rate of change is at its highest as the shaft crosses the zero axis, only to slow down as it approaches the peaks, then stops for an instant, before reversing direction.
Run a few numbers through the calculator embedded in the page. It is an interesting exercise.
http://www.rkm.com.au/ANIMATIONS/ani...sine-wave.html