Sex Machine on LetMeJizz
A motorized arm thrusting a dildo at a steady mechanical pace is exactly what the lens locks onto. The draw is that the machine never tires and never slows, so the camera can sit on one position while the speed climbs past anything a person could keep up with. Most clips cut between the contraption pistoning away and the face of whoever is strapped in front of it.
125 sex machine videos here, all HD and all free. New ones go up daily, so check back if you don't find your scene first time around.