The Chebyschev linkage is a mechanical linkage that converts rotational motion to approximate straight-line motion.
It was invented by the 19th century mathematician Pafnuty Chebyschev who studied theoretical problems in kinematic mechanisms. One of the problems was the construction of a linkage that converts a rotary motion into an approximate straight line motion. This was also studied by James Watt in his improvements to the steam engine. (Read more info about Watt Straight-line Mechanism)
O2O4 : O2A : AB = 200 : 250 : 100 = 4 : 5 : 2
Point P is in the middle of AB. This relationship assures that the link AB lies vertically when it is at the extremes of its travel.