Subject: Kinematics of Machinery

Topic: Special Mechanism

Difficulty: High

This mechanism is a modification of modified Scott-Russel’s mechanism with the difference that the point P does not slide along a straight line, but moves in a circular arc with centre O.

It is four bar mechanism and all the pairs are turning pairs as shown in fig.. in this mechanism , the centres O and $O_{1}$ are fixed. The link OA oscillates about O through an angle AO$A_{1}$. Which causes the pin P to move along a circular arc with $O_{1}$ as centre and $O_{1}$P as radius. For small angular displacement of OP on each side of the horizontal, the point Q on the extension of the link PA treces out an approximately a straight path QQ', if the lengths are such that

$OA=\frac{(AP)^{2}}{AQ}$