Drag Link & Quick-Return-Mechanism
One more type of Grashof-Linkage, shown in the diagram posted below, is formed when the shortest link is fixed. The unique features of this linkage are (a) there is no dead-centre phase, (b) Links 2 and 4 both rotate and (c) either 2 or 4 can be the driver link. Thus we have a double-crank-mechanism, referred to by a unique name Drag-Link-Mechanism.
You can see in the diagram, if the link-2 rotates with a uniform speed, link-4 will rotate with a non-uniform speed.
When the point C moves through the Arc C'CC" the link-4 moves through 180° whereas Link-2 has moved through an angle 'α'. However, if the link-4 further moves from point C" to C' it covers 180°, whereas the link-2 moves through an angle 'β'.
Since the angle 'α' is much larger than 'β' it is evident that even when link-4 has moved through the same angle i.e. 180° in both the cases, the movement of link-2 is not the same in both the cases. Consequently, we have a mechanism where we can impart dual speeds to the driven-link by rotating the driver-link at a uniform speed.
This type of mechanism gives a Mechanical Engineer an additional capability to manipulate his machinery, a little more and find at a solution to some of the very critical problems.
In a situation where you need a mechanism which can give you a slow working-stroke and a fast retracting-stroke you would need this mechanism, known a Quick-Return-Mechanism. An extension of this 4-Linkage mechanism is used in Shaping Machines.
From the Fig., posted above, you may note that by adding just one more link this 4-Linkage mechanism can be converted into a mechanism which is capable of transforming uniform rotation of a Crank (Link-2) into a quick-return reciprocating motion of the RAM (Link-6).
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