29-06-2012, 02:23 PM
Mechanisms and Machines
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When one of the turning pairs of a four-bar chain is replaced by a sliding pair,
it becomes a single slider-crank chain or simply a slider-crank chain. It is also
possible to replace two sliding pairs of a four-bar chain to get a double slidercrank
chain (Sec. 1.16). Further, in a slider-crank chain, the straight line path
of the slider may be passing through the fixed pivot O or may be displaced.
The distance e between the fixed pivot O and the straight line path of the slider
is called the offset and the chain so formed an offset slider-crank chain.
Different mechanisms obtained by fixing different links of a kinematic chain
are known as its inversions. A slider-crank chain has the following inversions:
Fixing of Link 2 of a Slider-crank chain results in the second inversion.
The slider-crank mechanism of Fig. 1.38(a) can also be drawn as shown in
Fig. 1.39(a). Further, when its link 2 is fixed instead of link 1, link 3 along with
the slider at its end B becomes a crank. This makes link 1 to rotate about O
along with the slider which also reciprocates on it [Fig. 1.39(b)].
Applications: 1. Whitworth quick-return mechanism, 2. Rotary engine.
Theory of Machines
it is a mechanism used in workshops to cut metals. The forward stroke takes a
little longer and cuts the metal whereas the return stroke is idle and takes a
shorter period.
Slider 4 rotates in a circle about A and slides on link 1 [Fig. 1.39©]. C is a
point on link 1 extended backwards where link 5 is pivoted. The other end of
link 5 is pivoted to the tool, the forward stroke of which cuts the metal. The
axis of motion of slider 6 (tool) passes through O and is perpendicular to OA,
the fixed link. The crank 3 rotates in the counter-clockwise direction.
Initially, let the slider 4 be at B¢ so that C be at C¢. Cutting tool 6 will be in
the extreme left position. With the movement of the crank, the slider traverses
the path B¢BB¢¢ whereas point C moves through C¢CC¢¢. Cutting tool 6 will
have the forward stroke. Finally, slider B assumes the position B¢¢ and cutting
tool 6 is in the extreme right position. The time taken for the forward stroke of
In a rotary engine, the slider is
replaced by a piston and link 1 by a
cylinder pivoted at O. Moreover, instead
of one cylinder, seven or nine cylinders
symmetrically placed at regular intervals
in the same plane or in parallel planes,
are used. All the cylinders rotate about
the same fixed centre and form a
balanced system. The fixed link 2 is also
common to all cylinders (Fig. 1.40).
Thus, in a rotary engine, crank 2 is
fixed and body 1 rotates whereas in a
reciprocating engine (1st inversion),
body 1 is fixed and crank 2 rotates.