Inversions Of Single slider Crack mechanism
When one of the links of a kinematic chain is fixed, the chain is known as mechanism. It may be used for transmitting or transforming motion e.g. engine indicators, typewriter etc
A mechanism with four links is known as simple mechanism, and the mechanism with more than four links is known as compound mechanism. When a mechanism is required to transmit power or to do some particular type of work, it then becomes a machine. In such cases, the various links or elements have to be designed to withstand the forces (both static and kinetic) safely.
A little consideration will show that a mechanism may be regarded as a machine in which each part
is reduced to the simplest form to transmit the required motion.
Single Slider Crank Chain
A single slider crank chain is a modification of the basic four bar chain. It consist of one sliding pair and three turning pairs. It is, usually, found in reciprocating steam engine mechanism. This type of mechanism converts rotary motion into reciprocating motion and vice versa. In a single slider crank chain, as shown the links 1 and 2, links 2 and 3, and links 3 and 4 form three turning pairs while the links 4 and 1 form a sliding pair.
|Single Slide mechanism|
The link 1 corresponds to the frame of the engine, which is fixed. The link 2 corresponds to the crank ; link 3 corresponds to the connecting rod and link 4 corresponds to cross-head. As the crank rotates, the cross-head reciprocates in the guides and thus the piston reciprocates in the cylinder.
We have already discussed that when one of links is fixed in a kinematic chain, it is called a mechanism. So we can obtain as many mechanisms as the number of links in a kinematic chain by
fixing, in turn, different links in a kinematic chain. This method of obtaining different mechanisms by fixing different links in a kinematic chain, is known as inversion of the mechanism.
It may be noted that the relative motions between the various links is not changed in any manner
through the process of inversion, but their absolute motions (those measured with respect to the fixed link) may be changed drastically.
Types of inversion
1. Four bar chain or quadric cycle chain
- Beam engine
- Coupling rod of locomotive
- Watt’s indicator mechanism
2. Single slider crank chain
- Bull engine or pendulum engine
- Oscillating cylinder engine
- Rotary engine
- Crank & slotted quick return mechanism
- Whitworth quick return mechanism
3. Double slider crank
- Elliptical trammels
- Scotch yoke mechanism
- Old-ham's coupling
1) Bull engine or pendulum engine:
In this mechanism, the inversion is obtained by fixing the cylinder or link 4 (i.e. sliding pair), as
shown in Fig. In this case, when the crank (link 2) rotates, the connecting rod (link 3) oscillates about a pin pivoted to the fixed link 4 at A and the piston attached to the piston rod (link 1) reciprocates. The duplex pump which is used to supply feed water to boilers have two pistons attached to link 1, as shown in Fig.
2) Oscillating Cylinder Engine;
The arrangement of oscillating cylinder engine mechanism, as shown in Fig. is used to convert
reciprocating motion into rotary motion. In this mechanism, the link 3 forming the turning pair is fixed. The link 3 corresponds to the connecting rod of a reciprocating steam engine mechanism. When the crank (link 2) rotates, the piston attached to piston rod (link 1) reciprocates and the cylinder (link 4) oscillates about a pin pivoted to the fixed link at A.
|Oscillating Cylinder Engine|
3) Rotary Engine :
Sometimes back, rotary internal combustion engines were used in aviation. But nowadays gas turbines are used in its place. It consists of seven cylinders in one plane and all revolves about fixed centre D, as shown in Fig. while the crank (link 2) is fixed. In this mechanism, when the connecting rod (link 4) rotates, the piston (link 3) reciprocates inside the cylinders forming link 1.
4) Crank & slotted quick return mechanism
5) Whit-worth quick return mechanism
These two Mechanisms are explained already here: