What is a mechanism?
A mechanism is a device which transforms motion to some desirable pattern and typically develops very low forces and transmits little power.
Some examples of common mechanisms are a pencil sharpner, a camera shutter, an analog clock, a folding chair, an adjustable desk lamp, and an umbrella.
What is a machine?
A machine typically contains mechanisms which are designed to provide significant forces and transmit significant power.
Some examples of machines are a food blender, an automobile transmission, a bulldozer, a robot, and an amusement park ride.
A machine not only imparts definite motions but also transmits and modifies the available mechanical energy into some kind of desired work.
Difference between machines and mechanisms:
There is no clear-cut dividing line between mechanisms and machines. They differ in degree rather than in kind. If the forces or energy levels within the device are significant, it is considered a machine; if not, it is considered a mechanism.
A useful working definition of a mechanism is: A system of elements arranged to transmit motion in a predetermined fashion. That is, it is a combination of a number of bodies assembled in such a way that the motion of one causes constrained and predictable motion to the others. This can be converted to a definition of a machine by adding the words: ‘work’ and ‘energy’ after ‘motion’.
For example, a slider crank mechanism converts reciprocating motion of slider into rotary motion of crank or vice-versa. However, when it is used as an automobile engine by adding valve mechanism, it becomes a machine which converts the available energy force on the piston into the desired energy, torque of the crank shaft. This torque is used to move a vehicle.
Mechanisms, if lightly loaded and run at slow speeds, can sometimes be treated strictly as kinematic devices; that is, they can be analyzed kinematically without regard to forces.
Machines (and mechanisms running at higher speeds), on the other hand, must first be treated as mechanisms, a kinematic analysis of their velocities and accelerations must be done, and then they must be subsequently analyzed as dynamic systems in which their static and dynamic forces due to those accelerations are analyzed using the principles of kinetics.