27-02-2013, 02:39 PM
BASICS OF MECHANISMS
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INTRODUCTION
Complex machines from internal combustion engines to helicopters
and machine tools contain many mechanisms. However,
it might not be as obvious that mechanisms can be found in consumer
goods from toys and cameras to computer drives and
printers. In fact, many common hand tools such as scissors,
screwdrivers, wrenches, jacks, and hammers are actually true
mechanisms. Moreover, the hands and feet, arms, legs, and jaws
of humans qualify as functioning mechanisms as do the paws and
legs, flippers, wings, and tails of animals.
There is a difference between a machine and a mechanism:
All machines transform energy to do work, but only some mechanisms
are capable of performing work. The term machinery
means an assembly that includes both machines and mechanisms.
Figure 1a illustrates a cross section of a machine—an
internal combustion engine. The assembly of the piston, connecting
rod, and crankshaft is a mechanism, termed a slider-crank
mechanism. The basic schematic drawing of that mechanism,
Fig. 1b, called a skeleton outline, shows only its fundamental
structure without the technical details explaining how it is
constructed.
Efficiency of Machines
Simple machines are evaluated on the basis of efficiency and
mechanical advantage. While it is possible to obtain a larger
force from a machine than the force exerted upon it, this refers
only to force and not energy; according to the law of conservation
of energy, more work cannot be obtained from a machine
than the energy supplied to it. Because work force distance,
for a machine to exert a larger force than its initiating force or
operator, that larger force must be exerted through a correspondingly
shorter distance. As a result of friction in all moving
machinery, the energy produced by a machine is less than that
applied to it.
Mechanical Advantage
The mechanical advantage of a mechanism or system is the ratio
of the load or weight W, typically in pounds or kilograms, divided
by the effort or force F exerted by the initiating entity or operator,
also in pounds or kilograms. If friction has been considered
or is known from actual testing, the mechanical advantage.
Levers
Levers are the simplest of mechanisms; there is evidence that
Stone Age humans used levers to extend their reach or power;
they made them from logs or branches to move heavy loads such
as rocks. It has also been reported that primates and certain birds
use twigs or sticks to extend their reach and act as tools to assist
them in obtaining food.
A lever is a rigid beam that can rotate about a fixed point
along its length called the fulcrum. Physical effort applied to one
end of the beam will move a load at the other end. The act of
moving the fulcrum of a long beam nearer to the load permits a
large load to be lifted with minimal effort. This is another way to
obtain mechanical advantage.
Simple Planar Linkages
Four different simple planar linkages shown in Fig. 8 are identified
by function:
• Reverse-motion linkage, Fig. 8a, can make objects or force
move in opposite directions; this can be done by using the input
link as a lever. If the fixed pivot is equidistant from the moving
pivots, output link movement will equal input link movement,
but it will act in the opposite direction. However, if the fixed
pivot is not centered, output link movement will not equal input
link movement. By selecting the position of the fixed pivot,
the linkage can be designed to produce specific mechanical
advantages. This linkage can also be rotated through 360°.
• Push-pull linkage, Fig. 8b, can make the objects or force
move in the same direction; the output link moves in the
same direction as the input link. Technically classed as a
four-bar linkage, it can be rotated through 360° without
changing its function.