1) Crank Mechanism
(Old)
The crank mechanism converts the reciprocating motion to a rotary motion, and vice-versa. A simple crank mechanism for a single-cylinder engine is shown in figure1.2. It consists of a piston which moves inside a cylinder, a crank of length r which rotates about a point O and a connecting rod of length L which is attached to the piston at point P and to the crank shaft at point C. The crank pin C follows a circular path while the wrist pin P oscillates along a linear path. Points on the connecting rod between C and P follow an elliptical path.
(P. Srinivasculu, C.V. Vaidyanathan, Handbook of Machine Foundations, p.4)
(new/better)
The “connecting rod-crank diagram” as commonly used,is simply a skeleton- or centreline-drawing of the mechanism, in one or more given positions. It is shown in figure, where O is teh centre of the shaft, OK the crank of length r, and HK the connecting rod of a length l=pr; p denoting the ratio l:r. A and B are the “dead-points” of the crank-circle; Ao and Bot he corresponding positions of the crosshead pin at the ends of its stroke; M a point mid-way between Ao and Bo: the mid-stroke positions of the extremely H of the connecting rod.
Such a diagram serves two purposes: first by fixing the corresponding positions of the piston, or the cross head, and the crank, and secondly by furnishing the necessary data, determining the transmission of the forces exerted in the cylinder, to the moving parts and to the frame of teh engine, for any given position of the mechanism.
(C. P. Holst, The connecting rod and crank mechanism and its inertia forces,p.3)
2) Lever Mechanisms
(Old)
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.
(N. SCLATER, N. P. CHIRONIS, MECHANISMS AND MECHANICAL DEVICES SOURCEBOOK 4th ed., pg 4)
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.
(N. SCLATER, N. P. CHIRONIS, MECHANISMS AND MECHANICAL DEVICES SOURCEBOOK 4th ed., pg 4)
(new/better)
The mechanism shown in figure, here for every complete revolution of link called as a crank, the link called as a lever makes a complete oscillation. Proportions of the links are
(l2+l3)< (l1+l4)
(l3-l2)>(l1-l4)
When the crank is very short, this mechanism can be used as an eccentric, in which case the connecting link or rod 3 called the eccentric rod.
(R V Dukkipati, Mechanism and Machine Theoryp, p.14)
3) Ratchet Mechanisms
(Old)
It consists of wheel calls Ratchet with saw-shaped teeth which engage with an arm called a pawl. The arm is pivoted and can move back and forth to engage the wheel. The shape of teeth is such that rotation can occur in only one direction.
(Onwubolu G.C.,Mechatronics:Principles and Applications,p. 380)
(new/better)
The ratched mechanism can only be turned in a counterclockwise direction. The ratched Wheel has many wedge-shaped teeth that can be moved incrementally to turn an oscillating drive lever. As driving lever AB first moves clockwise to initiate counterclockwise movement of the wheel, it drags pawl C pinned at B over one or more teeth while pawl D prevents teh wheel from turning clockwise. The amount of backward incremental motion of lever AB is directly proportional to pirtch of the teeth.
(Neil Sclater, Mechanisms and Mechanical Devices Sourcebook, p.10)
4) Pantograph Mechanisms
(Old)
A pantograph is a combination of links which are so connected and proportioned as to length that any motion of one point in a plane parallel to that of the link mechanism will cause another point to follow a similar path either on an enlarged or a reduced scale. Such a mechanism may be used as a reducing motion for operating a steam engine indicator, or to control the movements of a metal cutting. For instance, most engraving machines have a pantograph mechanism interposed between the tool and a tracing point which is guided along lines or grooves of a model or pattern. As the tracing point moves, the tool follows a similar path, but to a reduced scale, and cuts the required pattern or design on the work.
A simple form of pantograph is shown by the diagram, Fig. 12. There are four links, a, b, cand d. Links a and b are equal in length, as are links c and d, thus forming a parallelogram. A fifth connecting link e is parallel to links c and d. This mechanism is a free to swivel about a fixed centre f. Any movement of h about f will cause a point g (which coincides with a straight line passing through f and h) to describe a path similar to that followed by h, but on a reduced scale. For instance, if h were moved to k following the path indicated by the dotted line, point g would also trace a similar path.
( Franklin Day Jones, Mechanisms and Mechanical Movements, Elibron Classics, 2005, p. 19-20)
(new/better)
The pantograph is a linkage consisting of four bars arranged, to form a parallelogram as shown in fig. Link 1 of the pantograph is hinged at point O. Since AD is parallel to BC for all positions of the mechanism, the following relation for two points Q and P on links 2 and 4 respectively collinear with the hinge O, is satisfied.
(OP/OQ)=(OA/OB)
(R V Dukkipati, Mechanism and Machine Theory, p.101)
(Old)
A simple cam mechanism consists of three basic parts, a cam, a follower, and a frame. A cam is an irregularly shaped machine member which serves as a driving link by rotating with a constant velocity and imparting motion through direct contact to a driven link, the follower, which in turn moves in a desired motion. A cam (KA) is adjacent to a follower (KAf) and a frame (KF) with a cam joint (JA) and a revolute joint (JR), respectively. A follower, which is adjacent to the frame with a revolute joint or a prismatic joint (JP), is usually driven to move with varying speeds in a noncontinuous and irregular motion.
(Hong-Sen Yan, Reconstruction Designs of Lost Ancient Chinese Machinery, page 65)
(new/better)
Cam mechanism can be classified by their input/output motions, the configuration and arrangement of the follower, and the shape of the cam. Cams can also be classified by the kinds of motions made by the follower and the characteristics of the cam profile. The possible kinds of input/output motions of cam mechanism with the most common disk cams are shown in figure. They are examples of rotating disk cams with translating followers.
(Neil Sclater, Mechanisms and Mechanical Devices Sourcebook, p.15)
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