Engineering Class Notes | MechStudyWorld

Chapter – 01

Machines and Mechanisms

1. Introduction:

This subject given the brief study of synthesis and analysis in the study or motions and forces concerning different parts or an existing mechanism while synthesis involves the design of the different parts.it given the study of one part motions to other parts motion related to the ports which are connected by any means. Study of relative motion so the parts is in depended of their strength and absolute dimensions. 

To study the motions of different parts of mechanisms, the study of the forces is not necessary and can be neglected. For study of this mechanisms field can be divided into following 

1. Kinematics-

It deals with the relative motions of different parts of mechanism without taking into consideration the force producing the motions.

1. Dynamics-

It involves calculation if forces impressed upon different parts of mechanism. 

1. Machines and mechanisms:

If numbers of bodies which are usually rigid are assembled in such a way that the motion of one causes constrain and predictable motion to other remaining, it is known as mechanism. Thus such mechanism transmits and modifies the motions.

A machine which mechanism is a mechanism or combination of mechanisms which, apart from imparting definite motions to the parts, also transmit and modifies available mechanical energy in to sub kind of desire work. It is neither the source of energy nor the producer of work but it helps in proper utilization of same. Some examples of mechanisms are typewriters, clocks, watches, string toys etc. but the best example of mechanism is slider crank mechanisms which converts reciprocating motion of slider into rotary motion of crank or vice versa. Below fig. shows crank and slider mechanism.

1. Rigid and resistant bodies:

A body is set to be rigid if under action of forces, it does not suffer any distortion or if the distance between any two point on it remains same or constant.

Resistant bodies are the bodies which are rigid for the purposes they have to served. there are some semi rigid bodies which are flexible in nature but when under goes on any force acting on it these bodies act as resistant bodies, but if there is no loading conditions these bodies act as rigid body. 

Nowadays the resistant bodies are generally refer to be a rigid body.   

1. Link:

It is defined as a group of resistant bodies or resistant body with one or more rigid connections for preventing there relative movement is known as a Link it may also defined as a member or a combination of members of mechanism, connecting other members and having relative motion to them It consist of one or more resistance bodies.

A Link is also known as kinematic link or an element. 

It can be classified into binary, ternary and quaternary etc. depending upon its ends on which revolute can be placed. Below fig. shows rigid links and there no relative motion between the joints within the link.  

 

1. Kinematic pair:

A Kinematic pair or simply a pair is a joint of two links having relative motion between them. 

1. Types of kinematic pair-

These are classified according to following

1. According to nature of contact 
   1. Lower pair 

It is define as a pair of links having surface or area contact between members known as lower pair. Contact surfaces of two links are always same. For example universal joint, slider crank mechanism etc.

1. Higher pair

When a pair has a point or line contact between links it is known as higher pair. The contact between two link surfaces are not in similar manner. For example cam and follower, ball and roller baring 

1. According  to nature of mechanical constrain
   1. Closed Pair 

When the element of the pair are held together mechanically it is known closed pair. Generally these elements are geometrically identically.

1. Unclosed Pair or open pair

When two links of pair are in either due to force of gravity or some spring action, they constitute an unclosed pair.

1. According to nature of relative motion
   1. Sliding pair

If two links have a sliding motion relative to each other they called sliding pair. Bellow fig. shows example

1. Turning pair 

When one link has rotating motion or turning relative to each other they form turning or revolving pair. Bellow fig. shows turning pair.

1. Rolling pair 

When the links of a pair having a rolling motion to each other they form rolling pair. Bellow fig. shows example of ball and roller bearing.

1. Screw pair 

If two mating links having both turning and sliding motion between them is called screw pair. The below fig. shows the lead screw and nut of a lathe. 

1. Spherical pair

When one link is in the form of sphere turns inside a fixed link is called spherical pair. Bellow fig. shows same.

1. Degree of freedom:

If rigid body is unconstrained and moving independently  in  any direction and motions translational motions along with any three mutually perpendicular axis.

Generally every rigid body contains six degrees of freedom. The connection of a particular link with another link imposes constraint on their relative motions. Generally number of constraint cannot be zero or six. 

DOF of a pair is defined as the number of intendant relative motions, both translational and rotational, a pair can have. 

DOF= 6-number of restraint/constraint. 

1. Classification of kinematic pair:

Depending upon the number of restrains imposed on the relative motion of the two links connected together, a pair can be classified as given in Table which gives the possible form each class.

Different of each class have also been shown fig. 7.1 Remember that a particular relative motion between two links of a pair must be independent of the other relative motion that can have. A screw and nut pair permits translational and rotational motions. However, as the two motions cannot be accomplished independently, a screw and nut pair is a kinematic pair of the fifth class and not the fourth class.

1. Kinematic chain:

A Kinetic chain is an assembly of links in which the relative motions of the links is possible and the motion of each relative to the others is definite as shown in fig. 

1. Linkage, mechanism and structure:

A Linkage in obtain if one of the links of a kinematic chain is fixed to the ground. If motion of each link results in definite motion of the other such linkage is known as mechanism. The difference between linkage and mechanism is hardly followed and each one of them can be replaced to each other.

If links of redundant if chain is fixed it is known as structure. The DOF of a structure are less or zero a structure which is having negative DOF is called super structure.