2 Fundamental concepts, methods
and classification
2.1 Introduction
2.1.1 Definitions
In this book a mechanism is defined as the assembly of rigid members, also
known as links, connected by kinematic joints. A mechanism is sometimes
referred to as a kinematic chain as well.
A kinematic joint is formed by direct contact between the surfaces of two
members. Joints are the most important aspect of a mechanism in the view of
kinematics. They permit relative motion in some directions while constraining motion in others. The types of motion permitted are related to the number
of degrees of freedom of the joint which is equal to the minimum number of
independent coordinates needed to uniquely specify the position of a link
rela tive to the other constrained by the joint. Reuleaux (1875) published the
first book on theoretical kinematics of mechanisms, in which he called a kinematic joint a pair. He further divided joints into lower pairs and higher pairs.
A lower pair is the one in which contact between two rigid members occurs
at every point of one or more surface segments. A higher pair is one in which
contact occurs only at isolated points or along line segments.
Due to the requirement of surface contact, there are only six fundamentally different types of lower pairs classified by the types of relative motions
that they permit, all of which are listed in Table 2.1. There are, in contrast,
an infinite number of possible higher pair geometries. A couple of examples of the higher pairs are given in Table 2.2.
A kinematic chain is commonly known as a linkage if it is made from a
series of links connected by only lower pair joints. Whenever there is one
higher pair or more, it must be called a mechanism and should not be
placed in the linkage sub- class.
2.1.2 Mobility
The number of degrees of freedom of a mechanism is normally called the
mobility, which is the number of inputs required to determine the position
and classification
2.1 Introduction
2.1.1 Definitions
In this book a mechanism is defined as the assembly of rigid members, also
known as links, connected by kinematic joints. A mechanism is sometimes
referred to as a kinematic chain as well.
A kinematic joint is formed by direct contact between the surfaces of two
members. Joints are the most important aspect of a mechanism in the view of
kinematics. They permit relative motion in some directions while constraining motion in others. The types of motion permitted are related to the number
of degrees of freedom of the joint which is equal to the minimum number of
independent coordinates needed to uniquely specify the position of a link
rela tive to the other constrained by the joint. Reuleaux (1875) published the
first book on theoretical kinematics of mechanisms, in which he called a kinematic joint a pair. He further divided joints into lower pairs and higher pairs.
A lower pair is the one in which contact between two rigid members occurs
at every point of one or more surface segments. A higher pair is one in which
contact occurs only at isolated points or along line segments.
Due to the requirement of surface contact, there are only six fundamentally different types of lower pairs classified by the types of relative motions
that they permit, all of which are listed in Table 2.1. There are, in contrast,
an infinite number of possible higher pair geometries. A couple of examples of the higher pairs are given in Table 2.2.
A kinematic chain is commonly known as a linkage if it is made from a
series of links connected by only lower pair joints. Whenever there is one
higher pair or more, it must be called a mechanism and should not be
placed in the linkage sub- class.
2.1.2 Mobility
The number of degrees of freedom of a mechanism is normally called the
mobility, which is the number of inputs required to determine the position