OBJECTIVES:

– To understand the terms used in mechanisms,
– To understand and be able to use the degree of freedom of space and the degree of freedom of a joint,
– To be able to determine the degree-of freedom of a given mechanism.

1.1 Field of Study

The aim of mechanisms study is to determine the general motion principles which are common to all machinery and to describe the general synthesis and analysis techniques that can be applied for the design of machinery.

A machine may be defined as a combination of resistant bodies so arranged that by their means the mechanical forces of nature can be compelled to do work accompanied by certain determinate motion. Notice that in the above definition we are only concerned with the mechanical machines. The definition does not include electrical or heat machines (eg. a computer or a heat pump is not included as a machine with the above definition). The main characteristics of a mechanical machine is that there is force (or torque) accompanied with motion, Some exceptions to this characteristics are mechanical calculating machines, mechanical watches, indicating instruments, etc. In these types there is no actual work output. The work input is dissipated as heat due to friction within the system.

A mechanism may be defined as a group of rigid bodies connected to each other by rigid kinematic pairs (joints) to transmit force and motion.  

    We can state the following for a mechanism and a machine:

A machine structure is constructed to perform a particular task, such as a sewing machine, a lath, a packaging machine.

A mechanism is considered to be more general. It is an isolated group of rigid bodies through the study of which we can understand the basic structure of any machine and can design machines that are not in existence.

A machine may also involve a number of mechanisms and certain elements that are not rigid (although resistant). For example, in a machine we may have hydraulic drives, springs, dashpots, flexi-ble elements, etc. which are not considered as bodies that can be included in a mechanism (although we shall see how we can include these elements into the mechanism).