The term mechanism identifies an entity that has the constructive connotations, the mobility, the ability to transmit motion, force, energy, present in the definition of machine. The complex of several mechanisms connected together constitutes a machine. Mechanisms generally consist of moving components that can include:
- belt and chain drives;
- cam and followers;
- friction devices, such as brakes and clutches;
- structural components such as a frame, fasteners, bearings, springs, lubricants;
- various machine elements, such as splines, pins, and keys.
Indeed machine and mechanism could be considered synonyms. In fact, in Applied Mechanics the use of terms reflects not so much a distinction, but rather a preference, identifying with the term mechanism the levers and articulated homogeneous systems, and with the term machine the more complex devices and architectures, perhaps formed by more mechanisms; in the domain of fluid machines, however, where among the various members there is fluid as an essential agent for the transmission of energy, it is usual to speak of machines and not of mechanisms for identifying turbines, pumps, compressors and so on.
A distinction could arise precisely from the presence or not of the energetic story of a fluid, as the primary aim pursued. In fact, in mechanisms the transmission of motion and force can be an end in itself, while in machines it is in some way enslaved to energy; ultimately, it can be said that the mechanism develops laws of transmission, displacements, and forces involving and engaging energy, while the fluid machine creates laws of energy transmission involving and engaging displacements and forces. Based on the arrangement of the rotation axes of the pairs between members, the mechanisms can be divided into:
- plane mechanisms: when the rotation axes of the pairs are all parallel to each other;
- spherical mechanisms: when the rotation axes of the pairs converge at a point;
- spatial mechanisms: when the rotation axes of the pairs are arranged in space.
The connection between two or more mechanisms can be made in series or parallel. In a parallel connection, the work is transmitted from one member to another in succession, while in parallel connection the work is divided among the various members of the different mechanisms.