Machinery is diverse and can be categorized in several ways, such as: by function (power machinery, material handling machinery, crushing machinery, etc.); by industry (agricultural machinery, mining machinery, textile machinery, etc.); and by working principle (thermal machinery, fluid machinery, bionic machinery, etc.).
Furthermore, machinery undergoes several stages with different functionalities during its research, development, design, manufacturing, and application. Based on these different stages, mechanical engineering can be further divided into several interconnected and complementary subsystems, such as mechanical research, mechanical design, mechanical manufacturing, and mechanical application and maintenance.
These various sub-disciplines, categorized by different aspects, intersect and overlap, potentially leading to hundreds of sub-disciplines within mechanical engineering. For example, power machinery, categorized by function, has complex overlapping relationships with other sub-disciplines such as thermal machinery, fluid machinery, turbine machinery, reciprocating machinery, steam power machinery, nuclear power plants, internal combustion engines, and gas turbines, as well as sub-disciplines such as central power plant equipment, industrial power plants, railway locomotives, marine engineering, and automotive engineering. Marine steam turbines are power machinery, as well as thermal machinery, fluid machinery, and turbine machinery. They belong to marine power plants, steam power plants, and may also belong to nuclear power plants, etc.
