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Mechanics is an ancient science. Very long ago there were discovered the main principles of mechanics and invented such concepts as lever, wheel and axle, pulley, wedge and screw. All parts of a machine are mutually acting. There are stable (immovable) parts and mobile ones. Putting various parts into operation is called a drive. The drive of the parts may be employed mechanically (by different elements of a machine), electrically (by current), pneumatically (by air) and hydraulically (by fluid or gas). The sample of mechanical drive may be a shaft which rotates about its axis. In order to make this motion possible, it is mounted in bearings, such as journal, thrust or collar. Also to make everything move, gears should be engaged and disengaged. By the way, putting the parts into operation by means of disks or cones is known as the dry drive.

Speaking about electrical drive, it is necessary to say, that electric power installations incorporate generators and motors. Electric current generators transform mechanical energy into electrical energy, which can then be utilized to drive electric motors and for lighting purposes.

Electric motors convert electric energy into mechanical work. The principle of operation and design of electric current generators and electric motors forms the subject of general electric engineering.

As for pneumatic equipment compressed air is used to actuate air-operated machines and tools in boring and blasting jobs, rock excavation, earthwork and bridge construction. Compressed air is supplied by a compressor driven by an electric motor or, more frequently, by an internal-combustion engine.

According to how they operate, compressors can be divided into rotary, centrifugal and reciprocating types. Rotary compressors supply air in a uniform stream. In centrifugal compressors the air is compressed by the centrifugal force generated by rotating wheels. Machines of this type are also known as turbo compressors.

Reciprocating compressors are the predominant type used in road building.

Another type of machinery is hydraulic equipment. In road-making and construction machinery and in tractors and trucks hydraulic power installions are used mainly to impart translational or rotational motion to the operating members and control mechanisms. A hydraulic drive includes a pump, a distribution system, pipelines and hydraulic engines operating on the translational or rotational motion principle.

The hydraulic drive makes the parts move subjecting them to the elementary forces of fluid or water. The properties and action of water are studied by such science as hydraulic engineering.

Hydraulics is defined as that branch of science which treats water or other fluids in motion. The field of hydraulics includes hydrodynamics, which relates to the forces exerted by or upon fluids in motion.

Most of the early knowledge of hydraulics applied only to water, as the name indicates. With modern use and transportation of oil, gasoline, chemicals, steam and gases, it has been necessary to extand these laws of hydraulics both mathematically and experimentally to include these fluids.

Fluids are substances capable of flowing, having particles which easily move and change their relative position without a separation of the mass. Fluids may be divided into liquids and gases. Both liquid and gases are compressible.

If a rectangular vessel be filled with a liquid, then the total downward force exerted by the liquid on the horizontal base of the vessel will be equal to the total weight of the liquid. Let this weight = W. If “A” be the area of this base, then the force per unit area exerted on the base by the liquid will be W/A = F. This force per unit is known as the unit pressure or the specific pressure, or the intensity of pressure, or the hydrostatic pressure.

But any fluid may not be in static state, but in motion. Then it exerts on the plane and makes it move. This is the main principle of a hydrodrive. Hydraulic drives have become extremely popular and are used in general-purpose mechanisms.