Hydraulic machinery may be broadly divided into
1. Motor machines, and,
2. Pumps.
Water motors may be divided into
1. Water wheels,
2. Turbines, and,
3. Water pressure engines.
In hydraulic motor machines a quantity of water descending from a higher to a lower level, or from a higher to a lower pressure, drives a machine which receives energy from the water and applies it to overcoming the resistances of other machines doing work.
In the next general class, work done on the machine by a steam engine or other source of energy is employed in lifting water from a lower to a higher level. A few machines such as the ram and jet pump combine the functions of both motors and pumps.
The subject of water wheels is but a continuation of much that has been illustrated and defined in the historical introduction to which is now added the following summary.
In every system of machinery deriving energy from a natural water-fall there exist the following parts:
(1) A supply channel, leading the water from the highest accessible level, to the site of the machine; this may be an open channel of earth, masonry, or wood, or it may be a closed cast or wrought-iron pipe; in some cases part of the head race is an open channel, part a closed pipe.
(2) Leading from the motor there is a tail race, culvert, or discharge pipe delivering the water after it has done its work.
(3) A waste channel placed on or at the origin of the head race by which surplus water, in floods, escapes.
(4) The motor itself, which either overcomes a useful resistance directly, as in the case of a ram acting on a lift or crane chain, or indirectly by actuating transmissive machinery, as when a turbine drives the shafting, belting, and gearing of a mill. With the motor is usually combined regulating machinery for adjusting the power and speed, to the work done.