For truck-mounted hydraulic systems, the most common design in use is the gear pump. This design can be characterized as having fewer shifting parts, being simple to provider, more tolerant of contamination than various other designs and relatively inexpensive. Gear pumps are set displacement, also known as positive displacement, pumps. This means the same level of circulation is created with each rotation of the pump’s shaft. Gear pumps are rated in conditions of the pump’s optimum pressure rating, cubic ” displacement and maximum input speed limitation.
Generally, gear pumps are found in open center hydraulic systems. Gear pumps trap oil in the areas between the the teeth of the pump’s two gears and your body of the pump, transport it around the circumference of the apparatus cavity and then drive it through the outlet port as the gears mesh. Behind the brass alloy thrust plates, or use plates, 
a little amount of pressurized oil pushes the plates tightly against the gear ends to boost pump efficiency.