For truck-mounted hydraulic systems, the most common design in use may be the gear pump. This design is characterized as having fewer moving parts, being easy to services, more tolerant of contamination than other designs and fairly inexpensive. Gear pumps are fixed displacement, also known as positive displacement, pumps. This implies the same volume of circulation is created with each rotation of the pump’s shaft. Gear pumps are rated in terms of the pump’s maximum pressure rating, cubic in . displacement and maximum insight speed limitation.
Generally, gear pumps are used in open center hydraulic systems. Gear pumps trap oil in the areas between the the teeth of the pump’s two gears and the body of the pump, transportation it around the circumference of the apparatus cavity and then push it through the outlet slot as the gears mesh. Behind the brass alloy thrust plates, or wear plates, a small amount of pressurized essential 
oil pushes the plates tightly against the gear ends to improve pump efficiency.