According to gear mesh, they are external gear oil pump（eg. CAT336D C9 Oil pump） and internal gear oil pump.
External gear oil pump working principle:
It is composed of driving shaft, driving gear, driven gear and pump house.
Two same tooth of gears are engaged, this driving gear turns driven gear to rotate in reverse on working.
When two gears running, the oil in gear flow through suction filter from oil inlet to oil outlet to generate vacuum cause of.
The disengaged gear tooth is to force the oil flow into low pressure chamber under atmospheric pressure.
The oil is carried to friction surface of other engine parts by oil gushing from high pressure chamber cause of squeezing effect of gear engaged and oil.
Oil pump won't running until engine stop working, so it makes the route for lubrication oil circle round.
And the pressure limiting valve is open or close is controlled by the high or low oil pressure to steady the comprehensive performance.