Reduction gearboxes that let a fast petrol engine drive a slow triplex pump. A petrol engine wants to run near 3,400 rpm and a long life triplex pump wants 1,450 rpm. The gearbox bridges the two, which is why an engine driven belt free machine can still use a proper industrial pump instead of a high speed one.

Three things have to line up: the horsepower band, the drive shaft that goes to the engine, and the pump shaft that comes out the other side. Getting the horsepower band right matters - an undersized box will not survive, and an oversized one is wasted money and weight. Measure both shafts rather than assuming, because the same gearbox model is sold with several drive shaft diameters.