Many of the principals first mentioned apply equally to other industries. Soil conditions will generally determine the most efficient drilling method. When mud drilling, pushdown—also called pulldown or weight on the bit (WOB)—is key. The harder the formation, the more pushdown required. As more drill pipe is added to the drill string, more weight is applied to the bit and therefore less pushdown is required from the rig. The mud pump must supply adequate flow and pressure which is again determined by the bore depth, diameter and soil. Centrifugal pumps are cost effective and easy to maintain, but provide less pressure and volume than piston pumps. The rpm required from the rotation head is again determined by the material being drilled and the drilling method, with mud rotary generally requiring higher rpm than DTH hammer or auger drilling.