While the usage of laboratory tests for determining abrasiveness of rocks is of limited value, they might be helpful in understanding the wear mechanisms operating during rock cutting. One of the findings of the experiments with the pin-on-disc test was that with increasing normal pressure the rate of wear increases up to a maximum value, after which a decrease in wear rate is noted (Figure 11.15). At higher normal pressures crushing of rock occurs below the pin. Apparently when crushed rock material is present between pin and rock, less wear occurs. This led to the development of a hypothesis of different wear mechanisms, or wear modes, occurring. The decrease in wear rate at higher normal pressures could be explained by a transition from a two-body abrasion, or an adhesion wear type of mechanism, to a three-body abrasion mechanism.