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ZDDP (zinc dialkyl dithiophosphate or zinc diaryl dithiophosphate) - widely used as an anti-wear additive in engine oils to protect heavily loaded parts, particularly the valve train mechanisms (such as the camshaft and cam followers) from excessive wear. It is also used as an anti-wear agent in hydraulic fluids and certain other products. ZDDP is also an effective oxidation inhibitor. Oils containing ZDDP should not be used in engines that employ silver alloy bearings. All car manufacturers now recommend the use of dialkyl ZDDP in engine oils for passenger car service.
ZN/P curve - general graphic representation of the equation: µ = (f) ZN/P, where µ (the coefficient of friction in a journal bearing) is a function (f) of the dimensionless parameter ZN/P, (viscosity x speed) / pressure. This is the fundamental lubrication equation, in which the coefficient of friction is the friction per unit load, Z the viscosity of the lubricating oil, N the rpm of the journal, and P the pressure (load per unit area) on the bearing. The ZN/P curve illustrates the effects of the three variables (viscosity, speed, and load) on friction and, hence, on lubrication. See boundary lubrication, full-fluid-film lubrication.