The friction loss of gears and its quantitative estimation are fore, the authors carried out experiments to study the influences of important problem because of their relevance to energy comerlubricating oil viscosity and additives, as well as base oil type and vation and load-carrying capacity. Recent research results do not load and rotational speed on friction loss of spur gears. Base oil providk satisfactory estimates of friction loss of spur gears. Theretypes wed were paraffin mineral, poly-cu-olefin, and polyglycol with several oil viscosities. An EP and a mild EP additive were studied in these oils. Finally, the temperature rise of teelh of gears as a was derived.= center distance, mm = constant defined as Eq.[lo] = face width of the spur gear, mm = constant defined as Eq.[lo] = diameter of intermeshing pitch circle, mm = diameter of base circle, mm = diameter of generating pitch circle, mm = diameter of tip circle, mm = module, mm = rotational speed, rpm = torque of driver gear, N . m = transmitted power, kW = friction power loss of gear system including friction loss of bearings without load, kW = friction power loss of bearings without load, kW = friction power loss due to engagement of gear defined as Eq.[7], kW = increment of friction loss of gears due to load defined as Eq.[9], kW = friction power loss of gears without load, kW = increment of friction power loss of gear system including friction loss of bearings due to load, kW = power loss due to churning the lubricating oil, kW = total power loss including friction loss of gears and bearings, and churning loss of the lubricating oil, kW = Hertzian pressure, ~l m m~ = normal load on the tooth surface, N Rrnax = maximum height of roughness of tooth surface, k m Tm = average value of tooth temperature of driver gear and follower gear measured at the position of Fig. 2, "C Tmo = tooth temperature in the state of no load running, "C mean values of driver gear teeth temperature and follower gear teeth temperature Toil = temperature of lubricating oil at the position immediately ahead of oil nozzle, "C V = pitch line velocity of intermeshing pitch circle, m/s V I , V P = circumferential velocity of roller I and 2, respectively, Fig. A2, mls V.1. = viscosity index WI, W P . W = friction loss defined as Eq. [A3], [A4], and [A5], N . m x = addendum modification coefficient L = number of teeth a b = operating pressure angle, deg {g = friction loss due to the engagement of gears, percent &add