The permissible working temperature of fast gearbox shall be no more than 120℃. Gearbox can generate certain heat because of friction of moving member. The normal working temperature is about 38℃ higher than the ambient temperature. The heat is dissipated through gearbox housing. If heat dissipation is not normal, overheating will be caused. It is normal that vehicle gearbox is “too hot to touch” when the vehicle runs in hot summer. But if the coat of paint of gearbox is cracked and high temperature appears, please pay attention to inspection. There are many causes for overheating of gearbox. Too much filled gear oil or serious oil starvation will cause overheating. The machine part is abnormally worn and the bearing is marched too tight, which also can result in overheating. Gearbox runs with low speed or high speed, which also can cause overheating. When the brand of gearbox gear oil especially the viscosity number is wrong, it is difficult to lubricate and then the gearbox is overheating. The air ventilator of gearbox can cause overheating and also can make gearbox seriously leaks oil on the input and output end. Engine exhaust system is too closed to gearbox. Running speed is less than 32Km/h generally. Working angle of gearbox shall be more than 12°.