齿轮轴指支承转动零件并与之一起回转以传递运动、扭矩或弯矩的机械零件。一般为金属圆杆状，各段可以有不同的直径。机器中作回转运动的零件就装在轴上。

　　根据轴线形状的不同，轴可以分为曲轴和直轴两类。根据轴的承载情况，又可分为：①转轴，工作时既承受弯矩又承受扭矩，是机械中最常见的轴，如各种减速器中的轴等。②心轴，用来支承转动零件只承受弯矩而不传递扭矩，有些心轴转动，如铁路车辆的轴等，有些心轴则不转动，如支承滑轮的轴等。③传动轴，主要用来传递扭矩而不承受弯矩，如起重机移动机构中的长光轴、汽车的驱动轴等。

　　Gear shaft refers to mechanical parts that support rotating parts and rotate with them to transmit motion,torque or bending moment.Generally,it is in the shape of metal round rod,and each section can have different diameters.The rotating parts of the machine are installed on the shaft.

　　According to the shape of the axis,the shaft can be divided into two types:crankshaft and straight shaft.According to the bearing condition of the shaft,it can be divided into:①rotating shaft,which bears both bending moment and torque when working,is the most common shaft in machinery,such as the shaft in various reducers.②Mandrels are used to support rotating parts that only bear bending moment without transmitting torque.Some mandrels rotate,such as the axle of railway vehicles,and some mandrels do not rotate,such as the axle of supporting pulley.③The transmission shaft is mainly used to transmit torque without bearing bending moment,such as the long optical shaft in the crane moving mechanism and the drive shaft of the car.