Gear knowledge
Gear Train Knowledge
In machinery and equipment, in order to obtain a large transmission ratio, or speed change and commutation, multiple pairs of gears are often used for transmission, such as gearboxes, differentials used on machine tools, automobiles, and gear reducers widely used in engineering. Such a transmission system composed of multiple pairs of gears is called a gear train, or gear train for short.
According to whether the axis position of each gear is fixed during transmission, the gear train is divided into two basic types: fixed shaft gear train and planetary gear train. The geometric axis position of all gears is fixed during transmission. This kind of gear train is called fixed shaft gear train.
During transmission, the geometric axis of the gear g rotates around the common axis of the gears a, b and the member H. Such a train becomes a planetary train. According to different degrees of freedom, planetary gear trains can be divided into epicyclic gear trains and differential gear trains. The epicyclic gear train has one degree of freedom, and the planetary gear train has two degrees of freedom.
The role of the gear train
1. Realize the transmission of motion and power between two axes that are far apart
In gear transmission, when the distance between the master and slave shafts is relatively long, if only a pair of gears are used for transmission, the size of the gears is bound to be large. In this way, both the structural size and weight of the machine are increased, materials are wasted, and manufacturing and installation are inconvenient. If a gear train composed of two pairs of gears is used instead for transmission, the size of the gears can be made much smaller and the manufacturing and installation are more convenient.
2. Realize shunt transmission
By using a gear train, one driving shaft can drive several driven shafts to rotate at the same time to obtain various speeds required.
3. Realize variable speed transmission
When the speed of the driving shaft is constant, the gear train can be used to obtain a variety of working speeds. This transmission is called variable speed transmission. Many machines such as cars, machine tools, cranes, etc. require variable speed transmission.
4. Get a larger transmission ratio
Both fixed-axis and planetary gear trains can be used to achieve large transmission ratios. If a fixed-axle gear train is used to obtain a large transmission ratio, multi-stage gear transmission is required, resulting in a complicated and large structure of the transmission device. With the planetary gear train, only a few gears are needed to obtain a large transmission ratio. Because the planetary gear train uses multiple planetary gears to share the load, and often uses internal meshing transmission, the central space of the internal gear is reasonably used, and its input shaft and output shaft are on the same axis, which not only greatly increases the bearing capacity of the planetary reducer. Increase, and the radial size is very compact. With the same power and transmission ratio, the volume and weight of the planetary reducer is only 1/2 to 1/3 of that of the fixed-shaft gear train.
5. Realize the composition and decomposition of motion
A differential planetary gear train with two degrees of freedom is used in the machine to achieve the synthesis and decomposition of the motion. This is a unique function of the planetary gear train.
Motion synthesis: The differential gear train has two degrees of freedom, and the motion of the third basic component can be determined only after the motion of any two of the three basic components is given. That is to say, the movement of the third basic component is a combination of the movements of the other two basic components.
Motion decomposition: The differential gear train can also be used to decompose the rotation of one basic component into the rotation of the other two basic components in the required ratio.
