Gear mechanism is the most important and widely used transmission form in mechanical transmission. Its advantage is to ensure that the transmission ratio is constant and can transmit enough power; it has high motion accuracy and transmission efficiency, reliable work, long life, and structure. compact.

According to the positional relationship of the meshing gear axis, it is divided into parallel shaft transmission and intersecting shaft transmission. Parallel shaft transmission is divided into three types: straight tooth cylindrical gear transmission, helical cylindrical gear transmission and herringbone cylindrical gear transmission. The three transmissions have their own characteristics due to their different gear structures. Among them, the herringbone cylindrical gear transmission has obvious advantages, but it also has manufacturing difficulties.

1. Introduction to involute tooth profile

When the plane S is purely rolling along the base cylinder with radius b, the involute surface formed by the track of a line KK parallel to the base cylinder generatrix NN is the tooth surface of the involute spur gear. If the straight line KK and the generatrix NN form an angle b, the trajectory formed is an involute spiral surface, that is, the tooth surface of an involute helical cylindrical gear, and b is the spiral angle on the base cylinder.

2. Description of meshing characteristics

It can be seen from the forming process of the tooth profile surface that when the involute spur gear is meshed, the contact line of the tooth profile surface is a straight line parallel to the axis. During the meshing process, the entire tooth width enters and exits the mesh at the same time. The force is also suddenly added or removed, so the transmission stability is poor, and the impact and noise are large. When the involute helical cylindrical gear meshes, the contact line of the tooth surface gradually changes from short to long, and then gradually from long to short, until it is out of mesh, the force on the gear teeth is gradually from small to large, and then from Large to small, there are many pairs of gear teeth meshing at the same time, so the transmission is relatively stable, the impact and noise are small, and it is suitable for high-speed and heavy-duty transmission. However, the axial component force FX is generated when the helical cylindrical gear is driven, which is borne by the shaft and the bearing. In order to reduce the axial component force FX during transmission, the helix angle should not be too large. Generally, the helix angle on the indexing cylinder=8°～15°.

In order to resolve this contradiction,Herringbone gearIt came into being. Herringbone gear refers to a pair of helical gears with symmetrical helix angles and opposite directions to eliminate axial force. A part of the herringbone cylindrical gear has a right helical tooth and the other part has a left helical tooth. According to different manufacturing methods, there are two structures with undercut and without undercut between the left and right spiral teeth. In addition to the advantages of the helical gear, the herringbone gear has the same left and right helix angles of the two parts of the gear teeth, and the directions are opposite, so that the axial force FX can be offset each other; thus, a gear with a larger helix angle (generally Take the possibility of=25°～40°), so that the coincidence of gear meshing can be improved, so that the transmission is more stable while avoiding the adverse effect of the axial component on the transmission system, but its disadvantage is manufacturing It is more difficult, and the manufacturing precision of the gear has a relatively large impact on the stability and noise of the gear transmission.

3. Herringbone gear processing

Due to the special structure of herringbone gears, its processing is more complicated than ordinary helical gears, and the equipment requirements are higher than those for helical gear processing. In terms of technical requirements, in addition to meeting the accuracy requirements of helical gears, there are also requirements for the position of the intersection point of the cogging extension lines of the left and right gears (hereinafter referred to as the center line of the herringbone gear). Usually the intersection point is on the center line of the two positioning end faces of the gear. This is also a difficult point in the processing of herringbone gears. Here are several common processing methods.

3.1 Soft tooth surface gear (the tooth surface hardness of the meshing gear pair is less than 350HB)

The more common processing methods for soft tooth surface gears include profiling and generative processing.

3.1.1 The profiling method refers to the use of a CNC machining center machine tool and a finger milling cutter for milling. The accuracy of this machining method is greatly affected by the tool, and the accuracy of the gear teeth is low, usually reaching DIN8 Left and right accuracy, but the position of the center line of the herringbone gear is more accurate, and the overall processing efficiency is also low;

3.1.2 Generating method refers to the use of CNC gear hobbing machine for hobbing. Among them, if the gear hobbing machine control software has the herringbone gear processing function, the herringbone gear processing program can be used for processing. It is easier to ensure the center line position of the herringbone gear. The key is Just input the installation height of the gear processing accurately; if the herringbone gear processing function is not available, you need to draw lines on the tooth surfaces of the left and right-handed gears before processing to determine the position of the center line of the herringbone gear , And then perform the hobbing of the left and right gears according to the marking position respectively. The center line position of the herringbone gear processed by this method is not easy to guarantee, and the accuracy of the marking and the accuracy of the tool setting must be jointly determined; The gear teeth produced by the hobbing method have higher precision (up to DIN7) and higher processing efficiency;

3.2 Hardened gears

Hard surface gears are usually carburized or nitrided to the gear material, so that the surface hardness of the gear is about 60HRC. If the gear is nitrided, the teeth must be made live before heat treatment. The processing method is the same as that of soft teeth. Face gears are the same; if the gear is carburized, the teeth must be made alive after heat treatment. The processing method is usually gear grinding. The center line of herringbone gear can be adjusted during gear grinding. Due to the limited amount of hobbing, Therefore, the adjustment amount of the center line of the herringbone tooth is limited, and the position of the center line of the herringbone tooth is determined by the machining accuracy before heat, but the precision of the gear processed by grinding can reach DIN5 or higher.

Since the herringbone gear has the characteristics of automatically offsetting the axial force, increasing the helix angle has no effect on the force of the entire transmission system, but on the other hand, a larger helix angle can get a larger coincidence degree, making the transmission more stable . The gearbox with herringbone gear pair has the remarkable characteristics of stable transmission, low noise, small size, light weight, large transmission power and long service life, and has high comprehensive social benefits. It can be seen that the herringbone gear transmission is an important development direction of the gear transmission mechanism, and we need to further improve the existing manufacturing level.