1、 Concept and application of precision gear die
1) The precision gear refers to the very high requirements for parameter size and performance, including the outer diameter of the tooth tip circle, the outer diameter of the tooth root circle, the modulus, the diameter of the indexing circle (also known as the common normal), the tooth thickness of the indexing circle, the pressure angle, the inclined angle of the helical gear, the runout tolerance of the gear ring, etc; The performance includes the selection of materials and the accuracy requirements of other assembly dimensions.
2) In terms of application, representative products such as the internal transmission parts of printers and copiers, such as VCD disc drive brackets, are common. If the gear does not meet the precision requirements, the bracket will move in and out of the unstable motion, accompanied by vibration or swaying, with loud voice. Through this, we can have a perceptual understanding of the concept of precision and its necessity.
3) The precision of precision gears is divided into grades. The higher the grade, the more precise the national standard is. The smaller the grade, the more precise the Japanese standard is. According to JIS standards, most precision mold factories can only achieve grade 4 and grade 5 gears, and the level of grade 3 plastic gears is quite high.
2、 Precision plastic gear type:
1) From the angle of axial inclination of teeth, it can be divided into spur gear, helical gear, combined spur gear, combined helical gear and combined straight helical gear. The concept of combination is that straight or helical gears are distributed in the axial direction step by step and formed by one injection.
2) There are two main types of transmission: meshing transmission gear and belt transmission gear.
3、 Selection of mold:
The precision of the mold embryo will directly affect the manufacturing and injection precision of the mold, and will also affect the life of the mold. Experience shows that it is difficult to meet the requirements of precision gear die with general die precision.
4、 Selection of plastic raw materials:
There are two main types: POM and PA (PA+GF%). Other raw materials, such as polysulfone, are not excluded. The most fundamental reason for choosing POM and PA raw materials is that the surface of POM and PA products has good self-lubrication.
Other common reasons: good wear resistance, impact resistance and fatigue resistance; Secondly, POM products have high rigidity and excellent creep resistance; PA products have good toughness. If necessary, GF% can be added to enhance its rigidity. Generally, POM gears can be meshed with each other, and PA gears can be meshed with POM gears.
5、 Key points of mold structure design:
Gear mold is different from traditional mold design. Gear tooth thickness (arc length between tooth profiles on both sides of a gear tooth), modulus (a parameter used to measure the size of the gear tooth), pressure angle (refers to the sharp angle between the direction of force and the direction of motion without calculating friction) need to be modified in combination with empirical data, and cannot be directly processed according to the shrinkage rate, Some professional gear and gear reduction box manufacturing companies have developed cavity parameter calculation software for gear moulds based on the characteristics of plastic gear moulds and the cooperation with computer software manufacturers through long-term experience accumulation, which can directly generate gear parameters and profiles for gear profile modification and improve gear profile accuracy.
Post time: Jan-17-2023