Impressions about the construction of plastic gear presets
In the development of modern materials science, engineering plastics, as a new material, are increasingly used in the manufacture of mechanical parts, mainly in automotive, household equipment, industrial machinery, computers and electrical and medical equipment. This is because compared with metal devices, engineering plastics have low manufacturing cost, light weight, easy lubrication, low noise, corrosion resistance, stable chemical properties, simple manufacturing process, flexible processing design, etc.; and because of its flexibility, Therefore, it performs very well in shock absorption performance. Since engineering plastics have these advantages, they are of course used in the manufacture of gears.
1 Plastic gear and metal gear design and manufacturing difference The design and manufacture of plastic gear is different from the design and manufacture of metal gear. The design and manufacture of gears in teaching textbooks refers to metal gears and does not apply to plastic gears. Traditional gear tolerances and grading also do not apply to plastic gears. The calculation of the design of the metal gear does not apply to the function and life of the plastic gear. Many of today's plastic gears are designed with reference to metal gears, but in many cases metal design ideas are difficult to meet the design requirements of plastic gears. The details are as follows: 1.1 cutting: plastic gears are all injection molded with a mold. There is no undercut during the processing. Although the wire cutting is processed according to the tooth shape, it is a processing method of profiling, but with metal hobbing processing. There are differences.
1.2 Modulus: The metal gear forms a complete set of analog-to-digital series to become the standard modulus, and the establishment of this standard modulus is made by the convenience of processing, that is, it is necessary to use a tool to process a variety of gears as much as possible. However, for plastic gear molds, a set of gear molds can only be injection molded into one type of gear, and the machining process is wire cutting. Therefore, the modulus can be set freely, as long as the tooth shape is drawn, it is not necessary to consider whether the modulus is standard.
1.3 pressure angle: now the general pressure angle is 20 ° or 14.5 ° or 22.
5° and so on. However, considering the deformation of the plastic is relatively large, even if the design pressure angle is 20°, the actual meshing pressure angle of the plastic gear will not be 20°. The influence of the deformation on the pressure angle should be very large. It is not possible to simply simplify the plastic gear into a rigid body model for analysis. Therefore, how to consider the actual pressure angle is the most practical.
1.4 Displacement: The displacement of the metal gear is proposed from the perspective of changing the strength. It is based on the separability of the involute to propose the displacement of the gear. This kind of displacement idea can also introduce other problems such as strength and undercut after gear displacement. The idea is based on the simplification of the metal as a rigid body model, but now the various properties of the plastic gear and the metal are fundamentally different. The influence of the displacement of the plastic gear on the gear is mainly the installation, followed by the strength.
1.5 Analysis of non-standardized design of plastic gears: At present, metal gears have standardized the modulus and pressure angle, and the basis of standardization is to facilitate processing.
However, according to the condition that the gears are correctly engaged: m1=m2; α1=α2, it can be known that as long as the two equations are satisfied, the two gears that are correctly meshed can be designed.
The plastic gear is molded by the mold, so there is no difficulty in processing, so the plastic gear can be designed as a non-standard gear.
2 Plastic gear geometric parameter design Plastic gear geometric parameter calculation formula can refer to the calculation formula of metal gear geometric parameters in the Gear Manual. The following is mainly to analyze the selection of geometric parameters of plastic gears, and the calculation formula will not be introduced.
2.1 Number of teeth: Plastic gears are molded by injection molding. There is no forming movement of the tool and gear in this process, so there is no “root cutting†phenomenon in the processing of metal gears. Under the premise of ensuring that the meshing gear does not interfere with the meshing, if only the condition of the continuous transmission is satisfied (coincidence degree ε>1), the standard spur gear can be pushed out according to the calculation (ha3=1, a=20°). 3.
2.2 Modulus: Modulus is one of the most basic parameters of a gear. The larger the modulus, the higher the gear teeth and the thicker the teeth. If the number of teeth of the gear is constant, the radial dimension of the wheel is also larger. Modulus is an important indicator of the bending resistance of a gear. The choice of the modulus of a plastic gear also needs to consider the strength factor. Since the plastic processing of the plastic gear has little to do with the standard tool, and the plastic gear is mostly used in the low-power precision transmission system, the modulus can be selected by "fixing the modulus according to the known structure". The choice of the modulus of the plastic gear can be calculated by the following formula: m=2aZ1 Z2 where m is the modulus, unit: mm; a is the design center distance, unit: mm; Z1, Z2 are the number of teeth of the two meshing gears.
2.3 Pressure angle: At the tangent point of the two gear pitch circle, the acute angle between the common normal line of the two tooth profile curves and the common tangent line of the two circle is called the pressure angle, also called the meshing angle.
For a single gear, it is a toothed angle. In the design of metal gears, the pressure angle of standard gears can be 20 °, 14.5 °, 15 °, 22.5 °, 25 °, etc., where GB1356-88 specifies the standard pressure angle: α = 20 ° in the design of plastic gears, recommended Applicable α=20°. In addition, we can also choose other pressure angle values ​​according to the actual application. For example, in order to increase the coincidence degree, the noise can be selected to be α=14.5°; to improve the bearing capacity, the gear tooth strength can be selected as α=22.5° or α=25°.
2.4 Displacement coefficient: For the displacement gear, the gear displacement mainly considers the following factors: improving the root strength, adjusting the assembly center distance, and facilitating the correction of the tooth shape interference. In addition, in the selection of the plastic gear displacement coefficient, it is also considered to be combined with the modulus to prevent the tooth profile from being excessively deformed due to the displacement, and this is also advantageous for the shrinkage design of the tooth profile. Recommended range of displacement coefficient: 0 ≤ X ≤ m22.5 Other parameters design: The rest of the parameter design can refer to the metal gear design requirements.
3 Conclusion In all standard gear applications, plastic gears are rapidly replacing metal gears. With the deepening of the design concept of lightweight products, the application range of plastic gears has been greatly expanded. In the practice of the factory and the teaching content of the school, the design of the metal gear has been deeply rooted in the hearts of the people. The design and manufacturing technology of plastic gears is still in its infancy, and domestic plastic gear design standards have not yet been formed. The design of the plastic gear discussed in this paper is also limited to the design of the basic geometric parameters, and it is only used for throwing bricks.
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