The design of plastic gears and their transmission systems is different from that of metal gear transmissions. Plastic gear transmissions can be optimized by different methods, and the requirements for inspection and testing are also different. Full threaded rod produced by wire drawing and thread rolling threaded rod Shaoxing Grace International Trade Co.,Ltd , https://www.shaoxinggrace.com
The design of plastic gears and their transmission systems is different from that of metal gear transmissions. Plastic gear transmissions can be optimized by different methods, and the requirements for inspection and testing are also different. The following will introduce the new technologies in the design, mold manufacturing and testing of plastic gears and their transmission systems.
The difference between molded plastic gears and metal gears
(1) One fundamental difference between molded plastic gears and metal gears is the difference in manufacturing methods. Almost all metal gears are machined and ground, and molded plastic gears are machined from molds. With wire cutting, the spur gear cavity can reach an accuracy of 2.5μm. However, since the method is not developed, the cutting error may occur at any position. Therefore, the entire internal gear cavity must be tested, unlike cutting. The machined metal gear only needs to detect several representative teeth. On the other hand, since the shrinkage ratio of each part of the molded plastic gear is different and the modulus value may occur in any part, it is also necessary to detect each part of the gear. The advantage of molded plastic gears is that any special gear can usually be molded as long as it can be drawn in CAD. The challenge is how to measure and adjust the shrinkage anomalies and molding anomalies of molded plastic gears. Metal gears may also benefit from molded plastic gears in terms of the application of full profile inspection technology and the advantages compared to the development method. 
(2) Plastic gears and metal gears differ due to different processing methods. The metal gear is cut or ground and is rotary. Therefore, it has high coaxiality and the diameter accuracy is easy to guarantee. It is not necessary to consider shrinkage compensation in manufacturing. The plastic gear is molded, and the coaxiality is difficult to guarantee, but the tooth shape is more accurate than the metal gear, and the precision of the gear cavity of the wire cutting process is higher than that of the gear cavity processed by the rolling electrode. 
(3) Plastic gears are inferior to metal gears, but have the advantages of self-lubrication, light weight and low noise that metal gears lack. Large, continuous and repeatable shrinkage characteristics of engineering plastics in the mold cavity need to be considered and compensated for when machining the gears. In general, the diameter tolerance of plastic gears is larger than that of metal gears. 
The tolerances and transmission ratios of plastic gears are all based on the structure of metal gears, but these standards are unfair to plastic gears because they do not accurately guess the function and life of plastic gears, even if they are distributed according to resin materials. The plastic properties provided by the manufacturer cannot accurately determine the true parameters of the material when the plastic gear enters or exits at high speed. The properties of traditional plastics are obtained in the long run. 
Plastic gear design
Usually metal gears are designed for the basic racking process, and many plastic gear designers use a similar approach. The pitch circle defined by the metal gear describes the installation interval between the gear and its cutting tool, and the tooth top modification refers to the additional adjustment characteristics of the cutting tool to machine the required tooth shape. The full depth of the gear actually refers to the tool. How much into the gear blank. However, these concepts are not needed for plastic gears, and they often cause confusion and misunderstanding.
The biggest benefit of the basic rack method is that the gears that are allowed to be cut can be arbitrarily matched to each other, and plastic gears are usually designed for high volume applications. It should be designed so that the gear pair is stronger and stronger, rather than adapting the gear to a certain range of applications. The approach outlined below is to achieve specific drive requirements and maximize the optimal design of the gear function. 
At present, almost all straight-tooth plastic gears are molded, and the cavity is machined by wire cutting. Designers can design fully idealized digital gears that are then machined into solid gears by wire cutting. 
Involute gear drives are essentially equivalent to cross belt drives. The teeth use the same drive path to produce the same turning effect. The drive wheel pushes the driven wheel through the drive path, and the path moves away from one base wheel to the other based on the belt passing through the node. Many parameters of the cross belt drive are exactly the same as the gear transmission, such as the base circle, pitch circle, pressure angle and base circle tangent length.
Through the motion geometry and the involute principle, the size of the base wheel can be determined relative to the reduction ratio of the required gear pair. At this stage, the size is not important, because the final gear can be the required size. Then, select a base scallop thickness and draw an involute profile on the gear and the spacing from the gear to determine its working pressure angle. The outer diameter of the gear can be ignored. At this point, the gear has been determined, and other parts can be developed on its own. A portion of the structural gear rotates along the pitch circle of its mating gear to form a toothed profile of the mating gear. The crest is cut at the fair diameter and the second gear is rotated along the pitch of the first gear to form the root portion. This is to design the gear according to the maximum physical conditions. In consideration of eccentricity and molding tolerances, the teeth need to be thinned or slightly pulled outwards to have sufficient clearance, and the outer diameter tolerance of the gears is smaller than the maximum solid to avoid interference.
This self-developing construction technology allows the designer to maximize the role and performance of the gears when the plastic gears mesh. The teeth can be made longer to increase the meshing work area, or to increase the tooth thickness to increase the strength of the teeth. Still need to pay attention to the contact ratio and gear strength involved in the traditional gear. 
Another advantage of this design approach is that the geometry drawn by the CAD can be compared to a molded gear – an optical measurement or a scanning coordinate measuring machine for comparative measurements. 
The next critical step in the manufacture of plastic gears is the mold design. At this stage, it is necessary to estimate the shrinkage of the geometric shape of the plastic gear, otherwise it will cause many well-tested gear transmissions to be abnormal or not working at all. 
The shrinkage of plastic gears is complex and can be roughly divided into two aspects: macroscopic and local. The gear base and the simple symmetrical gear have the same amount of contraction of the main parameters, including the outer diameter of the gear, the diameter of the root circle, the base circle and the pitch circle. The local shrinkage of a single tooth is completely different, and the tooth thickness and other parameters hardly shrink. In some cases, it may also swell due to local effects, which is particularly evident in hollow crystalline materials such as nylon and acetal.
Plastic gear inspection
Due to the non-uniform shrinkage phenomenon, it is not possible to simply measure the medium diameter of the gear to determine the shrinkage rate, or to further mesh with the standard gear (measuring gear) to determine the shape error of the gear, and the entire gear must be detected. One possible method is to scan the involute profiles of all the teeth and best fit the profile based on the ideal tooth profile. The traces in the fitted graph represent the tooth profile error relative to the theoretical tooth profile, and the tilt variation along the gear tooth profile error trace represents the eccentricity of the gear. The result of the eccentric compensation shows that the gear has a deviation of 0.09 mm per 10 mm due to the shrinkage, resulting in a large radial runout, and the tooth thickness of the measured gear is much larger than the specified value. 
Plastic gear users can use CAD graphics to compare the molded gear shapes for inspection. When the shrinkage amount is correctly considered, a simple gear roll detection using the reference gear can be used for mass production. 
Design verification of plastic gears
Regardless of how well the design and inspection of the components in the plastic gear transmission are done (including the box, gears, shafts, etc.), it is necessary to carry out the transmission test on the plastic gear transmission system. Otherwise, it is impossible to guess the transmission of the plastic gear transmission system. Torque capacity, smoothness, noise and longevity. The best way to perform these functional tests is to directly measure the input, output torque and angular displacement/angular velocity using a drive dynamometer. It is best to install an accelerometer on the gearbox. Spectral analysis of input and output torque and/or speed will reveal incorrect gear geometry; the accelerometer's spectral analysis will not only detect bad tooth shapes, but also the vibration power that produces noise. Comparing the input output power (transmission efficiency) will reveal that the shafting parallel accuracy is not good, the size is too large, or the root is not cut to size, causing jamming and other defects. 
The size of plastic parts is subject to change during processing, such as mold cleaning, rework, molding compound changes, process changes, etc., which can cause dimensional changes. 
Regularly use the force gauge (power meter) to detect the product. By comparing the product with the dynamometer signal of the prototype, you can find the missing grid parts that were missing when detecting the geometry.
The structure of the transmission dynamometer is simple and complex. Many drive systems are driven by DC motors. The current of the DC motor is a good torque indicator and the EMF waveform can indicate the speed. Connecting a second motor to the output constitutes a complete and simple torque test system.
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