Molded Plastic Gear Transmission
The design approach of plastic gears and their transmission systems is different from that of metal gear transmissions. Plastic gear transmissions can be optimized in different ways, and the requirements for inspection and testing are also different. The following will introduce new technologies in the design, mold manufacturing and testing of plastic gears and their transmission systems.
Differences between molded plastic gears and metal gears
(1) A fundamental difference between molded plastic gears and metal gears is the manufacturing method. Almost all metal gears are cut and ground, while molded plastic gears are machined from molds. By wire cutting, the spur gear cavity can reach an accuracy of 2.5μm, but since this method is not a generating process, the cutting error may occur at any position, so the entire internal gear cavity must be inspected, unlike cutting Machined metal gears only need to detect a few representative teeth. On the other hand, since the shrinkage rate of each part of the molded plastic gear is different and the abnormal value of the modulus may occur in any part, it is also necessary to inspect each part of the gear. The advantage of moulded plastic gears is that any special gear can usually be moulded as long as it can be drawn in CAD. The challenge is how to measure and adjust for shrinkage anomalies and molding anomalies in molded plastic gears. Metal gears may also benefit from molded plastic gears in terms of the application of full profile inspection techniques and the advantages over generation.
(2) Differences between plastic gears and metal gears due to different processing methods. Metal gears are made by cutting or grinding, and are rotary processed, so they have high coaxiality, and the diameter accuracy is easy to ensure, so there is no need to consider shrinkage compensation in manufacturing. Plastic gears are molded, and the coaxiality is difficult to ensure, but the tooth shape is more accurate than that of metal gears. Because the precision of the gear cavity processed by wire cutting is higher than that of the gear cavity processed by rolling electrodes.
(3) Plastic gears are weaker than metal gears, but have the advantages of self-lubrication, light weight, and low noise that metal gears lack. The large, continuous and repeatable shrinkage characteristics of engineering plastics in the mold cavity need to be considered and compensated for when molding gears. Generally, the diameter tolerance of plastic gears is larger than that of metal gears.
The tolerances and transmission ratios of plastic gears are formulated and recommended according to the structure of metal gears, but these standards are not fair to plastic gears, because they cannot accurately predict the function and life of plastic gears, even if they are distributed according to resin materials. The plastic properties provided by the dealer cannot accurately determine the true parameters of the material when the plastic gears enter or exit meshing at high speed. The properties of conventional plastics are obtained in long-term practice.
Design of Plastic Gears
Usually metal gears are designed according to the basic rack principle of the cutting process, and many designers of plastic gears use a similar method. The pitch circle defined by the metal gear describes the installation interval between the gear and its cutting tool, and the tip modification refers to the additional adjustment characteristics of the cutting tool in order to machine the desired tooth shape. The full depth of cut of the gear actually refers to the cutting tool. How much into the gear blank. However, these concepts are not needed for plastic gears, and they tend to cause confusion and misunderstanding.
The biggest benefit of the basic rack method is that it allows the cut gears to mesh with each other in any pair, whereas plastic gears are usually designed for high-volume applications. It should be designed to make the gear pair as strong and robust as possible, rather than making the gears adaptable to a certain range of applications. The approaches listed below are the design methods to achieve the specific transmission requirements and maximize the gear function.
At present, almost all spur plastic gears are molded, and the mold cavity is processed by wire cutting. Designers can design fully idealized digital gears and then machine them into solid gears by wire cutting.
An involute gear drive is essentially equivalent to a cross-belt drive. The gear teeth use the same transmission path to produce the same rotary effect, the driving wheel pushes the driven wheel through the transmission path, and the path is moved from one base wheel to the other by the belt passing through the node. Many parameters of cross-belt drive are exactly the same as gear drive, such as base circle, pitch circle, pressure angle and base circle tangent length.
Through motion geometry and involute principle, the size of the base wheel can be relatively determined according to the required reduction ratio of the gear pair. Absolute size is not important at this stage, as the final gear can be made to the desired size. Then, select a base circle tooth thickness and draw an involute tooth profile on a 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 by themselves. Part of the structural gear rotates along the pitch circle of its mating gear to form the tooth profile of the mating gear. The addendum is cut off at a fair diameter, and the second gear revolves around the pitch circle of the first gear to form the root portion. This is the design of gears according to the maximum solid condition. To account for eccentricity and molding tolerances, the gear teeth need to be thinned or pulled slightly outward to allow for sufficient clearance, and the gear OD tolerance is smaller than the largest body to avoid interference.
This self-generating construction technique allows designers to maximize the gear's action and performance when the plastic gears are engaged. The teeth can be made longer to increase the meshing working area, or thicker to increase the strength of the teeth. Still to be aware of are the issues of contact ratio and gear strength involved with conventional gears.
Another advantage of this design method is that the CAD-drawn geometry can be used for comparison with molded gears—measured optically or with a scanning coordinate measuring machine.
The next critical step in plastic gear manufacturing is 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 gear transmissions that have been tested to be abnormal or not work at all.
The shrinkage of plastic gears is complex and can be roughly divided into two aspects: macroscopic and local. The shrinkage of the main parameters of the gear base and the simple symmetrical gear is basically the same, 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 gear tooth is completely different, with almost no shrinkage in tooth thickness and other parameters. In some cases there may also be swelling due to local effects, this is particularly the case in hollow crystalline materials such as nylon and acetal.
Inspection of plastic gears
Due to the non-uniform shrinkage phenomenon, it is not possible to simply measure the pitch 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, but the entire gear must be tested. A possible method is to scan and measure the involute profile of all teeth and best fit the profile based on the ideal profile. The trace in the fitting graph represents the tooth profile error relative to the theoretical tooth profile, and the inclination variation of the tooth profile error trace along the gear circumference represents the eccentricity of the gear. The results after eccentric compensation show that the gear shrinkage reaches an error of 0.09mm per 10mm, resulting in a large radial runout, and the tooth thickness of the tested gear is much larger than the specified value.