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Gear deformation problem during gearbox machining
Jul 02, 2018

It is well known that gears are prone to deformation during processing.


The heat treatment distortion of gears is very complicated, which is a comprehensive reflection of all the factors affecting the cold and hot processing steps. The residual stress generated by machining, the thermal stress and the tissue stress during the heat treatment will have a certain influence on the deformation. However, they have many influencing factors, including the design of raw materials, workpiece structure shapes, the entire process, technical requirements, machining methods, processing conditions, heat treatment processes (heating, cooling), and tooling.


Sometimes it is difficult to take measures to solve the deformation of the gears only from the heat treatment. It is necessary to consider various factors of the whole process of cold and heat processing, analyze the specific conditions and take corresponding measures to eliminate or reduce the influence of unfavorable factors. This is an effective way to reduce heat distortion in gears.


First, the common problems in the process of gear processing

1. The number of teeth is incorrect

The total number of teeth on the entire circumference of the gear is called the number of teeth. The principle and requirements for the determination of the number of teeth of the gears are to initially determine the gear module and the diameter of the drive shaft in order to meet the structural requirements. First of all, the hob is not selected correctly. Because the gear tooth shape is more complex, the factors affecting its machining accuracy are also very complicated and changeable. In addition to its own accuracy of gear hobbing equipment, tooth blank installation adjustment, gear material, heat treatment and other factors, the reasonable choice of gear hob is also very important.


The error in the number of teeth will cause an instantaneous change in the gear ratio during the meshing of each pair of gears. Second, the blank size of the workpiece is incorrect. The traditional blank dimension reference design ignores the differences in the process, and uniformly uses each dimension reference in the part drawing as the blank dimension reference, which results in the increase of the margin error in the subsequent processing and wastes the material. Third, the direction of additional movement is wrong. When the gear hobbing machine is processing helical gears, it is difficult to determine the direction of additional movement. In addition, the change of factors such as the milling method, the spiral direction of the workpiece and the spiral direction of the hob increases the difficulty of judgment.


2. Tooth asymmetry

First, the hob installation is not centered. The installation of the hob affects the radial and axial runout of the hob and ultimately affects the accuracy of the cutting. Generally, the cutter shaft should be corrected before installing the hob to control the radial runout at both ends to be less than 0.005mm. The verticality of the step and the nut end facing the axis should be less than 0.01mm, and the washer should be hardened and polished flat. The installation of the hob on the arbor needs to correct the radial runout of the bosses on both sides and make them “synchronous” as much as possible, ie the highest point of radial runout at both ends is in the same direction.


Second, after the hob sharpening, the helix angle or lead error is large. Commonly used tools for machining external gear spur gears and helical gears. During processing, the hob is equivalent to a helical gear with a large helix angle. The number of teeth is the number of hobs. The workpiece is equivalent to another helical gear and meshes with each other in accordance with a pair of helical gears, rotating at a fixed ratio. The tooth profile of each adjacent position cut in sequence is encased in the tooth profile of the gear.


3. Tooth error

Tooth error refers to the normal spacing between the ideal tooth profile (involute) profiles that contain the actual tooth profile within the tooth profile work section. The gear hob is the most commonly used tool for machining externally meshed straight teeth and helically toothed involute gears.


The axial profile of the reel side flank is a straight line. If it is used in place of an involute hob tooth, the tooth profile of the cut-out gear is not an involute, and thus theoretically results in a certain tooth profile. The error is called the shape error of the gear hob.


In addition, it is impossible to obtain a completely accurate involute profile in the actual machining process, and there are always various errors, thereby affecting the stability of the transmission. The base circle of the gear is the only parameter that determines the involute tooth profile. If the base circle produces an error during hobbing, the tooth condition will also have an error.

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