Gear shaving
The problem of shaved teeth is a problem that is often encountered in the gear shaving process. The generation of the concave recess makes the meshing of the gears less stable and the meshing noise is large, thereby seriously affecting the performance of the gears. Therefore, in the shaving process, how to effectively solve this problem has been a common goal of the automotive gear industry for many years.
Car gears mostly use shave teeth as the final finishing method for the teeth, but often encounter shaved teeth in the actual processing, that is, there is a concave amount in the middle of the teeth after shave (see Figure 1). This phenomenon is even more pronounced on the small number of spur gears. In the actual use of the concave tooth gear, due to poor transmission stability and large meshing noise, the occurrence of such a concave tooth shape should be avoided as much as possible. Because the cause of the concave tooth profile is more complicated, people have been controversial about the cause. The tooth concave problem has not been a very good solution in the process, and it is a difficult problem to improve the accuracy of long-toothed automotive gears. Here, the author summarizes the causes that have been approved by most people in the industry and verified by practice, and introduces the main solutions.
Fig. 1 The concave shape of the tooth
Causes of concave tooth formation
Shave teeth is the application of cross shaft helical gear meshing principle, the use of gear teeth with a small groove with the cutting capacity of the cutter gear and the workpiece is processed without clearance for free roll to mesh, by feed pressure and the use of meshing tooth surface between the relative slip generated The cutting action removes the metal for processing. This is the basic principle of shaving. For the ordinary shaved teeth, when the gears are machined with a standard involute tooth cutter, the tooth concave phenomenon often occurs near the pitch circle. The reasons are various. According to the shave-tooth meshing motion relationship and force analysis, it can be known from the shaving principle that the relative sliding speed near the pitch circle is the smallest when shave gear meshes, the oil film is not easy to form, the lubrication is not good, and it is easy to scratch; at the same time, the direction of the sliding speed near the node is changed. , To make the tooth profile produce rubbing effect; From the force situation, due to the shaved tooth engagement point is meshed, and the node at the single pair of teeth meshing opportunities, so that the pressure of the tooth surface when the single pair of teeth meshing increases, the margin removed at this place Also cut more than other parts, resulting in a concave tooth surface near the pitch circle. Practice has proved that excessive changes in the force conditions during shaving are the mechanisms and main reasons for the occurrence of tooth-shaped depressions. The main aspects are as follows:
1. Ordinary shaved tooth surface meshing is theoretically point contact. Actually, although the contact point is expanded to a small area contact, the contact point pressure is still relatively large, which makes the shaving shaving margin of ordinary shaving teeth more sensitive to changes in the number of meshing tooth contact points. .
2. The engagement of the workpiece with the tool is too small. Due to the small number of teeth in the workpiece or the unreasonable design of the shaver's parameters, the degree of coincidence when shaving teeth are meshed is too small. Due to the chance of a single pair of teeth meshing when the degree of overlap is small, the contact point of the tooth surface meshing suddenly decreases from the original multipoint, and the pressure at the contact point suddenly increases. Therefore, the single pair of tooth meshing contact points is shaved off. The remaining amount is more than the other points, and the tooth surface appears concave. And when a single pair of teeth meshes, the meshing point is just in the vicinity of the pitch circle of the gear, so in general, the tooth surface is formed near the tooth pitch circle. For the same workpiece, the amount of indentation is mainly related to the relationship between the degree of coincidence and the meshing motion. The greater the degree of overlap, the more conducive to reducing the amount of indentation. However, for a specific gear with few teeth, as the gear parameters have been determined, the design of the shaver is limited by the machine tool and the durability, and the increase in the degree of meshing is often limited, especially for the spur gears. Not easy to increase, so the use of ordinary shaving method is not easy to improve the design of the tool to achieve the purpose of solving tooth concave.
3. In the shaving and meshing process, the contact points on the tooth surfaces on both sides of the gear are not equal, and the force is unbalanced, resulting in different cuts resulting in concaves.
Shaving gear and workpiece teeth do no backlash meshing, both sides of the gear teeth have contact points, respectively, on the two meshing lines, set the shaver and the workpiece meshing degree of overlap between 1 to 2 At a value, the number of pairs of meshing teeth on the two meshes varies between 1 and 2. Sometimes it is a pair of gear teeth meshing, one contact point; Sometimes two pairs of gear teeth mesh, two contact points, so the number of contact points on both sides of the meshing gear teeth may not be equal, as shown in Figure 2, a, b. Because the shaving cutter acts on the meshing teeth, the moments in the forward and reverse directions generated by the forces are equal. Therefore, when the number of left and right contact points is not equal, only one contact point has a large pressure at one contact point. More metal was shaved; the pressure at the other contact with two contact points was less, and less metal was shaved. Because the single point contact is always near the pitch line (middle) of the tooth profile, the double point contact is always at the tooth tip and the tooth root portion, and this results in a concave phenomenon when shaved.
Figure 2 meshing contact points
Solution measures and problems
According to the causes of the shaved concave tooth phenomenon, the following problems are generally solved in solving the tooth concave problem:
1. Shaving cutter modification
The tooth profile of the shaving cutter is grinded into a concave curve according to the actual tooth profile record of the shaved gear, that is, the anti-reshaping method is adopted to ensure that the concave amount in the gear after shaving is reduced or eliminated. This method often requires repeated repeated grinding trials to determine the final concave and concave position of the shaving cutter modification curve.
The disadvantage of this method is that it takes a lot of trial and error to determine the tool modification curve, so it is very cumbersome and complicated, and it is also difficult to determine the starting and ending points of the best concave amount. As the number of tool dressings increases, the starting and ending points need to be corrected at any time, and the technicians are required to have a razor dressing equipment that can be accurately shaped and have rich repairing experience, and have poor quality stability for mass production.
2. Improved tool design increases coincidence
This method is mainly aimed at improving the design of tool parameters that affect the degree of overlap, taking into account the tool life and shaving quality, and increasing the degree of coincidence as much as possible. Such measures as reducing the engagement angle, increasing the number of teeth of the cutter, increasing the outer diameter of the cutter, etc., can reduce the amount of sinking in the shaved teeth to a certain extent.
The disadvantage is that due to limitations in workpiece parameters, machine tools, shaving accuracy, tool life, etc., the increase in the degree of coincidence is limited, and therefore, the improvement effect of gears in a certain number of teeth and a range of displacement coefficients is not ideal, and the applicable range is narrow.
3. Balanced shaving method
This method is to improve the design of the tool so that the number of contact points on both sides of the gear teeth is always the same during the workpiece meshing process. That is, the gear teeth have the same number of contact points on both sides of any meshing moment (see Fig. 3), and meshing occurs during meshing. The change of the tooth contact point is 4-2-4. In the position shown in Fig. 3a, there are two contact points on each side of the gear teeth; when the position shown in Fig. 3b is shown, there is one contact point on each side of the gear tooth. Therefore, the pressures at the contact points on both sides of the gear teeth are always nearly equal, ie, the pressure is balanced, so that as many teeth are shaved on both sides of the gear teeth, it is not easy to produce concave phenomenon.
Fig. 3 Schematic diagram of changing the contact points of meshing teeth
To achieve balanced shave, first determine the equilibrium conditions, that is:
α' = (Ïmax + Ïmin) / db-φb
Where: α' - balanced meshing angle
Db - base circle diameter
Ρmax - the maximum radius of curvature of the meshing profile
Ρmin - Minimum radius of curvature of meshing tooth profile
Φb - base circle tooth thick half angle
Calculate the equilibrium engagement angle α', and then design the shaving cutter according to this engagement angle. With such a shaver, the shaved teeth can be balanced.
This method can meet the limited balance of shave gears. However, with the shaving cutter, the tooth thickness of the cutter is gradually reduced, and the outer diameter of the cutter is also changed. As a result, the equilibrium engagement angle changes, and the concave phenomenon still occurs after the balance relationship is destroyed. Although the change of the balance meshing angle can be compensated by changing the outside diameter of the tool, this change is difficult to achieve at random, so the balanced shaving tooth is very effective, but its application is limited by some conditions.
4. Radial shaving technology
Because the profile of the radial shaving cutter has been specially modified, its working principle is: the tooth profile of the cutter and the tooth profile of the workpiece are in line contact during shaving and meshing. Compared with ordinary shaved teeth, on the one hand increasing the degree of coincidence, on the other hand also increases the load bearing area of ​​the tooth surface meshing feed pressure, which can make the shaving shave margin size of the meshing tooth contact points when the number of changes in the number of Sensitivity decreases, which can slow down the formation of tooth-shaped depressions.
The disadvantage is that the application of the radial shaving technology has certain limitations, such as the need for a dedicated shaving machine with a radial shaving function, and the need for a grinding and dressing device with a special radial shaving cutter. These conditions will restrict some manufacturers to apply this technology. However, with the rapid development of the automobile industry in recent years, many enterprises have no longer been constrained by their hardware facilities. Radial shaving technology has been widely used. In addition, since the radial shaved teeth cannot be used to process gears with too wide tooth widths and excessively large modulus, such large gears are not suitable for solving shave problems with radial shaving.
In actual production practice, all companies can use the above methods in a comprehensive manner according to their own circumstances, and will often achieve better results.
Practical application
In practice, we use radial shaving technology, and at the same time integrate the aforementioned concept of increasing the degree of coincidence and balancing shave to design a radial shaving cutter, which is applied to the reverse gear of a certain type of transmission and achieved good results. The following is a brief introduction as follows:
The reverse gear of the transmission is a typical low-tooth-number spur gear (see Figure 4, main parameters m2.5, z12, α20°). It has been using the ordinary shave method and there are many problems. The most important one is the resulting one. The concave tooth problem has plagued the normal production and quality of the gear. Due to the small number of teeth (only 12 teeth), in order to prevent the root cutting using positive displacement design, but also because of the use of shaved machine tools for Y4212/D, the smaller size, the tool diameter is also limited, can only use the nominal diameter of φ180mm or so razor. The number of razor teeth is small, and the degree of shaved teeth is relatively small. Therefore, the phenomenon of concave teeth during shaving is more serious.
Figure 4 Reverse gear
When machining with a standard involute tool, the amount of indentation is 0.035mm (see Figure 5a), which can't meet the requirement of use at all. The on-site processing has always used the anti-remodeling method. Although the amount of concave is improved, it is very unstable and cannot be eradicated. The amount of concave is generally between 0.01 and 0.02 mm (see Figure 5a). This is mainly due to the cumbersome and time-consuming nature of the reshaping method. It is very difficult to accurately determine the starting and ending points of the concave and concave quantities of the tool tooth curve. Due to the backwardness of the razor dressing equipment used at the time and the limited precision, even after several tests have been performed to determine the profile of the corresponding tool, it is often impossible to steadily wear out the specified modification curve; in addition, with the increase in the number of subsequent tool dressings, The tooth thickness of the tool is reduced more, and the engagement angle with the workpiece changes again when it is used again. The pit of the tool modification curve changes accordingly, and this change rule is difficult to grasp, and the grinding equipment used is also difficult to use. Adapt to this change. Therefore, although the anti-reform method was adopted, the actual processing results were not satisfactory and the product quality was unstable.
Figure 5 Reverse gear tooth profile comparison
In 2001, we tried to adopt the radial shaving technology. We asked a professional tool factory to design and manufacture a radial shaving cutter. In the design of the cutting tool, the consideration of increasing the degree of coincidence and the idea of ​​balancing shaved teeth was taken into consideration. Practice shows that the original Y4212/D shaving machine with radial shaving function is still used on the site. After the tool is used, it works well and basically eliminates the concave (see Figure 5b). At the same time, the effect of the knife after repeated grinding is still good. Since then, our company has further increased its investment in dedicated radial shaving machines and special razor dressing equipment, creating favorable conditions for reliably solving concave problems and improving gear accuracy.