Heavy-duty high-precision rotary tip and application on gear hobbing machine
Abstract: A heavy-duty high-precision rotary tip with double-row cylindrical roller bearings has been developed. The coaxiality of the tip relative to the tail cone is ≤0.005mm, and the radial taper of the tip cone is ≤0.003mm, which solves the problem of high-precision machining during heavy-duty machining. The top is easy to study and the rotation accuracy is low. The application effect on the P60S gear hobbing machine shows that the tip can significantly improve the tooth accuracy of the hobbing process .
1 Introduction
The tooth accuracy of the large helix angle and the oblique herringbone gear is an important precision index for determining the contact length of the tooth length direction and the stability of the contact zone when meshing. In hobbing, the radial runout of the tip of the machine tailstock is a key factor affecting the tooth accuracy of the machined gear. As a class of machine tool accessories, the top has formed a standard series of specifications. According to its structural form, it can be roughly divided into two categories: fixed tip and rotary tip. The fixed tip structure is simple, the precision is high, and the bearing capacity is strong. It is mostly used in the rotary cutting process where the positioning accuracy is high, but the phenomenon of grinding and pulling is easy to occur when using, thereby destroying the positioning accuracy. The high-speed rotation performance of the rotary top is good, it is not easy to grind and the positioning is reliable, but the positioning accuracy and carrying capacity are not as good as the fixed top, and it is mostly used in high-speed, low-precision rotary cutting. The P60S gear hobbing machine is mainly used for machining heavy-duty high-precision gears with low working speed and high machining precision, and the structural size and weight of the machined gear are very large. Therefore, the tip of the machine tool should have the combined advantages of high precision and high load capacity. To this end, we have developed a heavy-duty, high-precision rotary tip and successfully implemented the machining application on the P60S gear hobbing machine.
2 Peak radial runout and support stiffness analysis
The structure of the heavy-duty high-precision rotary tip is shown in Figure 1.
Analysis of factors affecting top radial runout
Bearing structure
Since the front support accuracy of the rotary tip has a greater influence on the tip radial runout than the rear support accuracy, the front support accuracy should be considered when designing the tip. Most of the ordinary rotary tops adopt a single-row ball bearing structure, and the support form can be simplified to two-point support (see Figure 2a). The relationship between the radial runout error d of the front support and the tip radial runout y is
y=(a+b)d/b
The heavy-duty high-precision rotary tip front support adopts a double row cylindrical roller bearing structure. Since the roller and the raceway are in double row line contact and have sufficient support width, when the span/shaft diameter ratio is less than 3, it can be regarded as Three points of support (see Figure 2b). The relationship between the radial runout error d of the front support and the tip radial runout y is
Y≈d (2)
Obviously, the use of a double row cylindrical roller bearing structure can reduce the influence of the support point error on the tip radial runout.
Bearing preload force
The radial runout error of the slewing front support is
d=dt+dj+dc (3)
Where: dt——elastic deformation of the rolling element and the inner and outer rings of the bearing
Dj - radial clearance of the bearing
Dc——the radial runout of the rolling elements and the inner and outer rings of the bearing
The double row cylindrical roller bearing adjusts the radial clearance of the bearing by moving the inner ring of the tapered bearing on the shaft diameter of the tapered surface. In order to achieve radial preloading, the radial clearance of such bearings is usually taken as a negative value (ie with an interference fit). When the pre-tightening force is appropriate, the two errors of dt and dj can be made zero, so that d=dc. Therefore, the radial runout of the tip can be effectively reduced by selecting a high-precision bearing with an appropriate preload.
Bearing orientation assembly impact
When assembling the top bearing, the directional assembly method should be used as far as possible, that is, the maximum radial runout point of the inner ring of the bearing and the maximum deviation point of the bearing axial line are placed in the same axial section, but in opposite directions, so that the final radial direction of the tip The minimum beating, ie
Dmin=dzj+dnj (4)
After the no-load static load processing
Scheme I Fine grinding ≤0.005 ≤0.005 ≤0.007
Scheme II Fine grinding ≤0.005 ≤0.005 ≤0.005
Rough grinding ≤0.005 ≤0.005 ≤0.005
For the same reason, the same method can be used when assembling the outer ring of the bearing.
Influence of the matching precision of bearing outer ring and hole
The inner bore of the double row cylindrical roller bearing is a tapered bore and can be mounted directly on the tapered shaft. The interference is the radial dimensional expansion caused by the axial movement of the inner ring on the tapered shaft. The amount is produced. To this end, the bearing outer ring and the hole are designed as a transition fit (Scheme I), but it is found that the tip made according to this scheme has a large change in the radial runout after finishing (see Table 1), and there is no tightness in the rotation. Uniform phenomenon. According to analysis, this is due to the use of heavy preloading in order to make the tip have sufficient rigidity and precision. Due to the heavy pre-tightening, the inner ring of the bearing is deformed too much, so the bearing tip is easily subjected to plastic deformation and wear when subjected to a large load, thereby destroying the accuracy. To this end, the design scheme was changed to make the bearing outer ring and the mounting hole have a small amount of interference without pre-tightening (Scheme II), and the 70°C temperature difference adjustment method was used in the actual assembly. The tip made by this solution has no change in the radial runout after the two processes of roughing and fine grinding (see the table above). By reducing the deformation and wear of the bearing, the working life of the tip can be significantly improved.
Support stiffness analysis
The roller of the double-row cylindrical roller bearing used in the heavy-duty high-precision rotary tip is in line contact with the track. Under the same load, the roller-to-rail compressive stress is smaller than that of the ball bearing, and the support rigidity and bearing capacity are remarkably improved. In addition, the tip uses a moderate pre-tightening process to reduce the amount of elastic deformation after loading.
The technical index of the heavy-duty high-precision rotary tip is as follows: the coaxiality of the tip relative to the tail cone is ≤0. 005mm, the radial runout of the top cone is ≤0.003mm, the minimum working speed is 4r/min, and the maximum weight of the workpiece is 1000kg.
3 application effects
On the P60S gear hobbing machine, the burr is processed with the tip of the radial runout of 0.01mm and 0.005mm, and the gear tooth error of the machined gear is detected by the CZE1200 gear checker.
According to the analysis and test results, when the burr is used for the burr of the radial runout of 0.01 mm, the tooth-tooth line of the gear is irregular one week, and even several irregular fold lines are formed. This is because the top radial runout is large, so that the tool-work cutting area (meshing point) changes continuously during machining, which directly affects the rotation precision of the machined gear axis, making the gear tooth direction irregular, resulting in tooth tooth error. The tolerance is different, which affects the proportion of the contact area in the direction of the tooth length. When the heavy-duty high-precision rotary tip with a radial runout control in the range of 0.005mm is used for hobbing, the toothed yaw direction of the machined gear is the same, and the toothing error value can meet the specified machining accuracy requirements.
The processing practice shows that the positioning accuracy and load carrying capacity of the developed high-precision rotary tip can meet the processing requirements of semi-finishing hobbing and fine hobbing on heavy-duty high-precision gears on the P60S gear hobbing machine.
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