Abstract By testing tooth profile accuracy and mechanical properties of 3-point feeding glass fiber reinforced plastic gears, combined with simulation results of MoldFlow, reasons for low meshing accuracy of gears and poor consistency of gear strength are analyzed. In response to this problem, a design scheme using central feeding was proposed to realize uniform arrangement of glass fibers. Results prove that this method can effectively improve meshing accuracy of glass fiber reinforced plastic gears and consistency of gear strength.

Compared with non-filled plastic gears, glass fiber reinforced plastic gears have advantages of high temperature resistance and high strength. They are widely used in automotive turbocharged throttle adjusters, electronic parking gear boxes and electric power-assisted brake systems. Glass fiber reinforced plastic gear adopting conventional 3-point feeding scheme has problems of low meshing accuracy and large strength difference between teeth, which causes transmission system to be noisy and has a short service life, which limits popularization and use of glass fiber reinforced plastic gears.
Taking a glass fiber reinforced plastic as an example, tooth profile accuracy and mechanical properties of a 3-point feeding glass fiber reinforced plastic gear are tested. Combined with results of MoldFlow simulation, reasons for low meshing accuracy of gear and poor consistency of gear strength are analyzed. Design scheme of adopting central feed is proposed to solve problem of uneven glass fiber orientation, result proves that method is effective and feasible.

Research object is a large gear in a glass fiber reinforced plastic double gear, material is PA66 Leona 1300G (glass fiber content 33%), its modulus is 0.7 mm, pressure angle is 20°, number of teeth is 63, tooth profile accuracy is required For JGMA level 4, initial mold gate design plan uses 3 uniformly distributed point gates for feeding, as shown in Figure 1.

 

Figure 1 Three-point feeding scheme
Based on 3-point feeding scheme for mold making and injection molding, double-sided meshing instrument is used to detect meshing accuracy of gear teeth near marked gate. Measurement result is shown in Figure 2. Single-tooth meshing error is 24.9 μm, accuracy is 4 JGMA-S, full-tooth meshing error is 80.1 μm, accuracy is 5 JGMA-S. It is found that meshing curve shows 3 crests and troughs, trough is at gate position, and the crest is between 2 gates.

 

Figure 2 3-point feeding double-sided meshing measurement results
Universal testing machine is used to test bending strength of gear. Test temperature is 25 ℃, and compression speed is 5 mm/min. Testing machine is shown in Figure 3. Stroke-load curve of gear under compression is shown in Figure 4. Maximum value during test is taken as gear breaking force. Measure bending strength of each gear in a clockwise direction starting from marked gate, record breaking force value of each gear separately, as shown in Figure 5. It can be seen from Figure 5 that flexural strength of each gear differs greatly. Flexural strength of gear at position of gate and joining line is relatively low, it changes periodically with position of gate. Average value of flexural strength of gear is 691. N, deviation between maximum value and minimum value is 164 N.

 

Figure 3 Gear bending strength test

 

Figure 4 Stroke-load curve

 

Figure 5 Measurement results of bending strength of 3-point feeding gear

In order to analyze reasons why 3-point feeding scheme affects tooth profile accuracy and gear strength, MoldFlow software is used to simulate and analyze injection molding process. Results of warpage deformation analysis are shown in Figure 6. From analysis results, it is found that warpage deformation results of mold flow analysis are consistent with actual results, warpage deformation values near gate and bonding line are quite different. Further analysis of forming mechanism that causes uneven shrinkage, analysis of glass fiber orientation results, as shown in Figure 7, can be seen from Figure 7, there is a big difference between orientation of glass fiber at gate and bonding line, showing a radial distribution from gate as center to periphery, resulting in a larger shrinkage rate at gate, a disorderly arrangement of glass fiber at junction line, so shrinkage rate is smaller.

 

Figure 6 Results of warping deformation of tooth tip circle with 3-point feed

 

Figure 7 Orientation results of glass fiber at 3 points
Arrangement of glass fiber will also have a greater impact on bending strength of gear. Analysis results of orientation tensor of glass fiber on gear at different positions are shown in Figure 8. It can be seen from Figure 8 that orientation of glass fiber on gear at gate and bonding line is relatively close, corresponding gear bending strength is low, gear glass fiber orientation at position between gate and bonding line is quite different. Corresponding gear has higher bending strength, influence of specific glass fiber arrangement direction on gear bending strength remains to be studied.

 

Figure 8 Analysis results of glass fiber orientation tensor on gears at different positions of 3-point feeding

Practice has proved that method of adjusting injection process cannot solve problem of inconsistent gear meshing accuracy and gear strength caused by uneven glass fiber orientation, changing gate design may be effective. In order to improve uniformity of glass fiber orientation, a design scheme of center feeding is proposed, as shown in Figure 9.

 

Figure 9 Design scheme of center feed
In order to verify effectiveness of scheme, MoldFlow software was used again to simulate molding process of center feed. Results of warpage deformation analysis are shown in Figure 10. From analysis results, it can be seen that uniformity of warpage deformation of gear with center feed is much better than that of 3-point feed. Further analysis of glass fiber orientation results, as shown in Figure 11, it can be seen from Figure 11 that glass fibers are evenly arranged in radial direction of gear with gate as center, so shrinkage is uniform. At the same time, because glass fiber orientation is consistent, it is predicted that bending strength of each gear is consistent.

 

Figure 10 Warpage deformation results of center feed

 

Figure 11 Orientation result of center-feed glass fiber

Based on central feeding scheme for mold making and injection molding, double-sided meshing instrument was used for meshing accuracy detection. Measurement results are shown in Figure 12. From Figure 12, it can be seen that meshing accuracy has been greatly improved, single-tooth meshing error is 9.2 μm, accuracy is 2 JGMA-S, full-tooth meshing error is 34.5 μm, accuracy is 2 JGMA-S. Compared with 3-point feeding scheme, tooth profile accuracy is greatly improved.

 

Figure 12 Measurement results of double-sided meshing of center feeding scheme
Gear bending strength test is carried out, test results are shown in Figure 13. From Figure 13, it can be seen that average value of gear bending strength reaches 737 N, deviation between maximum and minimum values 98 N, which is different from 3 point feeding. Compared with scheme, gear bending strength and consistency have been greatly improved.

 

Figure 13 Measurement results of bending strength of center feed gear

By testing tooth profile accuracy and mechanical properties of glass fiber reinforced plastic gears with 3-point feeding, reasons for low gear meshing accuracy and poor gear strength consistency due to uneven glass fiber orientation are analyzed. In response to this problem, a design scheme with a central feed was proposed to realize uniform arrangement of glass fibers.