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Section 5b: Developing the Contact Pattern Through Computer Modeling - Details of the Process |
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2) Understanding Contact Pattern and Gear Displacement 3) Conventional Methods for Contact Pattern Development 4) A New Method for Contact Pattern Development 5) Developing the Contact Pattern Through Computer Modeling: 6) Duplication of Operating Conditions with Universal Load Testers A Case Study of the PW6000 Project 8) Troubleshooting and Failure Analysis 9) Contact Arrow's Design Engineering Team
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In this section, we will present the details involved in the process of designing the contact pattern through computer modeling and how the software integrates with the machine tools. To begin, FIG. 5 is summary printout of a TCA study. This particular TCA is from the PW6000 upper tower or PTO (Power Take-Off) gear set. For the purpose of illustration, we will be looking at the concave side of the gear and addressing the loaded TCA phase of the design work, were the various displacement conditions were accounted for. In FIG. 6 you can see the contact pattern design that was created to meet the load requirements and the different displacements that the gear sets would encounter. The different
displacements which were presented are as shown in FIG.
7. At 3,140 inch pounds of torque, the pinion moves above the gear
by 13 thousandths,
The objective was to design a contact pattern that would have an acceptable shape and size - as well as never running off the ends, bottoming or running off the top lands while taking into account the different displacements the gears would experience under normal operating conditions. The contact pattern that was designed then met those requirements as shown in FIG. 7. Next to each requirement, you can see what the loaded TCA design contact pattern would produce as a contact. If you were to take all these contact patterns and overlay them or combine their areas of contact, you would, in essence, have a depiction of what the load zone will be for this gear set while it is in operation and encounters all of these different displacements at 3,140 inch pounds of torque. In addition to each one of the different displacements on the contact study, the study will also look at the various pressures that are occurring along the path of engagement. In FIG. 8, as the tooth comes into mesh, the path of engagement starts at point A. It then rolls through mess and exits at point B. Given a load of 3,140 inch pounds of torque, the table in FIG. 9 shows what the surface pressure is at the start of engagement all the way through to the end of engagement. You’ll see that the pressures at the start of engagement are low, which is a result of tooth sharing - due to the high contact ratio. The pressures then start to climb, and will reach a peak of 238,000 pounds per square inch in the center of the tooth. The pressures will then diminish – finally falling to nearly 84,000 where this tooth has exited from mesh. A key objective of this study in FIG. 9 is to verify that there are no hard spots occurring in the pattern. Hard spots would show as a spike in these surface pressure values. If a spike in these surface pressures is present, there is a strong indication that a failure mode may be present. As the teeth would come into mesh, the spike or ledge would create a nonuniform pressure, potentially causing pitting and subsequent failure. However, in this example, the pressure values do not include any spikes. There is a gradual increase to the center of the tooth, followed by an equally gradual decrease. These gears will move in and out of mesh very smoothly. This study will be performed for all of the displacement conditions, and when complete, the design will be ready for Finite Element Analysis. The results of the loaded TCA are first downloaded to the T-900 Finite Element Analysis software. The program then performs a real stress analysis of the tooth surface. A report is generated (FIG. 10), which through the use of different colors, shows the load distribution along the different areas of the tooth. In areas where you have heaviest contact on the tooth, you see a red area, and then as the stresses become less, the colors change and continue out until you have a base load which is the lowest surface pressures or stresses that will be seen on the gear tooth. A similar study is then performed on the root fillet (FIG. 11). Again, the varying levels of stress are indicated by different colors. On the reports for both the tooth surface and the root fillet study, a bar graph is generated (FIG. 12) which specifies the corresponding pressure value. If the maximum value exceeds the rating of the material being used, there is a high potential that the gear will fail. Another insight that is provided by the Finite Element Analysis is the potential for ledges or edge contacts. As was mentioned before, a red area is an indication of the highest pressure. If the study indicates any red areas outside the center of the contact pattern, it would suggest that a failure might occur in these areas. If the Finite Element study reveals any problems, the engineer can then go back to the CAGE software and modify the contact pattern as needed – and then perform a secondary Finite Element analysis. Once the TCA and Finite Element studies are performed, and the ideal tooth contact pattern size and location is achieved, the CAGE software creates the summary settings required by the Phoenix Cutters and Phoenix Grinders to machine the parts. In addition, the GAGE software is used to generate the inspection file for the Zeiss Hofler CNC inspection system. Using the Zeiss Hofler, electronic digital topographical plotting of the tooth surface is performed and the G-AGE software automatically changes the machine settings to match the computerized tooth shape desired. Through a hard wired network connection, both the summary settings and the inspection file are downloaded. Following all development activities, the production process begins. |
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Section 5b: Developing the Contact Pattern Through Computer Modeling - Details of the Process |
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the pinion going into mesh 13 thousandths and the gear
out of mesh by 12 thousandths. There is another set of circumstances where
it is nearly 2 thousandths offset, pinion into mesh almost 23 thousandths
and the gear is near stationary. We have another set of circumstances
where there is nearly 2 thousandths worth of offset, pinion into mesh
almost 29 thousandths and the gear into mesh 18 thousandths.