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Section 2: Understanding Contact Pattern and Gear Displacement |
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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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A critical attribute of a gear’s design is its contact pattern. Simply stated, the contact pattern is the area in which the gear teeth come in contact as they engage and disengage during their rotation. This area of contact is checked by the following procedure. The teeth are coated with a special marking compound and then run together in a tester. The area of contact can be seen in the disruption of the marking compound, and an experienced inspector is required to interpret the visual results. To document this contact, adhesive tape is then applied to the tooth surface and transferred to a piece of paper. (FIG. 1). When a gear is
installed in a gearbox and is powering the designated application, there
are
FIG. 2 shows the contact pattern from a gear with a very light load and a contact pattern from the same gear with a very heavy load. There is a general rule of thumb, which states that the heavier the load, the larger the contact pattern. Now here is where the issue of contact pattern becomes so important. For a gear to perform properly under load, the contact pattern must be a certain shape and at a certain location. Typically, an ideal tooth contact pattern should be an oval shape and be located centrally to the toe of the gear tooth (FIG. 3). There is another critical issue to consider when assessing how the contact pattern will perform in an operating gearbox. This issue is gear displacement. In the operation of many gearboxes, the gears and their shafts do not remain in a fixed orientation. Thermal forces and stress from being under load can cause significant movement of the gearbox components from their original position. There are typically four different types of movement which can take place – and these movements are described as Offset, Pinion in and out of Mesh, Gear in and out of Mesh, and Shaft Angle (FIG. 4). It is this movement that is referred to as gear displacement and it can occur in any combination of the four types. In aerospace gearboxes, where keeping weight to a minimum is a high priority, the mass of the gearing used is usually lighter, and these displacements can be significant. On the other hand, in commercial applications where the gearbox components are typically more rigid, there is not the same degree of displacement. |
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Section 2: Understanding Contact Pattern and Gear Displacement |
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varying degrees of pressure or load on the gear teeth. These
pressures include box deflections, bearing movement and temperature
changes. When the gear teeth
are subjected to these variables, the contact pattern will change.