Tip-to-root interference is a result of:
- Lack of tip (or root) relief – This can be due to poor design of the gear micro-geometry, or manufacturing error. In either case, there is not sufficient tip-relief to account for tooth natural tooth deflection and assembly tolerance. Figure 1 shows two LSS gears from different turbines at the same wind farm. In the left picture, there is no tip-relief and the edge is sharp to touch. In the right picture, tip-relief has rounded the edge. This variation indicates a manufacturing issue.
- Misalignment – Even without specific design or manufacturing error, operational conditions mean the meshing contact is unlikely perfect. An issue leading to misalignment may be enough to cause tip-to-root interference. Misalignment can be caused by high loading events, incorrect assembly and inadequate shaft support (due to spacers, lock-nuts, bearing damage), amongst other things. Nonetheless, such misalignments should largely be accounted for in the design process.
As the name implies, tip-to-root interference involves the tip of one gear tooth contacting and damaging the root of another. The damage at the tooth root will likely be most apparent and is of more serious concern. It will initially appear as a single, long horizontal grey-black line along the tooth root. This may have some depth, and appear as a scored trough. If it progresses, micropitting will form, either within the undercut trough on the tooth root, or just above it. In a severely progressed stage, point surface origin (PSO) macropitting may also form. This involves the formation of arrowhead shaped pits which fan out in the direction of gear sliding contact, above the root damage.
The mating tooth tip may appear plastically deformed, or appear scratched or gouged (see abrasive wear).
Figure 2 shows two mating gears where tip-to-root interference has occurred. The matching damage pattern between the tip of one gear tooth and the root of the other is clearly identifiable. In this case, as discussed above, the tip looks abraded while a wide band of micropitting has formed above the root scoring.
Tip-to-root interference may or mat not be progressive and needs to be treated on a case-by-case basis.
Progression of the wear depends on the contact pressure at the point of interference. If the initial undercutting and scoring of the tooth root results in an improved contact where this pressure is reduced, it is possible the wear will arrest. In such cases, only light markings would be produced and the gears would likely continue to operate normally for a long time.
Progression is always a strong possibility and any damage should be monitored closely. If the undercut becomes deeper, material will be removed and the high contact pressure will promote the development of micropitting. If micropitting develops at or above the wear line on the tooth root, then it becomes a cause for concern. This will be progressive. Damage along the tooth root will also act as an initiation point for point surface origin (PSO) macropitting.
Tooth bending moments are highest at the tooth root, where tip-to-root damage occurs. Surface damage which leads to stress concentrations in this area will therefore greatly increase the chances of tooth fracture. Any micropitting or macropitting which forms at the tooth root is therefore of more serious concern than it would be in other locations. It will lead to failure and mitigating action should be taken to ensure consequential damage is minimised.
As tip-to-root interference is primarily the result of design and manufacturing error, it may be a serial defect affecting multiple assets. Upon detection of tip-to-root interference on one asset, an investigation to establish the root cause should therefore be conducted. The development of a site-wide inspection strategy in case of serial defect may be deemed appropriate.
|Visual inspection||✓✓✓||Tip-to-root interference wear should be visible to the naked eye, particularly along the root at the start of the active profile. Markings on the tip of the mating gear may also be visible.|
|Borescope inspection||✓✓✓||The visibility of tip-to-root interference wear will be improved by use of a borescope. This will also allow inspection of the planetary stage gears.|
|Vibration analysis||✓✓||If it does not affect all teeth, tip-to-root interference may be detectable by vibration.|
|SCADA data||✓||Tip-to-root interference will not be detectable from SCADA data.|
|Oil debris sensor||✓||Tip-to-root interference does not cause substantial enough material removal to be detectable. A debris sensor will provide warning once it develops, although it will not indicate the source of the wear.|
|Oil sample analysis||✓||Tip-to-root interference does not cause substantial enough material removal to be detectable in early stages. Oil analysis will provide warning as it develops, although it will not indicate the source of the wear.|
Ensuring a good contact pattern during design (see edge loading) is paramount for avoiding tip-to-root interference. Doing this involves careful microgeometry modification. Microgeometry is the term used for very small modifications to gear teeth which can nonetheless have a large impact on the performance. In the case of avoiding tip-to-root interference, this involves tip-relief – removing just enough material from tooth tips to compensate for manufacturing error and operational deflections.
Romax has a strong background in wind turbine gearbox design and analysis, and our RomaxWIND software enables easy optimisation of gear tooth microgeometry.