GEAR FAILURES TUTORIALS
What are the types of gear failures?
Gearboxes, in general, are subject to a number of problems during their operation. These problems can be enhanced or accelerated if faulty situations happen. Faulty situations are undesirable conditions that may arise for various reasons.
For example, the lubricating oil leaks and the oil level drops, or improper oil is used. As a result of any of these situations insufficient lubrication and cooling takes place and parts become heated or hot spots develop, or other damage can occur. Any damage can lead to tooth breakage and the permanent failure of a gearbox.
There are two particular issues with wind turbine gearboxes. The first one, as mentioned earlier, is that in wind turbines the gearbox is not used as a speed reducer, as is the case in the majority of applications. The second issue is that, because of the nature of wind, the gearbox is subjected to frequent and sudden changes of the power it handles. Such power fluctuations, which in turn translate to load variation on the teeth, is not a desirable situation for the gearbox.
In this sense, the chances of damage and failure in wind turbine gearboxes are more than in many other applications. Moreover, changing a gearbox in a wind turbine is extremely expensive, since the operation is not on the ground. In summary, the failure in a wind turbine gearbox can be due to (a)
design and manufacturing, (b) mounting and usage, or (c) operation and maintenance.
Poor or wrong design defi nitely can lead to problems during operation. Similarly, defects in manufacturing create unnecessary stresses, wear, heat, and fatigue. In mounting a gearbox, alignment with the rotating shaft , in both the input side and the output side, is absolutely necessary. Alignment implies that the axes of two mating gears are completely parallel to each other.
Misalignment causes the teeth to go under fluctuating stress, meaning that the stress in parts of a tooth continuously changes from compression to tension and vice versa. Th is is called fatigue and, compared to a part under the same magnitude but constant stress, can tremendously decrease the useful life of a machine part.
Proper mounting of a gearbox implies taking care that the alignment remains within the recommended tolerance. Bad usage implies if a wrong gearbox is selected for an application; for instance, when a gearbox cannot handle the power or cannot transfer the torque it is applied on.
This leads to overload on the gear teeth.
Examples of faults in operation were just mentioned. Th ese are essentially based on the lubrication oil, overheating, and malfunctioning of an associated device such as a pump that must circulate the oil.
Other examples are contamination of oil, say if humidity gets to the oil; and if the oil fi lter clogs and oil pressure drops. Faults in maintenance refers to any mistake concerning prolonged delays in oil change, the wrong oil type, and so on.
Typical gear damage or failures are:
1. Fracture
2. Bending
3. Wear
4. Fatigue
5. Cracking
6. Scuffi ng
Fracture implies that a piece of tooth breaks apart. In such an occurrence, not only does the tooth with the broken part become weaker, the broken part has a grinding action on all other parts, because it can move around with oil into all the other teeth, until it is stopped by the oil filter.
Gear tooth bending is a deformation of teeth in the direction of load. Note that when two objects (here the gear teeth of the two meshing gears) push each other, the force on one is the opposite of the force on the other (see the discussion on force and stress).
This can happen due to overloading and/or high temperatures. High temperatures can weaken a metal part in terms of lessening its strength to loads. Thus, the part can bend or deform more easily.
Wear is normally in the form of abrasion, where two matching parts can grind against each other. In the case of two gear teeth, this happens if there is not suffi cient room for smooth rolling motion between the teeth. Th is can also happen due to contamination and debris in the oil. A physical abrasion action is normally called erosion, and if it happens due to chemical reactions (acid in the oil, for instance) it is called cavitation.
Fatigue, as mentioned earlier, occurs when a machine element is subject to frequent change in the applied force. As a result, the part is forced to compress and then pulled to extend. Consider a gear tooth that is pushed up, then pushed down, on a permanent basis. Th is causes fatigue. Vibration in gears causes such a phenomenon.
Fatigue can occur in the tooth roots due to the bending of teeth (which can be called bending fatigue), or it can happen at the points of contact where two teeth press each other and then are relieved. Note that we also have thermal fatigue, which happens in parts of devices that are subject to temperature fl uctuations (becoming very hot and then very cold, repeatedly). Th is does not normally happen in gears.
Cracking is self-explanatory. It is the starting point for eventual fracture, and it is due to localized stress. Cracks can start under the surface, and eventually expand to the surface at different parts of a tooth body.
Scuffing is the term used when tiny pieces of metal tear from one gear tooth and due to localized heat adhere (weld) to the body of the matching teeth. Th is causes the surfaces to become rough, instead of being smooth, which in turn causes the development of more stress in the teeth and vibration in the gears.
No comments:
Post a Comment