Minimising the contamination of gearbox lubricants
After studying a wide variety of gearboxes, conducting inspections and failure analysis, we found that there are three aspects that primarily affect the life of your gearbox – Assembly cleanliness, Design of the gearbox and Lubrication system. If these three aspects are properly handled, your gearbox will operate smoothly for years.
Understanding contaminants
Solid contaminants that enter your machines vary in friability, hardness and ductility; these qualities depend on their composition and they determine the amount of damage these particles will cause in your gearbox. Contaminants are rated on a scale of 1 to 10 (With 1 being brittle and 10 being the hardest), also known as the Mohs hardness rating. Diamond has the hardness rating of 10 as it is the hardest known material right after Aluminium oxide and Silicon carbide, which have a hardness rating of 9. Dust from the environment, which mostly contains Silica has the hardness rating of 2 to 8. Dust from Quarry has a hardness rating of 5 to 9. The steel from tools used in manufacturing processes has a rating of 6 to 7.
How do contaminants enter your gearbox:
- During its assembly
- Generated internally
- Through breathers and seals
- During maintenance
Minimizing built-in contamination
Built-in contamination can permanently damage the gears and bearings in just about 30 minutes during a run-in. The filter won’t help as the contaminants have already entered the gearbox during assembly; hence it is important to diligently clean the gearbox before assembly and run-in. There are plenty of contamination sources to eliminate long before you put your gearbox to use.
Paint the interior of gear housings with white epoxy sealer so that you have an easy to clean, hard, smooth surface. All the components that are to be assembled should be stored in a clean, dry environment away from the clean room. The final assembly and cleaning should be done in the clean room. Ensure that all components are rust-free and clean before assembly. Pay close attention to cavities like bolt holes and oil passages, they are likely to have dirt in them. Make sure that it is assembled in a clean room that is separate from other manufacturing processes like grinding, machining, deburring or welding. Seal all doors and windows; and include a filter in the ventilation system so that you have a draft-free, clean air. Seal the floor, have a painted ceiling and clean all the dust from it.
The technicians that carry out all this work should wear PPE’s and dust proof clothing. They should be adequately trained and skilled. Use tools that are - calibrated properly, appropriate for gearbox assembly, clean and in good condition.
Minimizing internally generated contamination
Wear debris from bearings, gears, splines or other parts of the gearbox are contaminants that are generated internally during its functioning. They may be generated due to abrasion, adhesion, fretting corrosion, micro-pitting or macro-pitting.
Micro-pitting is the most common source of internal contamination in your gearbox. It reduces the accuracy of gears, makes them noisy and it tends to develop additional issues in your gearbox like macro-pitting, bending fatigue or scuffing. If your lubricant gets contaminated with a large number of iron particles then your gearbox has micro-pitting.
Minimizing internally generated wear debris requires the use of smooth surfaces, surface-hardened splines and gears; use lubricants with high viscosity. You can prevent fretting corrosion by nitriding and force lubricating the internal and external spline teeth. The Annulus gears should be nitrided or carburized because if you get them through-hardened, the gears will become relatively soft and they would generate wear debris.
Have a separate lubricating system for wet clutches and brakes to prevent the gearbox from their gear debris. Do not park your wind turbine for an extended period of time, they are prone to develop fretting corrosion on splines, bearings and gear teeth.
Selection of a suitable lubricant for the gear box is also an important task as improper selection of the lubricant can lead to wear and shearing of the additive base. This can lead to the ingress of fine particles in the gear box, which can add to the contamination levels.
Minimizing ingressed contamination
Air within the gearbox expands and contracts periodically due to normal heating and cooling, this creates pressure inside the gearbox which is regulated by breathers. These breathers should be fitted with a good online filter and desiccant to prevent the ingestion of dust and moisture. Any ingressed contamination should be removed with the selection of online filters, which keep the internal environment clean. If the environment is especially harsh, seal the gearbox completely with an expansion chamber inside it that has a flexible diaphragm to regulate the pressure variation. Some heavy gear boxes also use accumulators which help in preventing sudden drop of pressure in the gear box due to line clogging.
Use V-ring seals rather than the commonly used labyrinth seals, because labyrinth seals allow containments to enter the gearbox. V-ring seals are effective but they should be protected using metal shields.
Maintaining the level of lubricant in the gear box to predefined intermediate levels will reduce the load on the lubricant and it will support to minimize the ingress of contamination due to wear of moving components.
Minimizing contamination added during maintenance
The location and design of breathers, drain ports, fill ports, filters, dip-sticks, sample ports, inspection ports and magnetic plugs should ensure that their maintenance can be performed in an easy and safe manner. Design of the gearbox, work platforms, lubrication system and nacelle housing should also be taken into consideration as the maintenance of wind turbines is a difficult and risky job.
Proper housekeeping procedures are of utmost importance while performing the maintenance work on the gearbox. Connect a filter cart to the gearbox using quick-connect couplings to minimize contaminants that can enter the new oil.
Modern filters do not create large pressure drops, they are compact and they provide fine filtration which prolongs the life of the lubricant. Make sure that the lubrication system is properly maintained so that gears get an adequate amount of clean and cool lubricant.
These were four points that will help you keep your lubricants contaminant-free and your gearbox healthy for a long time. For better reliability of your turbines, make sure to use high-quality lubricants from reputed companies like Castrol. Additionally, you can establish a lubricant-monitoring program which will prevent your gears and bearings from malfunction by notifying you about the health of your machinery.