New yaw ring repair method makes wind turbine lifetime extension viable


  • Patented system makes yaw ring repairs economically feasible 
  • Danish company CNC Onsite’s method can extend wind turbine life 
  • Repair versus replacement represents a significant CO2 reduction 
  • New tool is portable, compact and separable to allow in-situ uptower repairs  
                                                                 CNC control board

A new invention for repairing yaw rings in-situ is expected to make a major contribution to extending the lifespan of wind turbines. Developed by Danish machining expert CNC Onsite, the patented system means the yaw ring does not need to be replaced, eliminating the expensive, time-consuming and potentially detrimental disassembly of the rotor and nacelle. 

A crucial component in securing maximum power production from a wind turbine, the yaw ring, also called a “yaw gear rim”, is complex to replace. Yaw ring replacement costs are so

               Defekt yaw ring

high that broken or worn teeth can leave operators of older wind turbines with little choice other than scrapping them.  

“This inspired us to develop a repair method as an alternative,” Søren Kellenberger, Sales Director, CNC Onsite.  

“We can now offer a repair service for both onshore and offshore at a fraction of the cost of replacing the entire yaw ring, and that makes it viable to keep perfectly good wind turbines operating for longer,” he added. 

Patented portable precision tool 

The CNC Onsite method employs a portable precision machining tool to repair any broken or worn yaw ring teeth. Operated by a specialist engineer, the tool applies the patented method, working at extremely fine tolerances, to remove and reinsert machined teeth. The patent for the system covers the milling process by which the damaged teeth are excised, and the bed created for the new part, as well as its particular insertion method. 

“The process we apply when creating new yaw ring teeth for a wind turbine is similar in principle to a new dental crown that is first copied precisely then fitted by a dentist using precision tools,” Kellenberger said. “The aim is the same, and it should last for a long time.” 

                                                           Photo of patented milling machine

The machining tool has been designed to break down into component parts with a low weight that allows them to be transported in the tower elevator to the work area. After reassembly in the nacelle, the compact tool can be operated in the confined working space around the yaw ring. 

Not weather dependent 

The repairs are carried out inside the wind turbine tower so can be completed irrespective of weather conditions.  

“As long as it is safe to travel to and access the wind turbine, we can carry out the repairs. So there are far fewer days when we cannot work. This is also good for both work schedules and costings,” Kellenberger explained.  

                             Repair team on way to offshore wind farm

Scale of the problem 

Mounted at the top of the wind turbine tower, the toothed yaw ring is a gear that engages with motors mounted on the nacelle to align the rotor blades with the wind. CNC Onsite estimates that turbines on some 5 to 10 percent of wind farms will experience damage to their yaw ring teeth during their service life. Typical causes include unpredictable wind events or uneven loads sustained over time. 

Replacing the yaw ring requires the entire nacelle to be detached using a crane and specialist labour – a process that is expensive for onshore turbines and perhaps uneconomic for offshore. Across the lifetime of a wind turbine, maintenance can represent up to a quarter of all costs incurred, and decisions such as choosing a cost-effective yaw ring repair versus replacement are set to become an important trend. 

CO2 savings 

The system developed by CNC Onsite can usually carry out yaw ring repairs within a few days. This not only reduces downtime, but results in significant CO2 savings too. 

“We’re eliminating the need for manufacturing a new yaw ring and above all the huge logistical effort required to transport a yaw ring to the site, deploy cranes, which is particularly tricky offshore, and replace it,” Kellenberger said. 

“Such an operation requires a significant number of people and a lot of equipment with all the  associated CO2 emissions. With our repair method, this is no longer required,“ he added. 

“The fact that whenever you take down the nacelle there is a potential risk of damage to it and especially to the blades which is another factor that cannot be ignored,” Kellenberger concluded. 

The yaw ring repair service offered by CNC Onsite has already been used on a range of  turbines in wind parks, both offshore and onshore, since it entered the market in 2019 following many months of endurance tests, proving that the replaced teeth are robust over time.

The patented process and technical details 

The CNC Onsite’s patented method consists of three steps: 

  • Milling away the damaged yaw ring teeth  
  • Cool down the replacement yaw ring segment 
  • Mount and fix the new segment in place  

The company’s milling machine tool, which is also patented and is designed to be attached to the yaw ring, is a full CNC (Computer Numerical Control) machine, producing the required very precise milling in the limited space. 

The milling machine is split into three assemblies for easy transport to and up the wind turbine.  

Milling process and pocket design 

The machining tool is mounted to and aligned with the wind turbine’s yaw ring and is preprogrammed with all relevant parameters like dimensions of yaw ring and teeth.   

Once mounted to the yaw, the milling machine automatically identifies and cuts out the damaged area precisely, creating a pocket for the replacement teeth. The milling machine is designed to work sideways – part of the patent – on either inward or outward-facing yaw teeth, not from above as often there is often not enough room within the tight nacelle area.  

                                                                    Several new segments mounted

The machined pocket has precise dimensions that fits the exact size of the new segment to be inserted. The shape of the pocket is designed to hold the replacement part in place through grooves and ridges. 

The pocket structure also includes threaded holes, which are used to fix the new part in the next step of the process. Additionally, several raised ridges are created in the pocket which allows the new segment, equipped with corresponding indents, to be inserted like a jigsaw puzzle. This mechanical design improves the mechanical strength and will help prevent displacement of the replacement teeth, both axially and tangentially.  

Cooling pre-produced part 

The new segment due to be inserted into the pocket is first cooled down to marginally reduce its size, aiding the fitting process. Once re-expanded in the pocket, this further adds to the mechanical strength of the repaired structure.  

Slotting and fixing new part 

The new part is slotted into the pocket by the technician who then fixes it in place, using bolts in the threaded holes, created during the milling process. 

The new pre-produced yaw ring part is produced in advance by CNC Onsite. The parts match the profile of the yaw ring for that particular wind turbine brand 

Precise machining tool 

CNC Onsite’s machine has extremely fine tolerances of a few hundredths of a millimeter. This is a huge advantage as it allows the new part to be fitted with great precision, which ensures longevity and reliability.  

If required, CNC Onsite can repair all teeth on a single yaw ring.  

Compared to more conventional methods like welding and grinding, no heat is introduced to the yaw ring, meaning that there is no risk of changes in material properties, as well as being intrinsically safer by avoiding “hot works” in a confined space.  

A successful weld repair depends strongly on the experience and skills of the welder whereas with the CNC Onsite method, the machining is done automatically, and the teeth are prefabricated to high tolerances. The result is a repair with an expected lifetime of 10 years.