Moventas, an expert in wind turbine gearbox manufacturing, has been in the industry for 40 years and is passionate about innovating gearbox solutions that enable cost-savings & trouble-free operation. Over the past 30 years, starting from the very first Barkhausen system to the latest robotized system, Moventas has trusted their grinding inspection to Stresstech and their Barkhausen noise measurement systems.
As a key component in wind turbines, gearboxes need to adhere to extremely high requirements as large gearboxes come under extreme loads such as changing wind speed. Replacement of a gearbox is one of the most expensive maintenance operations wind turbines require and therefore the quality of the gearbox components must be flawless.
At Moventas, quality assurance is an integrated part of the daily operations and leadership of the company. All key manufacturing processes are performed in-house, and they are well known and sought after for their high-quality design and manufacturing.
Nowadays, wind turbine manufacturers require that surfaces are tested for the possibility of grinding burn. Grinding burn is a common name for thermal damages, which occur on the surface during grinding processes. Grinding burn causes local discolorations on the surface, it can soften or harden surface layers and cause unwanted residual stress.
Taisto Kymäläinen, Quality Manager at Moventas, recalls that in the mid-‘90s grinding machines were developed where the grinding speed increased dramatically, however with the development of new machines, it was possible to use a greater feeding rate thus increasing the risk of grinding burn.
Grinding Burn Detection & Process Optimization
Otto Lähdekorpi, Quality Engineer at Moventas, explains that Moventas runs a Barkhausen noise analysis on all ground surfaces with their gears and shafts. The measurements are done to newly manufactured components as well as components that are re-ground in rebuild purposes.
Moventas is an advanced Barkhausen noise analysis user and uses it beyond just sorting good samples to burnt ones. Taisto Kymäläinen explains that Barkhausen’s method allows for the early detection of damage, as Barkhausen noise analysis reacts in the smallest changes in a microstructure, therefore it can be used to optimize a grinding process to find correct grinding parameters. For example, the Barkhausen noise analysis can reveal flaws in cooling or grinding stone wear before actual burn appears.
Since the Barkhausen noise analysis is a comparative method, users need to determine acceptable levels for their products with the master sample procedure. The master sample procedure can be validated for example with X-ray diffraction measurements or nital etching. When the master sample procedure is set, Barkhausen noise analysis is an accurate method to detect microstructure changes.
For example, at Moventas they have set an acceptance to certain mp level (magnetoelastic parameter: RMS voltage of measured Barkhausen noise), when mp level is 20% more, you can already see the burn with the naked eye. Moventas used nital etching in their process to determine acceptable mp levels, however, Barkhausen noise reacts to grinding burn 10-20% before it can be seen with the nital etch.
The Barkhausen noise sensors require regular mastering procedures, the RoboScan has made it easy to do this as the master samples are measured hundreds of programs with the same results. If sensors need to be adjusted, the Roboscan allows the sensor to be run on the correct spot and adjustments can be made without fear of contact changes.
From Manual to Automated Grinding Burn Detection
Taisto Kymäläinen explains that over the years gear sizes have grown up to 600 mm tooth height and a manually operated Barkhausen noise measurement was becoming unproductive and unergonomic. A robotized Barkhausen noise system solved that problem, and now Moventas is using three XL-sized RoboScans to take care of the Barkhausen noise measurements.
With RoboScans, an operator does not have to be a Barkhausen noise expert as the system takes care of everything and the measurements are faster and more accurate. Otto Lähdekorpi recalls that measuring a gear with 180 tooths typically takes about two hours but would have taken twice as long with the previous manual measurement system in place.
The features and improvements provided by Stresstech’s Barkhausen noise measurement systems allow the measurement accuracy to remain at the highest level, ensuring Moventas can continue to provide and guarantee components that are of the best quality to their customers.