A Software Solution for Gear Noise Problems

The development of a Continuous Variable Transmission, and the precision required by its gearing, was enabled by the professionals at LMS International. Here's the story.

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Finnish-based tractor manufacturer Valtra’s continued drive toward innovation remains imperative for its future success. With tractor sales falling worldwide in 2009, they must continue to meet customer’s growing demands in the face of stiff competition from the likes of John Deere and Case New Holland, while at the same time making inroads into emerging markets. For these reasons the company recently embraced the use of the Continuously Variable Transmission (CVT) for their vehicles. In tractors this transmission was originally adopted by vegetable and specialist growers who recognized its potential. Increasingly, CVT has entered the mainstream and is being taken on by arable farmers and contractors who appreciate its flexibility. It’s a “step-less” transmission that doesn’t require a gear stick freeing up more room for the driver in the cab, at the same time offering better fuel economy than conventional transmissions since the engine can run at its most efficient RPM at a range of different speeds. From an engineering perspective, however, CVTs present quite a challenge. Noise issues are more prevalent, and also more difficult to solve due to the infinite number of effective gear ratios and the complex structure of the CVT drive, which contains a hydrostatic unit and an open planetary gear. 

Interesting Developments
In October 2007 Valtra embarked upon the prototype phase for their first CVT. “This was a completely new product for us,” according to Totte Virtanen, a senior technical specialist for Valtra, “and we were determined to get it right. We had been using LMS tools for over 10 years, and it just so happened that LMS was visiting when we started work on the unit. We noticed that we were experiencing gear whine and began discussing with LMS whether we could work together to solve this problem. They already had experience in similar cases and had greater measurement, analysis, and simulation capabilities than we did.” Figure 1, Figure 1b

The first phase of the project involved Valtra carrying out engine run-ups on a test rig. Using LMS Test.Lab, Valtra employed any number of combinations, including driving the transmission ratio from idle to maximum in order to isolate the circumstances under which transmission noise occurred and understand when it was most severe.
Wim Hendricx, technical manager of engineering for LMS International and supervisor on the Valtra project, says “This was a good start, as Valtra diagnosed which gears were causing the problems and under what circumstances. What we at LMS then did was instrument the gearbox on the inside to examine those problematic situations more carefully and identify the underlying dynamic phenomena of the gear whine.”

LMS Engineering Services approached this by making an operational deflection shape and alignment analysis of the transmission to observe how the parts inside the transmission moved in different operational conditions. This included placing 15 three-dimensional acceleration sensors and 17 position sensors inside the test transmission. In the end, more than 70 transducers—sensors that can measure the position of the shaft and gear wheels at a micron level—were deployed. This let LMS measure the pseudo-static alignment and vibration of several shafts and gears in operational conditions and therefore find a correlation between the physical movement of a shaft, the excitation of internal resonances, and the noise generated by the transmission unit.

“What we focused on was the precise position of the gears in an operational gearbox,” Hendricx explains. “We needed to understand if the position of the gears changed as a result of the operating parameters. By using inductive sensors we could measure how far the gear meshing surfaces were away from the theoretical position. Based upon these measurements we were able to understand more accurately the correlation between the alignment parameters and the noise.” Figure 2

LMS engineer Daisuke Nagahata, who worked closely with Valtra, adds, “It was important that Valtra and LMS discussed upfront where the sensors should be placed. In the end those sensors provided us with very useful information, not only with regard to operational alignment change, but also about structural resonance. This let us further understand the noise amplification mechanism, and eventually it paved the way for a solution.”

Diagnosis by Simulation
Based upon these measurements, LMS now knew the real physical position of the meshing gears. The next stage was a simulation to calculate the transmission error (TE) of the meshing gear pair. For this LMS implemented precisely the same misalignment which was measured in the operational gearbox. “When we looked at the measurement results we found in certain conditions we had more misalignment and vibration amplification,” Virtanen says. Then, with those measurements and simulation results, we knew what the problem was—that there was a combination of structural resonances excited by a larger than expected transmission error instigated by gear misalignment.” Figure 3

In other words, the excitation of the gear unit was due to a misalignment of the output gear, resulting in the gear teeth contacting at an imperfect position, in turn affecting the bending of the teeth and increasing the ripple on the gear ratio, with this ripple generating noise. LMS diagnosed the underlying problem as being a relatively high movement of a particular shaft and proposed to add stiffness in the bearing supporting it. Valtra did just this, modifying the design, paying greater attention to tolerances, and changing the structure to minimize misalignment. Valtra then built a prototype based on these modifications, and the validation measurements that followed were conclusive: transmission noise was reduced by more than 3dB in critical conditions, cabin noise by 1dB, and cabin sound quality was also significantly improved.

Hendricx sees solving Valtra’s transmission noise as being very much a joint venture between the two organizations. “The added value of LMS engineering is that we went inside the gearbox to pinpoint precisely the source of the noise, but some of the work—particularly the measurement preparation—was done by Valtra. They modified their gearbox by identifying the locations where the transducers could be installed and then prepared a prototype by mounting all internal transducers at the identified locations. This was a big contribution, and something we could not have done on our own.”

The Bottom Line
With eight weeks preparation time undertaken by Valtra, three weeks of measurements, and 10 weeks of analysis and simulation, the whole project was completed in approximately five months during the summer of 2008. Totte Virtanen is in no doubt about the contribution made by LMS International in their quest to reduce transmission noise rather than simply insulate the sources. “We saved at least one test cycle with new prototype parts,” he says. “This equates to at least three months, and might well be a lot more. Time is money. We learned a lot through this project and now have much better capabilities to work with these issues.”

Valtra launched its continuously variable transmission in October 2008. As part of an industry that is forever looking to make improvements to the bottom line, the introduction of CVT into Valtra’s portfolio of options has already pleased their customers as well as making an impact on their own cost-savings. “We are very happy with the results,” Virtanen says. “LMS has unsurpassable skills in analyzing data, and we are most certainly interested in future cooperation.”