Story ID: 
In-gearbox Sensor Detects Failures Earlier
By: Thierry Dubois
Caption: In EASA-funded research project, Cranfield University successfully tests crack detection system at full scale.
Word Count: 598
Researchers may have found a way of revealing damage inside a helicopter’s main gearbox sooner, thus possibly avoiding catastrophic failures such as the ones that crippled offshore rotorcraft between 2006 and 2013. A team from Cranfield University has managed to install a sensor inside the gearbox and transmit a signal to a receiver located outside.

The goal of the EASA-funded project was to improve the efficacy of today’s health and usage monitoring systems (Hums). “Hums was designed to prevent failures 50 hours in advance, but now we need to detect them just a few hours in advance,” Matthew Greaves, head of the safety and accident investigation center, told AIN. Early detection of failures is precisely the reason to locate a sensor inside the gearbox, rather than outside, where existing vibration sensors are located.

But the idea presents formidable challenges. The gearbox is not a hospitable place: it is hot, packed tightly with moving parts and awash in mineral oil. Greaves’s team found that “acoustic emission”–the capture of high-frequency surface stress waves–is the most suitable sensing technology. “The potential of this technology has increased dramatically over the last 10 years thanks to improvements in sensors and data acquisition,” he noted.

The researchers decided to focus on monitoring planetary gears and bearings. This is considered to be the most complex target, Greaves explained, so any monitoring solution that can be applied successfully to these elements stands a good chance of effectively monitoring less complex components, such as bevel gear shafts.

The monitor used in the trials was a piezoelectric-wafer active sensor 7 mm in diameter and 0.2 mm thick. In the event that the sensor detaches, it can be “chewed” easily by the gearbox without damage, Greaves said.

The most recent test, conducted at full scale on an Airbus Helicopters test bench, used an SA330 Puma gearbox. “Although this gearbox is an older design, it was the basis of the current EC225 main gearbox and they share many features,” Greaves said. Most important for the project, it has a final two-stage epicyclic reduction using a combined planet gear/outer bearing race design. Airbus Helicopters provided technical support and access to its test benches for the trials, which were conducted in May and June last year.

Researchers tested three conditions for a planet bearing: undamaged, slightly damaged and heavily damaged. The defect lengths were 10 mm and 30 mm, respectively, with a depth of 0.3 mm depth in both cases. The rig was run at power settings of up to 2,360 shp, 110 percent of the maximum takeoff rating.

In preliminary results, the outer race defect frequencies and harmonics were clearly visible, according to Greaves. Meanwhile, the undamaged case contained no such harmonics. Greaves predicts that the system, which might benefit from using more sensors, could be flying on production helicopters in 2020.
Ultrasound May Enable Pinion Imaging

Data Interpretation, a small UK-based company, is promoting an ultrasound method for monitoring epicyclic gear planetary pinions in the main gearbox. Founder Peter Sharp emphasizes that, unlike vibration sensing, the ultrasound scan technique yields images. Tested on an Aerospatiale Gazelle gearbox, the concept detected axial changes, as well as hunting and coasting.

“Our objective is to build a pre-flight advisory that, at the press of a button in the cockpit, will squirt a sequential stream of pulses at each pinion engagement in a ‘ring-good’ test,” said Sharp. He suggested his imaging system might complement existing Hums technology, estimated to be only 70 percent effective. Sharp is looking for investors.