Gears are complex parts that play a vital role in automobiles, aircraft, construction and mining equipment, food processing, watchmaking, and more. And companies are still trying to make them better — specifically, quieter.
As electric vehicles become more popular, the industry is pushing for gears that have tighter and tighter tolerances—in other words, smaller differences between the maximum and minimum sizes in a batch of gears that are considered acceptable for sale. Gears that fit better, make less noise, transfer power more efficiently and last longer.
“These gear noises have always been there in gas cars, but electric cars are so quiet that now you can hear them over the engine,” said Dennis Everett, a mechanical engineer at the National Institute of Standards and Technology (NIST). “Consumers don’t like to hear these noises.”
Now, after a hiatus of more than 30 years, NIST is once again conducting ultra-precise gear-related measurements for customers, starting with a major American company that supports gear manufacturing.
As the national laboratory for metrology (the science of measurement), NIST has performed these services for decades when requested by clients. In the 1980s, however, aging infrastructure made it impossible to maintain the required accuracy. A few years later, another government laboratory, Y-12, operated by the Department of Energy, began offering this service with measurements traceable to NIST.
But then, a few years ago, Y-12 could no longer make these measurements with the required level of accuracy. The facility metrology industry found itself in a situation where if NIST couldn’t do the measurements, it had to go overseas to another government’s national metrology institute, which would be expensive and time-consuming.
“It’s important that NIST is maintained as a resource for US gear manufacturers to use,” said Mark Cowan, senior systems engineer for Gleason Corp., the US company that commissioned the calibration to NIST and which maintains an accredited calibration laboratory for gears, which itself serves dozens of customers a year. Calibration labs must keep the uncertainty of their measurements as low as possible so they can keep the uncertainty acceptably low for their customers’ measurements—in this case, the measurements of the equipment the gear industry uses to verify their products. “There’s also the advantage of being able to say that the measurements are traceable to NIST,” Cowan continued. “Companies like to hear that something is NIST traceable.”
So, in 2019, NIST’s Everett was tasked with restoring the old calibration capabilities. After trial measuring one specialized part, Dan Sawyer of Everett and NIST completed measuring two more for the corporation this month with higher accuracy than before.
Among its other products, Gleason Corp. produces and uses a part called a cog artifact. Facility artifacts are metal objects of various sizes and shapes whose dimensions are known to a high degree of accuracy. The Gear Artifact is not itself a gear, but has parts that have the shape and size of a gear tooth. These artifacts are used to calibrate equipment that measures gears and are necessary for precision manufacturing and quality control.
Gear artifacts themselves must be calibrated to be useful. However, calibrating them is a highly specialized task. Gear teeth usually have a shape that follows what is called an involute curve, allowing gears of different diameters to mesh smoothly.
Gear artifacts also often have an involute curve. It’s a relatively complex shape compared to, say, a sphere or a block. Much experience and training, as well as precision measurement equipment, are required to make involute curve measurements of gear artifacts with sufficiently low uncertainty.
Everett first dabbled in this kind of calibration in the 1990s when he was a “rookie metrologist.” Since then, he’s become an expert at making difficult measurements of artifacts like those for thread gauges, which are relied upon for everything from nuts and bolts you can buy at a hardware store to threaded connections used in oil and gas drilling.
The NIST machine used to calibrate a gear artifact is called a coordinate measuring machine (CMM), which uses a sensing probe and specialized software to determine the distances between points on an object in three dimensions.
To set up the measurements in 2019, Everett was able to use a high-precision CMM purchased from NIST in the 1990s when it was used for a series of benchmark measurements of equipment artifacts between NIST and other institutions. However, the machine has since received a software update and new hardware, including a sensor probe and probe motion control system. So even though Everett started by reviewing his notes from 25 years ago, he also had to spend time evaluating the CMM’s improved performance.
With the CMM’s increased capabilities come unknowns about how the system works, and Everett can’t learn what else has changed without extensive testing. In addition, the gear artifact he measured was more complex than those he had calibrated in the 1990s.
“For probably a total of five months, I recreated what had been done in the past and then applied it to a unique artifact that we hadn’t seen before,” Everett said. “Then we went back and forth with the customer to make sure we were delivering what they wanted and expected, and that we were doing it right.”
For the 2019 measurements it performed for Gleason Corp., NIST was able to offer the client an uncertainty of 1.5 micrometers (µm, millionth of a meter). In new calibrations of two new Gleason artifacts completed this month, Everett was able to reduce this to between 0.6 and 1.0 µm depending on the parameter.
One way Everett improved the measurements this time was to use a technique called “measurement decomposition,” which involves introducing a series of simpler NIST-maintained artifacts to the calibration. By comparing these simpler objects to the sections of the gear artifact they most closely resemble, the NIST researchers reduced the uncertainty more than would have been possible if they had measured the artifact alone.
Now that NIST has regained (and improved) its former capability, it is possible that the institute will receive more requests for this special type of calibration.
“I believe there are some military facilities that, if they knew NIST could calibrate their facility artifacts, they would send their artifacts to them,” Cowan said.
“I wouldn’t be surprised if we get more inquiries once word gets out that we can do it,” Everett said.