Vol. 22, 2018
“Strive for continuous improvement, instead of perfection.” ― Kim Collins, Five Time Olympic Athlete
The mission of the Gear Research Institute (GRI) is to operate a technology resource that supports improvementin the gear and drivetrain industry, to address the technology challenges of performance and affordability in mechanical power transmission systems and to educate and train the next generation of scientists, engineers and technologists. The word improveis defined as, “to make or become better”. GRI has been striving to help our sponsors improvetheir products (gears, gearboxes, etc.) for 36 years and we are continually working to improvethe ways in which we do it. The most recent example of this was the development of a very cost effective way to characterize the bending fatigue performance of gear teeth made by 3D printing or Additive Manufacturing (AM).
AM has become the newest buzz phrase in the gear industry. In fact, it made it on to the AGMA’s Emerging Technologies list. The list contains new technologies that “may disrupt or significantly impact the power transmission industry”. AM is being utilized to produce parts in almost every industry, in almost every material system imaginable. Its continued infiltration into drivetrain system designs is inevitable. AM gears are not likely to replace conventionally produced gears any time in the near future, but one can certainly envision instances when the ability to (relatively) quickly produce a gear for replacement or prototype purposes would be invaluable. There are many challenges involved with building accurate, strong AM gears, but we are closer than you might think.
As manufacturing technologies evolve, so must the testing techniques. Metal AM gears can be built using either laser based or electron beam based methods. Both are essentially melting powders to build components one layer at a time. The resulting parts are very similar to a weld, with microstructures that would not be typical of a metal gear. Typical problems include high surface roughness, tensile residual stresses (cracking!), porosity and mechanical properties that are anisotropic (directional variation). These challenges can be overcome by optimizing material chemistry, heat input (or other process parameters) during production or through post processing. The number of variables involved with producing a good “gear” is quite large and the ability to test individual teeth is critical to keeping costs down. The following section outlines our newest test capability to do just that.
We strive to evolve to meet the needs of our sponsors. We are also in the midst of producing our first ever fully reversed stress, STBF test fixture. Stay tuned for more details on that in an upcoming newsletter.
Managing Director, GRI
Drivetrain Technology Center, ARL Penn State
Research into novel metal manufacturing processes, such as Additive Manufacturing (AM), has led to our sponsors asking for fatigue testing of single gear tooth test specimens. This test enables these innovators to begin development of materials and processes for gear applications, leaving the complexities and associated costs involved with fabrication of entire gears for a later date. In this early stage of AM gear development, a representative test specimen that has the integrity and strength to withstand the cyclical loads endured by gears teeth in operation is produced.
In response to this need the Gear Research Institute (GRI) has developed a design, for testing a single tooth test specimen in bending fatigue. Single Tooth Bending Fatigue (STBF) is and has been a standard gear test at GRI (and has been described in earlier newsletters) for many years. The test involves mounting an entire gear between loading and reaction anvils and subjecting individual teeth to cyclical loads until failure (or runout) occurs. The fixture to test a single tooth in bending fatigue uses a modification of the test specimen holding mechanism to a standard STBF fixture. The new test fixture is shown in Figure 1.
Figure 1: Overall AM Gear Tooth Test Fixture
While the left hand side of the fixture is the tooth loading mechanism, similar to a standard STBF fixture, the test specimen holding mechanism is specifically designed to rigidly hold the single tooth test specimen. Particular attention has been paid to the reference surfaces of the tooth, ensuring that each tooth is loaded identically. Figure 2 illustrates a close-up of the test specimen holding mechanism.
Figure 2: Close-Up
It consists of a rigid block of steel with a rectangular slot into which a parallel spacer supports the test specimen. The test specimen is clamped down in the holding mechanism by a tapered wedge, on the top, that locks the specimen in place. The load anvil is visible just above the test tooth specimen, on the left. Once the single gear tooth specimen is clamped into the fixture the rest of the testing procedure is identical to how the standard STBF test is conducted. Fatigue cracks initiate near the location of maximum bending stress in the root fillet. The cracks are detected by carefully monitoring the position of the loading device at maximum force. This method is quite sensitive. Cracks can be detected that are not yet visible to the naked eye.
After crack detection, the number of cycles to failure is recorded. The tooth is then separated completely and the fracture surface is examined to ensure proper fixture alignment and to study the material’s fracture characteristics. A typical broken AM tooth is shown in Figure 3.
Figure 3: Typical Failed AM Test Specimens
This test arrangement and fixture have been successfully implemented in a few test projects where steel gear teeth fabricated by Additive Manufacturing have been evaluated for bending fatigue properties. This fixture is available for other sponsors to evaluate bending fatigue properties of single gear teeth fabricated with other alloys and other manufacturing techniques.
Education and Training
In order to assist with replenishment of the gear industry’s aging work force, the Gear Research Institute has developed a hands-on education program for students at both the undergraduate and graduate levels. The results of the program are entry level engineers that have been trained in the basics of gearing. This involves incorporating engineering undergraduate students, at the junior/senior level and graduate students in the Institute’s research laboratory while being paid by a grant from the sponsoring industrial entity. Summer internships have also been arranged at the sponsor’s facility, so that the student and the sponsor have an opportunity to assess each other with future employment in mind.
Typically, students get hands on experience by setting up and monitoring gear test equipment with additional training topics such as gear metrology, failure analysis, metallurgical characterization, vibration monitoring for failure detection, statistical analysis of test data and more.
This newsletter’s student profile is of William McCreavy. Will is pursuing his Bachelor of Science degree in Biological Engineering with an option in Agricultural Engineering. Will’s position was made possible through a grant from John Deere. His primary focus is to conduct Single Tooth Bending Fatigue tests for John Deere. Will is learning how to operate the test equipment, document the testing properly, and he will perform fracture analysis, metallography and statistical analysis as the project proceeds throughout the next six months.
GRI would like to encourage attendance or participation in both the 2019 AGMA Fall Technical Meeting (Detroit, MI on October 14-16) and the VDI International Conference on Gears 2019 (Munich, Germany on September 18-20). Please click on the banners below for more information. Both are excellent conferences!
The Gear Research Institute is a non profit corporation. It has contracted with the Applied Research Laboratory of The Pennsylvania State University to conduct its activities, as a sponsor within the Drivetrain Technology Center. The Gear Research Institute is equipped with extensive research capabilities. These include rolling contact fatigue (RCF) testers for low- and high-temperature roller testing, power circulating (PC) gear testers for parallel axis gears with a 4-inch center distance (testers can be modified to accommodate other center distances), single tooth fatigue (STF) testers for spur, helical and spiral bevel gears, and gear tooth impact tester. Extensive metallurgical characterization facilities are also available at Penn State in support of the Gear Research Institute. For further details on our testing capabilities please go to www.gearresearch.org or call Aaron Isaacson, Managing Director, at (814) 865-5832.