Vol. 13, 2014
“The old order changeth, yielding place to new” is a line from the last of Lord Tennyson’s trilogy, “The Idylls of the King”. We wanted to take a little time to recognize GRI’s senior employee and the transition that is occurring.
Doug McPherson joined the Gear Research Institute in 1991 with a BS in Naval Architecture and a MS in Mechanical Engineering, after having worked in the marine engineering industry. He was the only Northwestern University employee to relocate to Penn State with GRI in 1996 and became its resident expert in gear fatigue testing, among other things. Due to personal reasons, he has decided to initiate a phased retirement, which started in July of 2013. He will continue supporting GRI’s research activities at a reduced effort amounting to 30% of full-time till the end of June, 2015. In anticipation of Doug’s eventual retirement, Aaron Isaacson has been taking over many projects, especially those related to the Aerospace Bloc, since the fall of 2012.
Aaron has a BS and MS in Mechanical Engineering and is currently pursuing his PhD in Material Science. He started working on GRI projects as an undergraduate senior in Mechanical Engineering and joined us full-time on the completion of his BS degree. Many of GRI’s current sponsors have worked with him on their projects over the last 15 years. While Doug’s eventual retirement will leave a void at GRI and he will be missed by his friends and associates, his continued participation over the next year will assist in making the transition as smooth and seamless to the outside world.
Suren B. Rao
Single Tooth Bending Fatigue of gears at GRI
The Gear Research Institute has been evaluating the bending fatigue strength of gear teeth through a well-established test called the Single Tooth Bending Fatigue (STBF) test. In order to make the gear community aware of GRI’s capabilities, this article briefly describes what has been accomplished.
Figure 1 shows the basic concept of this test. The test gear is held on a shaft and cyclical load applied to a test tooth through a load anvil (on the top in blue) on a Universal Test Machine while a similar load anvil reacts to the cyclical load (at the bottom in blue). As the gear tooth material reaches its fatigue limit one of the teeth will fail, normally with a break at the root where the bending stress is the maximum. Figure 2 shows a test fixture holding a spur gear for the purpose of conducting such a test.
The Gear Research Institute has conducted STBF tests and continues to conduct such test for a large number of sponsors. This test is a relatively inexpensive and rapid method for comparatively evaluating gear materials or the impact of gear manufacturing processes on bending fatigue performance.
Figure 1: Basic concept of STBF
Figure 2: STBF of Spur Gear
While conducting STBF tests on a spur gear is the simplest option, sponsors are interested in evaluating the bending performance of other types of tooth geometry and the Gear Research Institute has always risen to the occasion. Pictured below are test fixtures designed and fabricated for evaluating the bending fatigue performance of helical gears (Figure 3), spiral bevel gears (Figure 4) and face gears (Figure 5). The basic principle of loading a gear tooth while reacting at another tooth, as shown in figure 1, remains unchanged. The challenge is defining the gear holding portion of the fixture so that the test teeth are aligned in a manner that the vertical load and reaction load of a universal test machine can be applied in the appropriate manner. Design and fabrication of the load and reaction anvils (also called tups) to uniformly load the test and reaction teeth, especially on spiral bevel gears, are also a challenge.
Figure 3: Helical gear test fixture
Figure 4: Spiral Bevel gear test fixture
Figure 5: Face gear test fixture
As mentioned earlier, the STBF is an inexpensive test for screening for gear tooth bending strength, especially when a large variety of variables are to be compared. While statistical manipulation allows one to define bending strength “design allowable” from STBF data, running gear tests are generally recommended for that purpose.
Education and Training
In order to assist the gear industry augment its aging work force, the Gear Research Institute has proposed two initiatives to train more gear knowledgeable engineers at the undergraduate and graduate levels. This involves incorporating engineering undergraduate students, at the junior/senior level and graduate students in the Institute’s research efforts while being paid by a grant from the sponsoring industrial entity. Summer internships at the sponsor’s facility are also a part of the deal so that the student and the sponsor have an opportunity to assess each other with future employment in mind.
We had our first graduate of this program in the spring of 2013 who is currently employed by John Deere at their Coffeyville, KS facility. We now have a second student employee selected by John Deere, Byron Stuart, who will graduate this spring and who has been interviewed by John Deere. This program appears to be working for this large manufacturer of agricultural equipment as it is likely that GRI will receive funding to continue this internship for a third year with another student.
Another GRI contribution to education is a book by Dr. William Mark, which was made possible through a grant from the AGMA Foundation. The book titled “Performance-Based Gear Metrology” is a comprehensive compilation of methods and techniques pioneered by Dr. Mark, on gear transmission error, through over three decades of work in this field. It is available on Amazon.com.
Book Synopsis: Some gear-tooth working-surface manufacturing deviations of significant amplitude cause negligible vibration excitation and noise, yet others of minuscule amplitude are a source of significant vibration excitation and noise. Presently available computer-numerically-controlled dedicated gear metrology equipment can measure such error patterns on a gear in a few hours in sufficient detail to enable accurate computation and diagnosis of the resultant transmission-error vibration excitation.
The book explains how to efficiently measure, in detail, parallel-axis helical or spur gears using computer-numerically-controlled dedicated gear metrology equipment, and how to compute from such measurements the transmission-error vibration-excitation caused by geometric deviations of the teeth from equispaced perfect involute surfaces. It provides a complete explanation of how such deviations cause vibration excitations, including deviations causing tooth-meshing harmonics, sideband harmonics, ghost-tone harmonics (caused by undulation errors), and any form of tooth damage. It therefore should be of interest to anyone wanting to obtain a full understanding of how such deviations cause vibration excitations and noise, including makers of CNC gear metrology equipment, buyers of gear manufacturing equipment, buyers of gears, those interested in gear quality control, and researchers in gear noise and gear-health monitoring. The book contains many illustrations of measured gears and computed transmission-error tones caused by manufacturing errors, including one example correlating computed transmission-error tones with measured noise tones. Use of the technology in this book will allow quality spot checks to be made on gears being manufactured in a production run, to avoid undesirable vibration or noise excitation by the manufactured gears. Furthermore, those working in academia and industry needing a full mathematical understanding of the relationships between tooth working-surface deviations and the vibration excitations caused by these deviations will find the book indispensable for applications pertaining to both gear-quality and gear-health monitoring.
Provides a very efficient method for measuring parallel-axis helical or spur gears in sufficient detail to enable accurate computation of transmission-error contributions from working-surface deviations, and algorithms required to carry out these computations, including examples
Provides algorithms for computing the working-surface deviations causing any user-identified tone, such as ‘ghost tones,’ or ‘sidebands’ of the tooth-meshing harmonics, enabling diagnosis of their manufacturing causes, including examples
Provides explanations of all harmonics observed in gear-caused vibration and noise spectra
Enables generation of three-dimensional displays and detailed numerical descriptions of all measured and computed working-surface deviations, including examples
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 and helical gears, gear tooth impact tester, and worm gear testers with 1.75 and 4-inch center distances. 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 the Drivetrain Technology Center website or call Dr. Suren Rao, Managing Director, at (814) 865-3537.