Vol. 12, 2013
Looking to the future we ask the question, “Where are drive systems going?”
Customers and end users are always looking for increased throughput (power density), improved reliability, extended service life, reduced noise, and cost effective manufacturing. This is true in all of gearing used in transportation and everyday commercial use, from escalators to plastic gear drives in appliances. Gears are an integral part of our lives, yet only a small fraction of today’s universities provide more than a passing glance at gear design in their mechanical engineering programs. Computer sciences are the attention-drawing programs for educators today.
Gear technology research in materials and applied sciences is not a major focus in today’s mechanical engineering courses, but do not despair. All current research programs at the Gear Research Institute (GRI) are aligned and ready to achieve these goals of improved reliability, increased life and reduced weight and noise. The GRI has been in existence with specialized committees representing automotive, worm gearing, induction hardening, and aerospace drive systems research.
I am proud to say that I have been a member of the aerospace block since its inception in 1988. GRI’s programs in materials research have benefited the aerospace industry by providing a unified focused effort on the development of gear materials. The aerospace committee continues to advance the development of materials and manufacturing process improvements for use in aerospace gearing applications.
This newsletter shows a glimpse into some new technology developments underway at the GRI that are being achieved by today’s brilliant young engineers. Read this article to learn how they are working to improve the performance of drive systems that will benefit all future transportation vehicles.
Thomas L. Doubts
Senior Staff Engineer - Gearboxes
In all the fatigue testing, that is the forte of GRI, knowing when a certain consistent level of failure has occurred is vital. This is because in many of the experimental efforts we undertake we compare the life to failure of one set of test specimens against the life to failure of another set of test specimens, in order to establish the durability of one versus the other. Unless a consistent failure criterion can be identified during testing, we fall into the mode of “comparing apples and oranges” and the collected data is of marginal value.
The traditional mode of detecting failure, in most gear and gear material testing has been to monitor the vibration acceleration level, with a mounted accelerometer (figure 1) and assume that the failure criterion has been reached when this level exceeds a certain value. This has been found to be very arbitrary as the level of vibration is dependent on several factors such as the speed, load, tooth geometry, the dynamic response of the test rig system and even background noise.
|Figure 1: Mounted accelerometer
|Figure 2: Acceleration spectra
In recent years, with the availability of Fast Fourier Transforms (FFT) analysis hardware, GRI has begun to monitor the acceleration levels at specific fundamental frequencies and their harmonics, with significantly improved results. Figure 2 shows a typical frequency spectrum, after FFT, for a vibration signal from the 4 square gear test rig in figure 1. In this spectrum the fundamental tooth mesh frequency on the torque reversing gear pair is the large peak at about 1700 Hz. The peaks of the fundamental rotational frequency and its harmonics are on the left side of the large tooth mesh peak and the peaks to the right of the tooth mesh peak are further rotational harmonics, harmonics of the tooth mesh frequencies in the test box, harmonics of the tooth mesh frequencies in the torque reversing gear box and their accompanying side-bands.
By monitoring energy levels at the specific relevant frequencies much more consistent and repeatable results are being obtained, whether it is a running gear bending fatigue tests or a surface durability test. While we are still fine-tuning this methodology for surface durability failures, we are confident that we are detecting the initiation of bending fatigue failure of running gear teeth in an extremely consistent and accurate manner and long before any other technique known can detect this failure. This methodology developed at GRI could be an approach to gear health monitoring in gear boxes as the diagnostics and prognostics of mechanical machinery becomes an essential requirement of such systems.
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 this Spring (2013). Nicholas Lieberum, spent the 2012-2013 academic year working in our laboratory, supported by a grant from John Deere. He was involved in setting up gear fatigue test machines and collecting and analyzing data and communicating with the sponsor of the effort. He graduated with a Bachelor’s degree in Biological Engineering/ Agricultural Engineering option and accepted a position at John Deere’s manufacturing facility in Coffeyville, KS.
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.