Swinging into Gear

Automotive industry is evolving quickly, with advances in artificial intelligence and robotics increasing its capabilities faster than ever before. Yet, even the most cutting-edge internal combustion engine works only as beautifully as a synchronized work of a complex array of all its components.

With the transmission being one of the vital parts of a vehicle, damage to the gears in power transmission systems leads to vibrations that impede the vehicle’s overall functionality. There is a need for detecting and characterizing damage from vibrations, caused by compromised gears, as measured from the transmission — the task undertaken by Christopher Cooley, Ph.D., assistant professor of mechanical engineering.

While there are numerous applications for this investigation, Dr. Cooley’s most recent research has been focused on the tooth root crack damage in parallel axis spur gears. During normal operation, the total transmitted force is evenly distributed between multiple teeth of gears in contact; when a gear is damaged, the distribution of load alters, putting less load on the cracked tooth and making the healthy teeth in contact carry more load to compensate.

“When tooth forces change suddenly, they dynamically excite the gears, causing additional transient vibrations that further impact the dynamic tooth loads. Damage to the teeth of gears in power transmission systems impacts their noise, vibration and durability. If damage can be reliably identified, it could save costs from maintenance, allow components to be designed with less stringent design rules and increase the use of vehicles,” Dr. Cooley explains.

Thus, examination of the dynamic tooth loads caused by damage could give considerable insight into the evolution of damage on a particular tooth, its spread into adjacent healthy teeth, and the characteristics of the resulting vibrations.

Dr. Cooley and Suhas Gupta Thunuguntla, a graduate student in mechanical engineering, utilize a finite element/contact mechanics (FE/CM) model to offer an accurate analysis of the nonlinear static and dynamic response in spur gear pairs with tooth root crack damage.

To provide an accurate analysis of the nonlinear static and dynamic response in spur gear pairs with tooth root crack damage, Dr. Cooley and Yaosen Wang, a graduate student in mechanical engineering, utilized a finite element/contact mechanics (FE/CM) model that is often used to solve complex partial differential equations stemming from science and engineering. Specifically, their FE/CM models captured the elastic deformations on the gear teeth, tooth and rim deformations, vibration, and localized compliance increases due to tooth damage.

As a result, the researchers were able to measure the contact pressure distributions and the resulting elastic deflections. The study showed that tooth root cracks that have lengths larger than 75% of the tooth thickness can result in large enough vibrations to create nonlinear contact loss.

“At low speeds, the contact loss occurs when the damaged tooth is engaged. At higher speeds, tooth contact loss occurs several mesh cycles after the damaged tooth disengages from mesh,” Dr. Cooley adds.

Tooth root cracks damage in parallel axis spur gears is only one aspect of Dr. Cooley’s research. Alongside doctorate student Suhas Gupta Thunuguntla, the scientist is currently developing modeling approaches and characterizing the dynamics for another type of gear damage – pitting, i.e., tooth surface failure.

“In addition, we are looking to leverage what we have learned by applying this knowledge to planetary gears, which are commonly used in rotorcraft and car transmissions. Unlike spur gear pairs, planetary gears have multiple gear meshes interacting simultaneously, more vibrating bodies and additional vibration modes with unique structure,” Dr. Cooley says.

The new study, which received $321,479.00 from the Department of Defense Grants Awards, is a three-year project aimed to create a new predictive analytical framework for nonlinear dynamics in planetary gears with tooth root crack and surface pit damage.

“Dr. Cooley’s work in damage-induced gear dynamics is helping researchers identify when gear teeth fail and to determine the severity of the failure, which is especially important to the safety of helicopters,” says Brian Sangeorzan, Ph.D., professor and chair of the Department of Mechanical Engineering. “In addition, this project provides exciting research opportunities to our students who learn to use state-of-the-art software and to interact with researchers at the Army Research Laboratory.”

The project will develop a nonlinear tooth stiffness model for gear meshes, incorporating these types of damage; create predictive models for dynamic analyses of planetary gears with damage; characterize damage-induced dynamic response in planetary gears, and measure damage-induced vibration from actual rotorcraft transmissions.

Learn more about Dr. Cooley’s work.