A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Prognostics of Ball Bearings in Cooling Fans(2012) Oh, Hyunseok; Pecht, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Ball bearings have been used to support rotating shafts in machines such as wind turbines, aircraft engines, and desktop computer fans. There has been extensive research in the areas of condition monitoring, diagnostics, and prognostics of ball bearings. As the identification of ball bearing defects by inspection interrupts the operation of rotating machines and can be costly, the assessment of the health of ball bearings relies on the use of condition monitoring techniques. Fault detection and life prediction methods have been developed to improve condition-based maintenance and product qualification. However, intermittent and catastrophic system failures due to bearing problems still occur resulting in loss of life and increase of maintenance and warranty costs. Inaccurate life prediction of ball bearings is of concern to industry. This research focuses on prognostics of ball bearings based on vibration and acoustic emission analysis to provide early warning of failure and predict life in advance. The failure mechanisms of ball bearings in cooling fans are identified and failure precursors associated with the defects are determined. A prognostic method based on Bayesian Monte Carlo method and sequential probability ratio test is developed to predict time-to-failure of ball bearings in advance. A benchmark study is presented to demonstrate the application of the developed prognostic method to desktop computer fans. The prognostic method developed in this research can be extended as a general method to predict life of a component or system.Item Design, Fabrication, and Characterization of a Rotary Variable-Capacitance Micromotor Supported on Microball Bearings(2007-07-31) Ghalichechian, Nima; Ghodssi, Reza; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The design, fabrication, and characterization of a rotary micromotor supported on microball bearings are reported in this dissertation. This is the first demonstration of a rotary micromachine with a robust mechanical support provided by microball-bearing technology. One key challenge in the realization of a reliable micromachine, which is successfully addressed in this work, is the development of a bearing that would result in high stability, low friction, and high resistance to wear. A six-phase, rotary, bottom-drive, variable-capacitance micromotor is designed and simulated using the finite element method. The geometry of the micromotor is optimized based on the simulation results. The development of the rotary machine is based on studies of fabrication and testing of linear micromotors. The stator and rotor are fabricated separately on silicon substrates and assembled with the stainless steel microballs. Three layers of low-k benzocyclobutene (BCB) polymer, two layers of gold, and a silicon microball housing are fabricated on the stator. The BCB dielectric film, compared to conventional silicon dioxide insulating films, reduces the parasitic capacitance between electrodes and the stator substrate. The microball housing and salient structures (poles) are etched in the rotor and are coated with a silicon carbide film to reduce friction. A characterization methodology is developed to measure and extract the angular displacement, velocity, acceleration, torque, mechanical power, coefficient of friction, and frictional force through non-contact techniques. A top angular velocity of 517 rpm corresponding to the linear tip velocity of 324 mm/s is measured. This is 44 times higher than the velocity achieved for linear micromotors supported on microball bearings. Measurement of the transient response of the rotor indicated that the torque is 5.620.5 micro N-m which is comparable to finite element simulation results predicting 6.75 micro N-m. Such a robust rotary micromotor can be used in developing micropumps which are highly demanded microsystems for fuel delivery, drug delivery, cooling, and vacuum applications. Micromotors can also be employed in micro scale surgery, assembly, propulsion, and actuation.