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Encapsulated ball bearings for rotary micro machines

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1. Introduction2. Race design3. Encapsulated ball bearing fabrication4. Experimental setup and results5. Discussion6. ConclusionsAcknowledgmentsReferencesIOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERINGJ. Micromech. Microeng. 17 (2007) S224–S229 doi:10.1088/0960-1317/17/9/S03Encapsulated ball bearings for rotarymicro machinesCMikeWaits1,2, Bruce Geil2and Reza Ghodssi11MEMS Sensors and Actuators Laboratory (MSAL), Department of Electrical andComputer Engineering, Institute for Systems Research, College Park, MD 20742, USA2US Army Research Laboratory, 2800 Powder Mill Road, Adelphi, MD 20783, USAE-mail: [email protected] 26 February 2007, in final form 8 August 2007Published 31 August 2007Online at stacks.iop.org/JMM/17/S224AbstractWe report on the first encapsulated rotary ball bearing mechanism usingsilicon microfabrication and stainless steel balls. The method of capturingstainless steel balls within a silicon race to support a silicon rotor bothaxially and radially is developed for rotary micro machines and MEMS ballbearing tribology studies. Initial demonstrations show speeds up to 6.8 krpmwithout lubrication, while speeds up to 15.6 krpm with lubrication arepossible. Qualitative analysis is used to explain start-up behavior andinvestigate the wear of the stainless steel ball and silicon race.(Some figures in this article are in colour only in the electronic version)1. IntroductionMicro electromechanical systems (MEMS) fabricated siliconrotary elements for micro-motors, micro-generators andmicro-turbomachinery have received growing attention withapplications in power conversion and actuation. Withinthese technologies, the bearing mechanism is the primarydeterminant of device performance and reliability. Both activeand passive bearings have been investigated for rotary motion;however, no known successful commercial implementationsare known due to poor reliability and short lifetimes. Activebearings, such as magnetic or electrostatic have the advantageof being controlled during the operation, but at the cost ofthe accompanying circuitry [1]. Passive bearings have beeninvestigated heavily and span a large range of velocitiesthat include center-pin bushings with low revolution ratespossible [2, 3] and hydrostatic or hydrodynamic bearingswith high revolution rates possible [4]. Contact passivebearing mechanisms have poor reliability characteristics andare limited to low speeds due to the high frictional forcesof sliding motion [2]. In contrast, non-contact bearingsbased on active elements, or pressurized gas, have beendemonstrated to achieve high speeds but require complexfabrication procedures with tight tolerances [3].Tan et al [5] have investigated the tribology of a linear ballbearing mechanism and showed that the frictional propertiesbetween 440C stainless steel balls and silicon are quite lowwhen compared to pure sliding motion due to the rollingnature. The frictional properties of ball bearings allow thepossibility for higher speeds and better reliability than othercontact bearings relying on sliding motion while maintainingfabrication simplicity and stability. Although ball bearingshave been demonstrated in devices such as linear micromotors[6, 7] and rotary micromotors [8], they have yet to beintegrated into the microfabrication process to fully constrainthe dynamic element. In the cases of both Modafe et al[6] and Ghalichechian et al [7, 8] the dynamic element,whether it is a rotor or a linear slider, is held onto theballs by an electrostatic force. Their approach requires anadditional force to maintain contact between the dynamicelement and the stator. This imposes restrictions in themanufacturing process and assembly with the surroundingsystem components. Furthermore, a testing platform capableof investigating speeds greater than 100’s rpm for ball bearingshas yet to be developed.Hence, a rotary ball bearing mechanism is reported herewherein the rolling elements are encapsulated at the peripheryof the rotor to enable high speed rotation without relying on anyattractive force between the rotor and stator. An experimentaltest stand is implemented to demonstrate the encapsulatedbearing operation at rotational speeds up to 16 krpm andserve as a stepping-stone for future high speed and high loadtribology experiments. Results for both dry and lubricated,full compliment bearings are also reported leading towardthe development of micro turbomachinery. A qualitativeanalysis is given for the wear seen on the silicon race and the0960-1317/07/090224+06$30.00 © 2007 IOP Publishing Ltd Printed in the UK S224Encapsulated ball bearings for rotary micro machinesRotor w2w1hrpStator AA’Figure 1. Schematic drawing showing the design of the ball bearingmechanism. The rotational axis of the rotor is shown by the A–Adashed line on the right-hand side of the drawing.Table 1. Dimensions of the ball bearing mechanism.Parameter Dimensiondball285 µmrp4mmh = dball/√2+ δh 205 µmw1300 µmw2= dball/√2+ δw2205 µmstainless steel ball and correlated to the start-up behavior of thebearings.2. Race designThe design and fabrication of the rotary ball bearing is basedon commercially available 440C stainless steel balls with adiameter, dball, of 285 µm and a lot diameter variation of0.254 µm (see figure 1). The design features balls housedat the periphery of the rotor to enable a two-layer fabricationsequence for encapsulation via bonding. At the same time,this scheme of encapsulation allows features to be patternedon either side of the rotor while having minimal influence fromthe bearings.A square groove race was designed to encase the microball bearings. Alternative designs, nevertheless, may be ableto improve the performance and fatigue characteristics of thebearings. One can easily think of methods to incorporaterace designs which better mimic those from their macroscopiccounterparts, such as a rounded groove race or an angularcontact race; however, these may require more stringentfabrication processes. Housing the balls using a square grooverace fabricated by a dry anisotropic etch process, such as deepreactive ion etching (DRIE), allows control of the contactpoints and better repeatability when compared to other racedesigns and fabrication methods.The dimensions of the ball bearing mechanism are listedin table 1. The ball race was designed to have a pitch radius,rp, of 4 mm from the rotational axis to the center of the race.The width of the outer ring, w1, is


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