| Friction exists in all machines. Controlling the friction, either increasing orreducing, is crucial in modern machine design. Friction control based on external fieldsis one of the important methods. The large range of shear resistance modulation in2~5orders of magnitude upon the external electro/magnetic field of electrorheological(ER)/magnetorheological (MR) fluids implies great potential in such applications. Abetter understanding of the mechanism of ER/MR effect is very important for improvingthe performance of the devices. The mechanism of ER/MR effect has been researched inthis study based on the view point of tribology.First a new structure parameter is proposed according to the shear behavior of ERfluids of different materials. This parameter can be used to normalize the shear curves ofER fluids based on zeolite/silicone oil under constant electric field and ramped shearrate, resulting in a curve being analog to Stribeck curve, not only in shape, but also inphysics. So the lubrication regime defined in tribology has been introduced to theanalysis of structure revolution of ER fluids during the shearing, especially theinteraction between the aggregates of particles and the shearing plates along with thesource of the shear resistance in ER effect. In addition the shear thickening of ER fluidsunder low shear rates has been discussed. The magnitude of the critical structureparameters when the shear thickening occurs are all approximately10-6. Together withthe characteristic structure parameter values,10-4and10-2, the particle structureevolution of zeolite/silicone oil based ER fluids can be classified into four states: initialchain state; shear-thickening enhanced structure state; transition structure state and flowcompressed structure state.The stick-slip behavior of MR fluids below the shear rates of1s-1has beenresearched on a self-made MR shear test rig with uniform nominal shear rate. Based onthe observed spatial period of the stick-slip phenomenon it is speculated that the shearyield region locates in the boundary between the aggregates of particles and theshearing plates. The particles involved in the yield region are much fewer comparing tothe whole aggregates, most of which maintain a tetragonal chain structure perpendicularto the plates during shearing process. The images recorded during the shear process provided evidence to this speculation. The current test and acoustic emissionmeasurements are introduced to the ER and MR test. The variation of electric currentduring the shearing verifies the regime classification of shear curves based on theproposed structure parameter. The characteristic frequency component of acousticemission signals during the shearing of MR fluids decreases with the increase of themagnetic field strength and the shear rate, indicating the evolution of theaggregates-plates interaction.Then the influence of the boundary properties to MR effect is studied by changingthe roughness of the shear plates and lubrication ability of base oils. It has been foundthat the shear strength of MR fluids decreases with the increase of the surface roughnessand the improvement of the lubrication ability of its base oil. The stick-slip behavior ofthe MR fluids is diminished once the shear plates with higher surface roughness or thebase oil with better lubrication ability is used.Finally the prototypes of active friction control based on ER and MR effect aredesigned and tested, respectively. In order to enhance the local field and the interactionbetween the aggregates of particles and plates, the surfaces of ER device are coveredwith nylon textiles, while the boundary of MR clutch main part is modified with waveform boundary. Both of the two modifications show the improvement of theperformance of the device. |
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