ABSTRACT Magneto Rheological Fluid (MRF) Sahil Jitesh Department of Mechanical Engineering, G.K. Bharad Institute of Engineering, Gujarat Technological University, India International Journal of Research in Mechanical Engineering Volume 2, Issue 4, July-August, 2014, pp. 27-32 Online: 2347-5188 Print: 2347-8772, DOA : 06 August, 2014 IASTER 2014, www.iaster.com In the design of machinery and electronic using material standard the science and technology have made marvelous development. Material can have range of ability to change their shape and size simply by providing little bit of heat or to convert liquid to solid almost by providing the magnetic field, these material called Smart Material. Magneto Rheological Fluid (MRF) is a smart fluid whose properties can be manipulate with the help of magnetic field. Magnetic field can be controlled, thus it helps to improve the performance supremely in areas where controlled fluid motion is needed, some application of MRfluid are brakes, fluid clutches, damper, journal bearing etc. As a source we give electrical inputs and as a result we get mechanical output, comparatively faster and in controlled manner. Keywords: Behavior of Liquid Control Fluid, Magneto Rheological Fluid, MRF, MR Fluid, Rheological, Smart Fluid. Shear Stress. I. INTRODUCTION Rheology is the study of flow and deformation. Flow capability and deformation, which is either elastic or plastic, have common features and the study of both subjects must overlap. Magneto rheological fluid (MR Fluid) are a class of Smart Material whose rheological property such as Viscosity may be rapidly varying by applying a magnetic fluid. Under the presence of magnetic field, the magnetic particles interact to form a structure that resist the flow or deformation. This quick change in material appears as an increase in apparent viscosity or development of semi-solid state. Thus in the presence of magnetic field the MR fluid converts into a semi solid with an increase in the yield strength. This phenomenon takes just the fraction of milliseconds to occur. MR fluid thus acts similar to Bingham Fluid used in many engineering applications. In the absence of magnetic field the MR fluid behave like Newtonian fluids. Figure 1 a) No Magnetic Field Applied b) When Magnetic Field is Applied 27
The Above figure shows the typical relationship between shear stress and shear rate for a Bingham fluid and compares this with a Newtonian fluid. When a magnetic field is not present the MR fluid is behave like a Newtonian fluid. Some resistance is calculated around the zero shear rate. This results the plastic deformation, but there is no continuous movement. In this situation, the maximum stress which can be applied without causing continuous movement, is the yield stress and this is a function of the magnetic field strength. Here, by increasing and decreasing the magnetic field strength the yield strength can be controlled. Figure 2. Relationship between Shear Stress and Shear Rate of Bingham and Newtonian Fluid II. MAGNETO RHEOLOGICAL FLUID COMPONENTS MR fluid are basically non-colloidal suspension of micro sized magnestisable particles in an inert base fluid along with some additives, Thus there are three components in an MR fluid: 1. Base Fluid The base fluid behave as a carrier, and naturally combines lubrications and damping features. In order to get maximum effect of MR fluid, there should independence of temperature and small coefficient of viscosity. Due to the presence of suspended particles base fluid becomes thicker, commonly used base fluid are Silicon oils, Minerals oils and Hydrocarbon oil. 2. Metal Particles Small Size of metal particles approx. in order of 1µm to 7µm is used, for proper operation of MR fluid magnetic particles are navigate by the magnetic field to form a chain like structure. This chain like structure restricts the motion of fluid and therefore changes the rheological behavior of the fluid. The metal particles are usually made by carbonyl iron, iron powder and iron cobalt alloys. These metal particle form a strong magnetic chain because they have property to achieve high magnetic saturation. This concentration of magnetic particle in base fluid can be go up to 50% (approx.). 3. Additives Additives are suspending agents like anti-corrosive, friction modifiers, thixotropic and wear components. It also include the surfactants and stabilizers. Additives helps to control the viscosity of the fluid and the setting rate of the fluid particles. These all three components define the magneto rheological behavior of the MR fluid. By changing the any other property discuss above the property of MR fluid can be change and by mixing the optimum percentage of all three components, the desirable necessary proper of the MR fluid can be achieved. 28
III. PROPERTIES OF MR Fluid In absence of magnetic field, MR fluid is free flowing with consistency similar to oil having low viscosity. When the magnetic field is applied, the ion particles generate dipole moment along with the external field which cause particle to form a linear chain aligned with respect to magnetic field. Initially, ferrous particles area in an amorphous state as shown in figure (a), when a magnetic field is applied, the ferrous particles begin to align along the flux path as shown in figure (b). Figure (c) shows the ferrous particles aligned along the flux path creating chain in the fluid. These particles aligned along the flux path generate particles chain in the fluid. These particles chains resists and restrict the fluid moment. As a result, a yield stress is developed in the fluid. This change is related to the magnetic Field strength and may occur in a matter of fraction of seconds. Diameter of Magnetizable particle range from 3-5 microns, Functional MR fluid may be made with larger particles, but it become hard to form a stable suspension for the large particles. Smaller particles that are easier to suspend but the manufacture of such particles is difficult. Significantly smaller ferromagnetic particles are generally only available as oxides, such as pigments commonly found in magnetic recording media. It was founded that wall roughness on contact with the fluid is important for yield strengths especially low magnetic field. For low strains prior to yield, the shear modulus of MR fluid also shows a very large increase in applied magnetic field. Property Initial Viscosity Density Magnetic Field Strength Typical Value 0,2 0,3 [Pa s] (at 25oC) 3 4 [g/cm3] 150 250 [ka/m] Yield point Typical supply voltage and current intensity Reaction time 50 100 [kpa] 2 25 V, 1 2 A Few Milliseconds Table 1. Summary of the Properties of MR 29
MR material eventually reach a saturation point where increase of magnetic field strength do not increase the yield strength of MR material. This phenomenon typically occur around 300 ka/m. This effect of magnetic saturation on strength of MR material can be studies by using finite element analysis (FEA). MR Fluid is immediately reversible if the magnetic field is reduced or removed, the response time is just 6.5 ms. MR material that have been already developed are stable in temperature range -50 to +150, Also the size of distribution of the suspended particles effect the change in properties of the MR fluid when placed in magnetic field. IV. MRF MODES AND THEIR APPLICATIONS According to the type of fluid flow, there are three modes of MRF operation. 4.1 Valve Mode 4.2 Shear Mode 4.3 Squeeze Mode 4.1 Valve Mode In Valve mode of MRF operation, through the two stationary surface the fluid is allowed to flow and magnetic field is applied perpendicular to the direction of flow as shown in fig.6. Fig. 4 Valve Mode By controlling the intensity of the field, the resistance of the fluid can be controlled. This Mode of MR fluid is widely used in the various types of damper and shock absorbers and immense application in automobile industry. Fig. 5 Rheonetic Linear Damper The pressure drop in this mode is the summation of pressure developed due to fluid viscosity ΔPv and pressure developed due to the magnetic field ΔPm. ΣP = ΔPv + ΔPm The Rheonetic linear damper shown below is designed for use as a secondary suspension element in on- and off-highway vehicles. A magnetic coil integrated into the piston of the damper generates a magnetic field and this magnetic field regulates the MRF flow resistance within the damper. By using this damping arrangement becomes controllable and vibration transmission and frequency for a suspended seat can be adjusted. These MRF applications bring additional functionality whilst keeping the simplicity. Other possible MRF applications using this mode are dampers for knee prosthesis, vibration dampers and seismic dampers for civil industry. 30
4.2 Shear Mode The second operational mode is the shear mode. This mode is used in brakes and clutches The total force in the shear mode can be separated into a viscous (pure rheological) component Fr and a magnetic field dependent (magneto-rheological) component Fmr. The total shear force is defined through the following- Fig. 6 Shear Mode Fig.7 MRF Brake This brake contains just a few parts: shaft, bearings, sealing devices, and housing with coil, interface disc and MRF. The simplicity and easy control makes it a cost effective choice for controllable exercise equipment. An MR fluid brake is currently being manufactured and sold as a controllable resistance element for programmable aerobic exercise equipment. 4.3 Squeeze Mode The Third Mode is called Squeeze Mode. This mode is recent development compare to the other two mode and not been studied so thoroughly comparing it with the Valve and shear mode. Fig 8 Squeeze Mode For small vibration in damper, this mode seems to offer the possibility of very large forces, which can be managed by the MR Fluid. It believe that the yield stress could be achieved which would ten times as large as that which is possible with wither the direct shear or valve mode. V. APPLICATIONS OF MRF 5.1.1 The MR damper has a built-in MR valve across which the MR fluid is forced, the piston of MR damper acts as an electromagnet while the required number coils to avoid accumulator. 5.1.2 The MR Clutches, They are used in automotive power train to transmit torque from the engine to the transmission and the vehicle. 31
VI. DISADVANTGES OF MRF 6.1.1 The problem about the ER fluid are suspensions, it take time to settle out, to overcome this problem by means such as matching the densities of the solid and liquid components or by using the help of nanoparticles, which brings ER fluids into line with development of magnetorheological fluids. 6.1.2 The second problem is that that the breakdown voltage of air is ~ 3 kv/mm, which is near the electric field needed for ER devices to operate. VII. ADVANTAGES OF MRF 7.1.1 Speed of response is very small. 7.1.2 As magnetic field can be precisely controlled by current driven electromagnet. 7.1.3 It have higher magnetic yield stress. 7.1.4 Compared to ER fluids the simple construction of device. 7.1.5 Low power requirement to control. VIII. LIMITATION OF MRF MR fluid have many application but like other smart fluids it is limited in commercial feasibility due to following reasons: 8.1.1 Due to presence of iron, makes it heavy due to high density, however, operation volume are small, so while this is a problem, it is not insurmountable. 8.1.2 High quality fluids are expensive. 8.1.3 After Prolonged used, fluids get thicker so need replacing. 8.1.4 For some application setting of Ferro-particles can be a problem. 8.1.5 Commercial applications do exist, but will continue to be few until these problems (particularly cost) are overcome IX. CONCLUSION Magneto Rheological Fluid is a new gift for the future, many possibilities of application in automobile, aerospace and medical field. Simplicity and more intelligence in the functionality are key features of the MR Fluid. Immediate response and simple interface between electrical input and mechanical output and intelligent controllability gives the wide scope in the upcoming era. REFERENCES [1] Investigation and Control of Magneto Rheological Fluid Dampers by Andrzej Milecki, International Journal of Machine Tools & Manufacture 41 (2001) 379 391. [2] M. Kciu, R. Turczyn, Properties and Application of Magneto Rheological Fluid, Journal of Achievements In Materials and Manufacturing Engineering 18 (2006) 1 2. [3] Carlson, J. D., Weiss, K. D., A Growing Attraction to Magnetic Fluids, Machine Design, Vol. 66, No. 15, P. 61-64, 1994. [4] Jolly, M. R., Carlson, J. D., Munoz, B. C., A Model of The Behavior of Magnetorheological Materials, Smart Materials and Structures, Vol. 5, P. 607-614, 1996. [5] Milecki, Investigation and Control of Magneto-Rheological Fluid Damper, International Journal of Machine and Manufacture 41 (2001), Page 379-391. 32