29-10-2012, 11:58 AM
Magneto Rheological Fluids
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ABSTRACT
Magneto rheological fluids commonly known as MR fluids are suspensions of solid in liquid whose properties changes drastically when exposed to magnetic field. Magnetorheological (MR) fluids are materials that respond to an applied field with a dramatic change in their rheological behavior. The essential characteristic of these fluids is their ability to reversibly change from a free-flowing, linear, viscous liquid to a semi-solid with controllable yield strength in milliseconds when exposed to a magnetic field.
MR fluids find a variety of applications in almost all the vibration control systems. It is now widely used in automobile suspensions, seat suspensions, clutches, robotics, design of buildings and bridges, home appliances like washing machines etc.
The key to success in all of these implementations is the ability of MR fluid to rapidly change its rheological properties upon exposure to an applied magnetic field.
Introduction
Magneto rheological fluids commonly known as MR fluids are suspensions of solid in liquid whose properties changes drastically when exposed to magnetic field. It is this property which makes it desirable to use in different vibration controlling systems.
What are MR fluids?
Magnetorheological (MR) fluids are materials that respond to an applied field with a dramatic change in their rheological behavior. The essential characteristic of these fluids is their ability to reversibly change from a free-flowing, linear, viscous liquid to a semi-solid with controllable yield strength in milliseconds when exposed to a magnetic field.
Chemical composition
A typical MR fluid consists of 20%–40% by volume of relatively pure, soft iron particles, typically 3–5 microns, suspended in a carrier liquid such as mineral oil, synthetic oil, water, or glycol. A variety of proprietary additives similar to those found in commercial lubricants are commonly added to discourage gravitational settling and promote particle suspension, enhance lubricity, modify viscosity, and inhibit wear.
Physical properties
MR fluids made from iron particles exhibit maximum yield strengths of 30–90 kPa for applied magnetic fields of 150–250 kA/m (1 Oe . 80 A/m). MR fluids are not highly sensitive to moisture or other contaminants that might be encountered during manufacture and use. Further, because the
magnetic polarization mechanism is not affected by the surface chemistry of surfactants and additives, it is a relatively straightforward matter to stabilize MR fluids against particle-liquid separation in spite of the large density mismatch. The ultimate strength of the MR fluid depends on the square of the saturation magnetization of the suspended particles.
Applications of MR fluids
MR fluids find a variety of applications in almost all the vibration control systems. It is now widely used in automobile suspensions, seat suspensions, clutches, robotics, design of buildings and bridges, home appliances like washing machines etc. Before discussing the above said applications in detail it is desirable to go through the behavior of MR fluids on different types of loading and what are the design considerations provided to compensate this.
MR fluids on impact and shock loading
Investigations on the design of controllable magnetorheological (MR) fluid devices have focused heavily on low velocity and frequency applications. The extensive work in this area has led to a good understanding of MR fluid properties at low velocities and frequencies. However, the issues concerning MR fluid behavior in impact and shock applications are relatively unknown.
MR fluid in automobile clutches
MR fluids are increasingly being considered in variety of devices such as shock absorbers, vibration insulators, brakes or clutches. The activation of MRF clutch’s built-in magnetic field causes a fast and dramatic change in the apparent viscosity of the MR fluid contained in the clutch. The fluid changes state from liquid to semi-solid in about 6 milliseconds. The result is a clutch with an infinitely variable torque output.
Double plate MRF clutch design
Bansbach, proposed a double-plate and a multi-plate MRF torque transfer apparatus with a controller that adjusts the input current. The apparatus is proposed to be placed between the engine of a car and its differential. Gopalswamy suggested a MRF clutch to minimize reluctance for fan clutches. Gopalswamy also studied a controllablemulti-plate MR transmission clutch. This clutch was also designed to be placed between the engine and differential. Hampton described a design of MRF coupling with reduced air gaps and high magnetic flux density. Carlson proposed a MR brake with an integrated flywheel.