ATTRIBUTES OF LORD MR FLUID
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A. Although the formulation depends on the needs of the application, MR fluid typically contains three basic components:
- Liquid carrier - mineral oil, synthetic hydrocarbon oil, silicone oil, water, glycol, etc.
- Magnetic particles - carbonyl iron, iron/cobalt alloys, nickel alloys
- Other additives - suspending agents, thixotropes, anti-wear and anti-corrosion additives, friction modifiers
A. We use a variety of iron particles depending on the fluid formulation to optimize magnetic properties and application performance. Carbonyl iron, which is made from the thermal decomposition of iron pentacarbonyl, is most commonly used in making MR fluids. Carbonyl iron particles are highly spherical in shape with sizes in the 1 to 10 micron range with an elemental iron content > 98%.
A. The upper limit on force that can be created in an MR fluid is directly related to the amount of iron in the fluid. The more iron there is, the higher the attainable force. Depending on the volume fraction of iron particles, MR fluids can have maximum yield strengths ranging from 30 to 80 kPa for applied magnetic field of 150-250 kA/m. Our MR fluids typically have 20-40 percent iron by volume.
A. Density depends upon the liquid carrier and levels of iron content. For our standard hydrocarbon oil-based MR fluid the density is 3 g/cc. Densities can be as low as approximately 2 g/cc or as high as approximately 4 g/cc.
A. MR fluid is a mixture of iron, oil, and other proprietary additives and therefore does not have a molecular weight. See “Density of MR fluid” above.
A. The viscosity of an MR fluid depends on a number of factors, including viscosity of the liquid carrier, the volume fraction of particles, the amount and type of additives and the shear rate at which viscosity is measured. The additives in the fluid play a very big role in the viscosity at low shear rates, causing the viscosity to increase rapidly as one goes to lower shear rate.
All MR fluids will display a shear thinning character. This means that their apparent viscosity will drop as shear rates are increased, until eventually a steady-state value is reached. For a hydrocarbonoil-based fluid, this steady state value is reached at a relatively low shear rate of perhaps a few hundred sec-1. For the water-based fluids, the apparent viscosity starts higher at low shear rate and continues to drop over a much longer range of shear rate.
A. Many types of viscometers can be used with MR fluid, measuring both shear stress and shear rate to establish viscosity. Probably the most common type of viscometer used is a concentric cylinder arrangement, but some fluids are easier to load and measure (usually due to higher viscosity) if one uses a cone and plate or parallel plate configuration for the viscometer.
NOTE: The definition of shear rate is the fluid velocity through a gap (in units of cm/s, as an example) divided by the width of the gap (in units of cm, to be consistent). The length dimensions (cm, in this example) cancel each other to leave 1/s as the resultant unit.
A. In the absence of an applied field magnetic field, MR fluids are generally well modeled as Newtonian liquids characterized by their viscosity. When a magnetic field is applied, a simple Bingham-plastic model is effective at describing their essential field-dependent fluid characteristic. In this model, the total yield stress total is given by MR(H) is the yield stress caused by the applied magnetic field H, is the shear rate and is the field-independent plastic viscosity defined as the slope of the measured shear stress against the shear strain rate.
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A. No, LORD MR fluid is not classified as a hazardous material, but like all chemicals it should be handled with the appropriate care and equipment. Please refer to the MSDS sheets for more details.
A. Yes. The iron can be separated out and the carrier liquid can be treated as one would motor oil, anti-freeze, etc. We do not accept used MR fluid back – it should be disposed of properly.
A. MR fluid does not affect the ozone, as no CFCs are used.
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A. While particulate settling can occur, it can be controlled and has not been a barrier to the successful commercial application of LORD MR fluids. Most MR fluid devices such as dampers and shock absorbers are highly efficient mixing devices. As long as the MR fluid does not settle into a hard sediment, the normal motion of the device is adequate to cause sufficient flow to remix any stratified MR fluid back to a homogeneous state.
LORD MR fluids can be designed to have essentially no settling when required by applications where the device is inactive for a long period of time, such as in seismic damage mitigation or automotive crash energy absorption. However, a small amount of settling is typically an acceptable trade-off for other desirable properties. Much of the “art and science” of formulating MR fluids is striking a balance between low settling, low viscosity, high forces, good wear resistance, ease of filling, etc.
A. Settling is not important as the ability of the MR fluid to remix in the device (redispersability). Redispersability in a device depends on fluid properties and device geometry. Tests have shown that with as little as one stroke, MR devices will return to their original condition even after one year of settling.
A. Abrasion is no longer an issue due to our proprietary lubricating additives that coat and protect the surface of the particles and reduce wear on device components — much like the lubricants used in your car’s engine to prevent wear on close-tolerance sliding surfaces. For example, the MR fluid used in Delphi’s MagneRide dampers has been shown to last as long as the damper components, well past 150k miles.
MR devices and fluids are best designed together so that seals and wear surfaces are also optimized. It is possible to have dynamic devices that will sustain tens of millions of cycles or more and many hundreds of kilometers of cumulative seal travel. Expertise in both MR fluids and MR devices makes us the ideal partner for implementing MR technology.
A. In the off-state (no magnetic field applied), temperature effects are largely dependent on the liquid carrier fluid. Oil and silicone oil based fluids can typically operate from -40 to 150°C. Water based fluids are rated from 0 to 70°C. Glycol can extend water-based fluids to operate below normal freezing temperatures.
In the on-state (magnetic field applied), the magnetic viscosity effect is typically an order of magnitude greater than temperature viscosity effect, and so device performance is uniform and controllableacross a wide range of temperatures.
A. Operating life depends not just on the MR fluid, but also on the design of the device and the level of energy dissipation required of the fluid. LORD MR fluids have successfully passed the demanding qualification testing of the automotive industry and have logged many millions of cycles in real-world applications since 1998.
Storage life varies by fluid formulation, but in practice there have been no real-time shelf-life issues; accelerated aging tests confirm that our fluids will last years on the shelf.
A. The best way to be certain that a fluid is mixed properly is to check for consistent fluid density at various locations within the container. The simplest way to verify density is to use a weight-per-gallon cup or mini weight-per-gallon cup. Both are available (with instructions for use and reference to the test method) from Paul N. Gardner Co., Inc., 316 NE First Street, Pompano Beach, FL 33060; phone: 800-762-2478.
For mixing that is mistake-proof, a paint shaker is ideal. Five minutes in a Red Devil paint shaker should be plenty. A rotary paint mixer (like the type that fits on a drill), or any other type of prop mixer, is adequate, but it is probably overkill unless it is taking hours/days to fill the device. Be certain not to turn the mixer up too high and whip in a lot of air, since doing so will hinder the mixing process.
A. Depending on the conditions of the specific application, all MR fluids will eventually show some degree of deterioration. Such deterioration is usually manifested as a thickening of the fluid often referred to as “in-use-thickening” or IUT. We have spent a considerable effort in the mid-1990’s solving this problem so that today’s applications such as Delphi MagneRide shocks using LORD MR fluid show consistent performance over the life of the damper.
A. Response time of MR fluid is < .001 sec. In most MR fluid devices, the overall response time is limited not by the fluid but by the inductance of the electromagnet and the output impedance of the driving electronics.
A. Operation under high pressure is not a problem. In fact, we have operated MR fluids at static pressures up to 2500 PSI (17 MPa). The fluids should be fine at pressures at least an order of magnitude greater than this. However, at these pressures one will need to take into account the bulk compressibility, which will increase the effective volume fraction.
A. No, the fluid does not change volume when a magnetic field is applied.
A. This is not a problem with hydrocarbon oil based MR fluids. If the exposure time is not too long, water based MR fluids may also be exposed to air and then resealed.
COMPARING MR TO OTHER TECHNOLOGIES
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A. Our MR technology provides variable, resistive control of force in near real time. As compared to traditional electro-mechanical technology, MR has important advantages:
- Infinitely variable force resistance within a wide control range. For example, MR dampers can easily achieve large forces at very low speeds.
- Fast acting; almost instantaneous response. The response time of an MR fluid is much less than one milli-second, so the response time for most practical MR devices is controlled by the time it takes for the current source to establish the magnetic field in the fluid. With proper design, system response times of 2-10 milli-seconds are achievable.
- Mechanical simplicity. Unlike electro-mechanical valves there are no moving parts, which reduces complexity and improves cost and reliability.
- Low power requirements.
A. MR fluids are similar to ER fluids, but MR fluids are 20 to 50 times stronger. They can be operated directly from low-voltage power supplies and are far less sensitive to contaminants and extremes in temperature. Applied to a variety of devices, LORD MR fluid technology can provide flexible control capabilities in designs that are far less complicated and more reliable than with ER technology.
A. Ferrofluids are often confused with MR fluids. Both fluids are suspensions of iron containing particles in carrier oils. But where MR fluids use micron-sized, pure iron particles, ferrofluids consist of nanometer-sized iron oxide particles. Ferrofluids do not show the yielding behavior characteristic of MR fluids in a magnetic field as their particles are too small. Instead ferrofluid flows to and is attracted by the magnetic field. Even in a very strong magnetic field, ferrofluids always remain liquid.
Applications for ferrofluids include speakers and vacuum seals. MR fluids are not appropriate for those applications. For more information about ferrofluids, we suggest visiting the FerroTec Web site.
DESIGNING MR DEVICES
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A. Typical seals used in conventional hydraulic dampers do not work very well. We have developed a high-level of practical experience in selecting seals that operate well with MR fluids. It is possible to have dynamic devices that will sustain tens of millions of cycles or more and many hundreds of kilometers of cumulative seal travel.
A. Generating a sufficient magnetic field and a geometry that makes use of it is a critical aspect of any MR device design. The strength of the magnetic field depends on the force required, the amount of fluid used, and the size/design of the device (for example squeeze flow versus valve mode). We have developed a large number of proprietary spreadsheets and models to optimize MR valve magnetics.
A.The viscous effect of the fluid and the shape of the valve (direct-flow passage vs. indirect) also affect damping.
A. Yes, but such valves are not trivial to design. Rather than attempt to directly cancel the permanent magnet, it is generally much more efficient to redirect the magnet flux from the primary flow channel to a secondary, high-reluctance gap.
A. Yes, in principle one could use batteries or capacitors to energize the coil. One would need to carefully design the magnetic circuit to achieve the magnetic field strength within the desired time-scale. If the damper needs to turn on quickly, the time lag is mostly in the magnetic circuit.
A. LORD understands and has experience in complete system design- MR fluids, MR devices, controls and magnetics.
- Unparalleled MR fluid prototyping and testing capabilities
- Unique magnetic fluid property characterization equipment
- Lifetime, extreme temperature, and material compatibility testing
- Database of hundreds of past formulations
- MR device consulting, design, and manufacturing
- Engineering expertise covering linear and rotary MR devices
- Customized tools to quickly design and characterize MR devices
- Proficiency in many software packages including CAD (Solidworks), FEA (COSMOS, Ansys), Simulation (TruckSim), etc.
- Rapid controls prototyping (DSpace, Matlab/Simulink, etc.)
- Design and build embedded microcontroller-based electronics
- High bandwidth design knowledge required for fast MR system response
- Fault tolerant and high speed communication protocols (CAN)
- Algorithm development customized to specific vehicle platforms and customers
- Magnetic circuit optimization
- Proprietary MR valve design spreadsheets
- 2D and 3D FEA (Maxwell, Ansys)
SELECTING AND USING LORD MR FLUIDS & DEVICES
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A. The various carrier fluids and other components will affect seal compatibility, operating temperature range and other properties. See our MR fluid comparison sheet.
A. The first consideration is the carrier fluid: water, hydrocarbon (organic) oil, or silicone oil. Choose the liquid carrier depending on the environment of the application. A hydrocarbon based oil is always the best choice from a cost/performance perspective UNLESS the MR fluid will be in contact with an organic rubber like natural rubber, or the temperature extremes are great.
If broad temperature-range performance is desired below 40°C and above 100°C, a silicone-based fluid may be appropriate. Please note that silicone is very difficult to seal.
The second item to consider is iron content. LORD MR fluids can be made with different amounts of iron depending on the application. The iron content directly affects the yield stress of the fluid at a given magnetic field. Amounts range from approximately 20 percent to 40 percent iron. Viscosities vary with iron content as well. Higher iron-content fluids are “thicker”; in other words, their off-state viscosity is higher (as is their cost per liter).
A. MR technology was not able to become a practical reality until the age of fast, affordable microprocessors and sensors arrived. We have extensive commercial experience designing magnetic circuits, electronic controllers, and control algorithms for MR devices, all of which must be optimized to take advantage of the fast response time of MR technology.
For development purposes, MR devices can be supplied by the output current of the RD-3002-03, which in turn can be controlled with a 0-5 volts DC control analog signal input. An analog voltage can be generated from a digital device (such as a computer) with the appropriate software & hardware (Digital/Analog card). Other devices that create a 0-5 Volt DC analog output signal (i.e. a wave form generator) can also be used.
A. The MR damper contains a pressurized gas accumulator. Thus, the device acts as if it has a small spring that is mechanically parallel to the damper. The gas pressurization is necessary to accommodate the variable amount of rod volume inside the damper and to avoid cavitation. Unless some external means is applied to position the damper at mid-stroke, the damper will always extend to the limit of its travel.
A. As a general rule these MR dampers are rated for 2 amps with intermittent duty. The coils are about 4.5 ohms so that is (P=I^2 * R=4*4.5=) 18 watts. If they are left on too long (perhaps 15 minutes) at 2 amps, they will get very hot. If one limits the current to say, 1 amp, this would be better.
A. That depends on whether the damper is also being stroked. If the damper coil is energized to produce the magnetic field but the piston is not moving, no energy is dissipated and there is no substantial generation of heat.
When the damper is on and the piston is moving, energy is dissipated as heat. When stroking the damper on a test machine under constant current for extended periods of time, active cooling may be required to keep the damper from overheating. For example, the RD-1005-3 MR Linear Damper reaches 120-140°C (external body temperature) if it is continuously stroked at 2 Hz and +/- 0.5 inches at 0.5 amp current in a room temperature environment with no active cooling. The recommended upper temperature for continuous use is 160°F (70°C).
A. Although the dampers will have some of the same basic characteristics, there will be other differences in addition to overall force level:
- Does not have a pressurized gas accumulator that acts as a series spring element, thus there will be no force offset due to an accumulator. Unlike the RD-1005-3, the forces in extension and compression will be exactly symmetric.
- Is an unsealed device and uses a MR grease instead of a MR fluid.
- Will show less hysteresis (this is apparent in the force/velocity curves) and less velocity dependence in the force.
- Is a demonstration device, unlike the RD-1005-3 which uses production processes.
A. The syringe demonstrators are filled with a water-based fluid to make them as safe as possible. Some syringes seal slightly better than others, but those that do not seal as completely can experience some evaporation of the water that is in the fluid. This evaporation leads to high iron concentrations and the fluid becomes too thick to move. Some of these units are fine after many years, but some start to get stiff after about a year.