Performance of LORD MR Fluids
Q. Do LORD MR fluids settle?
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.
Q. How does fluid settling affect the performance of MR devices?
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.
Q. Iron particles in oil sounds abrasive. Are there issues with abrasion?
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.
Q. How does temperature affect MR fluid?
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 controllable across a wide range of temperatures.
Q. What is the operating and storage life of LORD MR fluid?
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.
Q. How can one be sure that the fluid is mixed properly?
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.
Q. How consistent is LORD MR fluid performance over time?
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.
Q. What is the time response of MR fluid?
A.Response time of MR fluid is <10^-3 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.
Q. What is the maximum allowable pressure for MR fluid?
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.
Q. Does MR fluid change volume during activation?
A.No, the fluid does not change volume when a magnetic field is applied.
Q. Can MR fluid be exposed to the air and then resealed?
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.