Selecting and Using LORD MR Fluids and Devices
Q. What are the different properties of LORD MR fluids?
A. The various carrier fluids and other components will affect seal compatibility, operating temperature range and other properties. See our MR fluid comparison sheet.
Q. What should I consider about my application before making a decision on which LORD MR fluid to purchase?
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).
Q. How can I control a MR Device?
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.
Q. Why is the force needed to compress the RD-1005-3 (MR damper) slightly larger than the force required to extend the damper?
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.
Q. How much power can RD-1005-3 (MR Linear Damper) take?
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.
Q. Is heat dissipation an issue if a MR damper is continuously on?
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).
Q. What are the major differences between the MR Controllable Friction Damper (RD-1097-01) and the MR Linear Damper (RD-1005-3)?
A. Although the dampers will have some of the same basic characteristics, there will be other differences in addition to overall force level:
The RD-1097-0:
- 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.
Q. I have a syringe demo that doesn’t work anymore. Why?
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.