Properties and Applications of Commercial Magnetorheological Fluids
Mark R. Jolly, Jonathan W. Bender, and J. David Carlson
Thomas Lord Research Center
Lord Corporation
110 Lord Drive
Cary, NC 27511
ABSTRACTThe rheological and magnetic properties of several commercial magnetorheological (MR) fluids arepresented and discussed. These fluids are compared using appropriate figures of merit based onconventional design paradigms. Some contemporary applications of MR fluids are discussed. These
applications illustrate how various material properties may be balanced to provide optimal performance.
Keywords:
properties.

magnetorheological fluids, MR fluids, magnetorheological fluid dampers, MR fluid
1. INTRODUCTION

Magnetorheological (MR) fluids are materials that respond to an applied magnetic field with achange in rheological behavior. Typically, this change is manifested by the development of a yield stressthat monotonically increases with applied field. Interest in magnetorheological fluids derives from theirability to provide simple, quiet, rapid-response interfaces between electronic controls and mechanicalsystems.
That magnetorheological fluids have the potential to radically change the way
electromechanical devices are designed and operated has long been recognized.
MR fluids are considerably less well known than their electrorheological (ER) fluid analogs. Bothfluids are non-colloidal suspensions of polarizable particles having a size on the order of a few microns.
The initial discovery and development of MR fluids and devices can be credited to Jacob Rabinow at the
US National Bureau of Standards (Rabinow, 1948a, 1948b, 1951) in the late 1940s. Interestingly, thiswork was almost concurrent with Winslow's ER fluid work. The late 1940s and early 1950s actually
saw more patents and publications relating to MR than to ER fluids. Except for a flurry of interest after
their initial discovery, there has been scant information published about MR fluids. Only recently has aresurgence in interest in MR fluids been seen (Shtarkman, 1991; Kordonsky, 1993; Weiss et al., 1993;
Carlson et al.,1994; Carlson, 1994; Carlson and Weiss, 1994). While the commercial success of ER
fluids has remained elusive, MR fluids have enjoyed recent commercial success. A number of MR
fluids and various MR fluid-based systems have been commercialized including an MR fluid brake foruse in the exercise industry (Anon., 1995; Chase, 1996), a controllable MR fluid damper for use in truck
seat suspensions (Carlson, Catanzarite and St.Clair, 1995; Lord, 1997) and an MR fluid shock absorberfor oval track automobile racing.
The magnetorheological response of MR fluids results from the polarization induced in thesuspended particles by application of an external field. The interaction between the resulting induced
dipoles causes the particles to form columnar structures, parallel to the applied field. These chain-likestructures restrict the motion of the fluid, thereby increasing the viscous characteristics of thesuspension. The mechanical energy needed to yield these chain-like structures increases as the appliedfield increases resulting in a field dependent yield stress. In the absence of an applied field, MR fluids
exhibit Newtonian-like behavior. Thus the behavior of controllable fluids is often represented as aBingham plastic having a variable yield strength (e.g., Phillips, 1996). In this model, the flow isgoverned by Bingham’s equations:τ = τy (H) + η γ& , τ≥ τy(1)

at stresses τ above the field dependent yield stress τy. Below the yield stress (at strains of order 10-3), thematerial behaves viscoelastically:τ = G γ , τ< τy(2)where G is the complex material modulus. It has been observed in the literature that the complex
modulus is also field dependent (Weiss, Carlson and Nixon, 1994; Nakano, Yamamoto and Jolly, 1997).
While the Bingham plastic model has proved useful in the design and characterization of MR fluidbased devices, true MR fluid behavior exhibits some significant departures from this simple model.
Perhaps the most significant of these departures involves the non-Newtonian behavior of MR fluids inthe absence of a magnetic field (Kormann, Laun and Klett, 1994).
This paper is organized as follows. In the next section, some common MR fluid device design
theory is reviewed. Simple MR fluid figures of merit based on this theory are then presented. Section 3
presents the properties of four commercially available MR fluids (Lord, 1998). Using some of theseproperties as a basis, figures of merit of the four MR fluids are then presented. In section 4, a few
contemporary device applications of MR fluids in linear motion devices are discussed. Conclusions arepresented in section 5.

2. MR DEVICE DESIGN AND FIGURES OF MERIT

2.1 Common Device Geometries

Figure 1: Basic operational modes for controllable fluid devices: (a) pressure driven flow mode, and (b)direct shear mode (after Carlson et al., 1994).

Most devices that use controllable fluids can be classified as having either fixed poles (pressure
driven flow mode) or relatively moveable poles (direct-shear mode). Diagrams of these two basic
operational modes are shown in Fig. 1. Examples of pressure driven flow (PDF) mode devices include
servo-valves, dampers and shock absorbers. Examples of direct-shear mode devices include clutches,brakes, chucking and locking devices. A third mode of operation known as squeeze-film mode has also
been used in low motion, high force applications (Jolly and Carlson, 1996).
The pressure drop developed in a device based on pressure driven flow mode is commonly assumed
to result from the sum of a viscous component ∆Pη and a field dependent induced yield stress component∆Pτ. This pressure may be approximated by (Phillips, 1969)
12 ηQ L c τ y (H) L(3)∆P = ∆Pη + ∆Pτ (H) =
+
3
g
g w
where L, g and w are the length, gap and width of the flow channel between the fixed poles, Q is thevolumetric flow rate, η is the viscosity with no applied field and τy is the yield stress developed in
response to an applied field H. The parameter c is a function of the flow velocity profile and has a value
ranging from a minimum value of 2 (for ∆Pτ/∆Pη less than ~1) to a maximum value of 3 (for ∆Pτ/∆Pη
greater than ~100). In a like manner, the force developed by a direct-shear device is:
ηS A+ τ y (H)A(4)
F = Fη + Fτ (H) =gwhere S is the relative pole velocity and A=Lw is the shear (pole) area.
2.2 Active Fluid Volume and Device Aspect Ratio
While Equations (3) and (4) are certainly useful in the design of controllable fluid devices, theyoften do not provide the best insight into the significance of the various parameters. It is often useful toalgebraically manipulate the above equations to provide a minimum active fluid volume (Duclos, 1988;
Carlson et al., 1994): η
V = k  2  λ Wm
τ 

(5)

where k is a constant and V = Lwg can be regarded as the necessary active fluid volume in order toachieve the desired control ratio λ at a required controllable mechanical power level Wm. For pressure
driven flow: k=12/c2, λ=∆Pτ/∆Pη and Wm=Q ∆Pτ. For direct shear: k=1, λ=Fτ/Fη and Wm=Fτ S. It is
important to note that in both cases the minimum active fluid volume is proportional to the product of
three terms: a term that is a function of fluid material properties (η/τ2); the desired control ratio or
dynamic range λ; and the controlled mechanical power dissipation Wm sought.
By noting that for both geometries, V = Lwg, Equation (5) can be further manipulated to givewg 2 =

12  η  λQc  τ η
g =   λS τ(pressure driven flow mode)(6)(direct shear mode)(7)

These two equations provide geometric constraints and necessary aspect ratios for MR devices based on
MR fluid properties, the desired control ratio or dynamic range, and the device speed (or flow). It isinteresting to note that while the active fluid volume depends on the ratio of viscosity to yield stresssquared (η/τ2), the geometric constraint depends upon the ratio η/τ.

2.3 MR Fluid Figures of Merit
The mechanical and electrical power densities of an MR device are given byŴm = τ γ&(8)

ˆ = BH
W
e
2 tc

(9)

where γ& is the fluid shear rate (or normalized flow rate, e.g., γ& =S/g), B and H are the magnetic flux
density and field in the fluid and tc is the characteristic time for the establishment of a magnetic field in
the fluid. A fluid efficiency can now be defined as the ratio of the mechanical and electrical power
densities given by:)Wmτα = ) = 2(t c γ& )
(10)
BH
We
where τ and B are defined at a common H.
From the above developments, several figures of merit can be conceived. The first is based on theactive fluid volume:
F1 =

τ2
η

(11)

which is seen to be inversely proportional to the minimum active fluid volume V and has units of Pa/s.
Maximizing
this figure of merit will minimize device size and electrical power consumption since)
We= We V. This figure of merit also reflects the dynamic range of a device based on a specified fluid
since, for a given active fluid volume, as F1 increases so will λ (Eq. 5). For weight sensitive
applications, this figure of merit may be modified to penalize large fluid densities ρ:
2

F2 =

τ
ηρ

(12)

which is inversely proportional to the minimum active fluid mass and has units of m2/s3. Finally, forhigh bandwidth applications, a unitless figure of merit based on power efficiency (Equation 10) is given
by:
F3 =

τ
BH

(13)

Maximizing this figure of merit will minimize electrical power consumption of an MR device for agiven delivered mechanical power at a desired bandwidth.
These three figures of merit provide a generic means of comparing the nominal behavior of various
MR fluids. Other factors such as stability, durability and temperature range will typically have asignificant impact on the suitability of an MR fluid for a particular application.

3. PROPERTIES OF COMMERCIAL MR FLUIDS

Magnetic, rheological, tribological and settling properties of four commercial MR fluids are
discussed. The basic composition of these four fluids is given in Table 1.
Table 1. Basic composition and density of four commercial MR fluids (Lord, 1998).
Commercial
Percent Iron
Carrier Fluid
Density (g/ml)
MR Fluid
by Volume
MRX-126PD

26

Hydrocarbon oil

2.66

MRX-140ND

40Hydrocarbon oil

3.64

MRX-242AS

42

Water

3.88

MRX-336AG

36

Silicone oil

3.47

3.1 Rheological Properties

The rheological properties of controllable fluids depend on concentration and density of particles,
particle size and shape distribution, properties of the carrier fluid, additional additives, applied field,
temperature, and other factors. The interdependency of all these factors is very complex, yet isimportant in establishing methodologies to optimize the performance of these fluids for particularapplications.
The magnetorheological effect of the four MR fluids was measured on a custom rheometer using a
46 mm diameter parallel plate geometry set at a 1 mm gap. In the parallel plate geometry, shear ratevaries linearly across the fluid sample with the maximum shear rate occurring at the outer radius. The
rheometer is capable of applying greater than 1 Tesla through the fluid sample. Figure 2 shows the shear
stress in the MR fluids as a function of flux density at a maximum shear rate of 26 s-1. At such a low
shear rate, this shear stress data is approximately equivalent to the fluid yield stress as defined in Eq. (1).
At low flux densities, the fluid stress can be seen to exhibit a power law behavior. The approximate
power law index of 1.75 lies in the range of low to intermediate field behavior predicted bycontemporary models of magnetorheology. Both linear models and models accounting for nonlinear
magnetic effects such as particle saturation (Ginder, Davis and Elie, 1995; Jolly, Carlson and Muñoz,
1996) predict quadratic behavior at very low flux densities. The non-linear model proposed by Ginder,
Davis and Elie (1995) predicts a power law index of 1.5 at intermediate fields. Beyond flux densities ofabout 0.2-0.3 Tesla, the effects of magnetic saturation are revealed as a departure from power law
behavior. The stress response ultimately plateaus as the MR fluids approach complete magnetic
saturation. As can be seen, the flux density at which this saturation occurs increases as the iron volume
fraction in the fluid increases.
In all MR fluid formulations optimized for a specific application or class of applications, the fluid
viscosity in the absence of a field is most significantly a function of the carrier oil, suspension agents,
and particle loading. The figures-of-merit described earlier are benefited by a low fluid viscosity, butmust be balanced with other fluid requirements such as temperature range and particle resuspendability.
Because of both the addition of suspension agents and changes in magnetic particle microstructureduring shear, most MR fluids exhibit significant shear thinning.

Figure 2. Fluid shear stress as a function of magnetic flux density at a maximum shear rate of 26 s-1.
Ascending order of the plots corresponds increasing iron volume fraction.

Figure 3. Viscosity as a function of shear rate at 25°C for MRX-126PD ({), MRX-242AS (•), MRX140ND (), and MRX-336AG (■).
The viscosity of the four MR fluids was measured on a TA Instruments CS2500 controlled stressrheometer using a cone and plate geometry (40 mm diameter and 1° cone angle). Figure 3 shows
viscosity of the four MR fluids a function of shear rate. It can be seen that the fluid viscosity does notstrictly scale with iron loading. Indeed, the fluid viscosity is significantly a function of the compositionand chemistry of the carrier oils. The MR fluids shown are seen to exhibit low shear rate power law
behavior with power law indices between -0.75 and -1. At high shear rates, the rate of shear thinning
decreases and will approach an asymptotic value dictated largely by the solids loading, carrier oil
viscosity, and extent of shear thinning of the suspension agents. MRX-336AG exhibits nearly an orderof magnitude greater viscosity at low shear rate as compared to the other MR fluids. This is a result of asignificant zero-field yielding behavior of this fluid, characterized by a -1 power law index at low shear
rates. The plastic viscosity ( ∂σ / ∂γ& ) beyond the yielding regime is actually quite low (~10-1 Pa-s).
3.2 Magnetic Properties

Understanding the magnetic properties of MR fluids is important for designing MR fluid-based
devices. In many such devices, the MR fluid represents the largest magnetic reluctance within themagnetic circuit. These magnetic properties may also prove useful in providing insight into thecharacter and formation of particle structures within the fluid.
.

Figure 4. Flux density within MR fluids as a function of applied field. Inset: Intrinsic induction as afunction of applied field. Ascending order of the plots corresponds to increasing iron volume
fraction.
Magnetic induction curves, or B-H curves, of the four commercial MR fluids are shown in Figure 4.
As can be seen, the MR fluids exhibit approximately linear magnetic properties up to an applied field of
about 0.02/µo A/m, where µo =4πe-7 T-m/A is the permeability of a vacuum. In this region, thedifferential permeability (the slope of B(H)) of the MR fluids is relatively constant. These permeabilities
vary between 5 and 9 times that of a vacuum. The magnetic properties of MR fluids vary significantly
from the properties of most bulk ferromagnetic properties in that ferromagnetic induction can typicallybe linearized over a much broader range of applied field and the corresponding permeabilities are several
orders of magnitude greater. MR fluids begin to exhibit gradual magnetic saturation beyond the linear
regime. Complete saturation typically occurs at fields beyond 0.4/µo A/m. The intrinsic induction orpolarization density (B-µo H) of an MR fluid at complete saturation is φJs Tesla, where φ is the volume
percent of particles in the fluid and Js is the saturation polarization of the particulate material (Jolly,
Carlson and Muñoz, 1996). For example, a fluid containing 30% iron (Js=2.1 Tesla) saturates at about(0.3)(2.1)=0.63 Tesla. Little or no hysteresis can be observed in the induction curves. This
superparamagnetic behavior is a consequence of the magnetically soft properties of the iron used as
particulate material in these fluids and the mobility of this particulate phase.
3.3 Lubricity, Settling Characteristics, and Material Compatibility

MR fluid is inherently somewhat abrasive. The degree to which it will effect the durability of aparticular part or component depends on a number of factors associated with fluid composition anddevice design. Wear bands and other bearing materials that are exposed to MR fluid in direct sliding
contact are often critical abrasion regions, particularly in linear device applications. The ability of MR
fluids to lubricate conformal sliding surfaces will reflect the propensity for wear of such surfaces within
MR fluid-based devices. The coefficients of friction for MR fluid lubricated iron-on-iron and nylon-oniron conformal interfaces are given in Table 2. These were measured with an Instron Model 4204 using
a standard sled geometry with contact dimensions of 7.6 cm by 7.6 cm. These results reflect the
averages of multiple runs at a sled velocity of 2.6 mm/s and with varying normal forces in the range of10 N to 20 N (no magnetic field was present). It can be seen that with MR fluid lubrication, the sliding
friction is reduced by a factor of two or three over the dry interface. However, it was found that themeasurement of MR fluid-lubricated sliding friction by this method exhibited somewhat poor
reproducibility. This is attributed to the complex rheological properties of MR fluids implying thatsliding friction properties will be a function of both sliding speed and interfacial fluid film thickness.
Table 2. Average coefficients of sliding friction for MR fluid-lubricated conformal interfaces.
Commercial
Iron-on-Iron
Nylon-on-Iron
MR Fluid
Dry

0.18

0.19

MRX-126PD

0.04 - 0.07

0.04 - 0.07

MRX-140ND

0.07 - 0.09

0.05 - 0.07

MRX-242AS

0.05 - 0.07

0.06 - 0.07

MRX-336AG

0.08 - 0.11

0.08 - 0.11

As with any micron-scale particulate suspension in which a density mismatch between theparticulate and surrounding fluid exists, settling in MR fluids must be considered when formulating.
Again, the specific application dictates to what extent settling must be controlled and how it is to bemeasured. For example, a seismic damper application may have very strict settling requirements,
whereas settling may be unimportant in a brake application. A simple, non-application specificcomparison of settling has been summarized in Table 3, which tabulates the initial rate of settling as apercentage of total height of a fluid column. In this experiment, the fluids were placed in a glass
graduated cylinder and left to settle for up to five weeks at room temperature. (Note that settling results
in a particle-free fluid on top of a particle-rich suspension, and as settling occurs, the settling rate slows.
Thus, the initial settling rate is the most rapid period of settling.) In general, settling is governed by therheological character of the suspending medium, surface properties of the magnetic particles, and thepresence of surface active agents. In a device, settling can be further influenced by the presence ofremnant fields, device orientation, and aspects of the device geometry. For the fluids studied here, thefluids ranked in order of viscosity. Those with the highest viscosity (Figure 3) exhibited the lowest
initial settling rate.
Table 3. Initial settling rates.
Fluid
Initial settling rate(%/day)MRX-126PD
1.0
MRX-140ND

0.3

MRX-242AS

0.2

MRX-336AG

0.0 (non-detectable)

Other criteria governing the composition of an MR fluid include the device operating
temperature range, compatibility with materials within the device, and whether or not the fluid will be
sealed from the atmosphere. Table 4 contains a list of typical seal materials with which the selectedfluids are checked for excessive swelling or loss in strength. (Note that this information is intended
merely as a general guideline.) Of the fluids listed, water-based fluid MRX-242AS is the only fluid thatshould not be used in an unsealed system.
Table 4. Temperature and compatibility properties.
Commercial
Temperature
Compatibility with...
MR Fluid
Range
NaturalNitrile Fluoro- Silicone EPDM Neoprene
RubberRubber elastomer
MRX-126PD-40°C to 150°C

poor

good

good

fair

poor

good

MRX-140ND-40°C to 150°C

poor

good

good

fair

poor

good

MRX-242AS

5°C to 90°C

good

good

good

good

good

good

MRX-336AG-50°C to 200°C

good

good

good

poor

good

good

3.4 MR Fluid Figures of Merit

Figure 5 shows the figure of merit F1= τ2/η as a function of shear rate for the four MR fluids. This
figure of merit reflects the dynamic range (or control ratio) of an MR device as well as required MR
fluid volume and power consumption. It is seen that as shear rate increases, the four fluids exhibit
significant improvements in F1 due to the substantial shear thinning character of these fluids. A broad
range of F1 is observed amongst the MR fluids resulting from the acute dependence of F1 on viscosity.
Figure 6 shows the figure of merit F2= τ2/ηρ as a function of shear rate. This figure of merit is closely
related to F1 except that it further penalizes fluid density. With the exception of vertical shifts in thedata, F2 demonstrates much the same character as F1. It is interesting to note that, as measured by F2,
MRX-126PD is particularly well suited for low shear rate applications and MRX-242AS is better suitedfor high shear rate applications. Both F1 and F2 illustrate that MR fluids in general exhibit better
rheological behavior at higher shear rates due to the shear thinning character.
The figure of merit F3= τ/ΒΗ as a function of flux density is shown in Figure 7 for the four MR
fluids. This figure of merit is related to the power efficiency of MR fluids and is important inapplications where high bandwidth is sought. The dashed lines in this data correspond to asymptotic
extrapolation generated with least-squares fits. This was done as an alternative to presenting theincreased level of measurement and computational noise present at lower flux densities. It can be seenthat there is a steady decrease in F3 as flux density increases with a plummet in F3 as the MR fluid begins
to magnetically saturate. Ultimately, F3 will approach zero as the fluid approaches the condition forwhich increases in magnetic input cease to result in increased yield stress. The MR fluids with higher
iron loading generally measure better by F3 since iron loading results both in higher magnetic
permeability and higher yield stresses. Interestingly, this proves strictly true only at higher flux densities
for the four MR fluids shown in Figure 7.

Figure 5. F1 as a function of shear rate at 25°C and 1.0 Tesla for MRX-126PD ({), MRX-242AS (•),
MRX-140ND (), and MRX-336AG (■).

Figure 6. F2 as a function of shear rate at 25°C and 1.0 Tesla for MRX-126PD ({), MRX-242AS (•),
MRX-140ND (), and MRX-336AG (■).

Figure 7. F3 as a function of flux density at 25°C for MRX-126PD ({), MRX-242AS (•), MRX-140ND(), and MRX-336AG (■).
4. APPLICATIONS OF COMMERCIAL MR FLUIDS
4.1. Heavy Duty Vehicle Seat Suspensions

Recently a small, monotube MR fluid-based damper (shown in Fig. 8) has been commercialized foruse in a semi-active seat suspension system for large on- and off-highway vehicles (Carlson, Catanzarite
and St.Clair, 1995; Lord, 1997). In this application the MR damper represents enabling technology for avariety of semi-active control schemes. This damper has also served as a testbed for developing
phenomenological device models (Spencer, et al., 1996; Pang et al., 1998).
Accumulator
N2 - 20 Bar

Diaphragm

Coil

Annular Orifice

Bearing & Seal

MR Fluid

Wires toElectromagnet

Figure 8. Commercial Linear MR Fluid-based Damper.

The controllable MR damper is capable of providing a wide dynamic range of force control for very
modest input power levels as shown in Figure 9. The damper is 4.1 cm in diameter, has a 17.9 cm eyeto-eye length at mid-stroke and has a ±2.9 cm stroke. The MR fluid valve and associated magnetic
circuit is fully contained within the piston. Current is carried to the electromagnetic coil via the leads
through the hollow shaft. An input power of 5 watts is required to operate the damper at its nominal
design current of 1 amp. Although the damper contains about 70 cm3 of MR fluid, the actual amount of
fluid activated in the magnetic valve at any given instant is only about 0.3 cm3.
Amps

2.5

1.5
1.0
.75
.50

2

Force(kN)

1.5
1

.25
0.0

.5
0
-.5
-1

-1.5
-2
-2.5
-40 -30

-20

-10

0

10

20

30

40

Velocity (cm/s)

Figure 9. Performance curves for the linear MR damper
This commercial linear MR damper has served as a test bed to demonstrate that MR dampers canhave long cycle lives with no significant abrasive wear caused by the MR fluid. As part of the standard
product test protocol these dampers are routinely operated through 5-10 million cycles without seal
failure. This corresponds to a cumulative linear travel of over 250 km. The key to this long life comesfrom a combination of dynamic seal design, material selection and MR fluid chemistry. MRX-126PD istypical of the type of fluid used in dampers having a dynamic shaft seal. In addition, this fluid isformulated to have a very low viscosity as a very low off-state is necessary in order to effectively
implement semi-active control in this application. The combination of carefully chosen particle
morphology and highly effective anti-wear ingredients allow MR fluids to be used reliably in
conjunction with dynamic seals. This linear MR damper has also served to demonstrate that the stabilityof the MR fluid suspension is not an issue for this class of device. Even though the MR fluid used inthis damper was formulated to have a very low plastic viscosity, particle separation and settling do notpresent a problem. The additives in the fluid are very effective at preventing caking or particle
sedimentation.
4.2. Control of Seismic Vibrations in Structures

To prove the scalability of MR fluid technology to devices of appropriate size for civil engineering
applications, a full-scale, MR fluid damper has been designed and built (Carlson and Spencer, 1996,
1997; Dyke et al., 1996). For the nominal design, a maximum damping force of 200,000 N (20-ton) anda dynamic range of ten were chosen. A schematic of the large-scale MR fluid damper is shown in Fig.
10.

The seismic damper uses a particularly simple geometry in which the outer cylindrical housing ispart of the magnetic circuit. The effective fluid orifice is the entire annular space between the piston
outside diameter and the inside of the damper cylinder housing. Movement of the piston causes fluid toflow through this entire annular region. The damper is double-ended, i.e. the piston is supported by ashaft on both ends. This arrangement has the advantage that a rod-volume compensator does not need to
be incorporated into the damper, although a small pressurized accumulator is provided to accommodate
thermal expansion of the fluid. The damper has an inside diameter of 20.3 cm and a stroke of ±8 cm.
The electromagnetic coil is wound in three sections on the piston. This results in four effective valve
regions as the fluid flows past the piston. The coils contain a total of about 1.5 km of magnetic wire.
The completed damper is approximately 1 m long and with a mass of 250 kg. The damper contains
approximately 5 liters of MR fluid. The amount of fluid energized by the magnetic field at any given
instant is approximately 90 cm3.
Unlike the vehicular seat damper described above which experiences a very lively dynamic
environment, the seismic damper is (hopefully) quiescent for most of its life. Such a damper could
reasonably be expected to do virtually nothing for many years. However, when a seismic event does
occur it must be able to function without hesitation. Such a mode of operation means that one cannot
rely on dynamic stroking of the damper to provide any mixing action to maintain a well dispersed fluid.
Rather, the fluid must not exhibit significant settling or separation over extended periods of time. MRX140ND is an MR fluid designed for use in such a seismic damper. Rather than being a totally freeflowing liquid, this MR fluid is purposely formulated to have a small but significant yield strength in its
off-state. This gives the fluid a custard-like or very soft grease-like consistency which prevents theparticles from gravitational settling. As the damper may only experience a few hundred cycles during aseismic event having a duration of one or two minutes, seal life and wear are minor considerations.

MRD-

Magnetic Flux

Thermal Expansion
Accumulator
Fluid
Flow

3-Stage Piston
MR Fluid

PistonMotionLORD
Rheonetic™ Seismic Damper
MRD-9000

Figure 10. Schematic of MR fluid seismic damper

4.3 “Seal-Less” Vibration Damper
A small, controllable MR fluid vibration damper that is being used for real-time, active-control ofdamping in industrial applications is shown in Figure 11 (Carlson, Catanzarite and St.Clair, 1995). The
damper functions by moving a small steel disk or baffle in a chamber of MR fluid. Primary controlled
motion is axial although secondary lateral and flexing motions may also be accommodated. Damping
forces of 0 to ± 125 N are produced in the primary direction. This damper may be also be used as alocking device. This damper does not require dynamic, sliding seals. The relatively small amplitudes
encountered (± 3 mm) allow the use of elastomeric rubber elements instead. Typically these dampers areused as controllable dashpots in parallel with separate spring elements. Stiffness may also be added tothe elastomeric elements so that the damper functions as a controllable mount.

Because this damper is totally sealed, a water-based MR fluid may be used. This is highly desirable
as the natural rubber diaphragms are not compatible with the more common hydrocarbon oil based
fluids. Water-based fluids can be made exceptionally stable because of the very wide choice of watersoluble stabilizers and surfactants that are available. Unfortunately, their use in long-stroke dampers
having dynamic seals is precluded however because the microscopic film of water that wets the shaft
evaporates slightly on every cycle and eventually leaves the damper dry. Water-based fluids having aconsistency of a light thixotropic paste such as MRX-242AS work exceptionally well in small sealed
devices of this type.

Baffle moves inchamber of MR fluid

Elastomeric
Diaphragms

Rheonetic™

5.1 cmCoil

MR Fluid

Figure 11. Schematic of small MR fluid vibration damper.
5. CONCLUSIONS

The rheological, magnetic, and material properties of four commercial MR fluids have beenpresented. Several figures of merit for MR fluids based on these properties were also computed andpresented. The examples presented illustrate that the properties and attributes of commercially available
MR fluids are wide-ranging. Some of these attributes are summarized in Table 5. It is evident that MR
fluids have evolved from laboratory curiosity to true engineering materials that involve engineering
trade-offs. The formulation of MR fluids involves the optimal balancing of properties for particularapplications or class of applications. Several applications were discussed to illustrate how various
material properties may be balanced.
Table 5. Ranking of fluids on the basis of various material properties,
with ‘1’ being best or highest and ‘4’ being worst or lowest.
Friction Temper.
Compatibility with...
Fluidτ2/η τ2/ηρ τ/ΒΗ Initial
Settling coefficient Range
Nat. Rubber Silicone
MRX-126PD
2
2
4
4
1
3
4
3
MRX-140ND

3

3

2

3

2

2

3

2

MRX-242AS

1

1

1

2

3

4

2

1

MRX-336AG

4

4

3

1

4

1

1

4

6. REFERENCES

Anon., 1995, “Brake cuts exercise-equipment cost,” Design News, Dec. 4.
Carlson, J. D. et al., 1994, "Magnetorheological Fluid Dampers", U.S. Patent 5,277,282. (1994) and"Magnetorheological Fluid Devices", U.S. Patent 5,284,330.
Carlson, J. D., 1994, "The Promise of Controllable Fluids", Actuator 94, 4th Int. Conf. on New
Actuators, eds. H. Borgmann and K. Lenz, Axon Technologies Consult GmbH, 1994, 266-270.
Carlson, J. D. and K. D. Weiss, 1994, "A Growing Attraction To Magnetic Fluids", Machine Design,
Aug. 8 61-66.
Carlson, J. D., D. M. Catanzarite and K. A. St. Clair, 1995, 5th Int. Conf. on Electrorheological,
Magneto-rheological Suspensions and Associated Technology, Sheffield, July.
Carlson, J.D. and Spencer Jr., B.F., 1996, “Magneto-Rheological Fluid Dampers for Semi-Active
Seismic Control”, Proc. 3rd Int. Conf. on Motion and Vib. Control, Chiba, JP, Vol. III, pp. 35-40.
Carlson, J.D. and Spencer Jr., B.F., 1997, “Magnetorheological Fluid Dampers for Seismic Control”,
Proc. of DETC’97, ASME, Sacramento, VIB4124.
Chase, V.D., 1996, “Cutting Edge,” Appliance Manufacturer, May, pp. 6.
Duclos, T.G., 1988, “Design of Devices Using Electrorheological Fluids” SAE Paper 881134, Future
Transp. Techn. Conf. and Exp., San Francisco, CA, Aug. 8-11.
Dyke, S.J., B.F. Spencer, M.K. Sain and J.D. Carlson, 1996, “Seismic Response Reduction Using
Magnetorheological Dampers,” Proc. of the IFAC World Congress, San Francisco, CA.
Ginder, J.M., L.C. Davis and L.D. Elie, 1995, “Rheology of Magnetorheological Fluids: Models andMeasurements,” 5th Int. Conf. on ERF, MRS and Thier Applications, July, 1995, Univ. Sheffield,
UK.
Jolly, M. R. and J. D. Carlson, 1996, “Controllable Squeeze Film Damping Using Magnetorheological
Fluid” Actuator 96, 5th Int. Conf. on New Actuators, eds. H. Borgmann and K. Lenz, Axon
Technologies Consult GmbH.
Jolly, M. R., J. D. Carlson and B. C. Muñoz, 1996, “A Model of the Behaviour of Magnetorheological
Materials,” Smart Mater. Struct. 5, 607-614.
Kordonsky, W., 1993, “Magnetorheological Effect As a Base of New Devices and Technologies", J.
Mag. and Mag. Mat, 122, 395-398.
Kormann, C., M. Laun and G. Klett, 1994, Actuator 94, 4th Int. Conf. on New Actuators, eds. H.
Borgmann and K. Lenz, Axon Technologies Consult GmbH, 271.
Lord, 1997, Lord Corporation Rheonetic Linear Damper - RD-1001/RD-1004 Product Information
Sheet, (Lord Corp. Pub. No. PS RD-1001/4).
Lord, 1998, www.mrfluid.com
Nakano, M., H. Yamamoto and M.R. Jolly, 1997, “Dynamic Viscoelasticity of a Magnetorheological
Fluid in Oscillatory Slit Flow,” 6th Int. Conf. on ERF, MRS and Their Applications, 22-25 July,
1997, Yonezawa, JP.
Pang, L., G.M. Kamath and N.M. Wereley, 1998, “Analysis and Testing of a Linear Stroke
Magnetorheological Damper,” AIAA/ASME Adaptive Structures Forum, Paper No. AIAA 98-2040,
Long Beach, CA, April, 1997.
Phillips, R. W., 1969, "Engineering Applications of Fluids with a Variable Yield Stress", Ph.D. Thesis,
University of California, Berkeley .
Rabinow, J., 1948, "The Magnetic Fluid Clutch", AIEE Trans., 67 1308-1315.
Rabinow, J., 1948, "Magnetic Fluid Clutch", National Bureau of Standards Technical News Bulletin,
32(4) 54-60.
Rabinow, J., 1951, "Magnetic Fluid Torque and Force Transmitting Device", U.S. Patent 2,575,360.
Shtarkman, E.M., 1991, U.S.Patent 4,992,360 (1991) and U.S.Patent 5,167,850 (1992).
Spencer, B.F., S.J. Dyke, M.K. Sain and J.D. Carlson, 1996, “Nonlinear identification of semi-activecontrol devices,” ASCE Eng. Mech. Conf., May, 1996.
Weiss K. D. et al., 1993, "High Strength Magneto- and Electro-rheological Fluids", Society ofAutomotive Engineers, SAE Paper # 932451.
Weiss, K.D., J.D. Carlson and D.A. Nixon, 1994. “Viscoelastic Properties of Magneto- and ElectroRheological Fluids,” J. Intell. Mater. Syst. and Struct. 5, 772-775.