Brakes – Internal-resistance motion retarder – Electroviscous or electrorheological fluid
Reexamination Certificate
2001-03-16
2003-02-11
Graham, Matthew C. (Department: 3613)
Brakes
Internal-resistance motion retarder
Electroviscous or electrorheological fluid
Reexamination Certificate
active
06516926
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to magnetorheological dampers and, in particular, to the surface finish of piston rods operating in said dampers.
BACKGROUND OF THE INVENTION
Magnetorheological (MR) dampening devices are used in various applications, such as dampers, shock absorbers, and elastomeric mounts, for dampening and controlling vibration. Devices utilizing the unique properties of magnetorheological fluids are used to control pressure in valves and to control the transfer of torque in brakes and clutches.
Magnetorheological (M) fluids are fluid compositions that undergo a change in apparent viscosity in the presence of a magnetic field. A typical MR fluid contains ferromagnetic microparticles suspended in a low viscosity carrier liquid which are capable of becoming polarized in the presence of an applied magnetic field. The particles become organized into chains of particles within the fluid. The particle chains increase the apparent viscosity (flow resistance) of the fluid. The particles return to an unorganized state when the magnetic field is removed, which lowers the viscosity of the fluid.
FIG. 1
shows a known monotube MR damper
10
for use in the suspension system of a vehicle having a piston
12
sliding within a hollow tube
14
filled with MR fluid
16
. The piston
12
is attached to a hollow rod
18
, referred to herein as the piston rod, that slides within a sealed bearing
20
at one end of the body of the damper
10
. The piston
12
contains a coil
22
, which carries a variable current, thus generating a variable magnetic field across a flow gap
24
between an inner core
26
and an outer shell or flux ring
28
of the piston
12
. A bearing
30
having relatively low friction is disposed between the flux ring
28
and the tube
14
. The flux ring
28
and the inner core
26
of the piston
12
are held in place by spoked end plates
32
. Terminals
34
of the coil
22
extend through the piston rod
18
and are provided with suitable insulation for connection to a source of electricity (not shown). One end
36
of the tube
14
is filled with inert gas which is separated from the MR fluid
16
by a floating piston or sealed gas cap
38
. The floating gas cap
38
accommodates the displacement of MR fluid
16
due to the varying length of piston rod
18
immersed within the MR fluid
16
of hollow tube
14
as the piston
12
moves and to accommodate thermal expansion of the MR fluid
16
. The circumference of the gas cap
38
includes an o-ring
40
that provides a fluid-tight sealing engagement with the hollow tube
14
. The hollow tube is sealed by end caps
42
,
44
and attachment eyes
46
,
48
are provided on the respective end caps
42
,
44
for installing MR damper
10
to a vehicle body (not shown).
In response to vibration-induced movement of the piston rod
18
, MR fluid
16
flows through the flow gap
24
. When the coil
22
is energized, the effective viscosity of the MR fluid
16
in the flow gap
24
is increased by the interaction of the microparticles with the applied magnetic field. Variations in the electrical current flowing to coil
22
can be used to modulate the strength of the applied magnetic field and, thereby, to control the apparent viscosity of the flowing MR fluid
16
. The modulation of the apparent viscosity affects the flow rate of the MR fluid
16
through the flow gap
24
to achieve a desired dampening effect.
The MR fluid
16
provided in the hollow tube
14
comprises a plurality of soft ferromagnetic microparticles that are dispersed and suspended in a base liquid, preferably in a low viscosity base liquid. Suitable microparticles include powders of carbonyl iron, magnetite, iron alloys (such as those including aluminum, silicon, cobalt, nickel, vanadium, molybdenum, chromium, tungsten, manganese and/or copper), iron oxides, iron nitrides, iron carbides, chromium dioxide, low carbon steel, silicon steel, nickel, cobalt, and other materials known to exhibit MR activity.
A suitable microparticle size exhibits multi-domain characteristics when subjected to a magnetic field. For spherical or near-spherical particles, a suitable size distribution for the microparticles ranges between nominal diameters of about 1 and about 25 &mgr;m, usually between about 1 &mgr;m and about 6 &mgr;m. The microparticles are preferably present in an amount between about 50 and 90 percent by weight of the total composition of the MR fluid
16
. Suitable base liquids include hydrocarbon oil, silicone oil, paraffin oil, mineral oil, chlorinated and fluorinated fluids, kerosene, glycol, or water. A particularly suitable MR fluid
16
comprises carbonyl iron powder suspended in a synthetic hydrocarbon oil.
Because the microparticles are quite small, they have a tendency to become trapped in valleys that are created in the surface of the piston rod during the superfinishing or microfinishing process. The trapped particles are then dragged past the damper seal
20
. The microparticles in the MR fluid
16
are highly abrasive and can damage the seal
20
. As a result, the MR fluid
16
can eventually escape through the degraded seal and, ultimately, the MR dampening device can prematurely fail.
There is thus a need to prevent the damper seal from being damaged by particle abrasion from the MR fluid and moving piston rod.
SUMMARY OF THE INVENTION
The present invention provides a piston rod for use in a magnetorheological dampening device that has a surface finish that renders the dampening device resistant to wear at the elastomeric seal/piston rod interface. To this end, and in accordance with the present invention, the piston rod has a surface finish of Ra<0.065 &mgr;m and &Dgr;a≦1.4°, as measured using a Gaussian filter with a 0.08 mm cut-off length. A magnetorheological dampening device operating with a piston rod having the surface finish of the present invention resists wear from an MR fluid having particles of less than about 16 &mgr;m in diameter in fluid communication with the piston rod and elastomeric seal. The present invention further provides a method of achieving the surface finish, including rotating the piston rod while moving an abrasive tape against the outer surface of the rod.
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Hopkins Patrick Neil
Kruckemeyer William Charles
Lisenker Ilya
Delphi Technologies Inc.
Graham Matthew C.
McBain Scott A.
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