DESCRIPTION
Title of Invention
GREASE COMPOSITION FOR ELECTRIC POWER STEERING DEVICE,
AND ELECTRIC POWER STEERING DEVICE
Technical Field
[0001]
The present invention relates to a grease composition
for electric power steering devices with which the occurrence
of stick-slip can be reduced, and to electric power steering
devices.
Background Art
[0002]
Automobile power steering devices are in wide use. The
majority of the power steering devices are hydraulic, whereby
a hydraulic pump providing a steering force is constantly
driven by the power of the engine. The constant driving of
the hydraulic pump, applied irrespective of the need for
steering, contributes to poor fuel economy. On the other hand,
electric power steering uses an electric motor to provide a
steering force, and drives the electric motor only when
steering the vehicle. The fuel saving effect is thus much
greater than that of the hydraulic power steering device.
Because the steering force to produce in comparison with the

hydraulic power steering is small as for the electric power
steering, the use of electric power steering devicehas limited
to relatively smaller and lighter vehicles. However, the
number of vehicles with the electric power steering system has
been increasing because of the better fuel efficiency.
[0003]
The sliding area of an electric power steering is
configured from a steel worm shaft and a worm wheel having resin
teeth. The resin typically uses nylon, a type of polyamide.
Some of the properties required for a grease as a lubricant
that is used in such a sliding area include a low friction
property for improved transmission efficiency, and an
anti-stick-slip property for stably maintaining a low torque
over extended time periods.
A number of greases mixed with various waxes have been
proposed as lubricants for use in electric power steering
devices having such a steel-resin sliding area. Examples
include a grease as a mixture of a thickener and a base oil
with a montan wax (PTL 1) , a grease as a mixture of a thickener
and a base oil with a polyethylene oxide-based wax (PTL 2),
and a grease as a mixture of a thickener and a base oil with
a carboxylic acid amide-based wax (PTL 3). Another example
is a solid lubricant, or a grease of primarily
polytetrafluoroethylene (PTL 4). Yet another example is a
grease mixed with an ionic liquid (PTL 5).

These greases all have low coefficients of friction or
a low torque. A problem, however, is that the grease tends
to gradually become removed from the sliding area under the
friction overa long period of use, and then the oil film finally
breaks down and possibly causes stick-slip. Stick-slip may
cause resistance in the movement of the steering wheel, and
may spoil the steering feel in situations where high output
is needed such as in parking a car.
Citation List
Patent Literature
[0004]
PTL 1: JP-A-2002-371290
PTL 2: JP-A-2003-3185
PTL 3: JP-A-2008-208199
PTL 4: JP-A-2002-363589
PTL 5: JP-A-2007-191523
Summary of Invention
Technical Problem
[0005]
It is an object of the present invention to provide a
grease composition for electric power steering devices that
can maintain a sufficiently low coefficient of friction at the
sliding area between steel and resin for extended time periods,
and with which the grease does not easily become removed from
the sliding area, and thus prevents the break down of an oil

film, and reduces the occurrence of stick-slip. The invention
is also intended to provide an electric power steering device
that uses such a grease composition.
Solution to Problem
[0006]
The present inventors conducted intensive studies to
solve the foregoing problems, and found that a grease
composition that can maintain a sufficiently low coefficient
of friction at the sliding area between steel and resin for
extended time periods, and with which the grease does not easily
become removed from the sliding area, and thus prevents the
break down of an oil film, and reduces the occurrence of
stick-slip can be obtained with use of a saturated aliphatic
amide compound, boron nitride, a glycerin fatty acid partial
ester, and a metal soap-based thickener.
The present invention has been made on the basis of this
finding, as follows.
[0007]
(1) A grease composition for electric power steering
devicescomprising ;
a synthetic hydrocarbon oil having a density of 0.75 to
0.95 g/cm3 at 15°C,
a saturated aliphatic amide compound,
boron nitride,
a glycerin fatty acid partial ester, and

a metal soap-based thickener.
(2) The grease composition for electric power steering
devices according to (1), wherein the synthetic hydrocarbon
oil is a poly-α-olef in, and is contained in 50 to 95 mass% with
respect to a total amount of the grease composition.
(3) The grease composition for electric power steering
devices according to (1) , wherein the saturated aliphatic amide
compound is contained in 5 to 20 mass% with respect to a total
amount of the grease composition.
(4) The grease composition for electric power steering
devices according to (1), wherein the boron nitride is
contained in 0.2 to 5 mass% with respect to a total amount of
the grease composition.
(5) The grease composition for electric power steering
devices according to (1), wherein the glycerin fatty acid
partial ester is contained in 0.1 to 5 mass% with respect to
a total amount of the grease composition.
(6) The grease composition for electric power steering
devices according to (1) , wherein the metal soap-based
thickener is contained in 2 to 15 mass% with respect to a total
amount of the grease composition.
(7) An electric power steering device which is configured
froma steering mechanism for turning wheels in response to a
steering operation of a steering wheel,a motor that provides
a steering force to the steering mechanism anda worm gear that

transmits a torque of the motor to the steering mechanism,
comprising;
the worm gear being configured from a worm wheel formed
of a resin material and a worm shaft formed of a metallic
material, and
a grease composition applied to a meshing surface of the
worm wheel and the worm shaft,
wherein the grease composition contains a synthetic
hydrocarbon oil having a density of 0.75 to 0.95 g/cm3 at 15°C,
a saturated aliphatic amide compound, boron nitride, a glycerin
fatty acid partial ester, and a metal soap-based thickener.
Advantageous Effects of Invention
[0008]
The grease composition of the present invention has
notable effects as follows. A boron nitride having cleavage
propertiesreduces the coefficient of friction at ' the
steel-resin sliding area, and the saturated aliphatic amide
compound and the glycerin fatty acid partial ester adsorb to
the sliding surface.As a result, thesemaintain a low
coefficient of friction at the steel-resin sliding area,
prevent the break down of an oil filmbecausethe grease does
not easily become removed from the sliding area, and reduce
the occurrence of stick-slipover extended time periods.
Brief Description of Drawings
[0009]

[Fig. 1] Fig. 1 is an explanatory diagram representing
a structure of an electric power steering.
Description of Embodiments
[0010]
[Synthetic Hydrocarbon Oil]
The synthetic hydrocarbon oil of the present invention
has a density in the range of 0.75 to 0.95 g/cm3 at 15°C. In
case the density is outside this range, dispersibility for
boron nitride lowers and the synthetic hydrocarbon oil cannot
sufficiently lower the coefficient of friction. Preferably,
the synthetic hydrocarbon oil is one with a density of 0.8 to
0.9 g/cm3.
The synthetic hydrocarbon oil has a kinetic viscosity
of preferably 1 to 500 mm2/s, more preferably 5 to 100 mm2/s
at 40°C. In case the kinetic viscosity is outside the 1 to 500
mm2/s range at 40°C, it becomes difficult to readily prepare
a grease composition of the desired cone penetration. For
preparing a grease of excellent lubricity, it is preferable
that the hydrocarbon oil has physical properties with a
viscosity index of 90 or more, particularly 95 to 250, a pour
point of -10°C or less, particularly -15 to -70°C, and a flash
point of 150°C or more.
[0011]
The synthetic hydrocarbon oil is preferably one with
excellent hydrolytic stability. Preferred for use as the

synthetic hydrocarbon oil are, for example, polyolefins (such
as poly-α-olefins, polybutenes, and copolymers of two or more
olefins), alkylbenzenes, and alkylnaphthalenes.
Poly-a-olefins are preferred in terms of availability, cost,
viscosity characteristics, oxidation stability, and
compatibility with the system members. For cost
considerations, the poly-α-olefins are further preferably
polymers of 1-dodecene, 1-decene and so on.
[0012]
The hydrocarbon oil either alone or as a mixture of two
or more may be used for the synthetic hydrocarbon oil. When
using a mixture of more than one hydrocarbon oil, the physical
properties of the individual unmixed hydrocarbon oils may fall
outside of the foregoing ranges, provided that these satisfy
the foregoing physical properties as an oil mixture. It is
accordingly not necessarily required that the individual
hydrocarbon oils satisfy the foregoing physical properties.
It is preferable, however, that the physical properties of the
individual hydrocarbon oils fall in the foregoing ranges.
The hydrocarbon oil content is preferably 50 to 95 mass%,
particularly preferably 60 to 85 mass% with respect to the total
amount of the grease composition. It becomes difficult to
readily prepare a grease composition of the desired cone
penetration when the content of the hydrocarbon oil falls
outside of the 50 to 95 mass% range.

[0013]
[Saturated Aliphatic Amide Compound]
The saturated aliphatic amide compound of the present
invention is a compound with at least one amide group (-NH-CO-) ,
and may be a compound with one amide group (monoamide), or a
compound with two amide groups (bisamide). Saturated
aliphatic bisamides are most preferred for their excellent heat
resistance, and for their ability to reduce the frictional
resistance of the sliding area even in relatively small
amounts.
The saturated aliphatic monoamides are amide compounds
of saturated aliphatic monoamine and saturated aliphatic
monocarboxylic acid. The saturated aliphatic bisamides may be
either amide compounds of saturated aliphatic diamine and
saturated aliphatic monocarboxylic acid, or amide compounds
of saturated aliphatic dicarboxylic acid and saturated
aliphatic monoamine.
Preferred for use is a saturated aliphatic amide compound
with a melting point of 100 to 170°C, and a molecular weight
of 298 to 876.
The saturated aliphatic monoamides are represented by
the following general formula (1) , and the saturated aliphatic
bisamides are represented by the following general formulae
(2) and (3), respectively.
[0014]

R1-CO-NH-R2 ... (1)
R3-CO-NH-A1NH-CO-R4 ... (2)
R5-NH-CO-A2-CO-NH-R6 ... (3)
[0015]
In the formulae, R1, R2, R3, R4, R5, and R6 each
independently represent an aliphatic hydrocarbon group of 5
to 25 carbon atoms. In the case of general formula (1) , R2 may
represent a hydrogen atom. A1 and A2 represent bivalent
saturated aliphatic hydrocarbon groups of 1 to 10 carbon atoms,
particularly preferably bivalent saturated chain hydrocarbon
groups of 1 to 4 carbon atoms.
[0016]
Specifically, preferred examples of the saturated
aliphatic monoamides include lauramide, palmitamide,
stearamide, and behenamide.
Specifically, preferred examples of the saturated
aliphatic bisamides represented by the formula (2) include
ethylene bis stearamide, ethylene bis isostearamide, and
methylene bis lauramide. Preferred examples of the saturated
aliphatic bisamides represented by the formula (3) include
N,N'-bisstearyl sebacamide.
Preferred bisamides are amide compounds in which R1 and
R2 in formulae (2) and (3) are independently saturated chain
hydrocarbon groups of 12 to 20 carbon atoms.
[0017]

The amide compounds may be used either alone or in a
combination of two or more in any proportions. The amide
compound content is preferably 5 to 20 mass% with respect to
the total amount of the grease composition.
When melted under heat in the presence of the synthetic
hydrocarbonoil, the amide compound transforms into a state in
which the oil is retained in the three-dimensional network
structure of the amide compound. This further lowers the
coefficient of friction at the steel-resin sliding area than
when the amide compound is simply dispersed and mixed in the
grease.
[0018]
[Boron Nitride]
The boron nitride used in the present invention may be
any of hexagonalatmosphericphase (h-BN) powders widely used
as solid lubricants. These may be used afterappropriately
selecting suitable-sized particles according to the intended
use. The particle diameter is preferably 1 to 10 μm.
The boron nitride content is preferably 0.2 to 5 mass%
with respect to the total amount of the grease composition.
[0019]
[Glycerin Fatty Acid Partial Ester]
The glycerin fatty acid partial ester used in the present
invention is a monoester or diester compound synthesized from
fatty acid and glycerine, and that has had one or two hydroxyl

groups of the glycerine esterified with the fatty acid. The
glycerin fatty acid partial ester is preferably a monoester.
Triester compounds are not preferable because these form a thin
film less efficiently and thus are less effective at reducing
friction than the partial esters.
In the present invention, it is preferable that the fatty
acid residue has 12 to 25 carbon atoms. The monoesters are
represented by the following general formula (4) or (5) . The
diesters are represented by the formula (6) or (7).
[0020]
R7-COO-CH2-CH(OH)-CH2OH ... (4)
HO-CH2-CH(OCO-R8) -CH2OH ... (5)
R9-COO-CH2-CH(OCO-R10) -CH2OH ... (6)
R11-COO-CH2-CH(OH)-CH2-OCO-R12 ... (7)
[0021]
In the formulae (4) to (7), R7, R8, R9, R10, R11, and R12
each independently represent a saturated or unsaturated chain
hydrocarbon group of 12 to 25 carbon atoms, and some of the
hydrogen atoms in the hydrocarbon group may be substituted with
a hydroxyl group.
[0022]
Specific preferred examples of such glycerin fatty acid
partial esters include saturated fatty acid monoglycerides and
saturated fatty acid diglycerides such as glycerol monolaurate,
glycerol dilaurate, glycerol monopalmitate, glycerol

dipalmitate, glycerol monostearate, glycerol distearate,
glycerol monobehenate, glycerol dibehenate, glycerol
monohydroxystearate, and glycerol dihydroxystearate; and
unsaturated fatty acid monoglycerides and unsaturated fatty
acid diglycerides such as glycerol monooleate, glycerol
dioleate, glycerol monoerucate, and glycerol dierucate.
[0023]
The glycerin fatty acid partial esters maybe used either
alone or in a combination of two or more in any proportions.
The content of the glycerin fatty acid partial ester is
preferably 0.1 to 5 mass% with respect to the total amount of
the grease composition.
[0024]
[Metal Soap-Based Thickener]
The metal soap-based thickener is a thickener based on
a carboxylic acid metal salt, and the carboxylic acid may be
a carboxylic acid derivative having a hydroxyl group or the
like.
The carboxylic acid may be an aliphatic carboxylic acid
such as stearic acid and azelaic acid, or an aromatic carboxylic
acid such as terephthalic acid. Preferably, the carboxylic
acid is a monovalent or divelent aliphatic carboxylic acid,
particularly an aliphatic carboxylic acid of 6 to 20 carbon
atoms. More preferably, a monovalent aliphatic carboxylic
acid of 12 to 20 carbon atoms, and a divelent aliphatic

carboxylic acid of 6 to 14 carbon atoms may be used.
Particularly preferred is a monovalent aliphatic carboxylic
acid containing one hydroxyl group.
The metal may be an alkali metal such as lithium and
sodium; an alkali earth metal such as calcium; or an amphoteric
metal such as aluminum. Preferably, the metal is an alkali
metal, particularly lithium.
[0025]
The thickener may be mixed in the form of a metal soap,
or may be prepared as a metal soap thickener by separately
mixing carboxylic acid and a metal source (such as a metal salt,
and a metal salt hydroxideetc.) and reacting these components
at the time of producing the grease.
The carboxylic acid metal salt may be used either alone
or as a mixture of two or more. For example, a mixture of
lithium 12-hydroxystearate and lithium azelate is
particularly preferred.
The metal soap-based thickener may be added in any
content, as long as the desired cone penetration is obtained.
For example, the content of the metal soap-based thickener is
preferably 2 to 15 mass% with respect to the total amount of
the grease composition.
[0026]
[Other Additives]
Additives may be appropriately added to the grease

composition of the present inventionas required, in addition
to the foregoing components. Examples of such additives
include common additives forlubricant and grease such as
detergents, dispersants, antiwear agents, viscosity index
improvers, antioxidants, extreme-pressure agents,
anti-rusting agents, and corrosion preventing agents.
[0027]
[Preparation Method]
The grease composition of the present invention may be
produced by using a common grease producing process. It is,
however, preferable to mix the saturated aliphatic amide
compound, and then once heat the mixture at a temperature equal
to or greater than the melting point of the saturated aliphatic
amide compound.
Specifically, the method may comprise; heating the
saturated aliphatic amide compound and the synthetic
hydrocarbon oil at a temperature equal to or greater than the
melting point of the amide compound, cooling the mixture, and
then physically mixing the mixture with a common grease
containing boron nitride, a thickener, and a synthetic
hydrocarbon oil. Alternatively), a mixture of all the
components containingthe thickener may be heated at a
temperature equal to or greater than the melting point of the
amide compound, and then cooled to prepare the grease
composition.

[0028]
[Subject of Lubrication]
The grease composition of the present invention may
preferably be used for lubrication of an electric power
steering device that includes a resin sliding member and a
metallic sliding member.
As illustrated in Fig. 1, such an electric power steering
is configured from a motor 1 for generating assisting power,
a reduction drive 2 for amplifying the torque of the motor,
a pinion gear 4 that transmits the steering torque and angle
of a steering wheel 3 through the torque of the reduction drive,
and a rack gear 6 that translates the steering torque of the
pinion gear 4 into linear motion to generate a force that turns
a tire 5.
The steering force from the steering wheel 3 is applied
to an input shaft 7, and a torque sensor 8 detects a torsion
in the torsion bar provided between the input shaft 7 and the
pinion gear 4. With the detected steering force and at the
detected steering timing, the motor generates the required
assisting torque under the control of the calculated current
value from a controller 9.
The reduction drive 2 serves to amplify the assisting
torque of the motor, and is configured from a worm shaft 10
with a metal gear, for example, such as a chromium molybdenum
steel (e.g., SCM415), and a carbon steel (e.g., S45C), and a

worm wheel 11 that includes a metal core and a toothed resin
gear, for example, such as nylon 6 and nylon 66 containing 30%
glass.
[0029]
Whenthe reduction ratio is about 18 and the maximum
assisting torque generated by the motor is about 5 Nm, about
a 90 Nm torque generates at the worm wheel. For reasons related
to characteristics of the worm shaft, a slipping velocity and
a surface pressure at the tooth surface are large. Therefore
the slipping velocity is about 3 m/s at the normal maximum
steering velocity of 700°/s, and the tooth surface pressure
is about 60 MPa at the maximum assisting torque.
Under these conditions, static friction increases at the
tooth surface, and the difference between dynamic friction and
static friction increases when the desirable lubricity is no
longer maintained, and tends to cause resistance upon
activation. This is called stick-slip, a problem associated
with the perception of resistance through the steering wheel,
and bad steering feel. The grease is applied to reduce
stick-slip.
Examples
[0030]
1. Synthetic Hydrocarbon Oil
(a) Poly-cc-olefin (Durasyn 170, INEOSLtd. )
Kinetic viscosity at 40°C: 68 mm2/s

Density at 15°C: 0.83 g/cm3
Viscosity index: 133
Pour point: -45°C
Flash point: 250°C
An amine-based antioxidant was added to the
poly-a-olefin.
[0031]
2. Amide Compounds
(1) Aliphatic amide
(a) Ethylene bisstearamide (guaranteed reagent)
(b) Stearic acid monoamide (guaranteed reagent)
[0032]
3. Boron Nitride
Average particle size: 2 μm (HP-P1, Mizushima Ferroalloy
Co., Ltd.)
Average particle size was measured by using a laser
diffraction method.
[0033]
4. Glycerin Fatty Acid Partial Esters
(1) Glycerol monooleate (guaranteed reagent)
(2) Glycerol monostearate (guaranteed reagent)
[0034]
5. Metal Soap-Based Thickener
(1) Lithium 12-hydroxystearate ("Lithium stearate" in
the table)

(2) A complex of lithium 12-hydroxystearate and lithium
azelate (a 2:1 mixture; "Mixed lithium soap" in the table")
[0035]
[Preparation Method]
Each component was charged into a container in the amount
(mass%) shown in Table 1, heated at 150°C (equal to or greater
than the melting point of the amide compound), stirred with
a magnetic stirrer, and then cooled to room temperature. The
mixture was subjected to a pressurized dispersion process with
rollers (three rollers were used) to prepare a grease
composition.
[0036]
[Lab Evaluation Method]
The grease was evaluated in a lab for its attributes and
performance in the manner described below. The worked
penetration and the dropping point of the grease representing
hardness and heat resistance, respectively, were measured
according to JIS K2220.
The friction characteristics of the grease were
evaluated in a test conducted with a reciprocating friction
tester using a ball and a disc.
As a model of the steel-resin sliding area of the electric
power steering, an SUJ-2 ball having a 1/4-inch diameter, and
a nylon 6 plate [N6 (NC), Toyo Plastic Precision Co., Ltd.]
were used as the metal sliding member and the resin sliding

member, respectively. The grease was applied to the disc, and
the presence o.r absence of stick-slip was evaluated from the
coefficient of friction and the waveform of the frictional
force generated by sliding under the test load of 2,000 gf,
the sliding rate of 10 mm/s, and the amplitude of 20 mm
(stick-slip was determined to be present when the frictional
force from unidirectional sliding.was not constant) . A lower
coefficient of friction improves the transmission efficiency
of the power steering device, and a more stable waveform of
the frictional force improves the anti-stick-slip property.
The evaluation results are presented in Table 1.
[0037]
[Lab Evaluation Results]
The mixture of the hydrocarbon oil with the amide
compound, boron nitride, and glycerin fatty acid partial ester
had a relatively low coefficient of friction, and a high
anti-stick-slip property. However, the dropping point was low
at 135°C because of the lack of the thickener (Comparative
Example 1).
The mixture of the hydrocarbon oil with the lithium soap,
boron nitride, and glycerin fatty acid partial ester had a high
dropping point of 250°C; however, the coefficient of friction
was high, the anti-stick-slip property was poor, and the
lubricity was insufficient (Comparative Example 2). The
mixture of the base oil with the lithium soap and the amide

compound had a high dropping point of 230°C, and a slightly
low coefficient of friction, but the anti-stick-slip property
was insufficient (Comparative Example 3).
The mixtures of the hydrocarbon oil with the lithium soap,
boron nitride, glycerin fatty acid partial ester, and saturated
aliphatic amide compound had a low coefficient of friction and
a high anti-stick-slip property while maintaining a high
dropping point of 200°C or more (Examples 1 to 6).
[0038]
[Actual Evaluation Method]
The grease compositions of Example 4 and Comparative
Examples 1 and 3 were subjected to a working durability test,
whereby the grease was applied to the steel worm shaft and the
toothed resin worm wheel of the electric power steering device.
SCM 415,. and nylon 6 with 30% glass fiber were used as the
materials of the steel worm shaft and the toothed resin worm
wheel, respectively.
Efficiency was determined from the input torque to the
worm shaft and the output torque from the worm wheel. For
evaluation, "average efficiency" was determined under varying
temperature conditions and varying load torque conditions.
The working durability test was conducted under maximum
output load conditions, and the presence or absence of
stick-slip after 100,000 runs was determined for evaluation.
The evaluation results are presented in Table 2.

[0039]
[Actual Evaluation Results]
The average gear efficiency was high, but the
anti-stick-slip property was poor, and the lubricity was
insufficient in the mixture of the hydrocarbon oil with the
amide compound, boron nitride, and glycerin fatty acid partial
ester, and in the mixture of the hydrocarbon oil with the
lithium soap and the amide compound (Comparative Examples 1
and 3).
The average gear efficiency, and the lubricity for
anti-stick-slip property improved with the mixture of the
hydrocarbon oil with the lithium soap, boron nitride, glycerin
fatty acid partial ester, and aliphatic amide (Example 4).
[0040]

[0041]
[Table 2]

Industrial Applicability
[0042]
The grease composition of the present invention is useful
for the lubrication of the sliding area of an electric power
steering device having a resin sliding member and a metallic
sliding member.
Reference Signs List
[0043]
1 Motor
2 Reduction drive
3 Steering wheel
4 Pinion gear
5 Tire
6 Rack gear
7 Input shaft
8 Torque sensor
9 Controller
10 Worm shaft
11 Worm wheel

CLAIMS
[Claim 1]
A grease composition for electric power steering devices
comprising;
a synthetic hydrocarbon oil having a density of 0.75 to
0.95 g/cm3 at 15°C,
a saturated aliphatic amide compound,
boron nitride,
a glycerin fatty acid partial ester,and
a metal soap-based thickener.
[Claim 2]
The grease composition for electric power steering
devices according to claim 1, wherein the synthetic hydrocarbon
oil is a poly-α-olef in, and is contained in 50 to 95 mass% with
respect to a total amount of the grease composition.
[Claim 3]
The grease composition for electric power steering
devices according to claim 1, wherein the saturated aliphatic
amide compound is contained in 5 to 20 mass% with respect to
a total amount of the grease composition.
[Claim 4]
The grease composition for electric power steering
devices according to claim 1, wherein the boron nitride is
contained in 0.2 to 5 mass% with respect to a total amount of

the grease composition.
[Claim 5]
The grease composition for electric power steering
devices according to claim 1, wherein the glycerin fatty acid
partial ester is contained in 0.1 to 5 mass% with respect to
a total amount of the grease composition.
[Claim 6]
The grease composition for electric power steering
devices according to claim 1, wherein the metal soap-based
thickener is contained in 2 to 15 mass% with respect to a total
amount of the grease composition.
[Claim 7]
An electric power steering device which is configured
from a steering mechanism for turning wheels in response to
a steering operation of a steering wheel, a motor that provides
a steering force to the steering mechanism anda worm gear that
transmits a torque of the motor to the steering mechanism,
comprising;
the worm gear being configured from a worm wheel formed
of a resin material and a worm shaft formed of a metallic
material, and
a grease composition applied to a meshing surface of the
worm wheel and the worm shaft,
wherein the grease composition contains a synthetic
hydrocarbon oil having a density of 0.75 to 0.95 g/cm3 at 15°C,

a saturated aliphatic amide compound, boron nitride, a glycerin
fatty acid partial ester, and a metal soap-based thickener.