TECHNICAL REPORTHUBLFT: Hub Unit for Low Friction Torque
S. YAMANE T. OGIMOTO

Hub Units are used worldwide as automobile wheel bearings. Through careful consideration of the function andperformance of components such as grease and seals, we have completed the development of a low torque HubUnit called "HUBLFT". This paper will introduce the factors that have been redesigned to give this product top
performance.
Key Words: Axle bearing, low torque, grease, seal, hub unitKnuckle

1. Introduction
As represented by COP (United Nations Conventionof the Parties for Climate Change), alively debate is
ongoing throughout the world regarding actions to protect
the planet’s environment. Year after year, the automotive
manufacturing industry is being required to comply withincreasingly stringent fuel efficiency requirements inline with CO2 emissions regulations aimed at preventing
global warming. Particularly with regards to Europe, aregulation requiring CO2 emissions to be reduced from130 g/km in 2015 to 95 g/km by 2021 is already in effectand there is talk of reducing it even further to 70 g/km.
The fuel efficiency requirements for countries and regionsother than Europe also show signs of becoming stricter,and it is believed that automotive manufacturers willcontinue raising the bar for their fuel efficiency targets.
In order to contribute to better fuel efficiency, JTEKT is
striving to not only reduce the weight of its hub unit, butalso to reduce their rotational torque. By determining theindividual factors that influence the rotational torque of
hub units and applying specific measures to reduce eachof them, JTEKT has developed a hub unit with superior
low torque called HUBLFT. This paper introduces thelow-torque technologies employed by HUBLFT.

2. Analysis of Rotational TorqueAs Fig. 1 shows, the hub unit is a unitized bearing
positioned inside a vehicle’s wheel comprised of a bearing
which includes rolling elements and inner/outer ring
raceways, and a flange section which joins the unit to thevehicle. This structure enables the hub unit to support thevehicle’s weight while facilitating smooth rotation of thewheel (Fig. 2). The bearing is also equipped with seals
to prevent muddy water thrown up by vehicle tires frompenetrating in to the bearing and to prevent lubrication64

Hub Unit

Drive Shaft
Brake Rotor
Wheel
Car Interior

Exterior
Environment

Fig. 1 Structure of axle assembly
FlangeGreaseBase Oil

Bearing Rolling Friction

Seal Sliding Friction

Fig. 2 Hub Unit

grease from leaking out of the bearing. These seals ensurethat the bearing is maintenance-free and rotates smoothly
for the life of the vehicle.
Two major factors contributing to rotational torqueare rolling friction of the bearing and sliding frictionof the seal. Depending on vehicle weight, the rolling
element PCD (pitch circle diameter) and other hub unit
specifications change, making the exact contribution ratio
between these factors vary slightly, however these twomajor factors are considered to each separately contribute
approximately 50% to friction loss.

JTEKT ENGINEERING JOURNAL English Edition No. 1014E (2017)

−HUBLFT: Hub Unit for Low Friction Torque−

3. Methods to Reduce Rotational Torque3. 1 Rotational Torque of the Bearing
The bearing section of the hub unit operates underan axial load called “preload”. Preload is necessaryto maintain stable bearing life and good vehiclemaneuverability, or in other words, to secure bearing
rigidity. Preload is applied by either clinching the shaft
end or tightening with a nut. One measure that would
be effective in reducing bearing friction loss would beto reduce preload, however this would significantly
impact vehicle steering. In order to reduce friction loss
without such a trade-off, and assuming no change tobearing specifications, the factors that determine bearing
rotational torque were analyzed. Figure 3 shows theresults of this analysis. Bearing rotational torque canbe partitioned into grease shearing friction, base oil
viscous friction and rolling-sliding friction. The following
sections introduce the measures used to reduce the torquecontribution attributed to each of these factors and thecorresponding results.
Reduction Measures

Grease Shearing
Friction

Reduce Shearing
Viscosity

Rolling Viscous
Friction

Lower Base Oil Viscosity(Thinner Oil Film)

Rolling-Sliding
Friction

Low Traction Base Oil(Optimization of Oil
Molecular Structure)c

Seal Sliding
Friction
Bearing Rolling
Friction

Ensure Oil Film Thickness
with Additives

Fig. 3 Influential factor analysis and measures for reductionof rotational torque3. 1. 1 Grease Shearing Friction
The friction generated when grease itself moves dueto the difference in the relative speeds of the rollingelements to that of the cage and inner/outer rings is called“shearing friction”. Conventionally, hub units use greasethat is parted by the passage of the rolling elements along
the raceway and has channeling properties which stopit from returning back to the raceway. However, due toback pressure from the passage of the rolling elements,
the expelled grease does return to the raceway and it isbelieved that the grease and rotating elements (rollingelements and cage) continue to make contact.
Shearing viscosity is a characteristic that describes
the shearing friction generated when grease moves.
One approach to reducing shearing viscosity would be
to change the grease consistency (soften the grease)
however this has significant disadvantages including
downwards flow when the grease is sealed into the hub

JTEKT ENGINEERING JOURNAL English Edition No. 1014E (2017)

g = s/cg：Shearing Viscosity, Pa・s
s：Shearing Pressure, Pac：Shearing Speed, s−1sGreaseu251 000Shearing viscosity ratio

Rotational
TorqueInfluential Factors

unit and grease leakage from the seals. As such, JTEKT
developed a grease with reduced shearing viscosity butwith the same consistency as that of conventional grease.
Figure 4 shows a comparison between the shearing
viscosities of the developed grease and conventional
grease, respectively. Shearing viscosity of the developed
grease was shown to have been reduced throughout all
speed ranges. The reduction of shearing viscosity at
low speeds was enabled by adopting a special thickener
while reduction of maintaining the same consistency wasachieved by reducing the amount of thickener overall.
Meanwhile, the shearing viscosity at high speeds isthought to have decreased due to the use of a base oil witha lower viscosity since it is widely recognized that thedynamic viscosity of the base oil is a dominant factor athigh speed, and this is explained further in the followingsection.

100
101Current

Developed

0.1
0.01
1.E+01 1.E+02 1.E+03 1.E+04 1.E+05
Shearing speed, s−1

Fig. 4 Comparison of shearing viscosity

3. 1. 2 Rolling Viscous Friction
According to EHL (elasto-hydrodynamic lubrication)
theory, the rolling of rolling elements is facilitated byan oil film which forms between the rolling element andraceway, and the friction of passing through this oil film isknown as viscous friction. Figure 5 shows the calculated
results of friction loss excluding grease shearing friction.
The rotational torque of hub units and other bearings
increases the faster they rotate and for that reason, viscousfriction which is the greatest contributor to rotational
torque at high speeds was reduced as much as possible.

65

−HUBLFT: Hub Unit for Low Friction Torque−

The dynamic viscosity of base oil is a dominant factor
in viscous friction, so lowering dynamic viscosity was apriority, however careful consideration was also given tomaintaining oil film thickness even under high bearing
surface pressure and with these factors in mind thereduction of viscous friction was successfully achieved.

①Base oil viscous friction
・Friction created
when the base oil
itself is churned

1.2②Rolling-sliding friction

1

Differential slip

0.8

Contact ellipse・Sliding friction
due to speed difference
Spin slide

0.6
0.4
0.2
0200800

Rotational speed, min−1

1.2
Traction coefficient ratio

Rotational torque ratio

1.4

reduce this friction at low speeds, or in other words, if ameasure was adopted to make oil film thicker only at lowspeeds, it is believed that there would be little impact onrotational torque during other times. Figure 7 compares
the oil film thickness of the developed grease with that of
the conventional grease. It is a well-known phenomenon
that the oil film thickens at low speeds less than S = 1 m/s
due to the thickener. However, additives able to thicken
the oil film more than the thickener alone could wereincorporated to ensure a thicker oil film at low speeds inthe developed grease, thus successfully reducing rollingsliding friction more than conventional grease.

・Sliding friction
due to ball spin
Elastic hysteresis
・Resistance in linewith elastic deformationof the raceway

0.6
0.40.2Mineral oil(Current)

Synthetic oil(Developed)

Fig. 6 Results of comparison of traction coefficient

3. 1. 3 Rolling-Sliding Friction of the Bearing

660.80Fig. 5 Breakdown of bearing torque (calculated)100
Hub Unit Area of Use
Oil film parameter ratio

Rolling-sliding friction is divided into spin slide
friction and differential slip friction however both of thesecan be categorized as sliding friction between the balls
and raceway. JTEKT focused on the friction coefficient,l, to reduce this sliding friction.
As a measure to lower the friction coefficient, thebase oil type, i.e. the base oil molecular structure
was revised. Figure 6 shows the results of a traction
coefficient comparison of different types of base oils.
Synthetic hydrocarbon oil and the conventional paraffinbased mineral oil were tested in a dual cylinder traction
coefficient measuring apparatus. Results show that thesynthetic hydrocarbon oil had a traction coefficientaround 30% lower than the conventional paraffin-basedmineral oil. In contrast to hydrocarbon oil which iswell known for its smooth linear molecular structure,mineral oil, a typical example of the paraffin-based oils
conventionally used, has an uneven three-dimensional
molecular structure. The researchers demonstrated that itsunique molecular structure lead to synthetic oil’s ability
to reduce its traction coefficient.
Apart from the molecular structure of the base oil,a correlation also exists between the l characteristic
and the thickness of the oil film that forms between
the rolling element and raceway. According to theaforementioned Fig. 5, rolling-sliding friction makes asmaller contribution to rotational torque during highspeed rotation than it does during low speeds. In order to1

Current
10Developed

1
Developed(without additives)
0.1
1.E−03

1.E−02

1.E−01

1.E−00

Rolling speed, min−1

Fig. 7 Comparison of oil film parameter

3. 2 Rotational Torque of the Seals
The seals used in bearings are critical components
required to maintain hub unit functions and consequently
reducing friction must be done without affecting
sealability. Equation⑴ is used to calculate seal sliding
torque. Here, T s is seal sliding torque, l is friction
coefficient, Pr is seal lip tension and D is the sliding
diameter of the seal lip. Seal sliding torque, T s , isproportional to the friction coefficient, l, and seal lip
tension, Pr . Below the measures used to reduce these twoitems are described.

JTEKT ENGINEERING JOURNAL English Edition No. 1014E (2017)

Axial lip

Radial lip
Conv. Seal

Double Axial Seal

Fig. 8 Comparison of seal lip

Meanwhile, efforts were also exerted to improve seal
grease in order to reduce the friction coefficient, l. In
the past, there was concern that mixing different types ofgrease would alter grease properties; therefore the samegrease was used for the seal as that used in the bearing.
However, it is now known that, due to the seal contact
surface pressure being small, and having a relatively
low rotational speed, the fluid lubrication zone is stable.
Independently from the bearing grease, an ultra-lowviscosity base oil grease was adopted specifically for theseal.
Regarding the aforementioned concern of mixing
different grease types, even when mixed there were noapparent changes to consistency or other properties when
the bearing grease and the grease developed for the seal.
This was confirmed in an infrared analysis (IR), where nochanges in composition were found. This has led to theconclusion that the affinity between the two greases is nota problem.

4. Results Verification and Vehicle
Compatibility
Results verification was done using an actual hub unit
and the extent of the torque reduction produced by thedeveloped bearing grease were confirmed. The results
are shown in Fig. 9 a). The hub unit used for testing

JTEKT ENGINEERING JOURNAL English Edition No. 1014E (2017)（1 W, Preload Mid）

1.25

Current

10.75
Developed
0.5

0.25

0200

400600 800min−11 000 1 200a) Bearing section
Rotational torque ratio (Hub assembly)

D⑴2First, the measure to reduce seal lip tension will beexplained. As shown in Fig. 8, conventional seals are
configured with a radial lip and an axial lip. In light ofthe space limitations imposed on seals, the radial lip
has to be short dictating a rigidity that leads to high seal
torque. For this hub unit development, the radial lip
was replaced with another axial lip to create a double
axial lip configuration comprising two long, soft lips.
Furthermore, by leveraging JTEKT’s own original superelastic analysis and using the stress-strain diagram forthe seal rubber material, seal lip tension was optimized
and seal torque was reduced. Finally, when tested under
JTEKT’s rigorous conditions, adopting the double axial
lip configuration and optimizing seal lip tension had thesynergistic effect of doubling sealability against muddy
water penetration.
Ts ＝ l Pr

Rotational torque ratio (without seals)−HUBLFT: Hub Unit for Low Friction Torque−−11.2（1 000 min , 1 W, Preload Mid）

10.8

Brg

0.6
0.4
0.20

GreasesealCurrent−25%

Double axial −15%
Grease−10%Developedb) Hub unit assembly

Fig. 9 Results of comparison of torqueemployed a seal with the tip of the lip cut in order toeliminate the impact of the seal’s sliding torque. In−1
JC08 mode, which has a maximum speed of 800 min(equivalent to 80 km/h), torque was confirmed to be
50% less than that of conventional bearings. Equivalent
reductions were also confirmed at other rotational speeds.
Furthermore it was observed that the improvement in
low-temperature flow point through the shift to synthetic
base oil was also effective in reducing start-up torque at
extremely low temperatures.
Additionally by adopting a double axial lip forthe seal and improving seal grease, as illustrated in−1Fig. 9 b), at a rotational speed of 1 000 min , the two
measures contribute to reducing torque by 30% and 20%
respectively when considering the seal in isolation (or15% and 10% when considering the hub unit assembly
as a whole). As was the case with the bearing grease,torque reductions as a result of these measures were also
confirmed at other rotational speeds.
Regarding vehicle compatibility, the developedgrease demonstrated equivalent or superior performancecompared to conventional grease for all criterion as
outlined by JTEKT’s robust tests guidelines, including
those for bearing fatigue life (as well as fatigue life given
water-choked grease, low-speed fatigue, high-speed
durability, muddy-water seal durability tests, etc.).

67

−HUBLFT: Hub Unit for Low Friction Torque−

5. Effects other than Torque Reduction
In line with a growing demand for passenger vehicles
on a global scale, more vehicles are being transported toextremely cold regions such as Russia. Vibration duringvehicle transportation may result in fretting wear on thebearing raceway and lead to problems such as noise andvibration during vehicle operation.
Under JTEKT’s test conditions low-temperature
fretting wear is reduced by 70% when the developed
bearing grease is used. Given that base oil acts as amedian to supply additives to the raceway and thatthe shift to synthetic base oil has enhanced the lowtemperature flow point, the grease rheology (flow) atextremely low temperatures has improved. Wear amountis believed to have been reduced due to this improvement
coupled with the selection of additives with goodabsorptivity and reactivity at extremely low temperatures.
By preventing fretting wear of bearings when vehicles
are being transported in extremely cold regions, it isexpected that damage during non-operational states will
be eliminated.
In regards to seal grease as well, maintaining therheological properties of the grease and its base oil at
low temperatures is important in sustaining sealability
in extremely cold regions. As was the case with thegrease used in the bearing section, it was found that by
improving the low-temperature flow point through a shiftto a synthetic base oil satisfactory rheological properties
were preserved even in low-temperature environments,and sealability was enhanced.

further improve fuel efficiency for all vehicles and protect
the planet’s environment.

References
1)

2)

3)

R. NAKATA: Grease Lubrication Technology for Sliding
Automobile Parts JTEKT ENGINEERING JOURNAL,
No.1011E (2014) 25-30
K. YOSHIZAKI: Grease Lubrication Technology ofRolling Bearings JTEKT ENGINEERING JOURNAL,
No.1010E (2013) 27-32
M. TAKIMOTO: Technical Development of Automotive
Wheel Bearing Seals (Muddy-Water Resistant Seal, LowTemperature Environment Seal, and Super Low-TorqueSeal) JTEKT ENGINEERING JOURNAL, No.1008E(2011) 64-69

6. Conclusion
In order to develop a superior low-torque ball hub unit,
JTEKT focused on the grease of the bearing section andseal, which had never before been investigated. Afterclarifying the factors influencing the rotational torque ofhub units a low-viscosity, synthetic base oil was adopted
as a reduction measure. As a result and with the tradeoffs associated with the adoption of low-viscosity baseoil being suppressed through optimized additive typeand amount, and by adopting a double axial lip seal andoptimizing seal lip tension as well, overall hub rotational
torque was reduced by 50%.
In particular this process of grease development, andthrough conducting various verification tests including
those for bearing fatigue, fretting wear and additive
coating efficacy, has provided new insight about thebehavior of grease during bearing rotation. Withemissions regulations growing increasingly stringent for
not only passenger vehicles, but also for SUVs, pickup
trucks and so on, JTEKT wishes to apply this technology
to the taper roller hub units used in such vehicles to help

68

S. YAMANE****

T. OGIMOTO**

West JAPAN Technical Center, Bearing Operations
Headquarters
Experiment & Analysis Dept., Bearing Operations
Headquarters

JTEKT ENGINEERING JOURNAL English Edition No. 1014E (2017)