TECHNICAL REPORT

Development of High Peripheral Velocity Seals for EV Drive Motors
Y. MOKUBO

H. KURODA

A new type of oil seal has been developed by improving the composition of the fluororubber and optimizing thedesign of the main lip shape in order to support the high peripheral speeds (up to 50 m/s) of EV drive motors. Seal
performance, including durability, has been evaluated under both poor and good lubrication conditions. It was
found that deterioration of the lubrication condition of the sliding part and occurrence of stick-slip were problems
with current products. These problems were solved by developing low-friction fluororubber, adding ribs that could
guide lubricating oil to the main lip sliding part, and devising a screw mechanism to collect a small amount of oilthat leaked.
Key Words: EV drive motors, oil seals, high peripheral velocity, fluororubber

1. Introduction

25 000Core volume
5.1 L

SmallerCore volume
2.2 L

60
50

20 000ity

ral veloc

eripheHigher p

15 00030

10 000

20HEV

5 000

10BEV01990 1995

402000

2005

2010

2015

2020

02025

Maximum peripheral velocity of the oil seal, m/sMaximum rotational speed of the motor, min1EV drive motors are using higher rotational speeds
in order to achieve more compact sizes and higher
efficiency, and it is expected that the maximum rotational
speed will reach 20 000 min−1 by 2025. At that time, theperipheral velocity of the EV drive motor seal will be 50m/s (Fig. 1).

Fig. 1 Trends in peripheral speed of oil seals

The seal is installed between the drive motor output
shaft and reduction gear, and lubrication is supplied byoil inside the reduction gear located next to the motor.
This means that the seal is lubricated using oil scatteredby gear rotation inside the reduction gear. However theoil amount is limited to the minimum necessary in orderto reduce reduction gear torque loss, and the oil level isbelow the seal installation position (Fig. 2).
When the amount of oil splashed onto the seal is small,a poor lubrication environment results. When the amountof oil splashed onto the seal is large, this produces a good
lubrication environment. Therefore, it can be expected

104that there will be differences in the lubrication conditionsof the seal depending on the unit structure and therotational speed.

Seal

Oil splashingfrom gear

Motor
Oil level
GearFig. 2 Oil level at seal

The seal in an EV drive motor is an important partthat functions to prevent oil inside the reduction gearfrom penetrating into the motor. If oil penetrates into themotor, it can cause degradation of insulation and lead tomalfunction.
In this report, we introduce the development of a sealthat can maintain good durability in both poor lubricationand good lubrication environments at high peripheral
velocity (50 m/s).

2. Performance Evaluation of Current
ProductIn order to identify the problems in EV drive motor
seals in a poor lubrication environment at high peripheral
velocity (50 m/s), we identified the performance of acurrent drive motor seal product that has been mass
produced.

JTEKT ENGINEERING JOURNAL English Edition No. 1020E (2024)

−Development of High Peripheral Velocity Seals for EV Drive Motors−

2. 1 Test Equipment and Evaluation Conditions

The evaluation conditions are shown in Table 1, andthe test equipment is shown in Fig. 3. Simulating acustomer application, the target number of cycles was
set at 600. During the test, video was used to observe
the condition of oil adhesion on the main lip when theshaft was rotating, and a thermocouple was installed tomeasure the temperature of the main lip sliding part.

lip sliding part rose to 165℃. It is considered that thisheat generation at the sliding part accelerated oxidative
degradation of the base oil, causing the oil to burn.

Air side

Burnt oil

Table 1 Test condition for poor lubrication
Rubber material

Fluororubber

Oil type
Target numberof cycles

Automatic transmission fluid(1)50 m/sTime

Objective

Yes

3 min

Check ofsealing
property

NoOperation
underpoor
30 minlubrication
conditions

Condition(2)

Fluid side

Below conditions 600 times
Peripheral Presence
velocityof oil

50 m/sFluid side face
Main lip sliding partFig. 4 Condition of main lip sliding area

At high speeds of rotation with peripheral velocity 30m/s or higher, transfer of the oil adhering to the main lip
nose face in the shaft rotation direction was observed(Fig. 5). It is considered that the more oil transfer
occurred at high rotational speeds, the stronger the flowof air became.

Nose face

Shaft rotation direction
Fluid side face

Main lip sliding part

Thermocouple
Rotating shaft

Fig. 5 Oil transfer on main lip nose face
Glass cover
Enlargedview

3. Improvement of Main Lip Sliding Part

Oil tank
Video camera

Seal

Fig. 3 Test equipment

2. 2 Test Results and Considerations

With the current product, oil leakage occurred at 18
cycles. Observation of the main lip sliding part at the time
when leakage occurred found uneven wear and burnt oilon the sliding part as shown in Fig. 4.
From the video, a small amount of oil was seen on
the cut face of the main lip at the time that condition (2)started, and splashing of the oil by centrifugal force wasseen when operation started. It is presumed that this oil
splashing adversely affected the lubrication conditions of
the sliding part, and resulted in uneven wear of the mainlip. It was also confirmed that the temperature of the main

JTEKT ENGINEERING JOURNAL English Edition No. 1020E (2024)

As a way of improving the problems of the main lip
sliding part in a poor lubrication environment at highperipheral velocity (50 m/s) (uneven wear caused bydeterioration in the lubrication conditions, burnt oil
caused by heat generation), we modified the rubbercomposition and lip design.

3. 1 Modification of the Rubber Composition

As a means of improving the wear resistance of therubber material and reducing heat generation, we focusedon lowering the coefficient of friction of the rubbermaterial. Possible means of improvement were changing
the design of the rubber composition, and rubber surface
treatment (Fig. 6).
However rubber surface treatment requires anadditional process during seal manufacture, and there
are concerns about the durability of the surface treatment
coating. In particular, when a combined treatment or
coating is used in a poor lubrication environment at highperipheral velocity (50 m/s), it will separate due to wearof the main lip sliding part, and the surface treatment
coating is expected to be lost quickly.

105

−Development of High Peripheral Velocity Seals for EV Drive Motors−

Combined treatment (application offluorinated thin film, etc.)
Surfacetreatment

Coating (polytetrafluoroethylene,MoS2, etc.)
Surface modification (etching,chemical vapor deposition, etc.)

Fig. 6 Coefficient of friction reduction method

Coefficient offriction ratio

Therefore in this development, we reviewed thedesign of the fluororubber composition. By changing thetype and amount of additive agent, we worked to lower
the coefficient of friction of the rubber. As a result, wesucceeded in reducing the coefficient of friction in the
improved fluororubber by 60% compared to the current
fluororubber (Fig. 7).
60% reduction

1.0

Test method: Ring-on-disc
Test time: 30 min
Lubrication: None
Temperature: Room temperature
Rotational speed: 500 min1
Load: 50 N

0.4
Current
rubberDevelopedrubber

Fig. 7 Comparison of friction coefficients

3. 2 Improvement of the Lip Design

To improve the lip design, we added a rib to the main
lip nose face, and utilized the flow of air to actively send
the oil adhering on the main lip nose face to the fluid sideface, aiming to produce good lubrication conditions on
the main lip sliding part over a prolonged period. The
video results confirmed that the flow of air moved the oiladhering on the main lip noise face along the rib on thenose face to the fluid side face. As a result, we succeeded
in ensuring lubrication of the main lip sliding part for alonger time than with the current product (Fig. 8).
Nose
Nose face face rib
Shaft rotation direction

200
150
100

165

500

Current
product

600

600
400
2000

Developed
product

18
Current
product

Developedproduct

In Section 3, we explained the large improvement result
for seal durability in a poor lubrication environment. This
Section performs a seal durability evaluation in a goodlubrication environment at high peripheral velocity (50m/s).

4. 1. Evaluation of Durability with Development
Seal Ver. 1

Using development seal Ver. 1 that achieved the target
durability in a poor lubrication environment at highperipheral velocity (50 m/s), we conducted a durability
evaluation in a good lubrication environment. The test
equipment was the same as in Fig. 3, and the evaluation
conditions are shown in Table 2. Simulating a customer
application, the target number of cycles was set at 10. A
high-speed camera was used to observe the conditions ofthe main lip during the test.
Table 2 Test condition for good lubrication
Rubber material

Developed fluororubber

Oil type
Target numberof cycles

Automatic transmission fluid
Below conditions 10 times
Peripheralvelocity

Fig. 8 Rib on main lip nose face

106

Improveddurability

4. Evaluation of Seal Durability in a Good
Lubrication Environment

Main lip sliding part

Using the “development seal Ver. 1” that combined
the developed fluororubber and rib on the main lip nose
face, we conducted a durability evaluation under poor

115

800

Fig. 9 Lip temperature and number of cycles

Fluid side face

3. 3 Verification of Improvement Results1 00030% reduction

Number of cycles

Reducing
frictionof rubbermaterials

lubrication conditions at high peripheral velocity (50 m/s).
The results confirmed a 95% reduction in main lip wear
width (no uneven wear), and 30% reduction in main lipheat generation (reduction in burnt oil), compared to thecurrent product. We also achieved the target number ofcycles of 600, confirming that the durability performanceof development seal Ver. 1 was improved more than 30×compared to the current product (Fig. 9).
Lip temperature,℃

Solid lubricant (polytetrafluoroethylenepowder, graphite, etc.)

Composition
designCondition

OilTime(1)15～30m/s

Yes

15 h(2)50 m/sYes6h(3)

StoppedYes

3h

Objective
Check of sealing
property in low andmedium velocity
rangesCheck of sealing
property at 50 m/s
and good lubrication
Cooling by stopping

JTEKT ENGINEERING JOURNAL English Edition No. 1020E (2024)

−Development of High Peripheral Velocity Seals for EV Drive Motors−

4. 2 Results of Observation of Lip Conditions witha High-Speed Camera

Oil leakage occurred under condition (2) at the firstcycle of the durability test. The high-speed camera
observed that stick-slip of the main lip occurred at thetime when oil leakage occurred.
Although the frequency of stick-slip occurrence during
the durability evaluation was low, when it occurred it
produced vibration of the main lip in the axial direction
and radial direction. It was confirmed that this produced
a minute gap of approximately 0.1mm between the endof the main lip and the shaft, resulting in oil leakage(Fig. 10).

m: Inertial force of lip(equivalent to radial load)
k: Lip modulus of elasticity
c: Lip coefficient of viscosity
h: Angle of rotation
T: Seal friction torque
T0: Seal static friction torqueT
N: Lip reaction force
l: Coefficient of friction
d2h: Acceleration
dt2dh: Velocity
dtSpring km

Damper cm・dhdt

d2h
dh+ c・ + k・h = − T = −（l・N ）= −dt2
dt

T0 + kdhdt

Formula (1)T
Slope kT0dh
T = T0 + kdt

Stick-slip is likely
to occur whenk<0dhdt

Fig. 11 Oil seal vibration model

Rotating shaft

Fluid sidefaceView
Nose faceFig. 10 Moment of stick-slip occurrence

From Formula １, the factors affecting stick-slip are
the coefficient of friction l, radial load N, stiffness of themain lip K, and viscosity of the main lip C. In order toprevent stick-slip, it is necessary to reduce the coefficientof friction l and radial load N, and to increase thestiffness of the main lip K and viscosity of the main lip C(Table 3).
Here as the rubber material, we used the developed
fluororubber with reduced coefficient of friction that wasdescribed before. Therefore, we studied improvements totwo factors: the main lip radial load N and stiffness of themain lip K.

5. Countermeasure to Stick-SlipTable 3 Factors affecting stick-slip

5. 1 Study of Methods to Prevent Stick-Slip

Idealcondition

In order to study the mechanism of stick-slip
occurrence, we converted the seal to a vibration model,
and studied methods of preventing stick-slip (Fig. 11).

JTEKT ENGINEERING JOURNAL English Edition No. 1020E (2024)

Coefficientof frictionlRadial loadN
Main lipviscosity
CMain lipstiffness
KSmallSmallLargeLargeMethod
Reducing rubbermaterial coefficientof friction
Reducing main lipradial load
Reducing rubbermaterial tan δ(viscosity/elasticity)
Changing main lip
shape (lip height, lip
included angle, etc.)

Improvement
necessary?
NoYes
No

Yes

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−Development of High Peripheral Velocity Seals for EV Drive Motors−

5. 2 Verifying the Stick-Slip Prevention Effects

We varied main lip radial load N and stiffness of themain lip K, and conducted a stick-slip reproduction
evaluation. The occurrence of stick-slip can be judged
from the size of the rapid temperature rise at the main lip
sliding part. The results from temperature measurement
confirmed that stick-slip was prevented with a certain
main lip radial load and special main lip shape (Fig. 12).
After change of main lip radial load and shape
Lip temperature

Lip temperature

Development seal Ver. 1

Test time

Test time

Frequent stick-slip occurrence(large temperature change)

Reduced stick-slip occurrence(small temperature change)

Fig. 12 Checking the effect of stick-slip prevention

The main lip radial load and main lip shape that wereobtained from the above evaluation were applied todevelopment seal Ver. 1 in order to produce “development
seal Ver. 2.” The results of a durability evaluation in agood lubrication environment at high peripheral velocity(50 m/s) using development seal Ver. 2 demonstrated that
although stick-slip could not be completely prevented, it
was largely reduced, reducing the amount of oil leakage
after 10 cycles to less than 1/30. The appearance of the oilleakage is shown in Fig. 13.

6. Countermeasure to Fully Prevent OilLeakage
In Section 5, we learned that a minute amount of oilleakage occurs in a good lubrication environment at highperipheral velocity (50 m/s) even with development sealVer. 2. We therefore studied a method for fully preventing
this oil leakage.

6. 1 Study of Oil Leakage Prevention Mechanism

The amount of oil leakage with development seal Ver.
2 was extremely small at less than 1 cc, and a flow of air
was present. We utilized these two points and studied amechanism for recovering the small amount of leaked oilon the sealed side.
As shown in Fig. 14, a space A for temporarily storinga small amount of oil leaked from the tip of the main
lip to the air side is provided by providing a minor lip.
We also added “leak recovery ribs” between the end of
the main lip and the minor lip, and utilized the pumping
action to collect the oil that had accumulated in space
A by sending it along the leak recovery ribs back to themain lip fluid side. This seal was named “development
seal Ver. 3.”

Minor lip
Leak recovery rib
Space A

Development seal Ver. 1

Development seal Ver. 2

Fig. 14 Shape of development seal Ver. 3

Oil pool

Oil leakage
Amount of leakage: More than 30 cc Amount of leakage: Less than 1 cc

Fig. 13 Appearance of oil leakWith development seal Ver. 1, there was enough oilleakage to create an oil pool on the testing machine.
However with development seal Ver. 2, only a small
amount of oil dripped down.

108

6. 2 Verification of Oil Leakage Prevention
Mechanism Effects

The results of a durability evaluation in a good
lubrication environment at high peripheral velocity (50m/s) using development seal Ver. 3 demonstrated thatoil leakage was fully prevented and the target numberof cycles of 10 in Table 2 was achieved. When theactual part was observed after the durability evaluation,although there were marks from the oil that penetrated as
far as space A, no oil leakage to the air side was found.
Therefore we can surmise that the minute amount of oilthat leaked from the end of the main lip was temporarilycollected in space A, and was then collected on the sealed
side due to the effects of the leak recovery ribs (Fig. 15).

JTEKT ENGINEERING JOURNAL English Edition No. 1020E (2024)

−Development of High Peripheral Velocity Seals for EV Drive Motors−Leak recovery rib

Fig. 15 Space A state after endurance evaluation

Using development seal Ver. 3, when we conducted adurability evaluation in a poor lubrication environment
at high peripheral velocity (50 m/s), the seal achieved
the 600 cycles which were the target number of cycles inTable 1. As a result, we concluded that development sealVer. 3 has good durability in both poor lubrication andgood lubrication environments at high peripheral velocity(50 m/s).

7. Conclusion
In this report, we improved the fluororubbercomposition and optimized the main lip shape so that theseal could support the high peripheral velocities (up to 50m/s) of an EV drive motor. As a result, the development
seal achieved the target number of cycles at durability
evaluations in both poor lubrication and good lubrication
environments.
In order to reduce uneven wear and burnt oil in a poor
lubrication environment, we improved the lubrication
conditions of the main lip sliding part, and worked tolower the main lip sliding part temperature. Improving
the fluororubber composition reduced the coefficientof friction by 60%, and improvements to the rib designon the main lip nose face ensured good lubrication
conditions over a prolonged period. These effects
succeeded in improving the main lip wear resistance by
95% (no uneven wear) and reducing heat generation by
30% (reduction in burnt oil).
In order to reduce oil leakage in a good lubrication
environment, we worked to reduce stick-slip. By
optimizing the radial load and stiffness of the main lip,
we reduced the frequency of stick-slip occurrence andreduced the amount of oil leakage to 1/30. By also adding
a minor lip and leak recovery ribs, we were able to collectthe minute amount of leaked oil from the end of the mainlip, and succeeded in fully preventing oil leakage.
The contents of this development will be applied toJTEKT design standards for oil seals, and the knowledge
gained here will be utilized in future seal development.

Y. MOKUBO**JTEKT ENGINEERING JOURNAL English Edition No. 1020E (2024)

H. KURODA*Engineering Management Dept., JTEKT SEALINGTECHNO CORPORATION109