TECHNICAL REPORT

Development of Warm Sizing Method for CVJ Outer Races
T. ABE K. KONISHI K. KURISU

S. FUKUDA K. MATSUMOTO

We have developed a warm sizing method that reduces CO2 emissions from the forging production line of CVJ
outer races. The conventional process took a long time to go from heating to cold sizing. We hoped to reduce theenergy consumed by consolidating processes, so we investigated the use of warm sizing to incorporate cold sizing
into the warm forging process. Eliminating cold sizing using this method reduced CO2 emissions by 24% andforging costs by 13%. Although based only on the results of our own research, this technology is unique in Japanand is considered to be extremely advanced.
Key Words: CVJ outer race, warm sizing, cold sizing, CO2

1. Introduction
As the entire world is working for decarbonization, theautomobile industry requires greater CO2 reductions thanever before, and the need for energy conservation throughproduction technology innovations has been growing year
by year. In order to meet these requirements, we havedeveloped a warm sizing method that emits little CO2
for the JTEKT forging material line. We are also using
this method to work for reductions in CO 2 emissions
by consolidating processes. This report introduces aninitiative at the forging process for the constant velocity
joint (CVJ) that is used in some passenger cars.
As shown in Fig. 1, the drive shaft is composed of an
inboard joint (differential-side CVJ), outboard joint (tireside CVJ), and an intermediate shaft that connects them.
The primary function required of a CVJ is to receive therotation speed that is transmitted from the engine to thedifferential and transmit it to the tires at constant speed
without dependence on the torque intersection angle.
The four primary components of the outboard joint
are the balls, inner race, cage, and outer race. We haveeliminated the cold sizing that produces large amountsof CO2 emissions at the outer race forging process, anddeveloped a warm sizing method that uses warm forging
to finish the material bore diameter and outside diameter.
Following is a description of this development.

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

Drive shaft
Inboard joint(differential side)

Outboard joint(tire side)

Outboard joint component parts

Balls
Inner race

Cage

Outer race

Fig. 1 Drive shaft and CVJ component parts

2. Development Objectives
With the conventional process, at the forging materialline where this development took place, the materialis shaped by warm forging. It is then cooled to theprescribed temperature and cold sizing is performed on a
separate line to finish the dimensions. It was an extremely
long process and required intermediate stock. We expectedthat it would be possible to reduce the energy consumed
by consolidating processes, and studied a change fromcold sizing to warm sizing for incorporation into thewarm forging process. As shown in Fig. 2, because
the required accuracy of the bore diameter and outside
diameter dimensions is high for the outer race material(hereafter referred to as the “workpiece”), previously cold
sizing was performed after four warm forging processes.
However with the change to warm sizing, we aimed toconsolidate the forward extrusion at the #1 process and15

−Development of Warm Sizing Method for CVJ Outer Races−

Conventional process

Warm forging

Heating

Controlled coolingand cooling waterIntermediatestock
Blasting

Single-component
Cold sizing
lubricantForging process flow diagram
Warm forging 1250T press

Cold sizing press

Warm sizing#1#2

Process consolidation

Warm forging

Heating

Controlled coolingand cooling water#3#4

Process shift Process shift

Cold sizing
Cold forging → Warm forging

Blasting#1#2#3 #4 (warm sizing)

Fig. 2 Explanation of development method#2 process, and thereby shift the #3 process (upsetting)and #4 process (forming) to incorporate the cold sizing
into the #4 process (warm sizing).

3. Technical Issues
3. 1 Warm Forging Process Consolidation: From
Four Processes to Three Processes

Previously forward extrusion was performed divided
into two parts, the #1 process and #2 process. Because
it is necessary to complete forward extrusion at the #1
process, the amount of required forming is larger and thesliding distance of the drawn part also increases. As aresult, as shown in the friction heat formula ⑴, there isthe concern of a rise in mold temperature.
Q =mmgs

⑴

Here,
Q: Friction heat, m: Coefficient, m: Mass,
g: Gravity, s: Sliding distance
From the Archard wear model ⑵, the mold hardness
is expected to decrease as a result of mold thermal
softening, and this may result in an increase in the amountof wear at the drawn part. Therefore, it is necessary to
reduce wear by reducing thermal softening of the mold.
w ＝∫

16

KP V
dt
Hc
a

b

⑵

Here,
w: Wear depth, K: Coefficient, P: Contact pressure,
V: Relative sliding velocity,
H: Hardness of the mold material

3. 2 Changing from Cold Sizing to Warm Sizing

This section first explains the sizing method. First as
shown in Fig. 3, a workpiece with a straight cup opening
shape is placed on the die. Then the cup outside diameteris gradually squeezed. During this process, the workpiecefills along the punch shape, forming a concave shape(undercut). Because this is an undercut shape, it is notpossible to release the punch from the workpiece.
Before forming Cup opening
Straight shape

MandrelLower dead point
Cup outside diameter is squeezed
Cannot releasepunch from workpiece

Workpiece
Die
Punch
Pre-process workpiece isplaced on the die

The workpiece fills
along the punch shape,
forming a concave shape(undercut) at the opening

Fig. 3 Sizing method

As shown in Fig. 4, the method of releasing the punch
and workpiece first uses a stripper to hold the cup end,
and then when the mandrel slides on the inside of thepunch, clearance forms between the punch and mandrel.
The formation of clearance causes a diameter reductionof the punch along the workpiece shape, allowing releaseby avoiding the undercut.

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

−Development of Warm Sizing Method for CVJ Outer Races−

Mechanism of mandrel
sliding inside punch

Diameter reduction of punch occurs along theworkpiece shape, avoiding the undercut

4. Development Contents and Results
4. 1 Reducing Wear by Reducing Thermal
Softening of the Mold

StripperClearance between
punch and mandrel

Punch diameter
reduction

Fig. 4 Punch and workpiece releasing

Although the use of this method for warm sizing was
also studied, the first concern with the change to warm
sizing was a decrease in the workpiece deformation
resistance. The workpiece temperature is higher at warm
sizing compared to cold sizing, and as shown in the load/
stroke diagram in Fig. 5, we learned that the change towarm forging reduces deformation resistance by 75%.
For this reason, there is the possibility of deforming theundercut shape when the punch is pulled out. Therefore, itis necessary to establish a punch design that can maintainthe workpiece shape.

Load, tLoad/stroke diagram

As shown in Fig. 6, the results from CAE analysis
of mold temperature showed that with warm sizing, themaximum temperature increases by 17% compared to theprevious process, raising the concern of increased moldwear caused by thermal softening.
Next in order to verify the mold hardness resulting fromthermal softening, the decrease in hardness was predictedbased on the tempering parameter. The prediction of moldhardness resulting from thermal softening shown in Fig. 7
is the result from hardness prediction when heat wasapplied to the mold. Because a decrease in mold hardness
with warm sizing compared to the previous process is
expected, a countermeasure is necessary.
Previous

Cold
Warm

Warm sizing

17% temperature rise with warm sizing
compared to previous process

Changing to warm forging
reduces deformation resistanceby 75%
75%.

Temperature
High

Low

Fig. 6 Mold temperature verification result from CAE
analysisFig. 5 Load/stroke diagram

Next, the second concern with the change to warm
sizing was the change in the punch temperature. With
warm sizing, a large change in temperature was expectedcompared to cold sizing. As shown by the formula forthe amount of thermal expansion ⑶, there is concern thatthe amount of thermal expansion will change due to theeffects of punch temperature change, resulting in variationin the workpiece bore diameter dimension. It is therefore
necessary to ensure the accuracy of the workpiece bore
diameter dimension by controlling the punch temperature.
k＝a L（0 t －t 0）

⑶

Here,
k: Amount of thermal expansion, a: Coefficient,
L 0: Initial length, t −t 0: Temperature change

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

Hardness, HV

Stroke, mm(2) Improvement ofmold hardness

Warm sizing(1) Improvement of cooling efficiency
Tempering parameter
Previous
P＝T（log t ＋ 20）
Mold temperature, ℃

Fig. 7 Hardness prediction results based on temperingparameters

Two countermeasures were implemented in order toreduce mold wear: (1) improvement of cooling efficiency,and (2) improvement of mold hardness.
(1) Improvement of cooling efficiency
At the warm forging process, a water-soluble
lubricating fluid is applied before stamping, however
the results of fluid analysis showed that the lubricating
fluid was exposed to the atmosphere and did not reachthe high-temperature part. For this reason, the lubrication
spray angle was changed, and was optimized to spraythe lubricant onto the high-temperature part. When17

−Development of Warm Sizing Method for CVJ Outer Races−

verification by fluid analysis was performed again, theresults showed that contact pressure where the lubricant
was sprayed onto the high-temperature part was increased
by 67% from before. Therefore we improved the coolingefficiency by increasing the amount of lubricating fluidthat is sprayed (Fig. 8).
(2) Improvement of mold hardness
By adopting a mold made of material with higher
hardness than the conventional material, hardness was
improved by 16% and the decrease in hardness was
reduced (Fig. 9).
Implementing the above two countermeasures enabled
us to reduce mold wear, and achieve the design target
mold lifetime.

thickness was changed consisting of a concentrated
load at a single point on the end of a cantilever. The
relationship between load and punch thickness wascalculated from the load calculation formula ⑷.
W＝6EId/l 3

⑷

Here,
W: Load, E: Young’s modulus, I: Moment of inertia,
d: Amount of deflection, l : Punch length

:t

ness

Thick
W: LoadPrevious

After countermeasure
Punch diameter
reduction

Spraycontact
pressure

Hightemperature
part
Exposed to atmosphere, preventing lubricationfrom reaching high-temperature part
Contact pressure, MPa

Analysis
resultTime, sHightemperature
part

Spray contact pressure
increased by 67%

Fig. 8 Fluid analysis result

Fig. 10 Punch structure

As shown in Fig. 11, the punch thickness was set sothat the load ratio compared to the current design was
the same as the 75% decrease in workpiece deformation
resistance. The use of a low rigidity design that reduced
the load (W) applied to the workpiece when the punch ispulled out allowed us to establish a punch design that canmaintain the workpiece shape.
Set punch thickness to produce
the ratio of load W that is thesame as the decrease in
deformation resistance[Material]
Countermeasurematerial
Previous material

Load W, N

Hardness, HV

Hardness increased by 16%

Current design

Low rigidity design

Punch thickness, mm
Mold temperature, ℃

Fig. 9 Hardness prediction result after countermeasures

4. 2 Establishing a Punch Design That Can
Maintain the Workpiece Shape

Because it was known that the change to warm
forging would reduce deformation resistance by 75%,
we studied the optimal punch shape that could maintainthe workpiece shape. It is necessary to reduce the load(W) applied to the workpiece when the punch is pulled
out in order to avoid deformation of the undercut shape.
As shown in Fig. 10, punch thickness (t) was changed inorder to reduce rigidity. As the thickness design method,
we created a model for the location where the punch

18

Fig. 11 Relationship of load and punch thickness

4. 3 Ensuring Workpiece Bore Diameter
Dimension Accuracy by Reducing the Punch
Temperature

Predictions of the workpiece bore diameter dimension
resulting from thermal expansion were conducted, andthe target was a punch temperature which produced the
median value between the bore diameter dimension upperand lower limits. Next for mold temperature change
prediction, a temperature rise is expected after one shotof stamping, and a temperature decline is expected
after lubricant application. Therefore if the temperaturecannot be fully lowered when the lubricant is applied,
the temperature will gradually increase. As a result, it

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

−Development of Warm Sizing Method for CVJ Outer Races−

is necessary to optimize the lubrication conditions tostabilize the mold temperature (Fig. 12).

Mold temperature, ℃

ture

Mold

era
temp

Punch

Stress, MPa
Contact
High

Closed space

MandrelriseStress: Low

Stamping

Low

Stamping

Workpiece

Die

Closed spaces
form during theforming process

Stamping
Lubricantapplication
Shot 1

Shot 2

Fig. 14 Sizing die analysis result

Lubricantapplication
Shot 3Fig. 12 Prediction of mold temperature transition

We implemented two countermeasures in order toensure the accuracy of the workpiece bore diameter
dimension: (1) optimization of the lubrication spray
position, and (2) monitoring of lubricant flow rate.
(1) Optimization of the lubrication spray position
The nozzle, which was previously not fixed, was fixedin place, and the spray position was changed to the hightemperature part of the punch (Fig. 13).

The mechanism which resulted in die cracking isshown in Fig. 15. After lubricant was applied to thedie, combustion gas was generated when the hightemperature workpiece was loaded. Closed spaces formed
during the squeezing process, and the combustion gas
was compressed in the closed spaces, causing erosion(physical attack by fluid).
Closed space
forms

Lubricantapplication

Combustion gas iscompressed during
squeezing, causing
erosionGas

Fig. 15 Sizing die cracking mechanism
High-temperature part
Change lubrication spray position tohigh-temperature part of punch
Change to fixed nozzle

Fig. 13 Lubrication spray position(2) Monitoring of lubricant flow rate
A flow rate monitoring system was adopted. Thissystem performs flow rate control, and stops the press if
the flow rate deviates from the set upper or lower limit.
Implementing the above two countermeasures enabled
us to stabilize the punch temperature and ensure that theworkpiece bore diameter dimension satisfies the processcapability.

4. 4 Issue after the Start of Mass Production

Although the technical issues were resolved and mass
production was started, early damage occurred in thesizing dies at the four warm forging processes. CAE
analysis was performed in order to address this problem.
The results showed that there was no problem with thestress applied to the dies, and that the damage was causedby closed spaces that formed between the workpiece anddie during the forming process (Fig. 14).

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

When the cause of the closed spaces was investigated,
we found that the workpiece angle was larger than thedie angle, as shown in Fig. 16. We learned that during
the squeezing process, the apex of the workpiece taper
contacted the die first, forming a closed space.
As a countermeasure to this, the workpiece taper angle
was optimized and the same angle was used for the tapersof the workpiece and die, preventing the formation ofclosed spaces. This eliminated the early damage to thedies and achieved the design target mold lifetime.
Workpiece angle > Die angle

Apex of workpiece taper
contacts the mold first

Workpiece
angle

Die angle

Closed space

Fig. 16 Causes of closed spaces

19

−Development of Warm Sizing Method for CVJ Outer Races−

5. Development Results

6. Conclusion

5. 1 Reduction in CO2 Emissions

As a result of this development, cold sizing waseliminated and the number of processes was reduced,
reducing CO2 emissions by 24% and forging cost by 13%.
Based on a JTEKT investigation, as of June 2023 thiswarm sizing method is unique in Japan and is considered
to be extremely advanced. The warm sizing technologythat was developed as described in this report has so far
been deployed to eight outer race part numbers. We areplanning to deploy it to other part numbers in the future,
helping to address environmental problems and further
improve product strength.

Intermediate stock, units

Following this development, CO 2 emissions werereduced by 24% compared to the previous emissions. The
CO2 reduction effect is shown in Fig. 17.
1

1
0.8

24%
reduction

0.76

0.6
0.4
0.2
0

Previous

Development

Fig. 17 CO2 emission reduction

5. 2 Reduction in Forging Cost

Forging cost (index)

Following this development, forging cost was reducedby 13% compared to the previous cost. The cost reduction
effect is shown in Fig. 18.
1

1
0.8

13%
reduction

References
1)

T. MISHIMA: Development of a Small-scale CVJ
Forging Line, JTEKT ENGINEERING JOURNAL, No.
1013E (2016) 77.

0.87

0.6
0.4
0.2
0

Previous

Development

Fig. 18 Forging cost reduction

5. 3 Other Eﬀects

Process count (index)

Following this development, the process count was
reduced by 33% and forging intermediate stock was
reduced by 100%. These effects are shown in Fig. 19 andFig. 20.
1

1
0.8

33%
reduction

0.67

0.6
0.4
0.2
0

Previous

T. ABE*

K. KONISHI*

S. FUKUDA*

K. MATSUMOTO*

Development

K. KURISU*Intermediate stock, units

Fig. 19 Process count reduction
5 000

5 000

4 000
3 000

100%
reduction

2 000
1 000
0

Previous0
Development

Fig. 20 Intermediate stock reduction

20*

Materials Innovation Engineering Dept., Production
Engineering Division

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