Development of a new high performance material to
improve vehicle efficiency by reducing frictional and
parasitic energy losses in transmission components.
Satoru Sekiguchi
Richard G. Van Ryper
Hiroyuki Suzuki
David J. Ritchey
Abstract:
Emission regulations, fuel conservation and green-house gas reduction have made vehicle
efficiency a driving force in driveline and transmission design. Literature and customer interviews
have identified only 10% to 15% of the energy in a liter of fuel makes it to the driving wheels. A
significant component (~10%) of this energy loss can be attributed directly to frictional losses
from moving or rotating parts. The largest component of frictional losses, outside the engine itself,
(1)
occurs in transmission and driveline components .
TM
®
To respond to the market need for improved efficiency, the DuPont Vespel Parts & Shapes
business has investigated the role materials and design play in reducing frictional and parasitic
losses within a transmission. This investigation led to the development of a new material and
design practices which demonstrate a 50% improvement in measured torque loss on a
transmission shaft. In addition, by careful design, the required oil flow rates can be reduced by
90%, significantly reducing the parasitic losses associated with the transmission oil pump.
This paper will discuss how the unique properties of a new composition provide opportunities in
motion- and fluid-control devices to reduce weight, improve dimensional control and drive out cost.
(1) ‘Parasitic Energy Loss Mechanisms Impact on Vehicle System Efficiency, Project 15171’, Argonne National Laboratory,
April 18-20-2006. Authors: G. Fenske, R. Erck, L. Ajayi, A. Erdemir, O.Eryilmaz.
1
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Development of a new high performance material to improve
vehicle efficiency by reducing frictional and parasitic energy
losses in transmission components.
Satoru Sekiguchi
Richard G. Van Ryper
Hiroyuki Suzuki
David J. Ritchey
Introduction:
TM
®
DuPont Vespel polyimide seal rings have been used successfully for over 25 years in
agricultural, military, truck, bus, and passenger vehicle transmissions. Emission regulations, fuel
costs and greenhouse gas reduction, have made improving driveline efficiency a major customer
requirement for new transmission designs.
In addition, our review of published papers and customer interviews led to a general, and
surprising, consensus that as much as 85% to 90% of the energy in a liter fuel never makes it to
the driven wheels. Several studies also have found as much as 10% of this ‘wasted’ energy goes
(1)
to overcome frictional losses from rotating or moving parts .
These observations, combined with specific customer requests to improve the operating range
and durability of new transmission designs, led us to look more closely at how something as small
as a seal ring could lead to very measurable gains in efficiency.
Situation Analysis:
We identified three main areas of potential efficiency improvement:
1) Direct: reduction of ‘system’ coefficient of friction of a lubricated polymer against metal surface.
Lower coefficient of friction results in lower rotational torque loss and less wasted engine power.
2) Indirect: reduction of variation in oil leakage across the seal rings as operating conditions such
as temperature and shaft speed change. Reduced variation in leakage results in lower pumping
losses and allows selection of smaller and more efficient pumps, which require less engine power.
3) Mass related: bearing materials which allow the use of light weight aluminum to significantly
lower the rotating mass and inertia of shafts and housings.
And because this is a cost-conscious industry:
4) Total system cost reduction: optimum gap designs which provide a robust assembly process
help to reduce broken rings and costly rebuilds. In addition, the ability to run polymer rings
directly against soft metals, such as aluminum, avoids the need for expensive steel inserts.
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Program Objective:
Figure 1 illustrates where oil leakage can occur in a shaft/bore/seal ring system:
between the ring face and the shaft groove;
between the ring outside diameter and the bore; and
through the seal gap.
seal rings
ring
Oil In
gap
shaft
Oil Out
housing
Oil leak paths between:
ring face and the shaft groove
ring outside diameter and the housing bore
seal gap
Figure 1
Material characteristics like modulus and robust wear properties address the first two potential
leakage areas. Gap design is typically used to control leakage through the third (the gap).
Improvements and innovations in gap design, such as “3D” gaps, have successfully
demonstrated reduced leakage. However in many situations, the complexities of these designs
prevent their use in small cross-section (thin) rings or where assembly of the seals to the shaft is
restricted by space availability.
Figure 2 – Common 3D Seal Ring Gap Design
Our objective was to develop a material which allowed reduced and stable leakage with a simple,
yet elegant, microcut gap design shown in figure 3.
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Closed
Open
TM
“Microcut” Joint
Base Resin Selection:
(All data in this report, unless otherwise noted, was generated in DuPont testing facilities.)
Figure 3 - DuPont
TM
®
®
The material characteristics of existing polyimides, such as DuPont Vespel SP-1 and Vespel
SP-21 have a history of success in high PV (pressure x velocity) seal ring applications. In figure
4, diamonds show the pressure and speed relationship from a DuPont field survey of seal rings in
actual commercial transmission applications. This field survey encompassed over 50 automatic
and continuously variable transmission designs produced over a 5 year period.
Field Survey of Automatic Transmission Seal Ring P x V Conditions
6
DuPontTM VESPEL® SP
P = Contact
Contact Pressure
Pressure(MPa)
(Mpa)
5
Experimental PV
limits by lab test
4
Actual PV condition of
commercial VESPEL® SP
seal rings from survey
PEEK
3
Not
Recommended
PTFE
2
OK
1
0
0
5
10
15
20
V = Sliding Speed (m/sec)
Lubricant:ATF D-III, Mating material:Carbon steel
Figure 4 - Pressure Velocity Survey
The three lines superimposed on figure 4 indicate the PV limits, measured by DuPont laboratory
testing, for three common seal ring materials. For each resin “below the line” is a “safe zone”
where the PV limits are not exceeded in the application. Above the lines are potential failure
zones where materials would not be recommended due to the possibility of degradation, melting
or high wear.
®
The tested PV limits of Vespel polyimide can be shown to cover the entire range of surveyed
operating conditions in automatic transmissions – with a comfortable safety margin. For this
®
reason, we selected Vespel polyimide as the base resin for our material development.
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Selection of Filler Type:
System coefficient of friction has a direct relationship on torque or energy loss. Selection of
optimum filler type and level is critical to achieving a low coefficient of friction in rotating systems.
This frictional energy loss is typically indicated by an increase in fluid temperature (thermal gain).
The data presented in figures 5 and 6 represent how coefficient of friction and temperature vary
with sliding speed in a standard DuPont lubricated thrust washer test.
Speed vs. Coefficient of Friction (at 1 MPa)
Coefficient of Friction
0.14
PI+Graphite
0.12
PEEK+Carbon Fiber
PTFE+Graphite
0.1
0.08
0.06
2
4
6
8
10
Speed (m / sec)
Figure 5 – Thrust Washer Coefficient of Friction Test Results
Speed vs. Surface Temperature (at 1 MPa)
Temperature (oC)
160
120
PI+Graphite
80
PEEK+Carbon Fiber
PTFE+Graphite
40
2
4
6
8
10
Speed (m / sec)
Figure 6 - Thrust Washer Surface Temperature Results
Test Conditions (both figures)
Oil: ATF -T4 Mating material: chromium molybdenum steel (JIS SCM420 Rz: 3.2 µm, Hardness: 450Hv50)
Figure 5 indicates the combination of polyimide resin with graphite results in a very low coefficient
of friction. Figure 6 shows how this low coefficient of friction corresponds to a reduced heat
buildup as sliding speed increases. For this reason we selected graphite as our primary filler
component.
5
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Mind the Gap: Controlling Leakage by Material Properties
Another factor for control of seal ring leakage is the thermal expansion properties of the seal ring
and the constraining bore material. If the seal ring material has a higher CTE (coefficient of
thermal expansion) than the bore material, the seal ring gap will close as temperatures rise.
Thermal Expansion
ring
large gap
ring
small gap
Gap closes
if ring CTE
> housing
CTE
cold
more oil leak
hot
less oil leak
Figure 7 – Thermal expansion effects on seal ring leak rate
If the cross-sectional area of the gap reduces faster than the corresponding reduction in oil
viscosity (due to higher temperatures), the oil flow is also reduced from the installed ‘ambient’
flow rates. In addition, if the gap closes completely, the seal ring can go into compression which
can lead to premature failure (either fracture or high wear).
To compensate for this closing effect, designers typically specify larger than desired gaps at
ambient conditions and low temperatures. These ‘over-sized’ gaps typically result in the overdesign of oil pumps and downstream components to provide for ‘worst case’ leak scenarios
(either too high or too low oil flow).
Material Design Targets:
To achieve both low leakage rates and consistent leakage rates over broad operating conditions,
we desired to closely match the seal ring CTE to that of the mating housing (bore) material.
In today’s transmissions, bore and shafts are typically made from steel or aluminum, with a noted
increase in the use of aluminum due to its low mass and easy machining. An observed issue with
the use of aluminum, and other soft metals, is the potential for increased wear due to the low
surface hardness of die cast and machined parts.
Therefore we selected two critical design targets:
o
1. CTE should have a target range of 20 to 25 µm/m/ C for final part CTE, which places the
CTE between that of aluminum and steel. This value would also represent a significant
improvement (~40%) over existing polyimide seal ring compositions.
2. Excellent wear properties to allow use of aluminum components.
®
Introduction of Vespel SP-2515
The remainder of this report discusses our newest polyimide product development,
®
Vespel SP-2515 and associated test results to confirm its performance in end-use applications.
®
o
Vespel SP-2515 achieves the thermal expansion target with a CTE of 23 µm/m/ C, allowing the
material to change dimension in a manner similar to the shaft and bore.
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Coefficient of Thermal Expansion
45
40
35
43% Reduction
in CTE vs SP-21
o
um/m/ C
30
25
Target
Range
20
15
10
5
0
SP-21
SP-22
Aluminum
SP-2515
Stainless
Figure 8 - Thermal Expansion Coefficients
Cast Iron
(2)
®
In figure 8 we see the reduction in CTE achieved with the Vespel SP-2515 composition. With a
coefficient of thermal expansion in the target range of aluminum and steel, a seal ring made from
®
Vespel SP-2515 can be designed with a much smaller gap at ambient (assembly temperatures)
and, as temperature increases, maintain a more consistent flow rate as the oil viscosity
decreases. This concept of performance improvement over a broad temperature range is
illustrated in figure 9.
®
Vespel SP-2515 Seal Ring Performance Improvement Concept
Step : Reduce CTE of ring to approximate CTE of housing material
Benefit: More consistent leakage over broad operating temperature
Oil Leak Rate (cc/min)
Step : Reduce initial gap (ambient) dimension
Benefit: Reduce overall leakage & achieve target leak rates
500
Same design as control using
Vespel® SP-2515
400
300
Control ring
Vespel® SP-21
200
100
0
20
Reduced gap design &
Vespel® SP-2515
40
60
80
100
120
140
160
Oil Temperature (°C)
Figure 9
Figure 9 demonstrates how a reduction in the CTE for a seal ring material can be used to control
the leak rate and leak behavior across the operating temperature range of an automatic
®
transmission. The solid line shows the leak rate of the control ring made from Vespel SP-21.
7
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The high slope of the leak curve indicates the gap opening and closing due to the expansion
differences between ring and bore.
The top dashed line indicates the same ring design (dimensions) as the control ring, now using
®
new Vespel SP-2515. The flat slope of the curve indicates more consistent leakage rates over
the design temperatures of the transmission.
Once this leak rate consistency is achieved, the designer can reduce the initial installed gap
®
dimension. This ”new design”, which takes advantage of the low CTE of Vespel SP-2515, is
shown as the lower dashed line. The combination of the improved material and new dimensions
allows both an overall very low leak rate (85 to 90% less leakage) and a very consistent leak rate
(flat slope) over a broad range of temperatures.
Confirmation of Improved Wear Against Aluminum:
The other performance target was improved wear against aluminum. Wear can occur both on the
seal and on the shaft when the two materials are in relative sliding contact. Laboratory tests were
conducted for both seal ring and shaft wear. Figure 10 illustrates the test conditions and
measured wear points.
Housing
Endurance Test condition
Ring OD:
59 mm
Pressure:
1 MPa
Speed:
8500 rpm
Oil temperature: 145 °C (ATF SP-III)
Test time:
300 hour
Shaft:
Aluminum Die Cast
(ADC-12)
Wear
of ring
ATF
Seal ring
Wear
of shaft
Shaft
Figure 10 - Seal & Shaft Wear Test Schematic
120
Wear (µm)
100
113
Ring Wear
total
Shaft Wear
81
80
total
75
60
41
total
40
32
33
20
0
total
Vespel SP21
38
12
8
®
81
20
®
Vespel SP2515
PEEK
Cast Iron
Figure 11 - Observed Seal Ring and Shaft Wear
Figure 11 indicates the total change in dimensions, due to wear, of four types of seal rings and
®
the aluminum shaft after testing at a typical transmission condition. The results indicate Vespel
SP-2515 has a very acceptable level of wear. In fact we saw a 60% to 70% reduction in total
wear vs cast iron and PEEK rings. The results also demonstrate both polyimide materials cause
very little wear on the mating aluminum surface.
8
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Evaluation of Wear in Contaminated Oil:
Contamination, such as particulate matter from machining, assembly or transmission operation,
can cause accelerated wear. As an additional confirmation test, DuPont evaluated the
®
performance of Vespel SP-2515 in the presence of contaminated transmission fluid.
1.4
1.15
Wear Volume (mm3)
1.2
Clean Oil
With Particles
1.0
0.8
0.6
0.50
0.40
0.4
0.2
0.08
0.08
0.07
0.0
®
Vespel SP-21
®
Vespel SP-2515
PEEK
Figure 12 - Contaminated Wear Testing
(oil contaminated with 14 µm particles of Al2O3 and tested for wear at
1.1 m/sec and 1 MPa for 10 minutes against aluminum die cast)
In figure 12, after steady state wear conditions were achieved for all materials (left side, solid bar),
®
particulate contamination was introduced. The Vespel polyimide resins show significantly less
material loss with the contaminated fluid (right-side, patterned bars). This is due to the inherent
®
property of Vespel polyimides which allows contaminates to safely imbed below the wear
surface. Other materials typically allow contaminants to remain on the surface, allowing wear to
continue (see figure 13).
A6061-T5 Aluminum
®
Vespel SP-2515
Figure 13 - SEM-EDX image of contamination particles after wear testing.
Left: Surface of aluminum, particles on surface, wear grooves observed.
®
Right: Surface of Vespel SP-2515, particle embedded in resin, no surface damage.
Test Conditions
Pressure: 2MPa
Speed: 0.05 m/sec
Time: 2 hours
Particles: JIS Z8901class 2, quartz
Grease: Calcium type 1-3
Mating material: chrome plated steel (Ra: 0.8 µm, Hardness: 1000 Hv50)
9
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Observation of Reduced Friction and Torque Loss:
Coefficient of friction (µ) between moving parts has a direct relationship on the efficiency of a
driveline component as measured by torque loss. Reduced torque loss directly relates to
improved gas mileage and lower emissions.
®
Using standard laboratory equipment, we measured the µ of Vespel SP-2515 seal rings against
steel at two temperatures. Multiple tests resulted in a range of µ indicated in figure 14.
Coefficient of friction
0.3
Test
equipment
23 °C
- 30 °C
Thrust load
ATF D-
0.2
55%
Reduction
Mating
material
45%
Reduction
Sample
ring
0.1
Revolution
0
Vespel®
SP-2515
PEEK
Vespel®
SP-2515
PEEK
Figure 14 - Coefficient of Friction Comparison
(Thrust load: 0.4MPa, Sliding speed: 1.67 m/sec, Lubricant: ATF D-III)
®
At -30°C, we observed up to a 55% reduction in the measured µ by using Vespel SP-2515
versus an identical ring made from PEEK resin.
To confirm a measured µ reduction results in an actual reduction in torque loss, we used an
actual transmission shaft and bore in a seal ring test stand fitted with a torque monitor.
Seal Ring Tester
Frictional Loss Torque
(N-m/ring)
0.5
0.4
Seal rings
Seal
rings
PEEK
0.3
Housing:
Housing:
Rotation
(w/Torque
detector)
Rotation
Oil inin
Oil
(w/Torque
monitor)
Oil out
Oil
out
50% Reduction
0.2
®
Oil
Oil leakage
leakage
Vespel SP-2515
0.1
40
60
80
100
120
140
G roove Surface Temperature ( oC)
160
Figure 15 - Measured torque loss on transmission shaft and bore
(Speed: 6000 rpm, Oil Pressure: 1MPa, Ring diameter: 59mm)
Figure 15 shows the observed frictional torque loss under typical transmission operating
conditions. The observed 50% reduction in torque loss was consistent with the previous bench
®
test comparison. This gives additional confidence that the low friction properties of a Vespel
SP-2515 seal ring will result in less heat generation and less power loss in a driveline component.
10
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Conclusions:
These DuPont laboratory results, combined with prototype testing underway at several customers,
have confirmed our material development goals have been achieved. We have demonstrated
significant efficiency improvements can be achieved in critical driveline components:
®
1) Direct: New Vespel SP-2515 demonstrates a low coefficient of friction in automatic
transmission fluid against metal. We have demonstrated ~ 50% reduction in measured torque
loss in a simulated transmission seal ring. In cold temperatures (-30°C), where efficiency losses
are highest, we have measured a 55% improvement in torque loss.
®
2) Indirect: The unique thermal expansion properties (CTE) of Vespel SP-2515, combined with
seal ring design principles, can allow significant reduction in oil leakage due to temperature
fluctuation. We have demonstrated an 85% to 90% reduction in leakage in our simulated
transmission testing. This can allow for use of lower capacity oil pumps resulting in less parasitic
energy loss.
®
3) Mass related: The excellent wear properties of Vespel SP-2515 parts may allow driveline
engineers to use aluminum as a mating bearing surface. The use of aluminum for shafts or
housings can significantly lower the rotating mass and inertia of the driveline components, leading
to higher efficiency, especially in stop/start conditions.
4) Total system cost reduction: The testing in this study confirmed a simple microcut joint can
achieve low and consistent leak rates. Use of a microcut joint has been shown to provide a
robust assembly process and reduce costly rebuilds due to broken rings during assembly. In
addition, the potential to run polymer rings directly against aluminum avoids the need for
expensive steel inserts.
®
The unique properties of Vespel SP-2515 may also allow efficiency gains in adjacent driveline
applications. Adjacent applications currently being evaluated are:
o Thrust washers to protect sun and planet gears in planetary gear sets. The high PV and low
coefficient of friction allow similar efficiency improvements, plus an extra margin of safety in
boundary or dry lubrication conditions – such as start/stop operations.
o Hydraulic system spool valves, which require a high level of tolerance control plus a
resistance to contamination particles to prevent sticking or jamming. The low CTE and
®
compatibility of Vespel SP-2515 with contaminated oil may allow the valve housing to use
light weight, cost effective aluminum.
o Bushing or bearings for on-engine components like EGR valves or turbo charger linkages
®
®
where the high thermal conductivity of Vespel SP-2515 (more than 4X higher than Vespel
®
SP-21) allows heat to be conducted away faster than most polymers. In addition Vespel
polyimide parts have demonstrated a resistance to soot or coke build up, resulting in long
bearing life in dirty exhaust environments.
Summarizing, the low CTE and the excellent wear and low friction characteristics of our newest
®
polyimide offering, Vespel SP-2515, provides a new material and design option for driveline
engineers who require improved vehicle efficiency and lower system cost.
___________________
(1) ‘Parasitic Energy Loss Mechanisms Impact on Vehicle System Efficiency, Project 15171’, Argonne National Laboratory,
April 18-20-2006. Authors: G. Fenske, R. Erck, L. Ajayi, A. Erdemir, O.Eryilmaz.
(2) Coefficient of Thermal Expansion data from following sources:
6061 Aluminum Alloy and G4000 Cast Iron from 1998 SAE Handbook, Volume 1, Section 10
316 Stainless Steel from data sheets published by Allegheny Ludlum Steel
11
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