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HOERBIGER SKS – the ideal Synchronizer for DCTs and AMTs
Dipl.-Ing. Ottmar Back, HOERBIGER Synchrontechnik GmbH &
Co.KG Schongau, Erol Ledetzky, HOERBIGER Synchrontechnik GmbH &
Co.KG Oberstenfeld, Dr. Michael Bergheim, HOERBIGER Synchrontechnik
GmbH &Co.KG Schongau
Abstract:
The revolution in the sector of automatic transmissions was
started by the DQ250 of
Volkswagen and continues unabated with the production start of
further DCTs from
various other manufacturers. For the first generation, primarily
the challenges of
clutch systems, either wet or dry, and their controls had to be
solved. With respect to
synchronizers, the developers relied on proven technology from
manual
transmissions. In the meantime it has become obvious that
adapted synchronizer
systems are also needed to open up further potential in terms of
function, reliability,
performance, efficiency and cost.
HOERBIGER is the synchronizer specialist with maximum system
expertise and is
equipped to serve customers needs worldwide. HOERBIGER supplies
synchronizer
systems and components for almost all DCTs featuring wet and dry
clutches in a
torque range of 200 Nm to 750 Nm.
The newly developed HOERBIGER SKS (Smart Key Synchronizer)
offers numerous
features which specifically meet the requirements of modern
Double Clutch (DCTs)
and Automated Manual Transmissions (AMTs).
The principle of the HOERBIGER SKS allows a robust synchronizer
function to be
achieved, without compromising the required shift quality.
Additionally, critical issues
such as noise and efficiency can be improved.
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The synchronizer rings used in the HOERBIGER SKS offer a higher
load capacity
and, in combination with the HOERBIGER sintered friction lining,
exhibit superior
characteristics.
This allows a reduction of system complexity from multi-cone to
single-cone
synchronizers, which not only results in cost advantages, but
also reduces drag
torque in the transmission. The modular concept rounds out the
advantages and
strengths of the HOERBIGER SKS.
Aside from the HOERBIGER SKS, this paper also presents the
supporting
developments in the area of sintered friction linings, which are
a key factor for
maximizing the entire cost reduction potential.
1. Introduction
The demands that are placed on synchronizers in DCTs and AMTs
differ from those
in MTs in a variety of respects. In vehicles equipped with
manual transmissions, the
driver experiences the gear shift primarily from the actuation
of the shift lever. The
driver feels the shift travel, the force path, and especially
the pressure points and
grating, directly at the hand. In addition, acoustic effects can
influence the shifting
perception. The shift simulator [1] allows different influencing
factors on the
subjectively perceived shift quality to be determined and
varied.
In contrast, gear shifts in DCTs and AMTs generally take place
without actions on the
part of the driver, with the exception of the manual mode, which
allows the driver to
issue specific shift commands. However, no direct mechanical
connection exists
between the driver and the transmission. The synchronizers to be
shifted are
operated by a hydraulic, electric or – in the case of commercial
vehicles – pneumatic
actuator, which receives its commands from the transmission
control. As a result,
shifting effects that would be perceivable at the gearshift
lever become less
important. The driver only perceives noise and potential load
change reactions.
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As far as the strain on the synchronizers in DCTs and AMTs is
concerned, two key
differences compared to MTs should be noted.
• Compared to the manually operated transmission, the shifting
frequency will
rise significantly, which is one of the reasons why DCTs and
AMTs result in
reduced fuel consumption in everyday driving.
• The shifting forces are typically higher than those applied
manually by the
driver, however incorrect shifts and overloads can be
excluded.
As a result, the synchronizers for DCTs and AMTs can be designed
with the loads in
mind. The use of multi-cone synchronizers can typically be
reduced, resulting in
lower costs and less drag torque.
The HOERBIGER SKS combines higher performance with a robust
shift behavior.
2. Requirements for synchronizers in MTs, DCTs and AMTs
Synchronizers must perform the same basic functions in DCTs and
AMTs as in the
classic manual transmissions. The friction system produces the
synchronization
between the gear wheel and the shaft, and the sliding sleeve
establishes the positive
connection between the gear wheel and the shaft for torque
transmission.
The different requirements for the particular applications are
outlined below.
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Table 1: Requirements for synchronizers – comparison between MT
and DCT/AMT
It is apparent from the table that comfort features are less
important for the
application in DCTs and AMTs. Accordingly, the design can be
geared more toward
functional reliability, load capacity, and efficiency. Special
areas of emphasis are the
pointing and back tapering of the sleeve and dog ring teeth as
well as the detent
elements.
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The influence of the particular clutch system should not be
neglected. AMTs and
DCTs equipped with dry clutches are comparable because the drag
torque of the
clutch depends only marginally on the temperature. Wet clutches,
in contrast, may
cause significant problems when engaging the first gear or
reverse while the
transmission is cold. Both blocking release and meshing are
carried out against the
drag torque of the clutch, which can be in the double-digit Nm
range. The pointing
should therefore be designed with the smallest angle possible,
which in turn
necessitates the use of coupled multi-cone systems (figure 1)
for these gears.
Figure 1: Standard synchronizer design for DCT and AMT
applications
The application of coupled systems hinders the use of common
parts for single-cone
and multi-cone systems. Both the coupling and the pointing angle
make variants
necessary.
Due to the shift actuator system, large clearances are provided
between the shift fork
and sleeve. In this way, losses from the frictional contact
between the shift fork and
sleeve are reliably prevented. In neutral, as is shown on the
right in figure 1, this
necessitates additional detents, which center the sleeve
relative to the hub. Aside
from the added complexity, this may also cause unpleasant noise
during
engagement.
3. The HOERBIGER SKS (Smart Key Synchronizer)
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In the conventional blocking synchronizer, the blocker ring
performs both the blocking
of the sleeve and the speed synchronization. The detent is used
only to position the
blocker ring during the indexing phase. The shifting force is
transmitted from the
sleeve to the blocker ring by way of the blocking teeth.
Figure 2: Design configuration of the SKS (Smart Key
Synchronizer)
The SKS blocker unit includes an indexing key and a blocking
key, and a spring is
located in the area of the previous indexing unit. The SKS ring
is coupled to the SKS
blocker unit by way of its recesses. The blocking key is
additionally guided laterally in
the sleeve.
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Figure 3: Shift operation with the SKS
The spring pushes the blocking key of the SKS blocker unit into
the detent groove of
the sleeve. During shifting, the sleeve is moved axially in the
direction of the gear to
be engaged and in the process carries the SKS blocker unit
along, thereby pressing
it against the blocker ring. The applied axial force produces a
friction torque between
the blocker ring and gear wheel cone, which results in a
rotation of the blocker ring
relative to the hub. The rotation of the blocker ring also
produces a rotation of the
indexing key relative to the blocking key guided in the sleeve.
As a result, the
blocking key is pushed radially outward by bevels into the
detent groove of the
sleeve, blocking it from further axial movement.
Now, the shifting force can be transmitted from the sleeve to
the blocker ring by way
of the SKS blocker unit. After the rotational speed differential
has been reduced to
zero, the friction torque in the cone collapses and the blocker
ring can be turned back
into the center position. This releases the blocking of the
sleeve, and the sleeve can
be moved axially and engage in the clutch gearing.
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Figure 4: Comparison between standard synchronizer and SKS
Figure 4 highlights the major differences between the SKS and
the standard
synchronizer. The installation space and shift travel remain the
same and thus
enable a replacement in existing transmissions. By eliminating
the blocking teeth at
the SKS ring, the pointings at the sleeve and at the dog ring
can be implemented with
smaller angles. Additionally, the external teeth of the hub can
be widened to improve
the guidance of the sleeve.
The HOERBIGER SKS can be implemented both as a single cone SKS
or as a
double or triple cone SKS. Figure 5 shows the corresponding
relevant
configurations:
Figure 5: HOERBIGER SKS kit
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A feature that is of particular interest for the DCT/AMT
application is hidden in the
blocking unit. The spring, which presses the blocking key
against the sleeve, is
guided on the inside in the hub and on the outside in the
blocking key. As a result, a
restoring force builds during the deflection of the spring in
the axial direction and
centers the sleeve in neutral, when no gear is shifted.
Figure 6: Integrated Neutral Detent Functionality of SKS
The integrated restoring function makes it possible to forego an
additional detent,
and thereby reduces the complexity and costs of the hub system,
while additionally
increasing the strength of the hub.
Figure 7: Comparison of hub system with neutral detent and SKS
hub system
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The use of synchronizers in DCTs and AMTs means that incorrect
shifts can be
prevented by the controller. However, in order to achieve the
shortest possible gear
shift times, the shifting forces will be higher than in the MT.
Furthermore, the shifting
frequency will rise, which is also a reason for the lower fuel
consumption of DCTs
and AMTs. The higher average loads and the high number of shifts
are not likely to
result in greater wear rates. On the contrary, the points in the
shifting sequence
should remain as constant as possible for the automated gear
shift. The SKS makes
an important contribution to this by distributing the load on
the friction lining more
evenly, thereby considerably decreasing wear.
Figure 8: Comparison of load distribution for standard
synchronizer and SKS
The introduction of the force at the small diameter of the SKS
ring generates a
uniform pressure distribution on the cone. Standard synchronizer
rings, in contrast,
always have a higher load in the area of the larger cone
diameter. The uneven
pressure distribution is compensated for by accordingly higher
wear.
4. The HOERBIGER SKS in practice
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In many respects, the design and the function of the SKS
accommodate the
demands of DCTs and AMTs. It is already apparent during the
conception and
design phase of the components that the SKS enables a maximum of
common parts
across all shifting points. The pointing of the sleeve and dog
ring teeth, which is
independent of the blocking function, allows the multi-cone
system to always be
uncoupled. Table 2 shows that theoretically the hub systems can
be designed
identically for all shifting points. In practice, however, a
differentiation is made with
respect to the inner teeth of the hubs.
Table 2: Common parts strategy with SKS
In addition, only the inner rings for the double cone and triple
cone systems are
different. The inner ring for the triple cone system is provided
with a friction lining at
the inside diameter which runs against the gear cone.
Aside from lowering the tooling costs and making volume effects
accessible, this also
gives the developer the opportunity to adapt the synchronizer
performance, where
necessary, with reasonable cost and equip different transmission
series with
common parts.
The measurements below show the benefits of the SKS in the
practical
implementation in the transmission.
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Figure 9: Comparison of wear results
The tests conducted on the HOERBIGER µ-comp component test rig
verify that with
its improved load distribution, the SKS also has the lowest wear
on the friction lining.
In addition to its functional benefits, the SKS also opens up
considerable economic
potential in conjunction with the HOERBIGER sintered friction
linings. This potential
includes, for one, the manufacturing costs, which were further
reduced compared to
systems with the standard design, and secondly the losses due to
drag torque in
non-shifted synchronizers.
Sintered linings have long been successfully applied in MTs,
AMTs, and dry DCTs. In
wet DCTs, however, it is not always possible to achieve the
necessary friction
coefficients with the oils that are geared toward wet clutches.
HOERBIGER
responded to this by enhancing its proven HS45 lining into the
new HS90 lining.
Adjustments to the formulation and limiting the process
parameters have resulted in
a lining that exhibits improved friction levels as well as
improved friction coefficient
characteristics across the entire operating range.
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Figure 10: Comparison HS45 and HS90 in DCT Oil FFL2
The curve of the friction coefficient shown in figure 10 is
based on experiments using
synchronizers in the standard design. The increase in the
coefficient of friction with
HS90 is obvious, however on the left a friction coefficient
minimum is still apparent
during the run-in period. This is essentially related to the
adjustment of the friction
surfaces to each other.
As can be seen in figure 11 the SKS, in contrast, is able to
reduce this minimum in
the friction coefficient curve. In conjunction with the HS90
lining, the level of the
friction coefficient remains consistently high right from the
start. Additionally, the rise
in the friction coefficient over the slip time is considerably
less and may be entirely
eliminated with some oils.
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Figure 11: SKS in practice test with DCT oils used for Dry
DCTs
As a result, the development results of the SKS have confirmed
the theoretical
assumptions and expectations.
5. Summary
HOERBIGER SKS
The ideal synchronizer for DCTs and AMTs
This objective entailed great challenges which had to be met
with the development of
the SKS. The development level that was achieved verifies that
with the SKS a
beneficial new synchronizer system was developed, which can
demonstrate its
strengths especially in DCTs and AMTs.
The functional and economic goals were pursued equally, and the
SKS has achieved
considerable improvements compared to the standard
synchronizer.
The elimination of the blocking teeth at the synchronizer ring,
an inherent feature of
the design, opens up the possibility of optimizing the pointings
at the sleeve and at
the dog ring for engagement. The use of coupled multi-cone
synchronizers has
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become redundant. The application of uncoupled systems further
opens up the
extensive use of common parts, both for the friction parts and
the torque-transmitting
components.
Special functional features of the DCTs and AMTs with respect to
the centering of the
sleeve are implemented by the SKS without additional
components.
The ideal introduction of the shifting force on the synchronizer
ring distributes the
load on the friction lining and thereby reduces wear.
The optimized pointings of the SKS and the elimination of the
neutral detents further
reduce any noise occurring during shifting as a result of
meshing and the
engagement of detents.
Combining modern metal forming technology with the enhanced
HOERBIGER
sintered friction linings leads to further cost reductions,
which is also supported by
volume effects from the common part strategy.
The HOERBIGER SKS in combination with the HOERBIGER sintered
friction linings
thus takes a leap in the direction of an ideal synchronizer
concept for DCTs and
AMTs.
[1] Schreiber,U.; Back, O..: Experiencing and definition of the
real shifting comfort
at the virtual synchronizer in the virtual power train, 8th
International CTI
Symposium, Berlin 2009