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.

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.

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.

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.

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)

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.

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.

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

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

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

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.

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.

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.

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

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