Potential of Sintered Friction Linings in Synchronizers

Dipl.-Ing. Ottmar Back, HOERBIGER Antriebstechnik GmbH Schongau, Peter Echtler, HOERBIGER Antriebstechnik GmbH Schongau Dr. Michael Bergheim, HOERBIGER Antriebstechnik GmbH Schongau

Abstract:

In transmissions, friction generally produces losses and wear. The effects of high friction in gear

teeth contacts and bearing can result in efficiency losses and, in extreme cases, lasting damage

with the risk of failure. For this reason, considerable efforts are in progress to reduce friction and

wear through suitable materials, surface and coating methods, and lubricants. The goal is to

increase the power density and improve the efficiency of the transmission. Gear wheel range

transmissions achieve the best efficiency and therefore play a key role in reducing CO2

emissions in vehicles. In this environment, the synchronizer faces ever greater challenges. In

order to ensure that gear shifts are reliable and comfortable, friction must be produced in the

synchronizer and the energy associated with speed synchronization must be absorbed. In the

non-shifted state, in contrast, the synchronizer should be as inconspicuous as possible and

generate neither losses nor noise.

Solving the dilemma of specific friction in a friction-minimized environment requires the

development of compatible friction systems. HOERBIGER is the specialist for synchronizers in

manual transmissions, AMTs, DCTs, and transfer cases. The product range encompasses all

sizes for use in passenger cars, trucks, and tractors.

In addition to extensive synchronizer system application and manufacturing know-how,

HOERBIGER also has the expertise required for developing sintered and carbon friction linings.

When developing friction linings, the application-specific requirements and manufacturing

engineering possibilities must be merged. In particular, the development of sintered friction

linings calls for a high degree of process development, because the cost advantages of these

friction materials can only take full effect in combination with the metal forming technology.

In contrast, the focus of carbon friction linings, in addition to the formulation, is above all to

develop the manufacturing process for the lining itself.

The complexity of the influencing factors can be controlled by employing statistical development

methods. The use of experimental design (DoE) illustrates the effects and interactions of

individual parameters.

In addition to their primary function of synchronizing the speed by friction, the friction linings also

influence the drag torque behavior of synchronizers. DoEs were also executed in this respect,

and the mechanisms of action were visualized.

While achieving the functional objectives is a necessity, developing competitive friction linings

requires more than that. The specification sheet is not satisfied until the friction technology can

be combined with cost-effective manufacturing methods for synchronizers.

With its latest designs, HOERBIGER has demonstrated that sintered friction linings offer high

performance and an excellent price-performance ratio.

Kurzfassung:

In Getrieben führt Reibung in der Regel zu Verlusten und Verschleiß. Die Auswirkung zu hoher

Reibung in Zahnkontakten und Lagern kann zu Wirkungsgradeinbußen und im Extremfall zu

dauerhaften Schädigungen mit Ausfallrisiken führen. Daher werden erhebliche Anstrengungen

unternommen, durch geeignete Werkstoffe, Oberflächen- und Beschichtungsverfahren und

Schmierstoffe, Reibung und Verschleiß zu reduzieren. Ziel ist es, die Leistungsdichte zu

steigern und den Getriebewirkungsgrad zu verbessern. Zahnradstufengetriebe erreichen die

besten Wirkungsgrade und sind somit wesentlicher Bestandteil zur Reduzierung der CO2-

Emissionen in Fahrzeugen. In diesem Umfeld fällt der Synchronisierung eine immer

schwieriger werdende Aufgabe zu. Zur sicheren und komfortablen Gestaltung von

Gangwechseln muss in der Synchronisierung Reibung erzeugt werden und die aus dem

Drehzahlangleich stammende Energie aufgenommen werden. Im nichtgeschalteten Zustand

sollte die Synchronisierung dagegen möglichst wenig in Erscheinung treten und weder Verluste

noch Geräusche erzeugen.

Um das Dilemma gezielter Reibung in einem reibungsminimierten Umfeld zu lösen, ist die

Entwicklung abgestimmter Reibsysteme erforderlich. HOERBIGER ist der Spezialist für

Synchronisierungen in Schaltgetrieben, AMTs, DCTs, und Transfer Cases. Die Produktpalette

umfasst alle Baugrößen für den Einsatz in PKWs, LKWs und Traktoren.

Neben umfassendem Anwendungs- und Fertigungs-Know How für Synchrosysteme verfügt

HOERBIGER auch über die Kompetenz zur Entwicklung von Streusinter- und

Karbonreibbelägen.

In der Reibbelagsentwicklung müssen die anwendungsspezifischen Anforderungen und

fertigungstechnischen Möglichkeiten zusammengeführt werden. Besonders die Entwicklung von

Streusinterreibbelägen erfordert ein hohes Maß an Prozessentwicklung, da die Kostenvorteile

dieser Reibwerkstoffe nur in Kombination mit der Umformtechnik voll zum Tragen kommen.

Demgegenüber liegt der Schwerpunkt bei Karbonreibbelägen neben der Formulierung vor allem

in der Entwicklung des Herstellprozesses für den Belag selbst.

Die Komplexität der Einflussfaktoren kann durch den Einsatz statistischer

Entwicklungsmethoden beherrschbar gemacht werden. Die Anwendung von DOEs

veranschaulicht die Wirkungen und Wechselwirkungen einzelner Parameter.

Neben der Hauptfunktion des Drehzahlangleiches durch Reibung nehmen die Reibbeläge auch

Einfluss auf das Schleppmomentverhalten von Synchronisierungen. Auch hierzu wurden DOEs

abgearbeitet und die Wirkmechanismen veranschaulicht.

Die Erreichung der funktionalen Ziele ist zwar notwendig aber nicht hinreichend zur Entwicklung

wettbewerbsfähiger Reibbeläge. Erst wenn die Reibbelagstechnologie mit kostengünstigen

Fertigungsmethoden für Synchronisierungen kombiniert werden kann, ist das Lastenheft erfüllt.

HOERBIGER zeigt mit seinen neuesten Entwicklungen, dass Streusinterreibbeläge hohe

Leistungsfähigkeit und ein sehr gutes Preis-Leistungsverhältnis bieten.

1. Introduction

Starting from brass synchronizers, in which the ring material is the same as the friction material,

a variety of coatings and methods for applying them onto support rings were developed over the

years. While in addition to the brass rings, molybdenum layers that are applied onto a variety of

support rings dominated the market for a long time, over the last few years sintered and carbon

friction linings have increasingly gained in importance.

It is difficult to say which is the best friction lining. The selection will always be made based on

the multifaceted functional and economical requirements.

The most expensive option is not always necessarily the best. Fulfilling the function is crucial,

and the implementation of the functionally required solution into a cost-effective synchronizer is,

at a minimum, desirable.

Below is an outline of how friction linings for synchronizers are developed and how sintered and

carbon friction linings differ from each other. An integral analysis reveals that sintered friction

linings, despite some weaknesses, have remarkable potential as it relates to their friction

coefficient behavior, which will keep them attractive in the future.

Aside from being used in manual transmissions, DCT and AMT applications equipped with

sintered synchronizers will become increasingly important.

2. Development of friction linings for synchronizers

Today, the speed synchronization in synchronizers for gear wheel range transmissions is

executed almost exclusively by cone friction clutches. Contrary to multi-disk clutches, which are

used in torque-converter transmissions, the shifting force that is applied increases in keeping

with the cone angle and the friction surfaces are subject to considerably higher specific

pressure.

Figure 1: Design of a multi-cone synchronizer

Figure 1 illustrates the design of a multi-cone synchronizer. The synchronizer rings are located

between the hub system provided on the transmission shaft and the clutch disk connected to

the gear wheel. Together with the respective steel mating surface, the friction lining layers that

are applied onto the rings form one to three friction surfaces.

In addition to the pressure, key load parameters are the maximum sliding speed and the mass

moment of inertia to be synchronized.

In sum, the friction lining is subject to thermal loads, which occur in the form of the peak

temperature on the friction surface shortly after start of the synchronization phase and are

reached in the form of the component temperature as a function of the shifting frequency, heat

capacity, and heat dissipation.

The loads must be determined accordingly when selecting the friction lining and compared to

the limits of use of the respective friction linings. Figure 2 provides a rough overview of typical

load limits for the sintered and carbon lining variants, however deviations from the values shown

here can be influenced by additional parameters.

Figure 2: Load limits of sintered and carbon friction linings

It is apparent that the durability of sintered and carbon friction linings is nearly identical. As a

result, this cannot be used to derive a selection criterion.

Other influencing factors that must be taken into consideration are the lubricant, lubrication

(installation situation), and grooving. So far no applicable theoretical model is known, making

screening in experiments necessary in the future.

Figure 3: Friction lining requirements

If the load capacity requirement is met, an evaluation of the effect of the friction lining on

functional reliability and shifting comfort is required. The locking reliability plays a key role in

optimal function, and the locking angle and cone angle must be matched to prevent shifting

through the gears before the end of the synchronization process. In addition to the influence of

loads and service life, this analysis must also include an examination of the temperature

behavior. While the cold shift behavior of metallic friction linings is more predictable, carbon

linings require specific examination.

In order to examine cold shifting capability, HOERBIGER has developed an experiment for the

component test bench, which allows an assessment of the function in a cold transmission.

Figure 4 shows on the left that the friction coefficient trend worsens when lowering the oil

temperature to -20ºC, while the right section of the figure shows that the friction coefficients rise

when the transmission is cold.

Figure 4: Assessment of cold shifting behavior

In sum, this creates a series of requirements for the lining developer, which must also be

evaluated from cost perspectives.

Another criterion to be considered in the selection of friction linings is production and

application. The following will explain the differences between sintered friction linings and

carbon friction linings in this respect.

The term sintered friction lining stems from the manufacturing method. A mixture of metallic and

non-metallic materials is spread onto stamped steel disks using a metering device. The disks

located on a conveyor belt then pass through an sintering furnace operated with inert gas,

followed by a cooling zone. The speed of passing through the furnace and the temperature

profile of the furnace are adjusted so that bridges can form between the coating materials in

quantities that are sufficient to ensure strength and the porous structure, which is important for

optimal function, is preserved.

Figure 5: Furnace for applying sintered friction linings

Figure 6: Metering device for sintered friction linings

Because sintering can only be carried out for level components, additionally a metal forming

step is required, turning the steel disk into a conical ring. The metal forming tools are designed

to apply the least amount of stress on the sintered friction lining and preserve the properties

which are important for the component to function.

Figure 7: Manufacturing sequence for synchronizer ring with sintered friction lining

Figure 8 shows a micrograph of a sintered friction lining. It illustrates the open structure of the

particles and the connecting bridges between the metallic materials of the powder mixture.

Figure 8: Micrograph of a sintered lining

The production of synchronizer rings using carbon friction linings differs fundamentally from this.

Depending on the technology, the carbon friction lining is produced in a pulp process or as a

fabric and then saturated with a resin.

Figure 9: View into a pulp processing machine for the manufacture of carbon friction linings

Carbon friction linings produced by a pulp process consist of a number of materials, which are

mixed into a pulp and then processed into raw paper-like sheets.

Figure 10: Materials of composite carbon linings

Strips are cut out of the lining sheets in accordance with the final dimensions of the cone

surfaces and glued into the conical support rings.

Both brass rings and steel rings (PM, forged, formed) can be used as support rings for carbon

linings. The use of formed metal rings is becoming increasingly prevalent. After forming, the

rings are deburred and then heat-treated. This step is primarily required to increase the wear

resistance of the blocking teeth. Before gluing, the surface of the ring must be activated. The

adhesive can be applied both onto the support ring and the lining. The adhesive is cured using

a hot pressing operation and the lining is calibrated to its final dimension.

Figure 11: Process flow for synchronizer ring with carbon lining

Figure 12: System schematic for the gluing of carbon linings

Figure 12 shows the schematic of a fully automated system for the mass production of glued

synchronizer rings. In order to ensure the bonding strength and lining properties, the individual

steps of the process are subjected to continuous monitoring and the components are inspected

in several stations, both dimensionally and visually.

3. Areas of application

The description of the different processes for the manufacture of synchronizer rings with

sintered friction linings and carbon friction linings illustrates that the manufacturing costs for

carbon linings must be higher than those for sintered linings. Accordingly, carbon friction linings

will be primarily used if the requirements in terms of the friction coefficient behavior of sintered

friction linings have not been met.

Figure 13: Comparison of carbon lining / sintered lining with respect to friction coefficient

behavior.

In essence, carbon linings exhibit a more favorable friction coefficient behavior in conjunction

with certain transmission oils containing specific additives to withstand increased loads. Another

field of application for carbon friction linings is the DCT with wet clutch, because the additives

are specifically matched to the clutch linings. Figure 13 shows that sintered friction linings

typically exhibit an increase in the friction coefficient toward the end of the synchronization step,

while carbon friction linings have a lower or no increase in the friction coefficient over the

shifting operation.

As explained above, both lining technologies are approximately comparable in their durability,

however sintered friction linings offer advantages in terms of their wear behavior and in

particular with respect to cold shiftability and the drag torque.

Figure 14: Drag torque sintered vs. carbon

Figure 14 shows that, under otherwise identical conditions, sintered friction linings produce

lower drag torque than carbon friction linings. This effect becomes more pronounced the

narrower the clearances are set.

As part of the advancement of its sintered friction lining technology, HOERBIGER has set out to

better meet the requirements specifically for use in DCTs. The methodology and the results

achieved so far will be presented below.

4. Methodology for the advancement of sintered friction linings

HOERBIGER employs the DoE method in its development efforts in a wide variety of fields. This

approach was also selected for the advancement of sintered friction linings.

Experimental design (DoE) is aimed at improving processes or formulations with the least effort

possible and to differentiate statistically significant influencing factors from those which are not

significant. Additionally, the goal is to detect and quantify interactions between different

parameters.

Standard methods often vary only one parameter at a time. Contrary to the analysis using DoE,

no interactions can be detected.

With DoE, additionally the analytical scope decreases. It would take 256 experiments if only one

factor at a time was varied and there were 8 factors to be varied at 2 stages (28 = 256). DoE

allows a reduction in the number of experiments, such as to 28-4 = 16.

In summary, the DoE tool considerably reduces the complexity of the development work and, in

some respects, only makes the detection of certain influencing factors possible.

Figure 15: DoE evaluation, significant factors on friction coefficient

Figure 15 shows, by way of example, the results of the DoE experiments after different numbers

of shifts. In addition to the variation of the lining components, the experiments also included

tests with two different oils. In each case, the friction coefficient amount and the friction

coefficient characteristics were evaluated. As expected, the influence of the oil turned out to be

significant, which is apparent from the fact that the ranges of fluctuation shown are smaller than

the bars and do not go beyond the baseline. It is also apparent which lining materials have more

significant effects and improve the friction behavior.

Using a suitable calculation method, these results can now be employed to determine an

optimized variant, which combines the positive effects.

5. Results

The optimization enabled the development of a new formulation, which exhibits considerable

improvements compared to the standard lining HS45.

The figures show that the friction coefficient was significantly increased with the optimized lining

variant. This is especially apparent from the trend lines for the starting friction coefficient µ1 and

the mean friction coefficient µ2 illustrated in figures 16 and 17.

Figure 16: Comparison of µ1 between standard lining and optimized lining

Figure 17: Comparison of µ2 between standard lining and optimized lining

In contrast, the final friction coefficient µ3 exhibits only a minor increase of the optimized lining

compared to the standard lining. The reason behind this is that the standard friction lining in the

analyzed oil is marked by an increase in the friction coefficient over the course of the shifting

operation. This increase was considerably reduced with the optimized variant, thereby achieving

a more uniform friction coefficient level over the course of the shift.

Figure 18: Comparison of µ3 between standard lining and optimized lining

Figure 19: Ratio of µ3/µ1 between standard lining and optimized lining

Another optimization result is apparent in the run-in period. During the first 500 shifts, the friction

components perform geometrical adaptations to each other, i.e. the specific loads will initially be

higher, until a more uniform contact pattern is achieved as a result of the adaptation of the

friction surfaces. In the process, the friction coefficients that occur can be lower, which

otherwise are only encountered in the range of higher load levels. The optimized lining variant is

able to configure this trough flatter and therefore offer better reliability of the design.

Figure 20: Comparison of µ2 between standard lining and optimized lining in the overload range

Associated with this is the ability of the optimized lining variant to record a lower drop in the

friction coefficients in the range of higher loads. Due to the decrease in the friction coefficient,

the standard lining reaches the functional limit sooner, while the optimized lining provides a

sufficiently high friction coefficient level even when subjected to high loads. In this way, it is

ensured that even extreme shifts can be carried out. Rapid gear shifts are common in DCT

applications in order to improve the responsiveness during kick-down.

6. Summary

The selection of suitable friction linings in synchronizers is primarily geared toward the

requirements of the respective applications. In general, there will be no ideal solution because

advantages in one area will be accompanied by disadvantages in other areas.

Because of their manufacturing process and in conjunction with metal forming technology,

sintered friction linings offer a robust and cost-effective solution for a wide range of applications.

These linings are marked by low wear and noncritical cold shifting behavior. In some oils,

however, the friction coefficient level is no longer sufficient. This is when the more complex

carbon friction linings must be used.

Using the method of design of experiments (DoE), HOERBIGER examined the formulation of

the standard sintered friction lining, which has been successfully applied for many years, and

optimized the friction coefficient level as well as the friction coefficient characteristics, while

preserving the strengths of the lining.

The test results were used to determine the significant parameters, and a new formulation was

computed. The new composition was also analyzed with respect to its robustness in mass

production, and it was manufactured on production equipment. The results confirmed the

findings gained during the prototyping process.

HOERBIGER will continue to employ the DoE method for the development of linings and is

intensively working on other interesting formulations.