2014 STLE Annual Meeting May 18-22, 2014 Lake Buena Vista, Florida, USA

Friction Screening Test For Wet Multiple Disc Clutches With Paper Type Friction Material

Robert ACUNER1, Hermann PFLAUM1, Karsten STAHL1

1 : Technische Universitaet München (TUM), Mechanical Engineering, Gear Research Center (FZG), Munich, Germany

Abstract: OEM’s rising requirements on friction performance of wet multiple disc clutches and brakes can only be met by development of new friction materials and lubricants. Due to the need of intensive testing the corresponding engi-neering process is costly and time consuming. This problem is picked up by a research project of Forschungsver-einigung Antriebstechnik e.V. (FVA) at the FZG (TU München) with the goal of a standardized short time test proce-dure which can be accepted both by suppliers and OEMs. A screening test for the determination of friction behavior of wet multiple disc clutches with organic paper type friction materials has been developed as well as specific values for the evaluation of the test. Within a few hours the friction behavior is tested under various load conditions and evaluated with an excellent differentiation. Influences of load conditions and additive package on level of coefficient of friction and tendency to shudder of the clutch are discussed by exemplary test results of proposed friction screening test. Category: Engine & Drivetrain Keywords: multiple disc clutch, wet clutch, friction screening test 1. INTRODUCTION

Wet multiple disc clutches are versatile power-shift de-vices in power trains. They are used as starting clutches in dual clutch transmissions (DCT), as shift and converter clutches in automatic transmissions (AT), in the industrial application as heavy duty brakes, slipping clutches etc. Friction behavior of each clutch is deeply influenced by the tribo-system that consists of the friction material, the steel plate and the lubricant and its additives [2],[3]. Paper type friction materials in combination with appropriate lubricants can provide smooth shifting performance for high comfort applications such as vehicle transmissions. SAE test specifications [4],[5],[6],[7] for wet clutches are complex and not suitable for actual friction testing. These tests mix up friction and endurance/durability behavior as they are run until the clutch fails. Furthermore they are not flexible because they are linked to one special test rig using absolute speeds and axial forces, and they are diffi-cult to handle as the test rig setup has to be changed with-in the test. The development of new friction materials and lubri-cants/additives requires an easy to use, flexible and fast test procedure. The proposed standardized short time screening test can be used to sort out a small number of good candidates within a range of fluids/materials for long time durability tests [1]. 2. TEST RIG, PARTS AND LUBRICANTS

The tests were run on the clutch test rig KLP-260, shown in Figure 1. The KLP-260 is an easy to handle clutch test rig in brake application. The central shaft has different fly wheels in a variable set up and is driven by an electric engine to set speed. The inner plates of the clutch are linked the shaft, the outer plates to the housing. By engag-ing the clutch the shaft is decelerated to standstill while axial force, friction torque and speed are measured and coefficient of friction (CoF) is calculated by evaluation software. Lubrication is done by injection lubrication into the center of the clutch. A typical paper type friction lining in a comparably small size of DKA 120/142.8 mm (inner/outer friction diame-ter), see Figure 2, was used. The test-setup included 3

steel and 2 friction plates (4 friction surfaces) with a spe-cific oil flow of 2.5 mm³/mm²s (2.8 liter/min) at 80°C. The plates are lubricated at 80°C for 1 hour before the test starts. The cycle time is set to 15 seconds with the clutch being engaged for 5 seconds. For the development of the test procedure a serial ATF fluid was used. Further modell-fluids with different addi-tive packages were investigated with the proposed friction screening test, see Table 1.

Figure 1: KLP-260 clutch test rig

Figure 2: Friction plates

Test-Clutch Fly wheel (shiftable)

Fly wheels (variable)

Name Additive package

ATF Serial

Modell-fluid 2 Dispersant

Modell-fluid 3 Rust Inhibitor

Modell-fluid 6 Friction Modifier + Detergent + EP/AW

Modell-fluid 10 Friction Modifier + Dispersant + EP/AW

Modell-fluid 12 EP/AW

Modell-fluid 13 Friction Modifier + Detergent + Rust Inhibitor

Modell-fluid 14 Dispersant + EP/AW + Rust In-hibitor

Modell-fluid 0 Basestock Table 1: Lubricants

3. FRICTION SCREENING TEST

The proposed friction screening test consists of a specially designed running-in procedure and a repeated variation of load stages for the actual determination of friction behav-ior. For the evaluation of friction performance target load parameters were set in advance, see Table 3. This pres-sure-speed-variation (p-v-variation) contains 9 load stages at 3 pressures and 3 sliding speeds. With friction behavior changing significantly during running-in, the task of the designed running-in procedure is to provide a finished steady state in the clutch regarding friction behavior. That means that the friction behavior must not change by re-peating the p-v-variation (p-v-1, p-v-2, …). The results of various tests clearly showed that this is only possible if the highest load within the test (LS9) is run already in the running-in. Otherwise conditioning effects on all load stages occurred in the second/third/… run of the p-v-variation; friction performance was not stable yet and could not reliably be determined. By using the developed running-in procedure, see Table 2, and the p-v-variation according to Table 3, a stable friction behavior in each load stage is ensured. The differ-ence in average CoF µavg (see below) between the repeti-tion of p-v-variations (p-v-1, p-v-2) is below 1 %, in higher load stages below 0.5 %. Reproduction tests with new plates also show results with a deviation in a range of 0.5…1 % in CoF.

Loa

d st

age

Cyc

les

/ -

Pre

ssur

e p

/ N/m

m²

Slid

ing

spee

d v g

/ m

/s

Fri

ctio

n lo

ss q

/ J

/mm

²

E1 100 0.5 5 0.09 E2 100 0.5 10 0.35 E3 200 1 10 0.35 E4 200 1.5 15 0.80 E5 20 1 5 0.09

Table 2: Running-in

Loa

d st

age

Cyc

les

/ -

Pre

ssur

e p

/ N/m

m²

Slid

ing

spee

d v g

/ m

/s

Fri

ctio

n lo

ss q

/ J

/mm

²

LS1 10 0.5 5 0.09 LS2 10 0.5 10 0.35 LS3 10 0.5 15 0.80 LS4 10 1 5 0.09 LS5 10 1 10 0.35 LS6 10 1 15 0.80 LS7 10 1.5 5 0.09 LS8 10 1.5 10 0.35 LS9 10 1.5 15 0.80

Table 3: Load stages of p-v-variation for evaluation of friction behavior For the actual screening test the running-in and 2 p-v-variations (p-v-1, p-v-2) are to be run directly one after another. Due to the very small deviations in CoF an excel-lent differentiation of results by the proposed screening test procedure can be reached. In order to ensure the ex-cellent differentiation the reproduction of p-v-1 and p-v-2 should be checked. Due to high cooling oil flow and me-dium loads damage of the clutch in the test can be ruled out. With a constant cycle time of 15 seconds the overall test-ing time is around 3.5 hours. 4. TEST EVALUATION

The evaluation of the friction curves produced by the test is done by 2 specific values. The definitions are summa-rized in Figure 3 and can also be found in e.g. [8], [9]. The average coefficient of friction µavg is defined as arithmetic mean of CoF between 0…60% of initial maxi-mum sliding speed vg,max. µavg can be used to determine the average transmittable torque at a set load condition and is therefore also an indicator for shifting time. The tendency of the clutch of inducing shudder-vibrations depends on CoF at low speeds. If the CoF rises at the end of the engagement (“high static friction”), the clutch is at risk, if CoF decreases (“low static friction”), the clutch is in a safe operation mode against shudder. The ratio µ2/µ5 can differentiate this behavior, where µ2 is CoF at 50 % sliding speed and µ5 is the maximum CoF at 0….10 % sliding speed at the end of the engagement. If µ2/µ5 is above 1.0, the clutch will be in safe operation mode, if below 1.0, the clutch will be potentially vulnerable to shudder.

Figure 3: Specific values avg. CoF µavg, µ2, µ5 and µ2/µ5

vg / % vg,max

100

µ / -

50100

µ5:max. 0…10% vg

µ2:

µ a

t 50%

vg

µ2/µ5 < 1,0

µ2/µ5 > 1,0

60

µavg:avg. CoF 0…60% vg

5. EXEMPLARY TEST RESULTS Average CoF µavg of tested lubricants are shown in Figure 4, ratio µ2/µ5 in Figure 5. The ATF with serial additive package is characterized as best fluid with high average CoF and relatively low static friction with µ2/µ5 around 1.0. The other fluids have either low friction level or high static friction. Average CoF depends also on load with high CoF at low pressures and vice versa. The CoF at low speeds is here not depending on load parameters because a systematic variation in ratio µ2/µ5 cannot be found. The single additives are strongly interacting. This can exemplarily be shown with the friction modifier additive (FM). FM is commonly used to decrease static friction. Modell-fluids 6 and 13 contain FM and show relatively low static friction (µ2/µ5 > 1.0), modell-fluid 10 contains also FM and leads to relatively high static friction (µ2/µ5 < 1.0). All 3 modell-fluids contain 2 additional additive components that clearly influence the performance of FM. That results in friction behavior and the interactions of additive components cannot be theoretically predicted what stresses the need for an easy and reliable screening test procedure.

Figure 4: Average CoF µavg of tested lubricants

Figure 5: Ratio µ2/µ5 of tested lubricants

6. CONCLUSION The friction behavior of wet clutches cannot be predicted and requires intense testing especially when new friction materials and lubricants are to be developed. In a research project at FZG a flexible, easy to use screen-ing test for the evaluation of friction performance of wet clutches with paper type friction material has been devel-oped. An especially designed running-in procedure en-sures an excellent reproduction of test results allowing precise differentiation of effects of friction materials or lubricants on friction performance of the clutch. The screening test can be run on different test rigs and clutch sizes as the load parameters are defined as specific values. ACKNOWLEDGEMENTS The displayed results were achieved in a research project sponsored by Forschungsvereinigung Antriebstechnik e.V. (FVA, http://www.fva-net.de ). The authors wish to thank FVA and the companies and their staff that sup-ported the project. REFERENCES [1] Acuner, R.: Öleinfluss Reibcharakteristik am Modell

nasslaufende Lamellenkupplung; Entwicklung eines Reibungszahlkurztest. FVA-Heft 1051, 2013

[2] Layher, M.: Untersuchung der Einflüsse von Grund-öl und Additivpaket auf die Reibcharakteristik und das Reibschwingverhalten von nasslaufenden La-mellenkupplungen. FVA-Heft 897, 2009

[3] Layher, M.: Untersuchung der Einflüsse von Grund-öl und Additivpaket auf das Reibungsverhalten von Synchronisierungen. FVA-heft 898, 2009

[4] SAE Specification J2487: SAE No. 2 Friction Test Machine 3600 r/min Stepped Power Test, USA, 2000

[5] SAE Specification J2488: SAE No. 2 Friction Test Machine 8000 r/min Stepped Power Test, USA, 2000

[6] SAE Specification J2489: SAE No. 2 Friction Test Durability Test, USA, 2000

[7] SAE Specification J2490: SAE No. 2 Friction Test Machine µPvt Test, USA, 2000

[8] Hämmerl, B.: Lebensdauer- und Temperaturverhal-ten ölgekühlter Lamellenkupplungen bei Lastkollek-tivbeanspruchung. Dissertation, TU München 1994

[9] Stahl, K., Pflaum, H., Hensel, M.: Methods for per-formance evaluation of lubricants and friction mate-rials in wet multiple disc clutches in axle brake ap-plication. International VDI Congress Drivetrain for Vehicles 2013, VDI-Berichte 2187, 2013

0.10

0.11

0.12

0.13

0.14

0.15

0.16

MF

0

MF

2

MF

3

MF

6

MF

10

MF

12

MF

13

MF

14

AT

F

µav

g/ -

LS1LS2LS3LS4LS5LS6LS7LS8LS9

0.7

0.8

0.9

1

1.1

1.2

1.3

MF

0

MF

2

MF

3

MF

6

MF

10

MF

12

MF

13

MF

14

AT

F

µ2/

µ5

/ -

LS1LS2LS3LS4LS5LS6LS7LS8LS9

0.5 N/mm²

1.0 N/mm²

1.5 N/mm²