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Investigations of Stick-slip Effects in Wet Friction Drive Elements

G. Polt1, T. Sebestyn2, A. Pauschitz1and F. Franek1

1

ACT research GmbH

Austrian Centre of Competence for Tribology,2700 Wiener Neustadt, Viktor Kaplan-Str. 2, Austria

2Vienna University of Technology, Institute for Micro Technique and Precision Engineering,1040 Wien, Floragasse 7/2, Austria

Keywords:stick-slip, friction disc, disc-on-disc tribometer, wet brakes, transmission oils.

Abstract. Up to now, the suitability of uniform tractor transmission oils (UTTO) for truck and

building machine drive, brake and hydraulic systems were often investigated in expensive on-

vehicle or specific laboratory rig tests. Present work describes the layout and adoption of a test rig

for application-oriented testing of wet brake system elements. The results gained from thetribometer experiments were compared with on-vehicle tests.

Introduction

According to building machine and truck industry, noise problems in wet brake systems already

occurred when they were launched approximately 30 years ago, and these problems havent beencompletely solved yet since then. Today, there are various friction couples - steel with brass,

bronze, molybdenum, carbon or paper - in use. Prosperous experiences at a certain system can cause

braking noise when being built in another model. In modern building machines and trucks this is

even amplified by the following factors:

! use of cheap components on economical reasons;! use of only one type of lubricant in all systems (hydraulic, brake and drive system);

! reduction of dimensions (cross sections) as a consequence of higher strength of new materialsand thus less torsional resistance;

! smaller brake diameter for more compact layout and ground clearance;

! resonance and dynamical interactions between different parts of the drive system;

! use of various brake systems in the same model series.

Although similar components are being built in, trucks are more affected by brake noises than

building machines. Generally, requests on use of UTTO are more extensive, where in many models

more friction systems belong to the same oil household. In the machines the following componentshave to be supplied by oil (Fig. 1.):! gear toothing as well as friction system synchronizing disks;

! hydraulic system;

! wet coupling friction system (prevention from coupling slip);

! wet brake friction system (prevention from noises and slip).


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Drive Train

Hydraulic systemWet Brake System

Fig. 1: One type of oil for all systems to be lubricated

Each of these systems possess different friction value requirements, which yet should be covered by

one type of oil. The occurring problems are of manifold kind and appear as follows:

! rough-running gear switch during slip through of the synchronizing disks;

! lack of torque during slipping of clutch;

! creaking, clattering, squeaking wet braking, mostly during the end of the braking process.

Braking noises are noticeable even for inexperienced drivers, which makes them most frequentlycomplained about.

Theory of braking noises

Noise occurs mainly during the end of the braking process, shortly before complete stop of thevehicle. (Fig. 2.). Although the noises can sometimes be quite loud, it is an obvious perception, that

it does not mean any danger to damage the system. A noticeable imperfection of the brakingfunction has so far not been observed. Anyhow, noise of this kind is unacceptable for the customers.

Fig. 2.: Schematic course of the braking process with braking noise


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During the braking process the oil heats up between the steel brake disk and brake pads

substantially, causing fall of oil viscosity. Where the counteracting parts relative velocity aroundthe end of the braking course is lower, the state of boundary friction with short stick-slip

phenomenon is reached. Braking noise arises due to sticking and releasing of the contact.

v

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v

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v

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Fig. 3.: Mechanical model and friction coefficient vs. gliding velocity.

A look on a mechanical model of a simple friction system (Fig. 3.) shows, that on one hand the

system elasticity (stiffness c) and mass (m) and on the other hand the velocity depending friction

coefficient (graph "-v) are necessary characters of a system with stick-slip sensitivity. Thus, these

parameters are of relevance for optimising a friction system to prevent stick-slip motion.

Braking noises can sometimes be reduced respectively avoided by raising oil viscosity withapplication of friction modifiers (FM). From the point of view of other parts in the system it might

mean disadvantages, as:

! problems during operation at low temperature due to too low oil viscosity;

! slipping clutch at higher viscosity and application of FMs;

! corrosion of non-ferrous metal at bronze synchronizing disks due to FMs (not Mo coatings);

! changed load carrying ability via FMs (for example not satisfying API GL-4 anymore).

Test Configuration

On-Vehicle Tests

Where results out of conventional laboratory or tribometer test dont allowany general conclusionon appropriateness of an UTTO, lubricants are being tested directly in certain tractor models. The

cost of such experiments is still justifiable, even though no general deduction can be drawn.

Assumed, that one will acquire information from vehicle tests, more than a simple squeaks doesnot squeakaccording to subjective driver observations, costs increase considerably.

Fig. 4. shows a test assembly in a STEYR tractor with tracking of velocity, oil temperature and

noise level. So far yet, there is no satisfying test method for a quick screening on wet friction

elements. A so-called disc-on-disc friction tribometer seemed to be useful to carry out scientific

investigations regarding the stick-slip characteristics of wet brakes.

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Fig. 4.: Sensor application for on-vehicle test

Tribometer Tests

The disc-on-disc tribometer of use (Fig. 5.), consists of a rigid tribometer frame and a turnable drive

shaft. This shaft is driven by an electric motor via a single stage worm gear and v-belt and is

connected to the friction disc sample. The counter disc of the tribo system is connected to the test

rig frame via a torsion sensitive element. This element is a specially shaped hard bendable, torsional

spring with a cross like cross-section (Torsionskreuz according to PAT 393167 B from26.8.1991). The spring deflection is transferred by the test sample adapter to elastic steel plates,

which are applied with strain gages and is proportional to the friction torque. The normal load onthe tribo system is carried out by single masses via a lever quasi statically.

Both elements of the tribo system are placed in a cylinder shaped sump, filled with the test oil.

Fig. 5.: Disc-on-Disc Tribometer

The dimensions of the torsional sensitive element (length, diameter and thickness of web) affect the

torsional stiffness and therefore the frequencies of torsional eigen oscillations. Thus, the variation of


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the element dimensions allow to tune the torsional oscillation corresponding to the characteristics in

the vehicle environment. This is a key point to provide useful correlation between tribometer

investigations and on-vehicle tests. Also the use of serial parts (Fig. 6.) for friction and counter disc

contributes to improve this correlation.

Fig. 6.: Friction and Counter Disc as used for tribometer tests.

Results and Correlation

All tests have been carried out with one type of friction and counter disc and a set of 6 differentlubricants (STOU: Super Tractor Oil Universal). The results with two different vehicles were

compared with the tribometer investigations and are shown in Table 1. (RRV: disc-on-disctribometer).

Table 1.: on-vehicle and tribometer test results.

Significantly high correlation between the different test procedures has been achieved, with 2 of 3tribo systems tested showing negative stick-slip evaluation.

Summary

The design process of stick-slip sensitive tribo systems is nowadays dominated by testing itscomponents whether in laboratory test rigs or complete systems on-vehicle. The component tests

show often insufficient correlation to on-vehicle tests, which are cost-intensive due to the effort oftime, material and measuring equipment.


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A disc-on-disc tribometer with adjustable components to tune the torsional vibration characteristics

as given in the vehicle application of a tribo system, improves the accuracy of the prediction of the

stick-slip characteristics.

The presented test results show a significant differentiation of the stick-slip behaviour of the usedbrake systems in presence of different lubricants (UTTO and STOU). The evaluation of the results

from the disc-on-disc test rig shows, that these lubricants behave similar as in on-vehicle tests.

For future requirements in the drive train engineering process, for example regarding wet friction

disc systems with dynamical normal load and the simulation of various lubrication conditions

(temperature, lubricant flow, etc.), an advanced disc-on-disc tribometer is already available at ACTresearch GmbH.

Literature

[1] Baumann, W.; Pauschitz, A; Franek, F: Ruckgleiten bei nasslaufenden Bremsen, Proc.Symposium 2000, The Austrian Tribology Society, Vienna, pp. 109 122, 2000

[2] Mikolasch, G.: Analysis and tribotechnical characterization of reaction layers on frictionmaterials for application in automotive components, PhD thesis (Vienna University ofTechnology), 2001

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