Contact temperatures in mixed friction

Contact temperatures friction

in mixed

P. Deyber” and M. Godett An infra-red technique for measuring the contact temperature in mixed friction is described. It can detect temperatures as low as 30°C above ambient. The shape of curves comparing the variation of the coefficient of friction, and the contact temperature with time are

The study of extreme pressure additives can be undertaken

on a wide variety of test benches. E.p. action is usually characterized either by an increase in load carrying capacity or by a change in the coefficient of friction with time. The contact conditions in these machines are such that a hydrodynamic film is not completely formed and that a ‘solid’ film with well defined mechanical properties (such as shear resistance u) is generated on the surface. The problem is to understand how the shear resistance, u, varies at the contact during a test. In mixed friction the load P is divided between the metal asperities Phi and the oil film P, . Thus: P=P,

+PH

+FH

where according to classical theory: FM

=!& PO

Hence: otFH At constant speed and load and for a given flow pressure, PO, Equation 2 becomes: F=F(PH,a)

(3)

where F,, usually very small, is neglected. Recent theory’ has shown that Equation 1 is only approximate, but the model that it interprets is accurate enough for this study. The hydrodynamic load PH has been shown’ to vary with viscosity, Q, and with changes in the geometry of the contact caused by wear. Thus Equation 3 becomes: F = F (PH [rl (T),

114

(4)

where r) varies with temperature. 1 is the distance of sliding contact. Hence to isolate u, it is necessary3 to isolate * Dow Coming Mol kote France BP 7 (67) Strasbourg-Neuhoss f Laboratoire de M%camque . des Contacts, Institut National des Sciences Appliqdes de Lyon, 69 - Villeurbanne, France

150

Contact temperature measurements Cdntact temperatures were measured using thermoelectric effects and infra-red techniques. Thermo-elec tric effect

Similarly the friction force F is given by the expression: F=F,

P ( q (T), 1). As a first step the temperature T which governs t Be contact viscosity and thus indirectly the friction force, F, should be known. Contact temperatures were therefore measured by different techniques. Data obtained on a much modified Timken machine showed that the temperature measured by infra-red radiation followed the variation of friction force closely.

TRIBOLOGY

August 1971

Two methods are commonly used; the thermo-electric potential created between two metals in contact is measured directly, or a thermocouple is lodged in one of the test pieces. The first method was introduced by Bowden and Ridler4 and later used to measure gear profile temperatures’-‘. The time response is good, about lO*s. Unfortnately the choice of material combination is limited and good results are only available when there is no oil film. Furey’ compared both methods and showed that the temperatures obtained with the first method are usually higher than those measured with the second method in similai applications. Classical thermocouples (iron-constantan, chromel-alumel, etc) are often used in tribology, but the interpretation of data is difficult as it depends on a knowledge of the exact position of the active part of the thermocouple usually mounted in a high temperature gradient area (40’C/mm). This led us to consider infra-red techniques.

In fra-red technique Infra-red techniques have been used amongst others by Bowden and Thomas’ in basic friction studies, by Parker and Marshall’ ’ in the study of brake shoe operating temperatures and by Reichenbach’ ’ , Chao and Trigger ’ * ’ ’ 3 in metal working studies. The temperature measured in most cases varied between 500 and 1200°C. In mixed friction however the temperature is believed to vary between 50 and 300°C and these low values led us to design an original system, and mount it on an existing bench.

I I

Diameter49-2

a magnetic pick-up triggered by a 60-tooth wheel mounted on the malnshaft. The dynamometer signal is amplified and directed to both and oscilloscope and a recorder. The contact resistance is also measured. The dynamometer signal can differentiate between three regimes: 1 A full (or complete) hydrodynamic film 2 A full (or complete) e.p. film with high electrical resistance 3 Mixed friction regime A general view of the machine is shown in Fig 2, and explained diagrammatically in Fig 3. Temperature measurements The infra-red system

Fig I Test pieces. The hardness of the block is 60 HR. All dimensions are in mm

T h e test bench

The machine which is described in detail elsewhere ~4 uses standard Timken test pieces (Fig 1). The test ring which is fixed on a shaft electrically isolated from the housing is driven by a variable speed d.c. motor. The test block is held in a cradle which forms the upper part of a hydrostatic bearing used for friction measurements. The lower part of the bearing is mounted on a load arm which is acted upon by a hydraulic jack. A strain-gauge dynamometer links the upper and lower parts of the bearing and thus transmits and measures the friction force. Oil is pumped to the contact and applied in a jet stream. It can be preheated to any desired temperature. The load is read on a precision manometer. The speed is measured by a counter which records the signal from

The contact temperature is obtained by comparing the number of photons emitted by the contact surface to the number of photons emitted by the reference surface of known temperature. Fig 4 shows the optics of the measuring system. A small cylindrical sapphire window, diameter 1.5 ram, height 1 ram, is glued in a radial hole drilled in the test ring. When the window sees either the contact or the reference surface, the infra-red beam emitted by the surface is first reflected by a plane aluminium mirror fixed in the main shaft onto a silvered parabolic mirror and is then concentrated on a light sensitive lead sulphide cell. A 130 k~2 load resistance is connected in series with the cell and with a 100 volt d.c. battery or a regulated voltage source. The output signal is magnified and then directed to one of the channels of the twin-beam five trace oscilloscope. The reference source temperature is measured with a thermocouple set well below the heated surface and very close the reference surface.

Fig 2 General view of the experimental test equipment

TRIBOLOGY August 1971

151

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Fig 3 Diagram of the measuring system

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Fig 4 Optics of the infra-red temperature measuring system -6 ._m c

Signa/ shape The c o n t a c t t e m p e r a t u r e signal shape (Fig 5) can be app r o a c h e d theoretically by assuming a c o n s t a n t t e m p e r a t u r e on the wear scar o f the block.

Emissivity corrections Errors introduced by variationsin surface emissivity between the contact and reference temperatures are usually overcome by using special filters or monochromators. Unfortunately in this case available energy is t o o small to use these 152

T R I B O L O G ¥ August 1971

".7. rY

/ 6

2 Time [ms]

3

Fig 5 Theoretical shape of infra-red signal at the contact area

methods. Hence the effect of emissivity difference was calculated between 100 ° and 400°C. The results are given in Fig 6 which shows the signal obtained for surfaces of different emissivities versus temperature. As an example a variation of 10% in emissivity for a relative signal of 5 introduces an error of +- 3°C at the reference temperature of 172°C.

Determination of the contact temperature Two infra-red signals were thus obtained (Fig 7). The narrow (Sc) and wide (St) peaks correspond to tlie unknown contact and the known reference source. The reference signal was calibrated; it was approximated by an exponential law S = Tm where S is the output signal in volts T is the reference temperature and m is a calibration coefficient. The values of m was determined by the method of least squares, and varied between 3.7 and 4.2. The contact temperature, T m, was obtained by drawing a line parallel to the calibration l i'he - - through a point corresponding to the i.r. signal at the reference temperature (Fig 8). T m is read from this line at the temperature corresponding to the contact signal. The difference in shape between m = 3.7 and m = 4.2 introduces errors smaller than l°C. A 10% variation in signal strength between the contact and reference temperature corresponds to a 2.5°C difference at 100°C and a 0.5°C difference at 200°C. The method is thus shown.to be extremely accurate at relatively high temperatures.

axis and about a pivot (four degrees of freedom) and the cell, along the shaft axis and vertically (two degrees of freedom). In practice, the ring is first fixed on the shaft. Then a light source is placed against the sapphire window and the plane mirror is fastened in position when the light spot, which is focussed by a lens on a screen, does not move during a rotation of the shaft. The screen and lens are then removed and the parabolic mirror is adjusted until the spot remains fixed on the lead-sulphide cell during a rotation of the shaft.

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Sr

Contact

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Fig 7 Measured infra-red signal during 540 ° rotation

t

t00

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Set-up procedure The setting up of an experiment is tedious as the ring must be positioned on the shaft (one degree of freedom), the plane mirror along the shaft, and around the shaft axis (two degrees of freedom), the parabolic mirror along the shaft

v

r-

0

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5

0:

100

'

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't4 o

'

Temperature [oc ]

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Fig 6 Influence of variations in emissivity, e, of materials on the relative intensity of the infra-red signal

TR

TIR

T mperoture

Fig 8 Determination of contact temperature TRIBOLOGY August 1971

153

Other temperature measurements

Thermocouples were also placed: 1 In the block at about 1.5 m m below the surface 2 In the inlet oil jet 3 Around the ring in small tubes connected to a vacuum pump. Here the oil is entrained in the tube and its temperature is measured at 0.2 mm from the ring surface The signals from these thermocouples were amplified, chopped and fed to One trace of the oscilloscope. Data treatment

recorded are unable to give any information concerning this variation. Hence in the model suggested by Equation 4 the temperature T which governs the friction force F is the temperature measured by infra-red technique. Further work 3 has shown that it is now possible to superimpose on graphs giving the coefficient of friction,/z, as a function of time, t, lines o f equal PH/P' from which the proportion o f the total load, P, which is carried by the oil film, PH' can be determined for all t. Certain implications regarding the testing of lubrication oils are discussed in this Reference.

Fig 3 shows that the contact resistance, the friction force, the different reference and oil temperatures, and the infrared signal appear simultaneously on the oscilloscope screen. This image is photographed with a low speed camera which takes one picture every 200 rotations of the shaft. Thus up to 1500 pictures can be taken during a four hour run. The information is then read off a curve reader and transfered automatically to punched cards which are processed by a computer.

Acknowledgement

Results and discussion

2

Results obtained from a friction experiment are shown in Fig 9. The high viscosity lubricating oil contained a high percentage of e.p. additive. The seven curves shown have the following significance: TIR = the contact temperature ~stimated by the i.r. technique /a = coefficient of friction TB = temperature of the block (thermocouple) TR = temperature of the ring (thermocouple mounted just outside the contact point) TH2 = temperature of the oil film near T R TH 1 = temperature of the oil film near the entrance of the oil, supplied by a jet stream TH = temperature of the oil in the jet stream, it is identical with the ambient temperature Fig 9 shows that only the contact temperature measured by the infra-red technique follows the variation o f the co efficient of friction closely and that the other temperatures

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References 1

3

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6

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This work was performed under Contract No 66.00.258 of the l~l~gation C~n6rale'a la Recherche Scientifique et Technique, 75- Paris Vile.

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Fig 9 V a r i a t i o n o f friction and temperature with time at a

load of 900 N and a speed of 1000 rev/min 154

T R I B O L O G Y August 1971

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Godet, M. 'Fondements mecamques de la tribologie', Journees d'etudes du GAMI sur l'usure, Paris 8 (Sep 1970) Godet, M. 'L'apport hydrodynamique dans le frottenment lubtifie, These soutenue a la Faculte des Saences de Parts (Feb 1967) Deyber, P. 'Contribution"a l'e~ude du coefficient de frottement et de la temperature en frottement mixte', These soutenue "ala Faculterdes Sciences de I'Universite"de Lyon (Sep 1970) Bowden, F. P. and Tabor, D. 'The friction and lubrication of solids. Vol 1', Oxford Clarendon Press (1950) Terauchi, Y. and Miyao, Y. 'On the measurement of temperature flash on spur gear teeth. Part 1', Bulletin o/the Japanese Society o/Mechanical Engineers, Vol 7, No 26 (1964) Terauchi, Y. and Miyao, Y. 'On the measurement of temperature flashes on spur gear teeth. Part 2', Bulletin o/the Japanese Society o/Mechanical Engineers, Vol 8, No 29 (1965) Niemann, G. and Lechner, G. 'The measurement of surface temperatures on gear teeth', Transactions o f the American Society o/Mechanical Engineers, Journal o/Basic Engineering Vol 87, Series D, No 3 (1965) pp 641-655 Furey, M. J. 'Surface temperatures in sliding contact', Transactions o f the American Society o f Lubrication Engineers, Vol 7, No 2 (Apr 1964) pp 133-147 Bowden, F. P. and Thomas, P. H. 'The surface temperature of sliding solids', Proceedings o f the Royal Society, Series A, Vol 223 (1954) Parker, R. C. and Marshall, R. P. 'The measurement of the temperature of sliding surfaces with particular reference to railway brakes', Proceedings of the Institution o/Mechanical Engineers, Vol 158 (1948) Reichenbach, G. S. 'Expermental measurement of metal cutting temperature distributions', Transactions o f the American Society o/Mechanical Engineers, Vol 80 (1958)

15

Cliao,B. T. and Trigger,K. T. 'Temperature distributionat tool-chip and tool-work interfacein metal cutting',Transactions of the A merican Society o/Mechanical Engineers,

Vol 80 (1958) pp 311-320 Chao, B. T., Li, H. L. and Trigger, K. J. 'An experimental investigation of temperature distribution at tool flank surface', Transactions o f the American Society of Mechanical Engineers, Journal of Engineering for Industry, Vol 83, Series B ( 1961 ) pp 496-504 Godet, M. 'Corretlations et divergences des me/thodes d'essais du pouvoir lubrifiant des hmles, Ingemeurs de l'Automobile, (Dec 1967) pp 625-636 Godet, M. 'La thetorie des deux lignes. La lubrification des ene!enages', Compte Rendu de l'Acad(mie des Sciences, Paris Vol 258 (Jan 1964) pp 71-74 •

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