The interaction between friction materials and lubricants

The interaction between friction materials and lubricants

Wear, 24 (1973) 69-76 9 Elsevier Sequoia S.A., Lausanne THE INTERACTION LUBRICANTS T. P. NEWCOMB December in The Netherlands BETWEEN FRICTION MA...

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Wear, 24 (1973) 69-76 9 Elsevier Sequoia S.A., Lausanne

THE INTERACTION LUBRICANTS

T. P. NEWCOMB

December

in The Netherlands

BETWEEN FRICTION

MATERIALS AND

and R. T. SPURR

Ferodo Lrd., Chapel-en-le-Frifh (Received

69 - Printed

(Ct. Britain)

21, 1972)

SUMMARY

Lubricants, beside removing the frictional heat from the contacting surfaces, also influence their friction and wear characteristics, and it is shown how the former alters with the viscosity and boundary properties of the lubricant and with the condition of the surfaces. Failure is thermal in origin, and analysis and experiment indicate that whether or not a given material will fail depends upon the rate of working for a given engagement time, or to the half power of the engagement time if the latter is varied.

INTRODUCTION

Friction materials are generally operated dry, as for example, in brakes and automotive clutches, but in some clutches and in all automatic vehicle transmissions they run in lubricant. Although many facings designed for dry use operate satisfactorily in fluid, special materials have been developed for particular operations, for example, resinpaper facings and resin-graphite materials for automatic gear boxes. Similarly special fluids have been developed to lubricate the friction material. The lubricant reduces wear, removes frictional heat from the surfaces and helps to give the required p characteristics, but in general at the cost of requiring a greater clamping force for a given torque than when operating dry. A great deal of work has been done to develop lubricants with the required properties, particularly lubricants for automatic transmissions. Both facings and lubricants have been developed more or less empirically. In this paper an investigation of the interactions between facings and lubricants is described, and the manner in which oil-immersed clutches and bands fail is also discussed. The investigation was mainly concerned with the particular and important case of automatic transmissions. FRICTION

MATERIALS

Automaticgearboxes(Fig. 1)contain clutches litted with facings in the form of annuli, and band brakes internally lined with friction material that tighten against

70

T. P. NEWCOMB,

(one

Rear

Front

Fig. 1. A typical

gear box showing

arrangement

of clutches

5””

of three

R. T. SPURR

sets)

sun gear

gear

and bands.

a rotating drum, to control torque reaction or to transmit power. Clutch facings for oil immersed applications can be made in a number of ways. Millboard facings are made by impregnating porous asbestos sheet with several different resins, partly polymerising the resins between impregnations. Paper based facings are made in a similar way. Paper facings can be either resin or graphite impregnated. Woven facings are made by impregnating woven fabric, which usually has brass wire woven into the fibre, with resins and polymerising them by heating under pressure: the facings are flexible and so conform well to the opposing surface, and can operate when oil starvation occurs. Moulded facings consist of tibre, various tillers, resin and. in one type, a considerable proportion of metal. The constituents are mixed together and the resin press-cured in a die, and polymerisation completed by further baking. These materials have a greater durability and can withstand higher facing pressures than the other types mentioned. For very severe conditions sintered metal facings are used. These are made by mixing metal powders and other tillers together and sintering the metals in a die. Resingraphite materials can also withstand severe conditions; they are made in the same way as moulded materials but they contain a large proportion of graphite, zis their name implies. Facings in automatic transmissions are generally only 1.52 mm (0.060 in.) thick and so have to be bonded to a supporting core plate to take the drive. Woven, moulded and resin-impregnated paper friction materials are widely used on band brakes in automatic transmissions. Physical properties of typical materials of the various kinds have been summarised elsewhere’. OPPOSING

SURFACES

The nature of the opposing metal surfaces can also have a considerable effect on the performance of the friction material and lubricant. The metal mating member should have good thermal conductivity so that it readily conducts heat away from the rubbing surfaces, as friction materials are poor conductors of heat. A material of high diffusivity is also desirable to minimise bulk temperatures. The material must have good wear resistance. In practice the choice of material depends mainly on its cost and ease of manufacture, and it has been found that ferrous materials such

FRICTION

MATERIAL-LUBRICANT

71

INTERACTION

as cast iron, En6 or En42 medium to high carbon steels, all of a pearlitic structure, and having a Vickers hardness of approximately 200, make very suitable mating materials for friction materials operating “wet”. A surface finish between 0.89 to 1.52 pm (35 to 60 pin.) obtained by surface grinding or by using cold rolled steel is considered satisfactory for most applications. FLUIDS

An automatic transmission fluid has to meet very exacting requirements. It has to have a satisfactory viscosity over the whole temperature range it may encounter, it must minimise wear and be resistant to oxidation, and not form sludge or lacquers. It should prevent corrosion of the metal surfaces and be compatible with the various components of the transmission, including the oil seals. It has to remove heat from the sliding surfaces and in addition it has to help give the required frictional properties to clutch facings and their opposing surfaces. The smoothness of the gear change is determined by the frictional properties of the fluid in conjunction with the properties of the friction surfaces. One widely used transmission (General Motors) specifies a fluid (DEXRON type) that will give a lower static than dynamic ~1in order to give a smooth gear change without audible squawk. The p nevertheless has to be high enough to lock up the reaction members. On the other hand Ford have designed their automatic transmission to use a fluid (type F) the static p of which is greater than the dynamic. FRICTIONAL

CHARACTERISTICS

OF MATERIALS

AND FLUIDS

Linear speeds in an automatic transmission may be as high as 35.6 m/s (7000 ft/min) but the smoothness of the engagement is largely determined by the frictional characteristics at low sliding speeds near the end of the engagement, and so special rigs have been built to examine the variation of p with speed, pressure and temperature at speeds from 100 ft/min (0.51 m/s) to rest. The rigs can also be run at constant speeds. Recently a high speed rig has been built which can be used not only to determine p but which will also compare the durability of different types of facings in various lubricants. A range of inertias and speeds are available and the clamping pressure is adjusted to give a constant slip time of 0.55 s. Facings (108 mm outside diameter and 79 mm inside diameter) are tested until failure occurs and so limiting conditions of pressure and speed for a particular facing in a particular oil can be determined. Linear speeds of up to 35.6 m/s (7000 ft/min) and pressures up to 20.7 MN/m2 (3000 lbf/in2) can be achieved. Figure 2 shows p uersus velocity curves for paper facings and sintered metals on a low speed rig together with curves for a much wider range of speeds on the high speed rig. It can be seen that the coefficient of friction of each type of material is practically the same on both rigs. The variation of ~1with velocity for a particular sintered metal material operating dry is shown in Fig. 3. Measurements were also made using mineral oils of two extreme viscosities, namely SAElO, and SAE140 (Fig. 3). The more viscous fluid had the lower p and with this oil the characteristic minimum appeared in the ,u/angular velocity curve, .H decreasing as a more and more complete lubricating

72

T. P. NEWCOMB,

R. T. SPURR

.16

36

Angular

velocity

Fig. 2. p of two materials curves). Fig. 3. Frictional

behaviour

Angular

(revlmln)

measured

velocity

(rev/min)

on low speed rig (left hand curves) and on high speed rig (right

of sintered

metal friction

material

dry and in various

hand

fluids.

film was set up, reaching a minimum and then increasing somewhat as the rate of shear of the fluid increased. These measurements were made with plain facings. Sintered facings are often grooved to increase their p. The grooves may be single or multi-start spirals, or they may form “waffle” or “sunburst” patterns made by machining square or diamond shaped lands on the surfaces. These grooves operate by reducing the amount of hydrodynamic lubrication; a grooved facing was run in the very viscous fluid-and it can be seen (Fig. 3) that the p/angular velocity variation was considerably reduced and the dynamic p much increased. Paper based facings behaved differently. The p/angular velocity variation in mineral oil was not very different to that dry, and even the very viscous fluid did not greatly reduce p (Fig. 4). Similarly when the one fluid was used at several different nominal temperatures, for example, 30”, 80” and 120°C the ~1traces were practically the same. Obviously there was little hydrodynamic lubrication occurring over the range of speeds and pressure used, and the reduction in p was largely a boundary lubricant effect. Grooving the facings therefore had no effect on their measured p. The fact that small concentrations of additives in the transmission fluids affect the friction properties also indicates that lubrication is of the boundary type. To show this, a facing was run in DEXRON and its characteristic p/angular velocity curve obtained (Fig. 4). The facing was removed from the rig, excess lubricant removed, and then run in a straight oil of the same viscosity. The static p was low and the p/angular velocity variation was similar to that of DEXRON, showing that boundary lubricant had become adsorbed on the surfaces. The paper facings have a matt finish on which it is presumably difficult to develop a fluid film. Sintered facings were given a similar finish by grinding, and this increased their dynamic p considerably (Fig. 3).

FRICTION MATERIAL-LUBRICANT

20

40

60

El0

INTERACTION

73

100

Angular velocity (rev/mint Fig. 4. Frictional behaviour of paper-based friction material dry and in various fluids.

FAILURE

The temperature reached at the friction surface predominantly influences the or:and wear behaviour, of the friction material and the ultimate duty it can withstand. In a short engagement involving high energy dissipation the cooling effect of the fluid is small, so that the thermal problem is identical to that in a dry application. The main contribution from the fluid is to cool the mechanism between engagements. With resin-asbestos friction facings incipient failure is usually indicated by a fall in p, and if the load on the facing cannot be increased to maintain the required torque throughout the engagement excessive slipping occurs and the resulting high temperature softens the bond between facing and core plate. The facing parts from the core plate which generally buckles. If the friction material is of sintered metal high temperatures can cause surface melting and friction material smears the opposing plate to give a low and erratic p followed by a high p at failure and excessive wear. As the temperature builds up distortion of the plates will aggravate the situation, for if there is preferential heating of the plates, and, for example, the temperature of the outer region of the annulus is higher than the inner region then on cooling the outer diameter shrinks and the stresses force the inner part outwards in an axial direction causing dishing of the plate. In this condition the clutch cannot easily be disengaged and prolonged running will give rise to severe thermal damage and failure. Band brakes mainly use a flexible paper-based material. Over-heating of the band causes bond failure similar to that described previously. In. these brakes very high loads are often experienced at one end of the band and the material becomes

T. P. NEWCOMB,

74

R. T. SPURR

crushed and fragments break away. Continued rubbing then results in loss of performance and severe grooving of the drum. Another mode of failure is for the material to become glazed and give a low p. As p is reduced so more slipping takes place and temperatures are increased. Most failures are therefore thermal in origin. An estimate of the temperatures attained in a clutch can be made if it is assumed that the heat is conducted away normal to the metal mating surface and that the metal opposing member is of sufficient thickness that its back face does not increase in temperature during the engagement, that is, the heat flow is taken as one-dimensional through a medium of infinite thickness. The maximum temperature rise’ 8,,, in “C in an application occurs halfway through the engagement and is given by e,,, = 82.2 x 10-6Ntf(0.12

x lo-%+)

(1)

for a resin-asbestos friction material sliding against a ferrous material. The quantity N = E/At, where E joules (ft lb f) is the energy dissipated in an engagement, A m2 (ft’) is the total area swept by the friction material and t seconds the duration of the engagement. The torque is assumed to be constant throughout the engagement. If contact is not over the complete annulus but confined to one smaller region which persists throughout the engagement, the maximum temperature is given by an equation similar to eqn. (1) so that f3,,, is proportional to Rt* where R is the rate of energy dissipation per unit of real contact area per unit time. Consequently if the maximum temperature is the criterion for failure, failure will occur at a particular value of Rti rather than at a particular rate of working. Since most engagements are however made in a constant time (of about 0.5 s) the nominal rate ofworking N can be taken as an indication of the temperature rise, and materials can be characterised by the maximum rate of working they can withstand. To determine the operating conditions for failure, measurements of p and wear were made on the high speed rig at different pressures and velocities but for a constant engagement time of 0.55 s. A sudden drop in .Dand a very rapid increase in wear rate were taken to be indications of failure. If the reciprocal of the torque at failure is plotted against the corresponding velocity for a given material the experimental points fall about a straight line (Fig. 5) suggesting that, for a fixed engagement time, failure is determined by the rate of working. For resin-impregnated paper facings sliding against thick metal members failure occured when 0.94 J (450 ft lb f) of energy were dissipated per mm2 (in’) of facing area in 0.55 s, that is, at a power rating of 1.68 W/mm” (1.5 hp/in2). In another series of measurements the rotational speeds at which failure occurred were determined for different engagement times using a fixed inertia on the rig. Knowing the energy dissipated the maximum power rating could be calculated for each engagement time. Figure 6 shows the variation of W/mm” with t -* for two widely differing friction materials and the linear behaviour is again consistent with thermal failure. These measurements were made with the facings, etc., immersed in oil. Changing from one oil to another containing different additives or of different viscosity had no effect on the failure point. Next the surfaces were run dry and again failure of the paper facings occurred at a power rating in the region of 1.68 W/mm’

FRICTION

MATERIAL--LUBRICANT

75

INTERACTlON

(1.5 hp/in’) so obviously the fluid does not play a significant part so far as the maximum power rating of the facings is concerned in a single application. Under these conditions the temperature rise at the rubbing surface calculated from eqn. (1) was about 128”C, assuming full contact between the surfaces, which was in reasonable agreement with the values measured by a thermocouple welded in one of the metal members about 1.52 mm (0.060 in.) from the rubbing path. The temperature of the circulating oil was 100°C so surface temperatures were about 228”C, sufficient for any boundary lubricant to be degraded.

0007

0006

0.005 1.6

I. E z 0

0.004

1.6

0003

1.2

1.4

$ b t

0002

ooole

0

1000

2ooo

Fig. 5. Reciprocal

of torque

Fig. 6. Rate of working

3000

velocity

Angular

4000

5000

at failure plotted

at failure against

1.2

0.4

1.6

ct secr”2

(rev/min)

against

corresponding

t -f for two different

angular

types of friction

velocity. material.

In calculating the temperature it was assumed that all the heat enters the metal surfaces, which is a reasonable approximation for resin-asbestos materials, but not for sintered metals which have a much greater thermal conductivity. When sintered metals slide against ferrous materials the proportion of heat entering the latter is about 0.75 for thick surfaces. Even for facings only 0.38 mm (0.015 in.) thick the higher thermal conductivity of sintered metal facings causes a significant reduction in temperatures. This reduction in conjunction with the intrinsic properties of the material, enabled the sintered facings to withstand nearly 1.12 W/mm2 (1 hp/in2) more than paper based facings, that is, 2.8 W/mm2 (2.5 hp/in2) in all. Plain facings failed at practically the same rating dry as wet. Rather higher ratings were obtained when the sintered metal facings were grooved. Very little fluid is pumped through the grooves during an application so that little cooling can occur; possibly the fluid is vapourised and the latent heat absorbed reduces the temperatures of the surfaces.

76

T. P. NEWCOMB,

R. T. SPURR

ACKNOWLEDGEMENTS

The authors wish to thank Mr. R. H. Gibbon for assistance, Mr. A. Jenkins for discussions, and the Directors of Ferodo Limited for permission to publish.

REFERENCES 1 A. Jenkins, T. P. Newcomb and R. C. Parker, Conference on Drive Line Engineering, Jersey, Proc. Inst. Me& Eng., 184 (31) (1969-70) 39. 2 T. P. Newcomb and R. T. Spur, Braking of Road Vehicles, Chapman and Hall, London, 1967.