I
S. R. SPRAGUE and R. G. CUNNINGHAM Wood River Research Laboratory, Shell Oil Co., Wood River, 111.
Chemical Additives Control
Frictional Characteristics of Lubricants Lubricant coolants for wet disk clutches affect lockup time, torque capacity, frictional vibration and fade. In the design of practical transmission fluids a modified four-ball wear tester has proved valuable in laboratory studies of important parameters:
h h h b
Antifriction additive and concentration effects Additive depletion Additive interactions Surface material effects
chemical control of friction is illustrated in the lubrication-coolant requirements of the wet disk clutch. These widely used elements of power transmissions are unique in that fluids ivith special antifrictional characteristics are often required, to prevent “squawk” and other frictional vibration phenomena. Shock intensity during initial clutch engagement and final lockup, time required for lockup, surface fade tendencies, and clutch wear are also affected by fluid frictional characteristics. A simple wet disk clutch arrangement with one drive plate and two driven plates is shown a t right. The frictional force resisting relative motion is directly proportional to the applied force? the coefficient of friction between the sliding surfaces: and the size and number of friction disks. Actuation of the wet clutch to stop relative motion between the driving and driven members is frequently accompanied by audible evidences of frictional vibration. Two rather distinct types occur : stick-slip and dynamic frictional vibration. The vibration or chatter of machine slides and the squeaking of door hinges are familiar examples of stick-slip sliding. Blok ( I ) , Bowden (Z)? hlerchant ( 3 ) . and others have shown that this phenomenon depends on the lubricantsurface frictional characteristics and the mass, damping, and rigidity of the mechanical system. If the static coefficient of friction exceeds the kinetic coefficient, the force required to initiate motion is T H E
0 Driven Plate I
A simplified wet clutch pack arr a n g e m e n t as e m p l o y e d in various automatic power transmission units
Y
2 Drive Plate
correspondingly larger than that required to maintain sliding. The moving element may thus undergo an acceleration from this stored energy immediately following “breakaway” from rest. Such motion is opposed bv the spring force of the mechanism as the mass overshoots its neutral position, as well as by friction ; the velocity drops, friction is higher, and the mass again comes to rest. This continual hunting is the source of stick-slip vibration. Dynamic frictional vibration describes a related but distinct phenomenon without sticking but with velocity pulsation. If during clutch engagement (high sliding speed to lockup) the friction coefficient increases sharply with a decrease in sliding velocity, an unbalance can arise which will amplify natural vibration and produce audible noises. An example is
a rotating clutch plate pressed against a grounded stationary plate. Either member may vibrate, depending on the mass, rigidity, and damping of the system. As a result of torsional vibration of either member, the relative sliding speed between the rotating and stationary clurch surfaces will not be constant but will vary according to the frequency and amplitude of vibration. Because of these speed fluctuations about the mean sliding speed, the reaction forces on both the stationary and rotating plates vary with the change in friction coefficient. p , with sliding speed, V . In the case of a rising coefficient with decreasing speed (large - dp,/dV) a self-energizing condition exists which amplifies the vibrational displacement to an audible level, and a transient squeal or squawk occurs during deceleration before final lockup. VOL. 51, NO. 9
SEPTEMBER 1959
1047
-
8
0
200
0
400
600
800
1000
Slidmg Speed, Ft;Per Mm,
Figure 1 . Polar-type additives markedly influence the coefficient of friction under the boundary lubrication conditions existing in full-scale wet clutches. The rising friction curve with decreasing speed promotes stick-slip and dynamic frictional vibrations
F t STRAIN
6%
-OIL
Three quarterinch disks fabricated from clutch-plate facing materials can be used as the stationary surfaces replacing the conventional '/2-inch balls normally used in the four-ball wear tester
LEVEL
_,AEROSTATIC BEARING
A modified four-ball wear tester was used in the laboratory study of the friction characteristics of lubricant chemical additives
If the friction coefficient decreases or remains constant as sliding velocity approaches zero, vibration amplification is absent and deceleration is noise-free. Rising friction coefficients with decrease in velocity (large - dp,'dV) are characteristic of refined mineral oils which exhibit relatively little surface activity-i.e., lack "oiliness." As indicated in Figure 1, the p-velocity curve exhibits a marked rise (large - d p / d V ) as motion ceases. The antifriction fluid typically exhibits a small dpL'dV value and pa (static coefficient) is a t or below the pk (kinetic) level. This p-velocity characteristic can be imparted to straight mineral oils by use of oil-soluble polar additives-e.g., fatty acids and sulfurized sperm oil. Surface Fade. The frictional properties of the lubricant in the wet clutch are important as affecting surface fade. This is a permanent decrease in the effective friction coefficient in use, which may result from the formation of carbonaceous deposits on the friction surfaces from degradation of the lubricant and/or friction facings proper. By suppressing high-temperature flashes a t the interfaces, antifriction fluids can prevent surface fade with some combinations of surface facings and operating conditions.
1 048
Lubricant Stability. Lubricant coolants for wet clutches also serve as hydraulic fluids for associated servomechanisms, pumps, etc. Improper operation or failure may result from oil degradation (viscosity increase. sludge: and lacquer deposits). Independent of these considerations, antifriction additive depletion in service can causc rnalfunctioning of the \$.et clutches. Depending on antifriction additive selection, inhibition, and operating conditions. the static coefficient of friction may increase, causing noise and/or a rough clutch engagement. Ideally, the antifriction additive loss should precede the cleanliness induction period to xvarn the operator to renew the transmission fluid.
variable-speed direct current motor is used to obtain spindle speeds between 20 and 2000 r.p.m. (1.5 to 150 feet per minute sliding speed). For friction measurements on organic and metallic clurch facings, a special holder containing three flat disks replaces the three steel balls (above) These disks (l 4-inch diameter) are easily punched or machined from clutch-plate materials. Considering the complexity of friction correlations, the modified four-ball apparatus does a good job of screening potential lubricant additive formulations and alloivs study of the parameters involved.
Experimental
For steel sliding on cork, the base oil curve (Figure 2) shows an essentially constant friction coefficient of 0.11 a t sliding speeds above 70 feet per minute.
In the design of practical antifriction fluids: laboratory measurements of frictional characteristics are of vital importance. A modified four-ball wear tester has been employed in laboratory studies of the important parameters affecting lubricant performance: sliding speed? bulk oil temperature, additive type and concentration, additive stability. and surface materials. Modifications included the addition of a flywheel (not shown) to the spindle carrying the rotating steel ball, a "frictionless" aerostatic thrust bearing. and friction force instrumentation consisting of a strain gage, amplifier, and recording oscillograph. The lower cup assembly consists of three stationary 1l *-inch balls immersed in the test oil and the heater elements. Spinning the flywheel by hand, friction is recorded continuously as the speed drops to zero. For higher speeds the flywheel is removed and a
INDUSTRIAL AND ENGINEERING CHEMISTRY
Effect
"R
0 30
O
of Sliding Speed
A - 0 6°C Sulf S p e r m 011 B 5 O b Sult Spcrm 0 1 1 C - 0 6% sat Long-Cham Carboxylic Acid hlvl Minerai 0 11
-
_-
I
0
20
40
I
I
60
80
I
100
120
Sliding Speed, Ft Per M i n .
Figure 2. Comparison of the influence of polar additives on the change in coefficient of friction with sliding speed. Practical automatic power transmission fluids often require friction characteristics in the intermediate range
L U B R I C A N T COOLANTS 0.4
r
0.4
Unsaturated Carboxylic Acid
0.3 0 6% Sat C a r b O q l i c 0 1 0
I
I
I
I
100
200
300
400
Bulk Oil Temperature, 'F
Figure 4. Concentration effects vary widely among antifriction additives
Sulf Sperm Oil
m
%
Figure 3. The transient effect of bulk oil temperature on the static coefficient of friction diminishes as the antifriction action is improved
0.1
Saturated Carboxylic Acid
0 0
I
I
I
0.5
1.0
1.5
1,-0
2.0
Additive Concentration, The increased coefficient at surface speeds below 70 feet per minute is attributed to boundary lubrication effects Xvith progressively greater surface-to-surface contact as velocity approaches zero. These cork specimens were punched from production Hydra-Matic transmission plates. Test conditions in Figure 2 were: bulk oil temperature 200" F., load 5 kg. (100 kg. per sy. cm.), and a highly refined mineral oil of 5.6-centistoke viscosity at 210' F. T h e high kinetic coefficient (0.11) with the cork surface is characteristic of resilient-type clutch materials. By contrast, steel-on-aluminum surfaces in the four-ball test a t similar unit loadings give kinetic coefficients in the 0.01 to 0.02 range. Possibly high hysteresis losses during sliding account for the high friction coefficients exhibited by "soft" clutch facings. -4. B , and C, Figure 2, show the wide range of friction levels that have been observed ( p a from 0.04 to 0.35). The reduction in friction coefficient resulting from the additives is attributed to both a multilayer chemisorbed film and an adsorbed film reducing surface-to-surface contact. Through additives and additive combinations, it is possible to span the 0.04 to 0.35 static coefficient range completely-important in the design of a lubricant to meet special field applications. The coefficients at higher sliding speeds (>70 feet per minute) are independent of lubricant additive effects and imply absence of boundary lubrication. The rate of change of coefficient with sliding speed is considered an important parameter in determining the magnitude of frictional vibration. and the shapes of speed curves differ widely, depending upon oil additive content. The static coefficient alone does not predict the rate of change of coefficient, although generally the higher static coefficients show the sharper coefficient change with sliding speed. The general shape of the antifriction fluid curves, B and C, is explained on the
basis of destruction and repair of the additive films on the sliding surfaces. Apparently fluid-film lubrication occurs down to a sliding speed of approximately 50 feet per minute. -4s the oil wedge collapses at lower sliding speeds. the surface films arr partially displaced and increased surface contact occurs. increasing thc Coefficient. As the sliding speed decredses further, repair of the chemisorbed film exceeds destruction, and belou 10 feet per minute the coefficient decreases. Bulk Oil Temperature
The influence of bulk oil temperature on the coefficient of friction of resilient materials is difficult to interpret because of simultaneous and independent changes in surface and oil friction characteristics ivith temperature. Such surfaces show both permanent and transient coefficient changes with temperature. The permanent changes-which in some cases correspond to a fade mechanism-can be caused by thermal degradation following short soak periods (1 to 5 minutes) at elevated temperatures (200' to 400' F.) or by oil-surface interaction. Oil deterioration was not a factor in the short tests discussed here. T o illustrate the magnitude of transient changes with temperature as observed in the four-ball rig, the test oil was heated to 400' F. and operated for 5 minutes at a sliding speed of 138 feet per minute. The cup was then allowed to cool. with friction measurements at 50' F. intervals. Subsequent heating or cooling would give repeatable friction cs. temperature results. The effect of bulk oil temperature is shown in Figure 3 for base oil. a 5yo sulfurized sperm oil blend, and a blend containing 0.6y0 of a saturated carboxylic acid. For the latter the static coefficient changed only 0.03 unit between 100' and 250" F.? whereas the base oil allowed a 0.08-unit change with temperature. The temperature effect can be considerably greater, depending on surface materials and
%wt.
operating conditions. Temperature often must be carefully considered in oil formulation work and has pronounced effects on noise-level in full-scale transmission tests. Additive Type and Concentration
Additive concentration effects are shown in Figure 4 for a sulfurized sperm oil and an unsaturated and a saturated long-chain carboxylic acid. The latter additive showed optimum effectiveness (0.04 static coefficient) a t approximately 0.257, concentration, whereas the unsaturated acid and the sulfurized sperm oil showed a decreasing static coefficient even above the 10%) lcvel (where ps = 0.19 and 0.12, respectively). Optimum additive concentration and the magnitude of the friction coefficient are affected by bulk oil temperature. In general, within a family of compounds the poorer the additive solubility, the less sensitive it will be to temperature change. Supplemental Additives
Supplemental additives. which in virtually all cases are required to obtain a finished blend, usually affect the friction characteristics of the formulation. For example. in Figure 5 the base oil static friction level is 0.38 and the unaltered 0.6Yc sulfurized sperm oil blend is 0.21,
50
r
lob-:7< 0
0
61,sat carboxylic
hcid
0 0
1
2
3
4
5
6
Cr Phenate, Cb
Figure 5. Supplemental additivese.g., the calcium phenate inhibitormay cancel the desired antifriction action, depending on choice of antifriction additive VOL. 51, NO. 9
SEPTEMBER 1959
1049
’. r 5e
o
40
1
Base 011 Level
- - - - - - - - -e
----
0.30 0 20
o io
. 200
0
ea0
400
Bonob Oxidation Test,Hours at 300°F
Figure 6. Depending on choice of inhibitor and antifriction agent, antifriction action can be greatly extended
exceed the useful antifriction additive life. Depletion characteristics of antifriction additives have been studied by running four-ball friction tests on small (5-ml.) used oil samples. from either full-scale field tests or laboratory-type oxidation tests. The typical depletion curves shown in Figure 6 were obtained from friction tests on used oil samples from a bench oxidation test conducted under the following conditions: Bulk oil temperature, Air flow, liters/hour Oil quantity, g. Catalyst
Addition of a calcium phenate-type inhibitor causes the coefficient to increase to the base oil level (0.38) at 2 to 37, phenate concentration. The saturated carboxylic acid a t 0.6% concentration was fairly insensitive to calcium phenate addition, showing a n increase from 0.07 to 0.16 at the 6y0supplemental additive level. Usually supplemental additives affect the frictional characteristics of a blend by “surface competition” with the polar antifriction additives. Supplemental additives normally are physically adsorbed and are more readily displaced by mechanical scrubbing. Studies of interaction betwecn the antifriction agent and other functional additives are very important, because most lubricants now possess multifunctional characteristics. T h e antifriction action observed with the antifriction additive d o n e is seldom indicative of that obtained with the finished blend. Additive Stability
The life of an antifriction additive in low-temperature applications-e.g.. machine-tool way lubricants-has not been a problem; however, antifriction additive depletion can be serious with automotive transmission fluids. Heavy sludge and lacquer deposition can accompany depletion of the antifriction additive. Depending on the method of inhibition, the cleanliness life may greatly
100
