rican
YP
C. F. Prutton and P. A. Asseff T H E LUBRIZOL CORPORATION, WICKLIFFE, OHIO
A brief report on hypoid lubricant development is presented with typical data on several of the more widely used presentday products. A general discussion of fundamental factors involved in hypoid lubrication makes i t apparent that w e are
sorely in need of more extensive fundamental research on each of the lubricating factors mentioned. Such basic theoretical knowledge would accelerate the development of new and improved hypoid lubricants.
sulfur-chlorine, and phosphorus-sulfur types. Table 1 shows X THE early 1930’s the hypoid rear axle was succcssfully incomparative test data on the VVL-761 and the newer 2-10913 troduced in passenger car and truck service, after exhaustive type of hypoid lubricants. preparation had been made by the automotive and petroleum industries t o provide adequate supplies of suitable lubricants, capable of withstanding the high unit pressures and high ratio of Mechanical and Metallurgical Characteristics of Gears rubbing to rolling action encountered in hypoid gears. Under intense loadings there must be a resulting deformation of N a n y special test machines and test procedures were developed the gears and possibly of the general surface contour of the confor the measurement of the load-carrying capacity of lubricants tacting gear teeth surfaces. If such deformation changcs lubricatfor hypoid use. Principal among these test machines were the ing surface conditions greatly, the lubricant may be unable to perAlmen, Timlren, SAE, Shell 4 Ball, and Falex. As field CYform its normal function. Even if gears, gear teeth, bearings, perience accumulated it became evident that these machine4 etc., are designed properly to prevent such major distortion, could not completely evaluate thc suitability of a lubricant for occasional units from product’ion may be deficient in this respect. hypoid service and today these test units serve principally as conIn establishing dynamometer or road test procedures on comtrol guides for specification uses and for screening purpose5 i n mercial axles, conditions are set considerably more severe than lubricant additive development. those usuallv encountered in normal service. If test Conditions It is now generally agreed that hypoid lubricants can be coincome close to the state of major deformation, it becomes difficult pletely evaluated only in full scale axle dynamometer tests and to reproduce test results and costs for lubricant development arid that road performance is required to give the final answer. evaluation rise t o eqorbitantly high values. A lubricant containing lead soap, sulfur, and sulfur compounds Another factor t h a t enters into deformation is the metallurgy of a a s the first type to be widely used in passenger car hypoid the gears and important axle parts. Hypoid lubricants can axles. Later, hou ever, the chlorine-sulfur type of lubricant found hardly be expected to lubricate gears where major deformations wider use in both truck and passenger car service because of better occur due to iricchanical or metallurgical wealrriesses, or misaligriperformance characteristics, cspecially in the more drastic operatmcnt. ing - conditions occurring in truck axlea. This type (VVL-761) served with excellent results during the reMechanism of cent war in militarv Action of Hypoid equipment and is Lubricants still in large scale usage. Although most of However, in 19-16, the development of new specifications (2hypoid lubricants has IOSB) established by proceeded with only the Vnited States a few general theoArmy Ordnance Deretical guiding principartment required ples, it is believed t h a t a hypoid lubrithat with greater cant should shorn subknowledge of thc stantially no staining mechanism 01 action, tendency on the gears, in “boundary” and and adequate lubri“extreme prcisurc” cating qualities in lubrication, the dogear tests conducted vclopmcnt of new undei both high and iniproved hypoid torque-lom speed and lubricants will br high speed operating greatly accelerated. conditions. UJith light and Hypoid lubricants moderate loadings, recently approved the hydrodynamic under this new specitheory of lubrication fication have been Figure 1. High Speed Axle Gear Surfaces has served as a n exprincipally of the cellent guide. Under Above. Test setup chlorine -sulfur-phosLeft. Base oil these load conditions, R i g h t . 2-105B oil phorus, lead soap-
960
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1949
Figure 2.
Endurance Gear Surfaces
Above. High torque dynamometer setup L e f t . Base oil R i g h t . 2-105B oil
shearing of a continuous fluid film of lubricant between bearing surfaces occurs with but little metallic contact. However, as loads increase, or speeds decrease, the thickness of lubricant film diminishes until the hydrodynamic properties of the lubricant become of lesser importance as compared with adsorption phenomena, chemical reaction films, the nature and contour of the metal surface, attractive forces between metal surfaces, and the viscosity characteristics of incomplete liquid films under high pressures. The lubricating importance of these complicating factors is in turn influenced by speeds, loads, rate of application of load, and bearing surface distortion. Adsorption of Polar Compounds. Mineral lubricating oil hydrocarbon molecules have but slight tendency to be adsorbed on iron or steel surfaces. Under moderate loads, these hydrocarbon lubricants are easily forced from between bearing surfaces and metal-to-metal contact occurs. Such metallic contact produces high-point temperatures accompanied by welding together of the contacting surfaces and subsequent separation to produce serious disturbances in the metal surfaces. Hydrocarbon lubricants have been repeatedly found entirely inadequate for hypoid service. I n journal bearings, the addition of polar molecules, such as a fatty acid, will provide lubrication under loadings somewhat higher than the upper limit for a hydrocarbon oil. This i s due to the attractive forces between the polar molecules and the metal surface, forming mono- or multimolecular adsorption layers which resist rupture under moderate loads. Many of such polar compounds are believed to react chemically with surfaces to maintain a tightly adhering surface film of a soap or other reaction product
(4).
961
I n connection with adsorption films in lubrication, the kinetics of adsorption is of great importance, as disturbed adsorption films must be rebuilt with sufficient rapidity to ensure maximum lubricating efficiency. Most polar compounds with mild chemical reactivity do not suffice for hypoid lubrication under all operating conditions, and it is therefore necessary t o have more efficient means of preventing metal-to-metal contact. Effect of Pressure on Lubricant Viscosity. Numerous investigators (6-8) have studied the effect of high pressures upon the viscosities of liquids. The increase of liquid viscosity with pressure differs with the molecular structure of the liquid. At higher temperatures this pressure effect is lower than a t ordinary temperatures. Certain lubricating oils have been found t o possess high coefficients of viscosity change with pressure, as compared with other mineral oils, and yet neither type of petroleum lubricant will adequately lubricate a hypoid axle. It is somewhat difficult t o conceive that high hydrodynamic pressures in the oil phase could occur between heavily loaded gear teeth unless the oil were partially or completely trapped (or adsorbed) in surface irregularities on the gear teeth. T o be effective, the oil pressure would be required t o reach high values, say 50,000 pounds per square inch or more, under the temperature conditions encountered on heavily loaded gear teeth surfaces. Surface Finish Factors. I n certain types of lubrication, perfectly smooth bearing surfaces are not ideal for maintenance of adeauate lubrication (9, 10). Connecting rod bearings, when in good operating condition, present a matte finish which has been considered to be a reservoir for lubricant that becomes available when loads are increased. Tests conducted in the author's laboratories (Table 11)on two different types of extreme-pressure test machines, indicate higher load-carrying capacities and better lubrication characteristics for a hydrocarbon lubricant when etched metal specimens are employed than when specimens with the normal finish are used. Surface finishing methods in industry have greatly reduced the average surface roughness of machined and finished steel surfaces. After grinding and lapping, the surface finish as determined by various profilometers shows jagged contours which generally give improved bearing conditions as the heights of these surface pro-
Table I.
Test Data on Typical Modern Hypoid Lubricants
Full scale axle tests High speed axle (AXS 1569) Endurance axle (AXS 1570) Bench tests film strength Timken OK load, lb. SAE a t 1000 r.p.m., Ib. SAE a t 500 r.p.m. (after heating a t 200' F. for 48 hr.), lb. Falex lb. Alme;, lb.
test (AX8 1572)
2-105B Type
VVL-761 Type
Pass Pass
Pass Borderline
77 208
33 325
373 4500+ 30+
425 4500+
Pass Pass
Borderline Pass
Pass
Pass
0.60 1.68
0.61 3.13 Nil
0.02
30+
962
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. 5
The reaction producing solid films must not occur until drastic loading conditions make this necessary, and then only a t the projections where cxcefisivc temprraturcs arc gencratcd. For example, if highly active chlorine compounds were used, which attacked iron a t ambient gear case temperatures, uniform attack on all exposed iron surfaces 11ould occur, thus changing clearances, depicting the lubricant of additivc, and producing other dct rimmtal corrosive action.
Table 11. Effect of Etching Metal Test Specimens on Film Strength
Figure 3. High Speed Axle Stained Surface Showing Ferrous Chloride
Metal test specimens as reccived Metal test specimens after etching
Load-Carrying Capacity, White 011,SS-180 a t 100' F.,Lb. S A E , 500 r.p.m., 3.83 to 1 rubbing Modified ratio, 125-lb. load, Falex 2250 6'. 100 Fail in 5 minutcs 200 Fail in 2 hours
Contact area of coast side of hypoid ring gear after high speed axle test shown b y treating gear surface with potassium ferricyanide to form Prussian blue
jections are diminished. V h e n the mean deviation reaches a value of about 2 microinches, further practical mechanical means of perfecting the lubricating surfaces do not seem to he available. On these "mirror" surfaces, differences betn-een the load-carrying capacities of a plain hydrocarbon oil and of an extreme-pressure oil are greatly reduced, a t least in teist machine operation. Chemical means of producing improved surfaces have been employcd with beneficial results (9, 1 0 ) by etching and electropolishmg. The ideal bearing wrfacne might \wll be oiie xyliere transverse grooves, or pits, occur, separated hy flat or slightly inclined smooth areas. The exact depth and width of these depressions and the width and inclination of t h c w smooth areas for efficient action would be difficult, even to approximate. However, with such a grooved or etched surface one could visualize how the absorption, chemical reaction characteristics, aiid pressure viscosity properties of a lubricant might better be utilized in obtaining more efficient lubrication under high loadings. Detailed results and conclusions from work on surface finish will be published shortly. Solid Surface Films. It has been shown that under rapidly applied loadings, point temperatures on bearing surfaces approach t,he melting points of the metals involved ( 5 ) . Incorporat,ion in t,he lubricant of chemical reagents capable of forming a solid Subricant film by chemical reaction with these high temperature projections 011 the metal, inay prevent the welding t,ogcther of such mctallic junctions, and subsequent, undesirable surface dist,urbances of the gear teeth. These chemical reagents may react with the metal surface to form a film of iron compounds (Figure 3), or t)vioseparate reagents in the oil solution may react at higher temperatures t o form a solid lubricant,, such as lead chloride or lead sulfide. Anot,her version of the solid reaction film concept involves the reaction of the high t.emperature metal project,ions with cheniical reagents in the lubricant to produce a low melting composition with improved load-carrying capacity of the lubricant. Certain phosphorus compounds apparently function in this manner (1-3). In each of the above cases, welding of the projecting metallic points is prevented and shear probably occurs either in a chemical reaction product film or in a low melting point phosphide of iron.
Table 111. Acceleratin Effect of Sulfur Compound on Reactivity on Iron of Chorine Compound in Mineral Oil Solution at 250' C. Blend Tested chlorinated paraffin wax f 2% benzyl
hIilliequivalcnt Weights of FeCL Formed 1 50 2 55
Therefore, it becomes necessary to select additive5 that have negligible reaction with iron below about 175" C , lyhich 15 well above the ambient temperature encountered in rear axles. I n the case of the sulfur-chlorine type of hypoid lubricant, a chlorine compound is selected with but slight action upon iron a t 175" C. I n order t o accelerate the action of this chlorine conipountl on steel a t higher temperatures, a carefully choscn sulfur compound is added to the lubricant which catalyzes the reaction of thc chlorine compound upon steel above 20" C. (see Table 111) ( 1 1 ) . At the same time, by proper selection of the chlorine and sulfur compounds, the otherwise objectionable reactivity of the sulfur compound inay be inhibited. Ferrous chloride is the major ingredient in the solid lubricant fihn formed by this type of lubricant.
Literature Cited Beeck, Givens, and Smith, Proc. Roy. Soc. ( L o n d o n ) , 177, 90 (1940). Beeck, Givens, Smith, a,nd Williams, Conference on Friction and Surface Finish, Mass. Inst. Tech., 1940. Beeck, Givens, and Williams, I'roc. Roy. SOC.(London), 177, 103 (1940).
Bowden, Gregory, arid Tabor, A-utwe, 156, 97 (1945). BoTyden and Ridler, Proc. Roy. SOC. (London), 154A, 640 (1936). Dibert el al., J . A p p l i e d Phys., 10, 113-15 (1939). Everett, H. A., S.A.E. Journal, 41, 531-37T (1937). Fenske et al., IND. KNG. CHEM.,29, 1078-80 (1937). Floyd, U. 9. Patent 2,266,379 (1941). Neely, Farington, and Borsoff, I b i d . , 2,266,377 (19411. Prutton, Turnbull, arid Dlouhy , .r, Inst. Petroleum, 32, 90-118 (1946). RLTMVED October 7, 1U48.
END OF SYMPOSIUM ON ADDITIVES FOR PETROLEUM
