884
INDUSTRIAL AND ENGINEERING CHEMISTRY
bearing corrosion but not to the estent desired for a bearing-corrosion inhibitor. The use of 0.25% Ortholeum 202 in conjunction with tributyl selenonophosphate reduces bearing corrosion to a satisfactory level. There is an indication t h a t the method of preparation of the selenonophosphates affects their performance as antioxidants and bearing-corrosion inhibitors (see Table I). The manner in which extraneous compounds are formed during the preparation of, and their effect on the performance of, selenonophosphates is not clear. -qdditional work along these lines is indicated ACKNO WLEDGRIENT
T h e permission of the Selenium Development Coniin1ttt.e to publish the results of this investigation is gratefully ncknowl-
Vol. 43, No. 4
edged. The authors also wish to thank €1 L. Hemmingrvay for his assistance. The cooperation of the Organic Chemicals Department, E. I. du Pont de Nemours & Co., Inc., in running a series of L-4 Chevrolet engine tests is acknowledged with appreciation. LITERATURE CITED
(1) Arbusoff, A , , Bcr., 38, 1172 (1905). (2) Denison, G . F., a n d Condit, P. C., IXD. EKG.CHEM.,41, 944-8 (1949).
(3) Heiks, It. E.. and Croxton, F. C., I b i d . , 39, 1466-74 (1947). (3) Mastin. T . W., Korrnan, G. R., and Weilmuenster, E. A&., J . Am. Chcm. SOC.,67, 1662-4 (1945). RECEIVED March 18, 1989.
Aqueous Nonflammable Hydraulic Fluids J. E. BROPHY, V. G. FITZSIRIJIONS, J. G. O'REAR, T. R. PRICE, AND W. A. ZISMAN Naval Research Laboratory, Washington,
D. C.
number of fires were reported during World War I1 in hydraulic systems of military equipment operating on petroleum hydraulic fluids. The flammability hxmrds involved were extensively investigated by the Kava1 Hesearch Laboratory during t h e war. Research on a wide variety of less flammable fluids led to the decision in 1913 to emphasize t h e development of aqueous base fluids. Two such fluids had been developed and were ready for production when the war ended. Research led to the development of nonflamninble hydraulic fluids designated as Hydrolubes, which are defined as polymer-thickened, corrosion-inhibited, aqueoiis fluids having one or more glycols as major constituents. Of t h e many classes of polymers investigated for thicliening and improving t h e viscosity-temperature properties, t h e most suitable were derivatives of t h e polyalkylene glycols. The large number of metals occurring in present aircraft hydraulic systems posed a difficult problem in cor-
A
rosion inhibition. However, suitable liquid phase and vapor phase inhibitors were found. Vear-reducing additives were developed nf t h e type forming hydrophobic films on steel. The results of extensive laboratory and flight tests have indicated t h a t Hydrolube U-4 i s the most promising fluid of this class developed to date. Unlike other fire-resistant fluids, i t presents no problem with respect to the deterioration of hydraulic paclcings. With t h e exception of highmagnesium alloys, i t is a safe medium for all the metals commonly found in hjdraulic systems. Its resistance to all types of fire hazards has been outstanding. Hydrolube L-4 has been employed in a large number of naval aircraft for three years and has been applied to a wide variety of industrial problems. This development has stimulated widespread interest and research on aqueous lubricants and synthetic organic liquids having high flammability resistance.
NUMBER of fires were reported during TT'orld War I1 in hydraulic systems of military equipment operating on petroleum hydraulic fluids. The worst fires were caused by the breaking or puncturing of high pressure lines by missiles. In some instances mechanical or packing failures led to oil leakage and subsequent ignition by electrical sparks, incendiaries, or spontaneous ignition on contact with hot parts of the engines. The numerous hydraulic lines distributed throughout the fuselage and wings of modern military planes increase the fire risks. The flammability hazards involved were extensively investigated by the Naval Research Laboratory early in the war. Petroleum oil sprays or mists such as those formed by the rupture of high pressure hydraulic systems were shown t o explode readily in the atmosphere, as did all available hydraulic fluids in aircraft and ship use. D a t a on the fire hazard involved in the use of such hydraulic fluids in military equipment were not easily collected. Few pilots lived through the serious fires t o report or to analyze what had happened. Existing flammability methods were unsatiefactory, making necessary the development of laboratory methods for measuring the flammability of oil mists. Hydraulic fluids in
usc and promising or new fluids were studied, using four laboratory methods of evaluation, and the conclusions relative to their flammability resistance mere verified by incendiary firing tests carried out a t Dahlgren Proving Grounds through the cooperation of the Bureau of Ordnance. The results of these flammability studies have been presented ( 1 4 ) . Research on a wide variety of fluids led to the decision in 1943 to emphasize the development of aqueous-base hydraulic fluids. Two such fluids had been developed and were ready for production for fighter planes when the war ended. Production was postponed in order t o attempt to improve the fluids for wider use. Following a series of serious fires in commercial and military planes in 1946, interest in these fluids increased, and a S a v a l Research report (8) summarizing progress in developing nonflammable fluids for aircraft was distributed widely to interested organizations by the Bureau of Aeronautics and the Civil Seronautics Administration. More recently a bibliographical survey of the problem and research activities related t o it has been made by the National Advisory Committee for Aeronautics ( 1 7 ) . The purpose of this paper is t o present a concise, more accessible, and up-to-date
April 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
account of the development of aqueous hydraulic fluids which has been described in detail in several Kava1 Research reports (4,10,
885
and in Figure 2 are plotted similar data for the fluids having the volume ratios 70-30, 6 0 3 0 , 50-50, and 40-60.
fa). POLYMER ADDITIVES FOR VISCOSITY INDEX IMPROVEBIEhT REASON FOR U S E OF GLYCOL-WATER SOLUTIOKS
Because of their lower bond energies, most organic compounds will decompose and eventually flash or support combustion under the proper conditions of time of exposure and degree of dispersion of the fluid in the atmosphere a t temperatures of 400 O to 800 O F. (14). It was recognized early t h a t the addition of such a heatresistant and efficient fire-quenching material as water t o an organic fluid would greatly improve its resistance to fire hazards. This led to a search for low-freezing aqueous solutions having the necessary physical propert,ies. I n the selection of the freezing point depressant t o be used with water, the lower alcohols and the low-boiling ethers and glycol ethers was eliminated because of their volatilities. The higher boiling alcohols such as 8-(methoxymethoxy) ethanol, ethylene glycol, diethylene glycol, and propylene glycol, and the glycol ethers such as the ethyl or butyl monoethers of ethylene or diethylene glycol were found more suitable. Considerations of ( a )the effects on pacliings, ( b )the volatility, flsshpoint, and incendiary fire hazard of the organic solvent alone (14),(c) the greater difficulty in making a fluid of the desired viscosity and viscosity index if the base stock has too high a viscosity, and ( d ) the availability for large scale use led to the choicc of mixtures of ethylene glycol and water. 4 graph of the freezing point against the percentage composition for ethylene glycol-water solutions (Figure 1) reveals that the eutectic mixture freezes a t -65' F. and contains approximately 67yoby volume glycol and 3370 by T-olume Ivater. I t is not desirable to use the eutectic mixture, however, because any evaporation of water from the hydraulic fluid will cause a rapid increase in the freezing point. A more reliable base fluid should contain somewhat more water than the eutectic mixture, for any small loss of water through evaporation r~-ouldthen merely cause an additional decrease in the freezing point. The spray flammability and incendiary fire tests ( 1 4 ) demonstrated that a t least 40% by volume water was necessary to asjure complete freedom from the propagation of flames in the liquid spray in the atmosphere. I n the former test, 40% oxygen was needed for an 80-20 glycolrynter mixture, and over 80% for a 50-50 mixture. These considerations led t o the decision to use a mixture of 55% by volume of ethylene glycol and 45% by volume of water as a base stock in developing a hydraulic fluid. Such a fluid has a freezing point of -55' F., a boiling point of 228" F., a specific gravity n t 77 F. of 1.085, a spontaneous ignition temperature (8.I.T.) of 507" F., and a spray flammability limit of between 67 and 80% oxygen. The viscosity-temperature charact'eristics of the 55-45 glycol-\vat,er solution selected for the base fluid are given in item 1 of Table I,
Linear polymers are frequently dissolved in oils t o decrease the slope of the viscosity-temperature curve or increase the viscosity index (17.1.). Such polymer additives also increase the viscosity of the oil, which creates one limit to the amount of viscosity index improvement obtainable. An early problem was to find or t o obtain water-soluble linear polymers suitable for increasing the viscosity index of the glycol-~vaterbase fluid. It was recommended b y the Bureau of A4eronauticsthat the viscosity of the finished aircraft hydraulic fluid should not be less than 10 centistoke. :it 130" F. and not over 2000 cs. a t -40' F., and that the freezing point should not be higher than -50" F. and as low as - G o 1'. if possible. For use above deck on ships, the Bureau of Ordnance recommended that the viscosity should not be less than 10 cs. a t 210" F. and not over 215 cs. a t 0" F., while the pour point should bebelow -40°F. The commercially available or n e d y developed water-soluble polymers studied included polyethylene glycols and a variety of other polyalkylene glycols and their derivatives, polyvinyl alcohol. methylcellulose, carbovymethylcellulose, Abapon, gum
GLYCOL-WATER SOLUTIONS
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TABLEI . YISCOSITIESOF POLYAIER-THICKEKED, GLE-COL-WATER illIXTVRES Item
Polymer i n Glycol-Water KO. Base Stock, K e i g h t yo 1 Base stock (55-45 glycol-water by vol.) 2 1.25% choline polyacrylate G-3360 3 2.76% trimethylbenzylammonium polyacrylate G-3380 4 5 76 8 9
3.707, ainruoninm tetraethanolammonium G-3381 0.1277, polyacrylatepolyacrylate G.A. 0.297, sodium polyacrylate G . S . 5.4.57, sodium polymethacrylate G-3667 3.6% potassium polymethacrylate G-1675 10.6870 polyalkylene giycol copolymer Ucon 75-H-69,400
Viscosity, Centistokes 210' P. 130° F. 100' F. 0' F. -40° F. Comments 0.79 1.74 2.60 23.0 134.0 Solutionclear . . 10.58 l5,19 .. .. 937.5 Partial precipitation of polyiner o n standing .. . 10,01 1.5.69 1 8 3 . 6 1083 Small amount of P (niethowymethoxy) ethanol used as solubilizer .. ,. .. . 9.92 15.63 197.4 1230 Partial precipitation of polyrncr 011 s t a n d i n g 10.07 lS.73 146.7 791 Solution clear .... 9.84 16.19 145.7 793 Solution clear .... 9,97 15.49 1 7 7 . 9 1125 Solution clear .. ,, ., ., 10.18 16.4 1 6 8 . 5 1025 Soiution clear 9.94 1 6 , 3 3 219.6 1476 Solution clear
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
886
Vol. 43, No. 4
T E M P E R A T U R E , DEGREES F A H R E N H E I T
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aratiic, g u m ghatti, agar agar, methyl starch, and the sodiuni potassium and ammonium salts of polyacrylic and polymethacrylic acids. T h e base fluid thickened with samples of these polymers was tested in the hydraulic bench test equipment described later. It was found that !Then the average molecular weight of t,he polympr exceeded 15,000 to 20,000, the viscosity and viscosity index decreased ox-ing to shear breakdown, the effect being larger the higher the average molecular weight. Fluids containing polyniers below 15,000 molecular weight showved no evidence of
shear 1)realidonn. This was in agreement with the predictions made from earlier theoretical research of this laboratory on the effectsof high rates of shear on the depolymerization of viscosity
index improvers (13). Hoyever, the higher the average molecular weight of the polymer, the smaller the weight concentration needed to obtain the desired improvenient in viscosity index; hence, as in the development of the high viscosity index petroleum hydraulic oils, a compromise was necessary between the conflicting requirements for optimum viscosity index improvement and
INDUSTRIAL AND ENGINEERING CHEMISTRY
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minimum rate of shear breakdonm in viscosity. Viscosity and solubility d a t a on the more promising polymers are given in Table I. The two most suitable were a polyalkylene glycol copolymer ' of et,hylene oxide and 25% propylene prepared from 75 mole % oxide and identified b y the producer as Ucon 75-H (items !) through 15, Table I), and sodium polymethacrylate (.4crysol G-3493B) (item 7, Table I). The final choice of a suitable polymer included the consideration of the tackiness and hardness of the residue left when the water and glycol had evaporated from a n exposed metal surface covered a.ith a thin layer of the hydraulic fluid. The Ucon polymer left a viscous oily fluid residue, while the poljmethacrylate left a hard and adherent resin. T h e latter type of residue is undesirable because it causes cementing of threaded fittings, seizing of sliding parts, and accelerated failures of rubber packings. -4 polymer Fhich is a liquid or a soft waxy solid over the temperature range of interest is more likely to be satisfactory than one , or crystalline. T h e viscosity indes iniprovement hy t,he former polymer was lew than by the latter and was caused principally by the difference in molecular w-eiglit s. Both polymers were studied further to learn if there were any important differences in the corrosion- and wear-preventive proIwrties. T h e resulting two types of glycol-water base fluids were given the names "Hydrolube U" and "Hydrolube A," the "I" and ".I" denoting t h a t the polymers added were the L-con copolymer and the -4crysol polymer, respectively. Evaporation of wat,er from t,he Hydrolubes during operation causes a n increase in viscosity; b u t so little occurs in practice that no significant viscosity increase of any Hydrolube has been t'ncountered in laboratory, mock-up, or flight tepts. Shear bw&down of the polymer during operation causes a decrease in viscosity, but no decrease in viscosity has been noted with the Cconthickened Hydrolubes. While considerable shear breakdown was noted in the Hydrolube .I fluids, it was within the limits tolprat ed in specifications for petroleum aircraft hydraulic oils. As all the Hydrolube fluids supercooled readily, approximate melting points were obtained after freezing the test mixture ill dry ice and acetone and in allowing it to st,and a d a y or two at the desired test temperature ( -50" or -60" F., usually) t,o ascertaiii whether or not it would thaw. T h e freezing point of unintiibitt~cl Hydrolube U \vas rhus estimated to be between -50' and -(io 1.' In general, fluids with freezing points approximately 10" 1.; lower could be obtained with sodium or potassium polymethacr,~~late than with the Ucon 75-H type. T h e freezing point of the 1111inhibited Hydrolube .In-as below -60" F. INHIBLTING CORROSIOIV IN LIQUID PHASE
Inhibiting corrosion in a n aqueous hydraulic fluid i u 1no1'1. difficult than in a n engine antifreeze coolant because of t l l ~ variety of metals present in hydraulic syetems and the close WI:Ition to the problems of wear prevention and vapor phase corrosioii inhibition. T h e difficult'ies are enhanced in aircraft because t h r hydraulic systems contain some or all of the following met:~ls: steel, copper, 52SO aluminum, 24ST aluminum, lead-bronze, tinbronze, brass, zinc-plated steel, and cadmium-plated steel. I n ship and industrial equipment the metals present are predominantly steel, copper, and bronze. I n addition, the handling of the fluid in small cans during storage and shipment will expose thta fluid to tin and 50-50 lead-tin solder. The electrical coupler likely t o be present are: steel us. brass, steel us. bronze, steel O S . copper, steel us. zinc-plated steel, steel us. cadmium-plated steel, steel z's. aluminum, solder us. copper, solder cs. steel, solder us. bronze, solder us. brass, cadmium-plated steel us. aluminum, and zinc-plat,ed steel us. aluminum. Occasionally aircraft hydraulic brake systems contain high magnesium alloys such as Dotmietals C and H, but effective inhibition against their corrosion has not been obtained without decreasing wear prevention and corrosion inhibition of more common metals ( 4 ) . I n the absence of prep-
INDUSTRIAL AND ENGINEERING CHEMISTRY
888
Vol. 43, No. 4
surized reservoirs it is evident that aqueous hydraulic fluids cannot be used near the boiling point. The only guides available on liquid phase inhibition were the patent literature and the assistance obtainable from industry based on experience in developing engine coolants. The stirring corrosion test used by the authors was a slight modification of the one used by Walker et al. (16).
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This involved stirring a test metal assembly a t 1800 r.p.m. in 400 ml. of test solution in a 600-ml. dye pot beaker fitted with a rubber stopper and maintained a t 160' F. for 336 hours. The test assembly x consisted of rectangular metal strips (1 X l j 1 8 inch with a 0.25-inch hole in the center) placed on a 6-mm. glass rod rotated through a hole in the rubber stopper and flattened a t the other end. The metal strips were crossed a t right angles t o one another to serve as blades of a stirrer and were bound securely with a fine Nichrome wire. The metals used in the accelerated corrosion tests \%ere: 52SO aluminum, cadmium-plated steel, mild steel, lead bronze, 50-50 solder, brass, copper and zinc-plated steel. Metallic contacts between them were in the order given in the top row of Tables 11, 111, and IV. All metals were polished with No. 150 alumina cloth, boiled in C.P. benzene, and dried before use. The p H of each fluid was determined before and after the corrosion test. Measurements were made with an electronic pH meter using standard glass and calomel electrodes. Where nietallic corrosion v a s measured quantitatively, the specimens were weighed before and after each test. Before final weighings, each specimen was cleaned of any loose corrosion products by rubbing it with a soft rubber eraser, followed by boiling in C.P. benzene and drying. No quantitative measurement was made of pitting corrosion, vhich nearly aln-ays occurred where aluminum was attacked, as well as in some other cases.
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Tlie corrosion rates so obtained are empirical, for they vary Tyith the technique of cleaning the metal specimens after the corrosion tests, with the speed of stirring, and with the degree of aeration of the fluid. Probably no single laboratory corrosion test can duplicate what happens in all types of hydraulic systems. I n the stirring corrosion test aeration was far from severe, because there was only a small clearance lietn-een the stirring shaft and the rubber stopper. However, the test has been helpful in eliminating a large number of potential corrosion inhibitors from consideration, and the results have given a fair idea of thc relative amounts of corrosion likely to take place in systems aside from where lubrication is a factor. Hundreds of combinations of corrosion inhibitors and metal deactivators were "screened" using this corrosion test. Among those studied were the sodium hydrogen phosphates, the alkylammonium phosphates, quaternary ammonium phosphates, alkylammonium carboxylates, alkylammonium salts of dicarboxylic acids, alkali metal salts ot' alkane sulfonic acids, boras, sodium nitrite, sodium silicate, sodium chromate, sodium tungstate, sodium niolyhdate, sodium fiuorophosphatc, sodiuni carbonate, sodium fluoride, potassium fluoride, sodium mercaptobenzothiazole (SaLIBT), and sodium mercaptothiazoliiie. Each corrosion inhibitor was tried first in a 55-4570 by volume "base stock" solution of ethylene glycol and water. Those showing the most promise were subsequently tested again in a reference fluid made by adding the same glycol-water solution t o a standard proportion of either Ucon
April 1951
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
or sodium polymethyacrylate polymers. Examples of the results obtained in inhibiting base stock are given $n Table 11. Compounds such as borax, monosodium phosphate, sodium molybdate, sodium tungstate, and sodium fluorophosphate were found t o inhibit the corrosion of most of the test metals. I n several instances sodium mercaptobenzothiazole in combination RTith such inorganic inhibitors adequately reduced the corrosion of copper. VAPOR PHASE CORROSION INHIBITION
Although the above inhibitors were satisfactory for corrosion inhibition in the liquid phase, all permitted serious corrosion of steel in the vapor phase. Inhibition of vapor phase corrosion of steel is essential in present hydraulic systems, as they were not designed for use with aqueous fluids. It is also possible t h a t some steel parts will not always be completely immersed in the fluid during storage, shipment, installation, adjustment, or repair of hydraulic components. Hence service testing of the early Hydrolubes was held up for over a year while a search n'as made for a vapor phase corrosion inhibitor or some means of circumventing the difficulty. Compounds such as morpholine and volatile amines were tried, but were not used because of their corrosiveness and the high pH of the liquid phase a t the concentrations necessary for vapor phase inhibition. In 1944 reports n'ere made available of the discovery by Kachter and Stillman (16) of the abilitj of certain crystalline amine nitrites to inhibit the rusting of steel objects wrapped in impregnated paper. The authors investigated the possibility of using these materials as vapor phase inhibitors in the Hydrolubes. The three most promising compounds were the diisopropyl-, diisobutyl-, and dicyclohe\ylammonium nitrite. The diisopropylammonium nitrite ( D I P A X ) was finally selected because of its higher vapor pressure and its high solubility in glycol-water solution. The preparation of such alkylammonium nitrites as pure crystalline solids was troublesome, but a good solution to the problem resulted from the work of coworkers, Wolfe and Temple (18). Diisopropylammonium nitrite melts a t 136" t o 137" C. and decomposes at a p H above 9 or below 7. At 100" F. the unbuffered compound is reported by the manufacturer to decompose a t a rate of approximately 0.1570 per day. Decomposition is accelerated a t higher temperatures and becomes so rapid a t 160" F. that this temperature appears t o be the upper limit of its usefulness. The effects of diisopropylammonium nitrite and dicyclohexylammonium nitrite on various metals in the vapor phase a t 100 O F. have been studied by this laboratory, and Baker ( 2 ) found that diisopropylammonium nitrite did not attack 1020 steel, 52100 steel, 8-18 stainless steel, chromium plate, Monel, and tin, but it tarnished copper, bronze, and silver, and it caused significant corrosion of 2S, 17ST, and 2451' aluminum, brass, antimony, babbitt, cadmium, zinc, lead, solder, and Dowmetal. Approximately O.O5$Zo diisopropylammonium nitrite in water was sufficient to protect steel against liquid phase corrosion in a 168-hour test a t 160' F.; however, many other metals were attacked-for example, ethylene glycol "base stock" containing 2.0% diisopropylammonium nitrite caused heavy liquid phase corrosion of solder, lead bronze, and cadmium in a corrosion test. Zinc-plated steel was somemhat attacked, brass and copper were tarnished, and aluminurn and steel were inhibited (see Table 111, run 2). Hence a search was necessary for corrosion inhibitors which afforded protection t o these metals in the liquid phase but a t the same time did not interfere with the vapor phase inhibiting action of diisopropylammonium nitrite. The simple vapor phase corrosion test used involved the suspension of a polished rectangular steel specimen (1.5 x 0.5 x 3/32 inch) in a vertical, closed borosilicate glass cylinder (20 mm. in outside diameter and 15 em. high) containing 20 ml. of the test fluid. The specimen wis suspended n-ith its long axis vertical, so t h a t the bottom end was about 0.25 inch above the liquid
889
level, and the whole system was left in an oven a t the desired temperature. A ground-glass joint gave access to the liquid and specimen. Tests for 168 hours on concentrations of 1.0% and 2.0% diisopropylammonium nitrite in distilled water showed there was complete vapor phase inhibition of steel a t temperatures of 120" and 140' F., respectively, b u t the use of even 3.0% diisopropylammonium nitrite did not afford adequate protection t o steel a t 160" F. Minimum concentrations required differed in going from one solvent t o another. T h e addition to base stock solution with or without the Ucon polymer of 1.0% diisopropylammonium nitrite gave vapor phase protection t o steel a t 140" F. for 168 hours. When tried a t lG0" F. rusting occurred in less than 48 hours. A concentration of 1.7% diisopropylammonium nitrite inhibited steel adequately a t 140" F. for 336 hours, but a slight corrosion resulted a t 160' F. Higher concentrations also failed t o inhibit adequately in the vapor phase at 160" F. because of the instability of diisopropylammonium nitrite a t t h a t temperature. The effect of other additives on the vapor phase inhibition of diisopropylammonium nitrite mas determined a t 160 F. using the same test. I n the absence of inhibitors Ucon-thickened base stock caused excessive vapor phase corrosion of steel (2.5 mg. per sq. em. per 336 hours). T h e incorporation of 1.7% diisopropylammonium nitrite reduced this weight loss t o 0.30 mg. per sq. em., while the further addition of 0.4% salicylal ethanolamine (SEA) and 0.1 % sodium mercaptobenzothiazole completely eliminated corrosion. Bntioxidant N-4-98 (0.1 %) and LV-(o-hydroxybenzyl)8-aminoethanol (0.1 %) were tried instead of salicylal ethanolamine with equally good results from the former and slight corrosion from the latter (0.08 mg. per sq. em.). In a subsequent test using O.4lO salicylal ethanolamine in Ucon-thickened base stock, some vapor phase inhibition (corrosion rate 0.75 mg. per sq. cm.) was observed, but the addition to it of diisopropylammonium nitrite was required for complete inhibition. The combination of inhibitors (diisopropylammonium nitrite, salicylal ethanolamine and sodium mercaptobenzothiazole) was so effective in the vapor phase either because (a)salicylal ethanolamine stabilized diisopropylammonium nitrite and increased its effectiveness as a vapor phase inhibitor, or ( b ) the vapor phase inhibition contributed by salicylal ethanolamine supplemented that of diisopropylammonium nitrite sufficiently t o give complete protection. It is believed t h a t (a)is the correct explanation. Because the antioxidant N - 1 9 8 was much less soluble in Hydrolubes a t low temperatures than was the salicylal ethanolamine, i t was not used so extensively t o stabilize diisopropylammonium nitrite. DEACTIVATION OF COPPER
The liquid phase corrosion inhibition problem was made more difficult by the decision t o use diisopropylammonium nitrite in the Hydrolubes. As shown in items 1 through 9 in Table 111, liquid phase corrosion tests demonstrated t h a t although diisopropylammonium nitrite did not attack steel or aluminum, it caused serious corrosion of copper, zinc, and cadmium. This necessitated a search for a suitable metal deactivator. Sodium mercaptobenzothiazole, which has been widely used for deactivating copper in engine coolant solutions, was also found in stirring corrosion tests to be a good deactivator, provided precautions were taken t o minimize exposure of its solutions to sunlight. Such fluids containing sodium mercaptobenzothiazole, sometimes become turbid, while precipitation eventually occurs after the fluid has been exposed t o sunlight. This is a well-known photochemical reaction in which the sodium mercaptobenzothiazole is converted into the mercaptobenzothiazyl disulfide, and it is accelerated by ultraviolet light. It is believed that oxidation also takes place in fluids containing sodium mercnptobenzothiazole after long operation in hydraulic pump systems. Many other compounds reported or considered as promising copper deactivators rrere tried in Hydrolube compositions ( I d ) , the follow-
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ing being the most effective: benzoin: a-benzoinoxime; @-benzoinoxime; 2,Pdiketoquinalinic acid; various isomers of naphtliylamine sulfonic acid ; 0-, m-, and p-phenylenediamine; thiodiethylene glycol : benzyl monorime : Z-mercapto-4-phenyIthiazole; potassium salt of disalicylal ethylenediamine; potassium salt of diealicylal o-phenylenediamine; potas8ium salt of Zmercaptothiazolinc,: and P-(2-niercaptobenzothiazyl) propionic acid. 11any of these cwmpounds were eliininated hecause of low solubility or attack on mrtals other than ro!,prr~-particularly solder. I n ;i(l(lition to sodium mercaptobenzothiazoie the most suitable c'ol)per deactivators for this fluid were: B-(2-mercaptobenzothiazyl~propionic acid, potassium salt oi 2-niercaptothiazoline, potassium 5alt of disalicylal-o-phenylenediamine,and a-benzoinoxime. It v a s finally concluded that sodium mercaptobenzothiazole was the most suitable copper deactivator for use in the Hydrolubes. The improved corrosion results obtained by the addition of sodium mercaptobenzothiazole are shown in items 11 through 13 of Tat~le 111.
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WEAR PREVENTION
Early tests on aircraft hydraulic pumps at 1000 and 1500 pounds per square inch shovied t h a t the rates of wear of the gears of Pesco gear pumps were considerably higher with the aqueous solutions than with petroleum fluids. On the other hand, well inhibited fluids such as Hydrolube U and Hydrolube A caused less wear to the bronze bushings than the petroleum fluid. Because none of the usual aqueous corrosion inhibiting combinations were able to decrease greatly the rates of wear (or spalling) of the gears, research was directed toward improving boundary lubricating properties through the use of special additives. After a numlier of wear preventives were discarded because of their marked corrosive action on the various metals of interest, it was coneluded that some compromise would have to be made between the maximum corrosion inhibition and t,he maximum wear prevention. Polar compounds appeared especially promising as oilincw or mild wear-reducing agents. Particular attention was given to aliphatic compounds containing a carbon chain of at least eight, carbon atoms (preferably unbranched) and a hydrophilic group capable of adsorbing on the metal surface. Such flexible long carbon chains extending from the metal surface form a surface of low shear strength and at the same time t,hey form a harrirr ablr to decrease t,he probability of metal-to-metal contact. The more promising wear preventives of this type were selerted from the large number of compounds available, by using :I simple laboratory test.
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ob
HYDROLUBE G-4 ON T'ARIOL-s TYPES OF KUBBER
Tline, Days
Temp.,
z
158 158 138
I
F.
Volume Change, 70 Hydrolube AK-VV-0u-4 366ba +:.5 5.3 +'.3 13.5
+
+, . Y
-+ 1-13
Petrdeuiu-baae liydrauiic fluid, Specification hN-VV-0-366b.
Foanling characteristics of the Hydrolubes have been exanlined ( I P ) and compared with those of the oils now specified. This a-as done by passing clean dry air nt a constant rate through a sinteredglass disk filter and into the fluid. The height to which the foam rose in a fised interval of time and the rate of collapse of the column of foam after turning off the air were noted. Hydrolube A appeared, in fact, considerably better than the present petroleum aircraft hydraulic fluid, whereas Hydrolube U and Hydrolube U-4 were slightly worse. In an attempt t o determine the effect of altitude on the Hydrolube U ( I s ) ,a beiich test was made in which the reservoir was connected to a vacuum system and operated at an absolute pressure of 10 inches of mercury (corresponding t o 29,000 feet of aititude). This vas the maximumpressure differential (external to internal) at which the pump seals n.ould fmction. The temperature of the fluid in the reservoir \vas varied from 100" to 160" F. KOsigiiificant change was observed in the flow rate, and o ~ i l ya slight hazing of the fluid was noticed. The iiigrcdierite in I-lytlrolubes are not irritat,ing to the skin nor are tiie xiporz poisollous. However, frequent and prolonged breathing u! tiie vapors has undesirable physiological effects due and aniine nitrites present in the vapor. Rubber sirell t a t s were made to determine the effects of Hydrolube I;-4 011 types of rubber commonly found in aircraft hydraulic equipiiierlt. These results are given in Table VII, together with coniparabie data. ou the aircraft petroleum Iiydraulic fluid ( 1 ) . The types of rubber used were Linear O-ring stock, Coodyear O-ring stock, and F41Y natural rubber. The per cent volume changes observed oil the three types of rubber investigated shorn that in most, instances Hydrolube U-4 caused less sir-elling than did the petroleum fluid. Other testa were made ( l a ) using l o a acrylo-Euna N-type synthetic rubber containing various coinniercial plasticizers. Measurements of such properties as volume sn-cll, modulus, tensile elongation, and compression set were made on teri typical O-ring compositions a t 75 and 158' 1;. during various interv:& of a 30-day test period. These results show that Hydrolube U-4 caused significantly less deterioration of the various rubbers than did tlie aircrnft petroleum fluid (AS-VV-036ijt1). I n a simulated performance test, made by the Battelle llleniorial Institute ( 3 )O-rings were cycled in Hydrolube U-4 a t a O
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1951
T E S T S~ S I S GPEsCo TABLE 1'111. BENCH Pressure, Run
No. 1 2 3 4
Flujd CompositionO Hydrolube C Hydrolube U Hydrolube I; Hydrolube I;
\{-ear Additlre Sone Sone None Sone
Model of
Pump
1P349-N 1P349-S 1P349-S 8-12276
Lb./Sq. Inch 1000 1000 1000 2700 (max.) 1000 1000 1000 1000 1000 1000 1500 3000 3000 1500 3000
Pump Speed,
R.P.M. 3600 3600 3600 3600
GEaR
Time of Temp., F. 100 100 140 140
Test, Hours 50 100 100 1
100 100 100 100 100 140 140
400
1P319-S 1800 0 Hydrolube A Sone 1800 6 Hydrolube U-2s 0 57, DIAL 1P349-S 3600 7 Hydrolube U-2s 0 5So DIAL 1P349-S 3600 Hydrolube E-2b 0 8% D I A L lP349-?i 8 3600 DIAL 1P349-N 9 H q drolube 3600 DIAL IP349-S Hydrolube 10 3600 Hydrolube u-3 11 1 0% DIAL 1P3.19-?i 3600 Hydrolube u-3 1 oqc DIAL 6.122ib 12 3600 Hydrolube u-3 1 0% DIAL 6-12276 13 3600 Hydrolube L-4 14 1 0 5 DIAL 1P349-S 3600 1 o q DIAL J-1227C Hydrolube r - 4 15 See Table V b Manufacturer's babbitt bushings replaced by S R L i!-ith special bronze Report 3536 ( 4 ) . c Aluminuin bushings and nitrided gears furnished by manufacturer.
E:; ; :$
PuarP
140
140 140 140
100
100 100 100 100 100 100 2000 100 3s.3
Total Weieht LOSS, aIg, Four Two gears bushings 183 226 11.2 141 248 10 3 378 236
372 107 121.0 29 125 201 22 1 155 278 207 63
27 17 39 78 75 84
62 21 36 51 118
.\IECH.AVIC.4 1. PERFORRIANCE
The scvi.rc,st dc~iiiarid-o n the IubricutiIlg properties of hydraulic fluids are generally t:ncountered in decreasing frictio~ibetn-eeri sliding or rotating metallic parts in hydiaulic pumps :ind motors This can involve the friction betweeri steel against steel, steel against tin bronze, or steel against lead bronze. As the probleni of lubricating pumps is identied v i t h that of hydraulic motors, the laboratory screening tost$ were concerned with the \war i r i hydraulic pumps. G w r , pkton, and vane pumps are used \r.itlc.ly on shipboard and in industrial installations, xhereas in aircraft, vane pumps are seldom employed. In industrial and shipboard applications, pressures exceediiig 1000 pounds per square inch are not common. I n aircraft, maximum pressures Tvere 1500 pounds per square inch during the Far, but since then the trend in military aircraft has been t o x l r d pressures of 3000 pountl.5 ])cI square inch. Various aircraft hydraulic pumpa uscci in thv p(~rforiiiiiiicc~ tcsts on the Hydrolube? were: Borg Karner Corp. Peaco gear pumps, 11odel 1P349-S operated a t 1500 pounds per square inch and .\Iode1 d-1227 operatcd at 3000 pounds per square inch Vickers piston pumps, Alodel PF9-2713-10ZE operated :it 1500 pounds per square inch anti JIodrl PF17-3011-10Zk~ operated a t 3000 pounds per square inch New York Airbrake e o . piptoil punip, \Iodr~l 67('1$IO o p c ~ . t l (1 < at 3000 pounds per square inc,h Vickers vane pump, Jfodcl 77105-.~1T,:i single-stiigt. t ! p , ope1'ated a t 1000 pounds per squaw inch Experimental Hydrolubes w r e "bcuc,h trsJed" for evidence of pump wrar, corrosion, foaming, and cavitation. E3sential cwniponents of the hench tcst assembly were ( 4 )the rewrvoir anti f i l t i ~ r combination, the teat pump, the relief valve, teniperatuw controls, florv meter, pressure controls, and the connecting tubing and fittings. The drive motor w e d was either a constant e p e e ~ l (3600 r.p.m.) 5-lip. motor or a variable speed 15-lip. motor. Tliv reservoir-filter combination, which is the same as that used iri the Grumman F8F fighter plane, ronaisted of an aluminum tank divided into two compartments. The outlet compartment served as a reservoir for the test fluid, while the inlet compartment was fitted with a niicronic filter capable of removing part,icles as small as 3 microns in size. The mtire hydraulic assembly usually contained b e t m e n 3 and 4 gallons of test fluid, and the fluid was cycled through the system approximately once per minute. Pressure \vas maintained at a constant value by a T'ickers (Model C-167E) relief valve. The hydraulic lines and fittings were either 2iST or 52SO aluminum, and the heat exchanger consisted
of a spiral of this tubing cooled by water and jacketed by a concentric copper coil. BefoSe each test, the pump, reservoir, and valves which could not bc cleaned by a simple flushing operation w r e dismantled and washed Tvith suitable solvents. The tubing component8 were flushed with hot water and hot solvent+ before each test. With the. exception of the pump, tht, entire system was flushed ~ v i t l i warm acetone and then bloirti dry with clean air. -4 nc'\\filter element was used for eat311 test.
R e a r in gear pumps \ ~ : I P m e a s u r e d kJy t h e w e i g h t changes of the gears arid the p l a i n b e a r i n g s . Kew gears and bearings - w r e used t o assure initid uniformity of niechanical conditions. ltesults of typical bench bests on the gear pump ure summarized in Tabltb VIII. The rate of wear increased with the speed, temperature, :ind pressure. In the first j 0 hours of operation of new gears, the rate of wear n-as aln-ays gnxatcr t,han in any subsequent 50-hour period. For instance, in run 13 a t 140' F. and 3000 pounds per $quare inch, the weight loescs of the gears and bearings were 125 ant1 5.4 mg., rtaapectivrly, aftc~rthe first 100 hours. After 1000 hours the total weight losses were oiily 200 and 26 mg., after 2000 hours they n w e 278 and 36 mg. Thc high init,inl wear rates r e r e greatly reduced by using "broken-in" or specially polished gears. In :I 2000-hour run under the same conditions, t,he weight, lossw obtained with Specification .4S-VV-O-366L petrolvum oil w r i ~ comparable. I n all tlie early brnch tests thy g'ar teeth had a greater tendency to "apall" or flake ofjc in spots and t,o w a r more unevenly than is usua11y characteristic of gears run n-ith petroleum fluids. This niny be caused by intergranular corrosion accelerated by streses, rvhich occurred beneath the coating of the nitrided steel gear teeth surface. I t is inore likely due t o the inadequate load carrying citpacity of the :tbsorbtd film. It is of interest that changes in the mauufacturer'v niet;tllurgical techniques have productd gears which arcs not a? susceptible to spalling and pitting ne wr'rc thow uscd in this invwtig:it,iori. Sot\vithrt:Liidiny, a t the end o f all the tests the pumps were operating satisfactorily wit11 little 10s; i n the origirid volumetric efficiency. Rririr 6 through 1-4 :is comparcd with runs 1 through 5 show t,hat diisopropylaniinoniuni lauratr eflected a considerable retluction in the wvur rates of gears and beuritigs. The effect of increased pressure in the abseiice arid presence of a wear preventive is shown by comparing run 4 for Hydrolube L with runs 12 and 13 for Hydrolube U-3 and run 15 for Hydrolube U-4. I n run 15 failure occurred in 63 hours. This ivas due to the. introduct'ion by the manufacturer of a special aiuminum alloy bearing. Replacing this Jvith a bronze bearing cared for the difficulty and the resulting \\-ear rates were as small as those obtained in run 13. Siniilarlj-, the usc of babbitt bearings in 3000 pounds per square inch pumps caused immediate seizure. Replacement of the babbitt m-ith bronze led to the excellent ptsrformance of run 13. Hence, alunlinuni alloy and babbitt bttarings should not be used in such pumps operated with IIyclrolubes. It is concluded that Hydrolubes U-3 and U-4 will operate satisfactorily for prolonged use in gear pumps a t pressures from 1000 to 3000 pounds per squarc inch and temperatures a3 high as 140" F. All failures a t 3000 pounds per square inch have been traced to inadequately tested new bearing materials. Indicative bench test results using T'ickers piston pumps are given in Table IX. The main difficulty encountered in the bench tests o n the Vickers piston pumps was with ball bearing failures.
buahing S o . 1 as described i n N R L
driving prcssure of 5000 pounds pcr sc;u:rrc inch and :i t?niperature of 90" F. for 316,000 cycles n-itii satisfactory performance. Under the same conditions, the aircraft petroleum hydraulic fluid ( 1 )had previously been found to f:iil at, between 40,000 and 90,000 cycles. The O-rings ~ m p i o y e drvcxr(' Linear LT-2-70 (8pcifii7:itions AS-6;;-28 and .4S-667-19 ).
e93
INDUSTRIAL AND ENGINEERING CHEMISTRY
894
TABLE 19. BENCHTESTSUsma
Kone
PF9-2713-10ZE
1000
3600
100
None Pione Pione None 1.0% DIAL 1.Oy0DIAL 1 .O'Zo DIAL 1. O h D I A L 1.0% DIAL
PF9-2713-10ZE
1000 1000 1500 3000
3600
100
IIydrolube U-4
1 . 0 % UI.11.
PF17-XYll-lOZH
1800
31300
140
IIydrolube L--4
l.Oyo DI.%L
PI'li-3illl-lOZC
:1OL10
3tiW
1-10
1
Wear hdditire Xone
2
Hydrolube U
3 1 5 6 7 10 11
Hydrolube U Hydrolube U Hydrolube U Hydrolube U Hydrolube 72-3 Hydrolube U-3 Hydrolube IJ-4 Hydrolube U-4 Hydrolube U-4
I2
13
Yo.
8
9
b
'
Model of Pump PFQ-2713-10ZE
F1ui.d Composition" Hydrolube A
RUll
\-ICKERS
Pressure Lb./Sq.' Inch io00
140 140 1-10
140 110 I10 110
r a t c of 11.33
These usuully occurred in the axially aiid radially loaded thrust bearing 011 tho drive shaft end of the pump and sonletinies in t,hc. small thrust and radially loadvd bearing supporting the cylinder l ~ l o c k . Using Hydrolube U or Hydrolube h at 1000 pouiids per square inch, bcaring failurc occurrcd after about 100 hours oi continuous opwation when the reservoir teiiiperature n as 100" F. (see runs 1, 2, and 3 of Table IX), However, the pump life was increased by renioi-iiig the line-type inicronic filter and the cyliiidrical reservoir, by irplacirig thein ivith a combination reservoir and filter unit, and by adding a linc directing the fluid leaking by the punip piston+ back to the filter instead of through the inter1i:iI relief valve into the pump. As a result of those changes, the average pump life nt 100" F. increased to 500 hours. Wlit~nthe. temperature was increased to 140' F. usilig this system, the colttinuous ruiining life of the punips dropped to 235 houre (run 4;. Iininersion corrosio~itesta a t 160" P. with the ball bearings used iii this type oi pump revealed slight corrosion in 168 hours anti very little increase a t the end of 1 nioiith. I t \vas iiifcrred that the ball bearing failures involved stress corrosion. The use of Hydrolube U-4 estendcd the puiiip life to :Lpproxiiiintely -LOO hours a t 1500 pounds per square irii-h and 140 I:. h.fore failure of the thrust ball bearing (see ruus ti and IO). At 3000 pounds per q u a r e iuch wid 140' F. the punili life was reduced t o 74 hours jiun 13). Using the modified c a x feed described abovi,, the pump life could be doubled (run 11 ). In run 12 a pump tit,vigncd for 3000 pouucls per square iiicli was opcrliteti :it 1500 pouiicls per scjuai'e iricli ~ n 140' d F.and the liic \vaso111~-sliglitly iiicreaaed (conyare runs 9 and 10). Heiicc the iailurv of the. thrust bearings in those pumps does not appear to be iiiflueuced significantly by tlie luted load-carrying capacity of thc beaiiligr; It can be concluded that I€ydrolubr U, -i,:tnd L-4 do not fuuctiori \vel1 in I-ickers piston puiiips vr-hen operated a t 3000 pounrlr pi:r square irich. However, lifetinles of 1000 hourr may be vbtailied :it 1000 pourids pcr square inch. The limited life a t high pressures is believed to be due to stress corrosion of the ball bcariiig?. Early attcmpts to operate the S e n . York Airbrakc piston punip a t 3000 pounds per square inch and 140" F. CJII Hydrolube L7-4 led to breakage of pistori spriiigs after 20 hours of opt,ratioit. Such breakage did not result, in a coinplete failure of the pump b u t caused excessive seal leakage and a significant drop in volumetric efficiency. Such difficultiea with the helieai springs do nut occur \\.hen the pump is operated on petroleum-base hydraulic fluids. Eliminating diisopropylammonium nitrite from the Hydrolube L--4 formulation decreased the life of the springs to orily 5 hours, ivhile increasing it to 4.0% increased the life t o 100 hours. A iiuniber of different coating nlaterials for these springs were tried iticluding rubber, silicone resin, chromium, gold, lead, and cad-
' F.
I40
1,500
at
Temp., 100
3600
1000 1500 1500 1300
See Table V. circulated through bearing cane E8F reservoir e ~ n ~ i l o y e i l I'luid .
PISTON Pc~rs
Pump
$,y,\j,.
Vol. 43, No. 4
Time of Test, Hours Remarks 100 Shut d o w n , no p u m p failure hall and socket joints in Tickers p u m p very'stiff considerable insoluble material deposited on' pump parts 96 Ball bearings failed, green deposits over inside of pump 104 Ball bearings failed 235 P u m p shut down. ball bearings beginning t o fail 3 36 Pump shut down, ball bearings bewinning t u fail 15 P u m p s h u t down, ball bearings bezinning t o fail 1000 S h u t down, pump had not failed 382 Shut down, ball hearings beginning t o fail 374 Thrust bearing failed 399 Thrust bearing failed 929 Thrust bearing failed. SIodiiied ) ~ r i i i i j i iised in this run also equipped with extra cl!ainber t o remove gns rapors from p u m p housing 423 Thrust bearing failed. Comparison of run> 9, 10, and 12 shorva that bearing failure is caused priniarily h y fluid and not by bearing design i4 Thrust bearing failed. Longest 3000 lb./sp, inch r u n on IIydroluba L'-4 in !inmodified Iurrnp
gal,,'iiiiii.
..
iiiiuiii. 1Iic. first three costiiigb did not help, but gold, lead, aiitl i.:tdriiiuiii plating gave average life tinics of BO, 110, and 1100 liours, respectively. The best cadmium coatings were protluccd I\ ith :I cadmium cyanide bath following a mechanicid cleaning ~ i i ' ~ ~ ~ ' d uThe r e . customary ctioniical treatment not oiily protli~c,c:detching of the springs but d~~rwultcd iii f:tulty boilding of I tie cadiniuiii platiiig. .ift,er 1100 hours of opcratioii a i 3000 pJuiidS per srluart' inch .uid 140 Y.in this pump, H~clrolut.)eL-4 evidenced iio significant l c u ~ nhydraulic fluid (1 ) of owration and caused izure. It is concluded tli:tt IiioMications of the springs d o n g t,he lines indicated \\-auld niiiliti this pump satisfactory for I".oimigcd operation witti Iiydrolui~eL--4. 1 Iic \-icliers v ~ n epump cshibited con~itkrsble\ v c ~ riii the v:iiic7s and riug i v h e ~ iopt.r:tted on loiv i osity petroleum oils ntive such a s tricrcsg-l phosphate was used. of the vai,iuus Ilydrolubes in this pump were The pe~~ioi~i~i:iiiccs vcsIy v;irixl)le a t 100' F. FrequPntly after 100 hours of operation tile w i g h t loss of t,he ring and vanes was a low as 20 a i d 10 !iig., rcyicctively. In tlie worst run3 tilts w i g h t loss was B M high , ' I - 500 i i i q . on tlie ring a i d 200 nig. 011 the vanes. The lo\v and liigli ~ v e rates ~ ~ r are about tlie saiiie :is those obtained 1);trabk petroleum hydraulic fluid n-ith and without tric iili:iti,. It >,vasfound that the best pcriormiiig batches !til)v$ L-4and U-7 on-ed their ability to fuiiction so well in the vaiiv I)unip t o iiiipuritics prcserit in the p o l ~ . m c rused. It \ Y ~ Z E curlcludecl I tiat to obtain good pcrforiiinticc: in t,he vaiie pump :t ntivc than di;~niylaiilii~oiiiuui laurntc \rill lie rt>. ?
cluircd. APPLICATlOZ. EXI'ERIESCE
l given long tests in a mock-up ut : i r i airCraft hydraulic system consisting of :i motor-driven pump, it haud pump, relief valve, filter, uiiloader valve, selector valves, actuittiirg cyliiidcr, accumulator, and reservoir. The cylinders were' autoniaticidly operated through five coniplete cycles a t the e n d of 20 minutes. Such tests werr caonducted for 8 hours ever?clay for 1000 hours of operation. The total time the fluid remained in the system n-as 9 months. I1 operation was satisfactory a t the erid of the test and if no significant corrosion, Wear, or leakage x-as evident during the tcet or in tht: final disassembly of thc system at. the end, t,he fluid xae reconiniended for service tests. The first Hydrolube tests made outside of this laboratory were conducted a t the Kava1 .\ireraft 1\Itlt6riel Center in Philadelphia, Pa. A complete mock-up of :in aircmft hydraulic system with a capacity of zpprosimately 3 gallons \v:ty operated for 1-30hours OII
April 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
Hydrolube A at 150" F. This mock-up was then moved into a cold room and operated successfully for 150 hours at, -40" F. air temperature and -20" F. hydraulic fluid temperature. Upon disassembly, no signs of excessive corrosion were evident. Several threaded fittings wetted with Hydrolube A before assembly seized when an attempt was made to disconnect them a t the end of the test. Flight tests on the same fluid were then conducted on a Grumman FGF plane xhich had been previously operated on a petroleum hydraulic fluid, and it was flown for a total of 100 hours over a 4-month period. During this time the various hydraulic units were operated approximately 600 times. d t the end of the test the various components were removed from the plane for observation; all parts were iii excellent condition. Although 110 seizures of threaded fittings lvere encountered in disconnectirig the components, several were found unduly tight owing to tlic ;iccuniulation of a hard deposit of t,he ;icrysol polymer. -4s the Hydrolube U showed all the advantages of Hydrolubc, .i i n mock-up tcsts a t this laboratory plus the fact that it \\-asnot so tacky, a Chance-Vought,F4C plaiie was converted to use i t in tests at the Chance-Vought factory a t Stratford, Conn. -4series ( ~ Hights f (some to altitudes of 35,000 feet) over several months showed such promiw that nine additional F4U planes were convrrted by Chance-Vought for use by the Xavy a t the Naval Air Station, Brunswick, Maine. Comparison with ten planes of the same type operating on the AN-T'V-0-366b specification hydraulic fluid showed the Hydrolube U t o be a satisfactory fluid in every respect. By 1916 14 months of flight testing in planes equipped ivith piston pumps operitt'ing a t 1000 pounds per square inch had bren completed and a tentative naval Specification 5lF22 (Aer) had been issued. Hydrolube U-4 was then submitted to Hight tests in several planes equipped TTith 1-ickcrs piston pumps operating at pressures u p to 1500 pounds per square inch. These early flight tests, which lasted 1.5 years, were considered satisfactory. On the basis of these and more reccnt results i t has been conciudcd that IIydrolube U-4 is satisfactory for use in year or piston pumps operating at, pressures as high i+s 1500 pounds per square inch, provided thc plane is not equipped with high-magnesium alloy brake assemblies or other components. The various service tests demonstrated that the flight life of hydraulic piston pumps is ~ipproximntelyten tinwa tlie life obtained in bench tests. Hydrolubes are being further developed under contract by two iudustrial organizations. Meann.hile, considerable routine use o f Hydrolube U-4 is being made in several different types of naval planes. The Hydrolubes lixve already aroused much industri:tl interest. Hydrolubes designated by one manufacturer as "U-4," ',3OO-K," and "500-S" having viscosities a t 100" F. of approximately 85 (18.3 cs.), 300 ( 6 5 cs.)) and 550 (119 cs.) S.C.S., respectively, are now available coniniercially (6). Hydrolubes 300-X and 500-S are generally similar to Hydrolube U-4, except that they are more viscous. Ifydroluhes have already proved suit:ible for use in die casting machinery, hydraulic presses, materials furnace and combustion controls, mining machinery, handling (9), and similar applications where a nonflammable fluid is required. T h e advantages of using a Hydrolube fluid in die casting applications has been illustratrd by it recent report ( 5 ) . FUTURE TRENDS
Despite the large variety of fluids studied, Hydrolubes U-4 and U-7 are the only nonflanimable hydraulic fluids known which are available in large quantity and do not require the installation of new types of packings. However, the wear prevention is not Adequate for one type of pump. Improvement in the performance characteristics can be expected when the metals exposed t o it, are so chosen as to minimize the probleni of corrosion inhibition. I n other words, the hydraulic Eysteni eventually should be redesigned to avoid metals such as cadmium, zinc, solder, and magnesium, which are difficult to inhibit. At present none of the Hydrolubrs
895
has been inhibited satisfactorily for prolonged use a t temperatures above 160' F. It is also believed that research on other polymer additives will improve the viscosity index of the Hydrolubes and decrease the viscosity of U-4 a t -40" F. to much beloiv 1900 cs. Through further research a lower freezing point can be obtained by either decreasing the freezing point of the glycolwater base stock with the addition of another high boiling glycol or alcohol, or by the development of anot,her rionhardening watersoluble polymer which will have a freezing point depressant actioti. Great,er freedom from packing deterioration and leaking can h, obtained with the Hydrolubes if the present elastomers, which w ~ r e developed for petroleum fluids, are replaced by available materials such as natural rubber. Progress in the development of t h i w fluids has k e n hampered by the lack of basic information on thtx relation of wear prevention and corrosion in aqueous solution.; and by the esseritially empirical state of the present. knowledge ( i f t,he mode of action of aqueous corrosion inhibit,ors. Further ri,search probably \vi11 bring improvement in the lubricating propctrties. ACKNOW LEDG-MENTS
The authors wish t o acknowledge the cooperation and essentid assistance of the following orga.nizations in evaluatirig the Hydrulubes: The Chance-Vought .lircraft Corp., Stratford, Cotin., the Saval -4ir Station, Brunsvick, >Id., the Naval Air S t a t i m , Patuxent, N d . , and the Airborne Equipment Division, Bureau 01 Aeronautics. Sincere thanks are due to I
