Chemical Analysis for Metals in Lubricating oils

These methods of chemical analysis are intended  for the  determination of barium, tin, silica, zinc, aluminum, calcium, magnesium, sodium and potassium in  new  and used  lubricating  oils. Other metallic elements, sulfur, phosphorus and  chlorine in  amounts  commonly  found  in lubricating  oils  do not  interfere in this  method.

Essentially,  the  method  consists  of  igniting  the sample, dissolving  the  residue  in  mineral acid and  then  separating  the various  metals by  conventional  methods.  

Chemical Changes in steel surfaces During Extreme Pressure Lubrication

Analysis may aid in explaining the mechanisms by which gear lubricants, particularly EP oils, function. Which  it is  generally  accepted  that  iron sulfides, chlorides and  phosphates are  possible reaction products  of  EP additives  under extreme operating conditions, this  may not  be the  complete explanation. Godfrey^5 considered that there was a lack of understanding of the above actions. Consequently, EP films and  fragments  were  obtained  and  analyzed  by : electron  diffraction, X-ray diffraction, emission  spectrograph, proton  scattering, chemical spot tests, and  volumetric  analysis. For one wishing to study the chemical mechanisms of extreme pressure lubrication this article should provide suggestions as to analysis. Suggestions have also been made that changes in EP additives while in service may be measured by infra-red spectroscopic technique.

Wear Prevention Agents

A number of investigators have distinguished between wear prevention and EP agents in lubricants. The  former are  effective  by  a  chemical  polishing action, which  takes  place  at a  lower  temperature  than  does  the  formation of antiweld  films by EP agents. A very  extensive  investigation  of  wear  prevention  agents,  which are  effective  in  lubricants for  two  steel  surfaces was  reported by  Beeck, et  al^9. Calhoun and  Murphy^18, who  reported  on both  anti wear  and EP  additives  for  lubricants found that it  was  possible to  blend  two  or  more  types  of  additives  and  attain both  properties in  the same  composition.

Typical  of  anti wear  additives  are  tricresyl  phosphate  and  Zinc  dialkyl  dithiophosphate.  Such  agents  are  seldom  used  in  industrial  gear  oils  but  are  valuable  in  lubricants  such  as  those  for  jet  turbines  where  the  oil  serves  several  mechanisms  including  gearing.

Color stabilizers for gear and transmission lubricants

While there is little likelihood of the necessity for the use of color stabilizers in gear transmission lubricants, information should be available if desired. In a sense, oxidation inhibitors are color stabilizers for lubricating oils because oxidized oils often  turn  dark  or black. However, what  is in mind  here  are  compounds  which may  stop or  change  chemical   reactions which  tend  to  form color  bodies.

For  the  purpose , Bart Maas^7  recommend  the  addition of 0.0001 to 0.01 per cent of an oil soluble  sodium hydrocarbyl  phenate to  lubricating oils. Kalil^47 lists as color stabilizers: Certain  hydroquinones, dithiocarbamates, aliphatic amines and dicyclohexylamines.

Prevention of Contamination of Gear Lubricants

Gear sets operate under such varied conditions that it is difficult to give general suggestions for prevention of contamination of the gear lubricant. The two contaminants most often encountered are water and dust. In the latter are included large particles such as scale in steel mills. Less trouble is encountered where a circulating  system  provides  the  lubricant  rather  than  a  splash system, since the former  can  be  supplied  with settling  tanks  or filters. Even here regular inspection is necessary and accumulated water should be drawn off whenever noted. Such accumulations may  be quite  large  in the case of  gear  oils  for  ship  propellers or in oils servicing  paper  mill  machinery  or steel  mill  gears.

Where  gear cases  are  vented, the location  of the vent or a pipe  connection to the  same  should  be so  located  that  the least  dirt  possible  can enter. One automobile  manufacturer reduced  contamination in  differential cases by  extending  a  line  from  the  vent  and  securing  it  forward  under  the  car  frame  with  the  opening  toward  the  side  of  the  vehicle.

Prevention of contamination of lubricants by chemicals must have individual consideration. In  extreme  cases it  may  be  necessary  to  provide  pressure  on  gear  cases to  prevent  entry  of contaminants.

Air-Oil Devices for Lubricating Gears

By injecting or pumping oil drop-by drop into an air stream the air will carry the lubricant to the points of application, such as gears. A similar mixture can be obtained by the aspiratory action of compressed air. With such systems, provision can be made to start the oil flow with the start of the machine and stop it when the gears stop. The delivery is positive and there is little chance for contamination because the gear box is under some pressure which prevents entry of dust. Also,rate of oil feed can be regulated.

On the negative side, it is necessary to provide compressed air and a considerable flow of air escapes through the vent of the gear case. While this air will have some cooling value, it may carry some oil mist. The oil is used on a once through basis and provision has to be made to drain the sump of the gear case at times.

Shear Stability of Gear and Transmission Lubricants

While most mechanisms containing gears will tolerate a considerable variation in viscosity of the gear oil used for lubrication, a radical change in viscosity at a stated temperature, while in use is not desirable. Further, if such a change is due to a partial breakdown of an additive the purpose of the agent may be defeated. Such changes may occur due to shear while in service. The components most often affected are polymers such as V.I. improvers.

At present, products used in vehicles are the lubricants most often influenced by shear. Such changes will become increasingly important as a single fluid is used for several purposes such as a hydraulic fluid, for ATF, and perhaps as an axle lubricant. Further, a multirange gear and transmission oil has advantage in cars and trucks. That is, oil which will cover two or three SAE viscosity ranges.

Under present formulations some of the lubricants offered for the above services contain polymers as V.I. improvers. However, the action of gears or even pumps tends to change the polymers by shear. The shearing action causes either a chemical or mechanical breakdown of the large polymer molecules so that their value is largely lost in the oil. In some cases a viscosity decrease in service is temporary and in such instances there may be an alignment of the polymer molecules at high rates of shear. Of course some polymers or additives are more resistant to break down with shear than others but unfortunately those of high molecular weight, which prove the most effective V.I. improvers, are also most susceptible to loss of viscosity with shear.

Where high V.I. is necessary or desirable in gear oils, tests should be made to determine the viscosity after shear tests. This is most often done by using a test where a pump forces the lubricant through a sharp edge orifice for a stated time at a given temperature. A similar breakdown of polymers occurs with sonic shear, and a method using this procedure has also been used to evaluate the shear resistance of V.I. improvers.

Klaus and Fenske^34 tested fluids containing about 7 per cent of polymer for their permanent decrease in viscosity due to shear. After 5000 cycles in a pump at 100 degree (F) and a pressure drop of 800 psi, decreases of 25.5 to 30.5 per cent occurred. At a pressure drop of 1500 psi, the decreases were 38.5 and 40.5 per cent. The time required to stabilize viscosity will vary both with the mechanism and the fluid used. 

Oxidation stability of gear lubricants

Once a proper gear lubricant is selected for a given application it should suffer a minimum chemical and physical change during use. One of the changes most likely to occur is oxidation  of  the oil  which ultimately  will  lead  to the formation of undesirable  products  and  changes in the characteristics  of the oil. Such changes may result in the formation of acidic  products which  may corrode  the metal  surfaces, in an increase  in viscosity  of  the  oil, or in production  of  insoluble  materials. Oxidation  of  lubricants  is  accelerated  by  high  temperatures or  by  the  presence of  certain catalysts, particularly  soluble  metals. The  immediate  effects  of  oxidation  may appear  beneficial  in  that  petroleum  acids formed  function  as  oiliness  agents, perhaps by  the formation  of  monolayers  of metallic  soaps. Ultimately, as oxidation of oil proceeds, the harmful effects become evident. The degradation of the oil by oxidation may result in not only the formation of acidic products but also asphaltenes, resins, or other polymers. Changes in the lubricant will  probably  be  accompanied  by  increase  in  viscosity , darkening  in  color, and  the  formation  of  sludge. Cases have been noted where gear oils became almost solid due to oxidation.

However, oxidation of gear lubricants can be retarded by addition of antioxidants or oxidation inhibitors. The use of such agents in most gear oils is wise since the environment for the lubricants is favorable for oxidation in that both air and heat are present and thin films of the oil are in contact with the air.

The mechanism of the action of antioxidants is generally considered to be that of chain breaking as the additive reacts with a “hot” molecule, thus being itself oxidized. In this process the oxidant molecule is destroyed, but with dissipation of the energy possessed by the “hot” molecule, so that the chain reaction is broken. Thus, the oxidation of hundreds or perhaps thousands of molecules of hydrocarbons is prevented, since the energy would be passed on from one molecule to the next in the normal chain reaction.

The suggestion was made by Larsen and Diamond^35 that antioxidants may be either inhibitors or retardants, the former acting to break reaction chains and the latter being converted into an inhibitor during the oxidation process. Three possibilities were given by Murphy et al.^42  for the possible disposition of such inhibitors after they had reacted: (a) the inhibitor is oxidized to a compound which is incapable of further antioxidant action; (b) the inhibitor is oxidized to a compound which still exhibits antioxidant action, but generally to a reduced extent; (c)  the inhibitor is capable of regeneration. The latter type of additive is the most desirable, provided the rate and degree of regeneration are high.

Specific compounds suitable as antioxidants will be suggested in a later section, but most of these agents fall in the following bellows: (a) various types of phenols, (b) certain sulfur bearing compounds, (c) numerous organic phosphites, and (d) certain of the amines. A number of additives function as dual purpose agents and thus, in some cases, a specific antioxidant may not be required.

Freedom from separation in gear oils

Precipitation or settling of some components in gear oils is sometimes noted. This most often occurs in mixtures containing EP additives. The separation may be due to lack of solubility or to reaction of ingredients resulting in formation of sludge. Since such additives are often present in concentrations of 9 per cent or more, the base oil must keep a high proportion of heavy chemical compounds in dispersion or suspension. Both additive manufacturers and oil blenders select ingredients which will keep any separation of such agents at a minimum.  

Fletcher^23 selected three SAE 90 hypoid gear lubricants and three multipurpose SAE 90 oils meeting MIL-L-002105A specification. By precipitation tests, the first three oils showed some sludge in the unused oil which increased after a service test in two of the lubricants. In the multi-purpose oils there was only a trace of sludge before use but measurable amounts up to 7 per cent after the tests.

Of course, settling or sludge formation in gear lubricants results in loss from the action zone of valuable active ingredients, but the greatest concern is the effect sludge may have on operating mechanisms. Thus, Fletcher^23 mentions that precipitation of sludge out of oil due to centrifuging in transmission cases may result in carbon like deposits in pocket bearing positions, internal clutch teeth, and in some cases in oil grooves  and synchronizer grooves. It is conceivable that such deposits could adversely affect the operation of the unit. This fact was probably recognized by one tractor manufacturer who specified that oils used in their equipment should be filterable, thus, indicating that sludge should not separate during  normal operation.

Where sludge is formed in EP gear oils the action is accelerated by increasing temperature. It is therefore probably a result of reaction of the chemical compounds which constitute the EP additives. Detergent agents do not seem to be a correction for such sludge separation, and any improvement in the condition probably lies in selection of the EP additives.