Identification of Gear oil Additives

Verification of the presence of  various additives  in gear oils is often of  interest and different methods have been  suggested for the purpose. To identify dialkyldithiophosphate additives in lubricating oils, lewkowitsch  isolated  the  compounds in the form of copper salts. Paper  chromatography  was applied to the  identification of certain  anti-oxidants by Delves. Among the compounds  distinguished  were  diphenylamine, phenothiazine, and  phenyl alpha and  phenyl  beta  naphthylamines.

Missile and Space Vehicles Gear Mechanisms and their Lubrication

While details of space vehicles are not publicized, it can be expected that gears may enter, even if only for small instruments. These will no doubt be of a nature which will not require fluid lubrication. However, Hartman^24 mentions that there may be a gear drive between the turbine and the shaft on certain liquid rocket engines. Where kerosene is the fuel used, this also provides lubrication for the gears. However, kerosene alone allowed scoring of gears and consequently additives were included. Use of 2 per cent by volume of zinc dialkyldithiophosphate  in the fuel, decreased gear wear. This combination also improved the rust resistance of gears. Such a kerosene additive mixture is suggested as a break in lubricant no matter what type of lubricant may be used in service. In this connection, an article by Watson^51 entitled “Materials and Ratings for Dry Running Gears” should be of interest. After  experimenting  with  various  materials for  gears, it  was  found  that  under  light  loads, spur  gears, made of case  hardened  En  steel, Phosphate  prior  to  coating  the  flanks with  molybdenum  disulfide, would run  continuously  in a dry state  without measurable wear.

Limitations on Heating of Lubricants for Application

Heavy bodied lubricants, particularly residual types used on exposed gears, are often heated in order to make application easier. If such lubricants are straight mineral oil products, the amount and intensity of heat should not harm them. However, if additives are included, only a very moderate heat should be used. Otherwise some change in the composition is possible. A supplier of the lubricant can advice the limitations on heating. A similar caution is necessary in case lubricating greases are used as gear lubricants. The thickeners for such products may be soaps which upon the application of considerable or prolonged heat will separate from the oil present. 

Turbine oils for Automotive Vehicles

Since  gas  turbine  engines  for  automobiles  have  a  potential  which  no  doubt will  soon  be  realized,  mention will  be  made  of the  lubricant  requirements  for  the  drives. The oil will be used to lubricate both bearings and gearing. The  bulk oil  temperature will  probably  be at least  300 degree F during  operation, but  the oil will  not  be  subjected   to hot combustion gases  and thus should remain    comparatively   clean.

Whether   the oils  used will preferably  be  petroleum or  synthetic or  blends  of  the  two  is a  question. Whatever type, they will   no doubt contain various additives, such as   antifoam    agents, oxidation inhibitors, metal deactivators, and antiwear agents.

Speculation as  to  consumption  and  renewal  of  such  oils   indicates that  the oil  reservoir  will  have  a capacity  of about  3 quarts  and that  the   consumption will be  almost  nil  but  that  an  oil  change  yearly  can be  expected.

Storage Solubility of Universal Gear Lubricants

In view of the almost  universal  use  of  additives  in gear  oils there  arises  the  possibility that  separation  of some  of the  ingredients may take place  during  storage. With  this  in mind , Federal  Method  3455, is intended  to  determine the  storage stability of Universal   Gear  Lubricants in  particular, but  may  also be  used  to  evaluate  other gear oils. The  method consists of placing  100 ml  of the  compounded  oil in a centrifuge  tube  and  storing  in a dark  room  for 30 days. The tube is then centrifuged    for 5 minutes and examined for a deposit.  

Suppliers will  seldom distribute  gear oils  which will show settling in storage, but  if  the oil is suspect separation may be due to.

Precipitation Number of Gear Oils

The  precipitation  number, as  determined by  ASTM  D 91-52 (Federal  Test Method 3101),  is  the  amount  of solid  sediment  in an  oil. This  is  defined  as  the  number  of  milliliters  of  precipitate formed when  10 ml  of  the  oil are mixed  with 90 ml of ASTM precipitation naphtha  and  centrifuged. Unless the  base  oil is a cylinder  stock  or a black oil, in which  case some  asphaltic  material  may  precipitate, the  sediment should  be  nil in  unused oils.

Used gear oils, whether  straight  mineral  oil  or one  containing  additives, may  have  high  precipitation  numbers. These  may  consist of  metal  particles, water, sludge, or  material  arising  from degradation of oil  or  additives.

Polymers as components of gear lubricants

Polymers find application in gear lubricants both as additives and as actual fractions of the base fluids. Specific types of polymers used as additives will be mentioned later. Since the fluids in gear lubricants consist  primarily of mineral  oils  or of  asphaltic materials, any polymers  added  should  be compatible  with  the  major  ingredients. Likewise, polymerization  of  the added compounds  should  be complete  so that  no  further  thickening  occurs  in  service  even  if  the  mixture  is  subjected  to  some  heat. Most  polymeric  materials will  break  down  under  shearing  action  of gear  to  products  of  lower  molecular   weight  and  hence  any  initial  bodying  effect  may  be  partly  lost  after  use.

Although the polymers may be miscible with lubricating oils, special procedures may be necessary to introduce them into gear oils. Thus the grade of  polyisobutylene  which  is used  to  give  tackiness  or stringiness to oils is about  the  nature  of, but tougher than, crepe  rubber. Consequently this polymer is dispersed by a rubber mill in lubricating oil and the solution known as paratac is available to compounders. Also  most  grades of  polyethylene  are  of  such  a  nature that  only  prolonged  heat and  agitation  will  disperse them  in  lubricating oils.

The following polymers have been  suggested or  actually  used as components of  fluids for  gear lubricants: polyisobutylene  of various molecular  weight; styrene-polyisobutylene;  polyethylene ;and resins recovered  from  mineral oils. With the continued  investigation and  production of new  or  modified polymers it  seems  reasonable that  increasing  use  of  such  materials can  be expected in gear  lubricants.  

Odor Control Agents

In general there has been no apparent  attempt  to cover up the odors  of gear  oils  which  arise  from EP  additives. Occasionally pine oil or pine tar will be detected in lubricants designed for exposed gears. Sulfur  compounds  perhaps  have  the most  disagreeable  odor  of any additives  and  consequently  Jonach^46  suggested  treating  such  agents  with  lead  peroxide  to  react  with  any  free hydrogen  sulfide  which might  be  present.

Influence of Additives on Viscosity Tests

Erratic viscosity results can be obtained in some cases with oils containing additives. If the gear oil contains diluents, some of this fraction might escape before a test is completed, particularly if the determination is conducted at 210 degree F.

Some  additives  which  are sensitive  to  heat  may change  under similar  conditions  or two agents  may  react and thus change  the final viscosity. Also, in case  the  lubricant contains a polymers, such as “Paratac”, heat  may tend  to  reduce its  effectiveness  and  in turn  the  viscosity of the  mixture.

High and Low Temperature Stability

While  separation  of  additives at  low  temperatures  might  influence  either  pour  points of low  temperature  fluidity, fed. Test Method 3460 is primarily concerned with the stability of blends. The method is used  to  determine  the  tendency  of the  components of  blended oils to  be  incompatible  when  subjected to  temperature cycling. It  consists  of heating  the  sample  of  oil to  205 degree F for  24 hours, cooling it  to  0 degree F for  16  hours,  storing  it at  room  temperature for  10  days and  examining  it  for  changes in appearance.

Gas, Fluid, or Solid Lubricants and Their Future

Thought is being given to experiments and   perhaps usage of gear lubricants in each of the above states. Application will be found  for   both  gases  and  solids as gear lubricants, the  major  portion of operating  gears  in  the  future  will  continue to be lubricated with fluids.

Naturally these fluids will be improved for many applications. For such purpose, the first thought concerns synthetics. However, the abundance and relative cheapness of petroleum oils rules out synthetics for most application in the immediate future. It can  be expected  that  mineral  oils will continue to be  upgraded  by  greater  selectivity , more  chemical  processing, and use  of  improved additives.        

Dispersion of Silicone Antifoam Agents

Most often  the  additives  employed  in   compounding  gear  oils  are  readily  soluble  in the  base  oil; therefore , simple  mixing, as described  above, is  sufficient  to obtain a  satisfactory  blend. On the other hand,  since  silicone fluids which are  frequently  used  as  antifoam  agents have  limited  solubility in oils, they  must  be  thoroughly  dispersed, or they  will settle  out. In view  of this  Beerbower  and  Barnum  made  a study  of  dispersions  of  silicone  defoamers  in oils  using  radioactive tracers  to  determine  dispersion  and  separation of these  additives. They  concluded  that  to  form  stable  dispersions  of  silicones  in oil  the  droplet  size  of the  additive  should  be  in the  10  micron  range. This  could  be arrived at by  heat  and the use  of  high  intensity  mixing  approaching colloid  milling. In laboratory mixers, a speed of 4800 rpm and a temperature of 200 degree F “gave blends in which the silicone was uniformly dispersed after 70 days of storage”.

Additives for Future Gear lubricants

Since the limit  to which  an  unstable  lubricating oil can be  upgraded by  the  use of  additives is restricted, the first  point  of  attack  in gear lubricant improvement is the base oil. However, even the most stable oils are often improved by the use of additives. Such agents, like most things dealt with, are not ideal and thus there is a wide field for improvement in gear lubricant additives.

Not only  can it be  expected that  individual additives will  be perfected  which  will  give  better response, but  combinations  of such   agents  will  also  be improved. In this connection, the functioning as synergistic will be increasingly important. That is , many  compounds will provide  mutual  aid to  another compound to a  greater extent than  if either  were  used in  a much  large  proportion.

Further, there  will also  be increasing recognition  that there  is perhaps  a very  critical  optimum proportion  of additives  to  give the  maximum response. 

Solids as Gear Lubricants

Dry solids, such as graphite or molybdenum disulfide, have had very limited use as lubricants for gears. More often such solids are used in conjunction with fluids for gear lubrication. Such use is not general and data as to their value is meager.                                                     

                                                                                     
A large gear on a radar  installation was  said  to  be  difficult to  move  until  molybdenum  disulfide was  mixed  with  the oil used. Vago^50 cites  gears operating  in excess  of 200 degree (F)  where  both automotive gear oil and  a molybdenum disulfide  fortified  lubricant were tested. After three years, inspection of the gear teeth indicated no advantage of the latter lubricant over “the chemically fortified gear oils”.

If  solids  are  included  in gear oils, the particle size of the additives  is  of  importance and should  preferably  be of  colloidal size. Thus, Kyropoulos^28 in citing  claims  for a 20 per cent increase in efficiency  with  a  worm gear  unit after  adding  colloidal  graphite, also stated  that powdered  graphite was not  effective. While settling of the solid occurred in this case there was said to be no  clogging of oil ducts.

No mention is made of the proportion of solids  desirable, but one  per  cent  of a  colloidal  suspension, which  in turn  contains 10 per cent  of  solids, is a  normal dosage. This holds true for either graphite or molybdenum disulfide. The latter  material  has  been  recommended where  there  is  impact  loading, or  oscillating or  reversing motion.

Nonferrous Gears and Their Lubrication

While the tendency is to think only of gears made of ferrous metals, we find in Dudley^16 the statement: “A wide variety of bronzes, aluminum alloys, zinc alloys, and nonmetallic plastics and laminates are used to make gears.”

Fortunately, well refined mineral oils have little action on most of the combinations. We say  combinations  because in  many  cases the nonferrous  gear is driven by a  steel pinion, but when the loads are light and the  gears are small, both members  may be of the nonferrous  material.

Nonreactive oils should be used with nonferrous gears unless specific recommendations state otherwise. This is illustrated in the case of worm gears where the gear is normally made of bronze. The  general  recommendation for  such units is a mineral  oil  containing  tallow, although  often  such  oils  contain  lead  soaps  and  occasionally certain EP agents.

Synthetic fluids, both diesters and “Ucon fluids, have been used as lubricants with nonferrous or ferrous and nonferrous combinations of gears. Where a problem of lubricating  an  unusual combination of  gear  materials is  encountered, the manufacturer of  the gear set should  be  able to  make a safe  recommendation. However, lacking a  suggestion, a well  refined  lubricating  oil  with no reactive additives  present  can  generally be used with safety in the  case of  nonferrous  gear sets.

Reasonable Costs of Gear and Transmission Lubricants

Cost is a quality just as are physical characteristics, and by discussing it immediately following characteristics, the reader will have in mind all of the requirements of gear oils and can thus visualize the fact that considerable effort and precise compounding goes into the manufacture of a satisfactory lubricant.

At one time gear oils consisted, in the main, of black oils which were not too well refined. While some residual products are now sold for the purpose, most gear lubricants are on par with motor oils in quality and price. As further demands are made on gear lubricants for specific applications, more expensive additives and even synthetic fluids will be required in some cases. Thus, the trend of cost of such products can be expected to increase to some extent. In return, the consumer will obtain a better product and in many cases a longer service life.

It is appropriate, when considering the cost of a lubricant, to also keep in mind the cost of the equipment to be serviced. The proper lubricant, applied as needed in the right amount, will protect and prolong the life of machinery many years.

Foam inhibition in gear oils

The rapid movements of gears tend to aerate oils and produce foam. This tendency may be aggravated by the presence of some additives, such as EP agents. Also, the higher the viscosity of the oil the more permanent the foam as a rule. Cases have been noted where foam became so great that it filled the gear case to overflow and long before this the gear teeth failed to obtain sufficient lubricant. While  foaming of gear oils might  result  from vaporization of entrained  water or driving of air  out  of solution, the general  cause in gear sets is churning  of air  into  the oil  by  agitation.

Little trouble is experienced from foaming of gear oils in service because the use of foam inhibitors in such lubricants is almost universal. It has been suggested that there is a difference between a foam inhibitor and a foam depressant, the latter being an insoluble material. Robinson and Woods^44 use the term “antifoaming agent” to embrace all aspects of the destruction, elimination, or prevention of foams. These investigators state that a foam inhibitor may act:

“ (1) by causing  coalescence of smaller bubbles into large bubbles at or below the surface, (2) by causing the rupture  of  individual bubbles at the surface, (3) by destroying the inherent stability of the liquid  films, or (4) by causing any or all of  these actions simultaneously.”

McBain et al.^39 found that the most complete defoamers for oils are generally, but not always, insoluble. This is true of silicone fluids which have wide usage for this purpose. There is an optimum amount of antifoam agent required which is quite low. Consequently most suppliers furnish defoamers as dispersions or solutions so that low dosages will be more accurate. While Woods and Robinson^50, in testing varying proportions of  DC 200 fluid in two oils, found that 0.01 per cent gave  the greatest  foam  inhibition, actual  usage  in most  gear oils is only a fraction  of this proportion. Thus, Klaus  and Fenske^34, using two oils which foamed badly with no additive, found  that both responded to silicone  antifoam additives at concentrations as low as 0.00001 weight per cent and that maximum effectiveness  was achieved with 0.00005 per cent  or greater.

Detergent qualities of gear and transmission lubricants

Detergent additives are not required or included in most gear oils. However, in mechanisms where the lubricant serves both gears and control devices, such as clutches, some of the moving parts will not tolerate deposits. In such cases detergent additives are included in the lubricants. A good example of such usage is in ATF.

The term detergent dose not properly describes the function of such compounds which are really dispersants or peptizers for materials that would otherwise appear as deposits on parts of mechanisms. Antioxidants which may be present in the gear oils are not completely effective in preventing formation of gum and varnish constituents, hence, the need for dispersants.

Detergents now used consist largely of phenates or sulfonates with a lesser amount of polymers. Either the phenates or sulfonates are added as metal salts, most often of barium or calcium. While neutral salts are satisfactory dispersants, the tendency is to use basic compounds since they will neutralize any acidic products formed during oxidation. A further advance is the use of non-ashing detergent additives, most of which are specific polymers, with the suggestion also of nitrogen containing soaps

Oiliness of gear lubricants

As lubricating conditions in gear sets change from that of thick film to boundary lubrication, the oil benefits by the presence of additives. For conditions with spur gear lubrication, some agent which will provide increased lubricity or oiliness may prevent film rupture and thus maintain a low friction. Oiliness additives consist of polar materials such as fatty acids or even animal or vegetable oils. One end of such fatty acid molecules will adhere to the metal surface and resist removal by shear of the gear teeth.

Sulfurized fatty oils have also been used for oiliness additives but have not always prevented a stick slip condition in automatic transmissions. By proper choice of materials and also of the sulfurizing methods, oiliness additives are provided which are said to satisfy the requirements of automatic transmissions and yet prevent “squawking”. Also, certain Phosphorus compounds have found application in ATF as lubricity agents.

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.