Lubrication of Non-Reactive Surfaces at High Loads

By non-reactive is meant that the  surface will not react with  what  are  customarily  known  as EP elements such as  chlorine, phosphorus, or sulfur compounds. While there is little likelihood of extensive use of  some metal  combinations, it is well  to mention the possibilities. Thus,Antler suggests that 0.03 to 10  per  cent of trimeric  tin  sulfide compounds in  either  oils  or lubricating greases will  increase  the anti wear  qualities of the lubricant on surfaces such as  titanium-on-titanium, stainless steel-on-stainless steel, or gold-on-gold. The same  additives are also  said to be  effective on plastics, such as “nylon”, polyvinyl  chloride, polyethylene, etc.

Chromium is inert to most sulfur compounds but  will  react  with  most  chlorine compounds. Consequently, in  lubricating stainless steel  gears  under  heavy  loads, the  use  of chlorine compounds is  dictated.

Manufacturers  of gears  from  special  metals or  other  materials will no  doubt have  suggestions as to  the  proper  lubricants. Since EP agents are not  effective on  Babbitt, use of such  compounds in lubricants for  certain journal  bearings will  not  increase  the  lubricating  value.

Nonferrous Metal Rolling Gear Lubrication

Metals such  as aluminum, brass, copper etc., are  rolled  into  sheets or  other  shapes in  continuous rolling  mills designed  somewhat like those  found in steel  mills. Modern mills use circulating oils to lubricate the gear drives, pinion stands, and journal bearings. Either mild EP or MP gear oils of a noncorrosive nature can   be employed in such service. The  grade  of  oil  will depend  upon the speeds  of the gears  and  may  vary  from an  SAE  80  to an SAE  140. In  screw down  equipment  either of the above  type  of oils  or a straight  mineral  oil  can  be  used. Where  any  of the above  drives  are  through open gears, a residual  type  of gear  oil  containing  a  rust  inhibitor  and having  a viscosity  of 1000 to  2000 SUS at 210  degree F  should  prove  satisfactory. The  best  practice  is to  apply  such a lubricant automatically so as  to  insure a coating on  the  gears  at all  times. 

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.

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.

Dissipation of heat by gear lubricants

Under the most ideal conditions of lubrication of two moving metal surfaces heat is developed. In fact Bowden and Tabor^11 found that, even though lubricating films are present, surface temperatures of metals  may exceed several hundred degrees Centigrade  at  relatively small loads and sliding speeds. Blok^6, 7 first postulated and then verified conditions of “temperature flashes” between operating gear teeth. The temperature at the points of contact was shown to be proportional to CfP^3V, where C is a constant, f is the coefficient of friction, P the mean pressure, and V the gear engaging speed. This formula holds for both spur and hypoid gears, but the action of the latter type develops the greater amount of frictional heat. Since the contact points are small with respect to the overall dimensions of the gears, this heat is conducted into the two moving metal surfaces. A lesser amount of heat may also be developed by churning friction where gears are bath lubricated. 

Gear oils are an aid in dissipating this frictional heat. How effective this action is depends upon the amount of fluid coming in contact with the gears as well as the temperature and viscosity of the oil and the manner in which such oil is flushed over the gear teeth. Oils are not the ideal coolants since the specific heat of petroleum products is about half that of water.

Design and application influences heat dissipation in that the size of the gear case determines the total gear oil present and radiation from the fluid and the metal depends upon the surface exposed. If the oil application is by spray, the jets can directed at the points where the greatest heat is present, perhaps on the leaving side of the gear teeth. Circulating systems permit not only placement of oil streams but also adjustment of quantity. In case heat dissipation is not rapid enough, additional oil storage or settling tanks can be used to provide more radiation.

The lower the viscosity of  the lubricant the more effective it is in transferring heat from the tooth surfaces to the bulk oil and then to the gear housing and thence to the atmosphere. The value of low viscosity gear oil in dissipating heat was shown in certain truck operations. Here the differential oil ran about 35degree ( F) lower  in  temperature  when an SAE 90  lubricant was  substituted for an SAE 140 gear oil.

Abrasive or cutting wear

Gear oils are not correctives for abrasive wear because here the action is due to hard particles between the gear face as they mesh. If the abrasive is due to loose metal, sand, etc; gear oils may wash the foreign particles from the moving areas, but unless the abrasives settle out they will continue to act as lapping compounds. However, if the viscosity of the gear oil is low, the large foreign particles may be deposited in areas where the velocity of the oil is low, and thus they will be harmless.

The best corrective for abrasive wear of gears is to drain and flush out the gear case and refill with clean oil. Circulating oil systems used for gear oils can be equipped with filters or strainers. Likewise, a settling period can be provided in the storage system for the fluid. Some gear cases in automotive  vehicles  have magnetized  drain plug  so that  most  iron or steel  particles will  become attached  as  the  gear oil  circulates. Where vehicles operate under conditions promoting  dust, as do many  tractors, it is wise to  drain  gear  cases  frequently  so  that  abrasives  filtering  into  the  gear  oil  will be removed.

Corrosive wear

Corrosive   wear  in  the  presence of a gear  lubricant may  be  due to the  environment if air, water , or electrolytes  are  present. If gear cases are not tight and high humidity prevails, rusting may occur, not only on idle gears above the oil line, but also on the walls of the gear case. If necessary, rust preventive compounds can be added to gear oils to counteract the action of moisture. Such additives may be polar compounds, often containing long chains, which will be adsorbed at the metal oil interface to form hydrophobic films. Prevention of corrosion due to electrolytes may be more difficult than prevention of rusting. However, if the contaminant is salt, the same types of additives as mentioned above will aid in corrosion prevention. If water soluble acids entering the gear case cannot be prevented, ordinary gear oils will not serve to prevent corrosion. In this case it may be necessary to use gears of different composition. Stainless steel will resist most acids and some electrolytes. High silica irons, while somewhat brittle, also have this faculty.

The corrosive wear most apt to occur in gear operations is that due to chemical additives, known as EP agents. The secret of a satisfactory EP gear oil is to obtain controlled  corrosion so that welding  of  the  metal  surfaces  will not take place  and  yet  asperities will  be  reduced. In  the  case of most EP gear oil  compositions  corrosive  wear  should  not  be excessive and  is actually  beneficial in  that it extends the life of the gears under extreme operating  conditions

History related to gear lubrication

We  are  little  concerned  with  the  first  gears, which   were   said  to  consist    of  wooden  wheels   with  wood  pegs   for   teeth, since  speeds   and  pressures   were   low  and  lubrication was  not  much  of  a   problem    at  that  time. However metal    gears of cast iron required   a lubricant   to   reduce   both noise   and wear. For the purpose, animal fats were   used, followed   by   petroleum fractions when   the latter   became   available. The  first  mineral  gear  lubricants  were residua  which  were  quite    sticky  and  therefore   resisted  displacement  by   tooth   pressure.  While   such products still   have some usage, high speeds   and closer   tolerance   led   to the use   of   lower    viscosity   gear   oils.

In  factories  the  transition   from    steam   drives, with   line   shafts,  pulleys, and  belts, to the  use  of  electric    motors    for   specific   apparatus   led   to    the   use   of  gearing   to  reduce  or  change   the  direction   of    drive.  Further   changes  in  industrial   gear  sets   has   been   largely   due   to  both  increased  power    and  speed  of   the  driven   units.  This trend has increased   to the point where   5500 hp   and   higher   rolling   mill   drives have   been    installed   in   steel mills. On  the other  hand , gears  in  watches  and , no  doubt, in  some   instruments   have  decreased  in  size.  Therefore,  when  we  speak  of  gear  lubrication  we  think  in  terms   of  power  delivery   varying  from  a  fraction  of  a  hp  to  several  hundred  hp.

The  wide  use  of  automobiles   and  the  development  of  gearing   for  all  automotive  vehicles   has  been  responsible  for  the  greatest   changes   in   gear   lubricants   over  the  last thirty  or  forty   years. The  Society  of  Automotive   Engineers (SAE)   has  been  a  large  factor  in  improvement  of  automotive  gear  oils. The  SAE   fuels  and  lubricants   committee, which  consists  of  technical  men  from  both  the  motor  car  manufacturers  and  the  suppliers  of  lubricants,  has  been  a  meeting   ground  for  ironing  out  differences   and  arriving  at  a  solution  of  many  technical  problems. While  people  from  governmental  departments  entered   the  picture  a  little  later   than   the  above  two  groups, their   suggestions  and  help  has  aided  in  standardizing   gear  and  transmission   lubricants.    

One  cannot  discount  the  efforts   of   the  American  Gear  Manufacturers  Association  (AGMA)  who  have  suggested  and  tabulated  standard   oils  for  use  in  industrial  gearing  under  various    operation  condition . AGMA   was founded in  1917   and  consists  of  a  group    supplying  about  75  per cent  of  the  cut  gears  marketed   in  the  United  States  and  Canada.

Since  that time this  organization  has issued  certain  engineering   standards  and such  specifications, relative  to  gear  lubricants  and   gear  lubrication, have  been  an  aid  to  the  lubricants  industry  and, therefore, will  receive  further  reference. One of  the first  steps  of  the  SAE   group  was  to  establish  viscosity  ranges  for  transmission  and  rear  axle  lubricants  so  that  the  consumer  would  secure  a  material    within  the  same  viscosity  range  no  matter  who  the  supplier  might  be. The designations  were  in  terms  of   the  approximate viscosity SUS at  210 degree F, thus  No.90, No. 110, and No. 160.Naturally  , a certain  range  was  permitted  in  each  grade, and other grades  have  been in use at various time, such as SAE 80,SAE 250, etc. An  SAE  report , adopted  in  February  1924, indicated  that  at  that  time  transmission  and  rear  axle  lubricants  were  made  from  mineral  oil  with  or  without  the  addition  of  animal  or  vegetable  oils, soaps, etc. The purpose  of  the  soaps  was  to  decrease  the  tendency of  the  lubricant  to  leak   from  the  housings. Such  addition  was  said  to  have  little  or  no  effect  on  the  load  carrying  property, nor  did  it  prevent  ease  of shifting  of  gears . The introduction  of the hypoid  differential drive  changed  the requirements  for  gear  lubricants  for  automobiles  and  led  to the  use  of what  are called extreme  pressure (EP)  gear oils. This change started in 1925 when  the Gleason Gear Works perfected gear generating  machines  which  would  produce  gears  of  the hypoid  type  with  improved  standards of accuracy, strength, and quietness  of  operation. The Packard  Motor Car  Company adopted these  gears  for  final  drives  in  their  1926  models. Other  motor car manufacturers  started  to  consider  the  use  of  hypoid  gears  and  to  change  over to such use  until, by 1937,  practically  the  entire  U.S. passenger automobile  industry  had  adopted  the  hypoid rear axle. A number of truck manufacturers in this country likewise converted to this type of differential. The change in the type of gears in the final drives of automobiles abroad was more gradual. Thus, Towle^10  mentions that the  first use of hypoid  gears  in production cars  in England was  in 1929  and that  it was not until 1934  that further models appeared using  this type of  gear. In the 1951 Motor show in the United Kingdom

                                                                                             

Ninety nine models were equipped with the hypoid axles as compared with forty one with spiral bevel gears. On the continent, the change to hypoid   gears has been even more gradual.

Since  such gears subject  two metal surfaces to a sliding  action  as  well  as  to a rolling one, the problem  of  lubrication  is  more  severe  than  with  involute  gear types and, yet, is as important as  the production  of the gears. Experience quickly demonstrated that hypoid gears could not be lubricated with straight mineral oil particularly under severe operating conditions. However, as early as 1869 a “plumboleum’’ lubricant consisting  of  lead soap and sulfur^4  had been found  satisfactory in one model of  spiral  bevel  gears  where all  other  lubricants failed. Gear  oils  containing  lead  soaps  were being  used  in  industrial  applications  at  the  time  hypoid  gears were introduced  in automobiles. It also  happened  that  the oils used  with such lead  soaps  contained  sulfur  compounds  which  became active  at relatively  low  temperatures. Consequently, such gear  lubricants  were  tried  in the  differentials  of vehicles  equipped  with  hypoid  gears and found useful.   

This  type  of  gear compound  was  used  for  hypoid axles  from  1925 to 1932, but all  such compositions  did  not  prove  satisfactory. At  about  this time  it was found that other compounds might  be  desirable  in  hypoid  lubricants  and Wolf  and Mougey^11 listed  three  general  types  of  gear  oils for  the purpose, namely:

               (a) Sulfur chlorine treated saponifiable oil base with petroleum oil or sulfur petroleum oil;

                (b) Sulfur treated saponifiable oil base with mineral oil or sulfur treated petroleum oil;  

                (c) Lubricants containing lead soap and sulfur.

At this period the motor car manufacturers were appealing to the distributers of lubricants to provide the necessary EP gear compounds. Thus, Wolf and Mougey^11 stated: advances in gear design were urgently awaiting the development of satisfactory extreme pressure lubricants. In1933 Mougey^7 said:  EP lubricants are at the cross roads. Many  of the refiners  are  assuming  the  attitude that EP lubricants are not needed  at the present  time, and  if and  when  required, they will  produce  them, while the automotive  manufacturers  are  hesitating  to introduce gear designs which require satisfactory performance in service  until these lubricants  are universally  distributed  and are available at all filling stations.

 During this development  period  in perfecting  satisfactory  hypoid gear  lubricants the problem  was not only availability  and  composition  but also methods  of evaluation of EP  oils. For this purpose thought was given to testing machines which, by bench tests, would determine the quality of the lubricant quickly. Unfortunate of the value of  EP gear oils did not prove simple.

While  several  EP test  machines  have been  proposed  and  are  still  in use, none  of  these  give sufficient  information  or correlation  to  permit  approval  of  EP  gear  formulations  based  on  such  tests  alone. Initially the Gleason Gear Works set up a testing procedure using hypoid gears, and lubricants were  approved  on the basis  of  this “Four –Square Test.’’ Later, any laboratory  tests, even if on full  scale  axles, were  supplemented  by  use  in  cars  on  the  proving  grounds  of  automobile  manufacturers.

Specifications  under  which  hypoid  gear  lubricants  have  been  manufactured  and  sold  have  changed  frequently  over  the  period  from  the  introduction  of  such  gears  up  until  the  present. Using  the  experience  of  motor  car  manufacturers  and  of  oil  companies, the  Federal  Government  set  up  such  specifications  in  1942.Since  products  meeting  these  requirements   did  not  prove  entirely  satisfactory  for  high  torque  low  speed  performance  of  heavily  loaded  axles, a  Coordinating  Lubricants   Group, under  the  Coordinating  Research  Council  was  formed. Under  their  direction  further  standardization  of  test  methods  was  arrived  at   and  some  suggested   changes  in  government  specifications  for  EP  gear  oil  could  be  produced  which  would satisfy  all  automotive  vehicle  requirements, whether  the  operating  conditions  be  one  of  high  speed  and  low  torque  or  low speed  and  high  torque. At the time of  writing, formulations  are  available  which  satisfy  both  conditions, but a  few  consumers  are  somewhat  dubious. 

Automatic  Transmission  Fluids (ATF)  have  somewhat  the  same  history  and  resulting  solution  as  in  the  case  of  hypoid  lubricants  at   an  earlier  date. Since  the  type  of  fluid  used  is  rather  critical  for  proper  performance  and  there  was  no  wide  distribution  of  a  suitable  fluid, the  motor  car  manufacturers  at  first  provided  the  lubricant  under  a parts  number. Within  a  matter  of  a  couple  of  years  after  the  introduction  of   automatic   transmissions  on  various  cars, the  oil  companies  were  able  to  offer  approved  ATF  quite  generally.