Moisture Corrosion Characteristics of Universal Gear Lubricants

A full  scale axle  gear  unit is used  for this test. Before the test , the  parts  are cleaned as follows: immerse   in a 1 per cent solution of Sodium Hydroxide  for  1 hour. Rinse  the  parts with water  until free  of caustic. Dry  the parts  by  rinsing  with  methanol, followed by  air-blowing to  remove  the  methanol. After  the assembly, 28 ml of  distilled  water  is put in the axle housing followed by  filling with  the gear lubricant to the  level plug. The  propeller  shaft is  rotated  at a speed of 2,400 rpm  for  4 hours while  the  oil  is maintained  at a temperature  of 180 degree F. At the  end of the  4 hour period, the  drive is stopped and  the  axle  test  unit  is placed on the  storage  rack  without  draining  the  lubricant. There  it  is allowed to remain for a  10 day period at room  temperature.

At the  end of the 10 day period, the lubricant is  drained and  the  axle  disassembled  for  inspection. The  rating  of the  lubricant shall  be based  upon  visual  inspection for  rusting  of the  gears, the  pinion-gear  bearing,  the  differential-carrier  bearings and  the  differential-gear  thrust surfaces.     

Deodorizing Gear Lubricants

Straight mineral oils should have no offensive odor. Lubricants containing EP agents will  often  have odors which  are not  too pleasant. However, when in an  enclosed gear case  this will  not  be  particularly  noticeable  unless the  operating  temperature is high. If a problem of this  nature is encountered, the  first  step will be to  see if addition of another  odoriferous substance will cover up the first  scent. Thus, pine oil might serve for this purpose. Addition of chemicals which might  change  the  odor of EP lubricants  should  be made  with  caution because  they  might  interfere  with  the EP value  of the  composition.

Aniline Point of Petroleum Products

Aniline point is the minimum equilibrium  solution temperature, generally given  in F although C is also used, for  equal  volumes  of  aniline  and  oil. Aromatic  compounds in oils  contribute to a low  aniline point  which  in  turn  indicates that  such  an  oil will  soften or  swell  both  natural  and  most  synthetic  rubbers. Therefore  to  insure  minimum  action  on rubber  gaskets, seals, etc., oils  with high  aniline  points  should  be  selected. Solvent refined oils  as a  rule  satisfy  such  requirements. 

SAE Extreme Pressure Lubricant Testing Machine

Since  investigators  often  refer  to wear  tests using  the SAE EP Lubricants  Tester, this  apparatus  deserves  mention at  this point. The machine  was  constructed  under  the  sponsorship  of SAE  by  the  Bureau  of Standards. The  action  is  presumed  to  imitate  the  rubbing  of  gear  teeth  by  rotating  two  Tim ken  test  cups (T-48651)  in line  contact  with  each  other  and  in  opposite  directions, under  controlled  speed, slipping  velocity,  temperature  and operating  pressure.

The  conditions selected by  Calish  for wear  tests  on an SAE  machine  were: 500 rpm; 3:4:1  roll ratio; 180 lb  load; 225 degree F; 500 ml /min  flow  rate; and  4  hours  test  duration. This author makes the following comment relative to this test:

“ Experience  in the  laboratory  and  from  service  indicates  that  it  is  desirable  to  hold  the wear to less  than  30 mg  in the  test  consistent  with  other  desired  oil  properties.”     

The Timken test cups can be weighed before and after a run and thus the weight loss in mg determined. For  wear tests on an  SAE    EP Tester  the  conditions  should  be  modified over  those  used  to  determine EP values. Such conditions are speed, rubbing ratio,     pressure and perhaps temperature.                              

Water in Petroleum products and other bituminous materials

This  method  is  intended for  use  in the  determination of water in bituminous  products  which  would  include  asphaltic  residua  used as  exposed gear  lubricants. A sample  of  the  product in  question is  diluted  with  a solvent  and  distilled  the  water  being  caught  in a  graduated  trap. Water in gear oils should only be present as a contaminant. A qualitative test  for  moisture  in gear  lubricants  consists of  holding  a  sample  in  a  beaker or  can  on a  hot  plate  and  when  the  temperature  reaches  about  212 degree F, observing  if  foaming  takes place.

Tests  for Other Characteristics of Lubricating Oils. Before  lubricating  oils  are  selected for  use  in  compounding  specific  types  of gear lubricants,  other  characteristics may  need  to  be  considered. For example, compatibility with   the other ingredients of a composition is very important. This and other qualities will be mentioned as need arises.

Viscosity Index Improvers

While  the change in viscosity  of lubrication oils with  change  in temperature  can  be  reduced  by  the  addition  of  certain  long  chain  polymers, such  use  in gear  oils  is not  common. Such  polymers when  subjected  to   the  shearing  action of  gear  sets  degrade  and  form  shorter  chain  compounds  which are  less effective  than  the  original  additives. These agents may also function as pour depressants. The  reason  the  polymers in  question  are  effective  is that  at  low temperatures  they  are  coiled  up  and  only  colloidally  dispersed. As  the  temperature  increases  the  polymers  uncoil and  go  into  solution  to  increase  the  viscosity  of the mixture.

Various Fluids as Gear Lubricants

While a  number of  viscous fluids have no  doubt  been used  as  gear lubricants, most of these, other than the  petroleum or synthetic  oils, are  deficient in  desirable  characteristics. However, glycerol has been   suggested as a lubricant and carrier for molybdenum   disulfide. The  specific  application in this case  was  on  small gear sets of  the  worm  and  hypoid  types. Also, molasses was used as a transmission gear lubricant in France during World War II.

The London Transport Company has experimented with   an inhibited castor oil in the axles of certain buses. The thought behind this use was to reduce the fuel consumption of the vehicles. Low viscosity oils or  synthetic fluids were  previously tried but  with  such  lubricants the  necessary  damping  effect was  absent so that  proper shifting was  not  possible. Caution  is given that  this  application of  castor oil is only  possible  under  the  stop and go  operation of busses and  would not  be possible  in over the  road  vehicles. The axle in this case consists of a worm drive with a bronze worm wheel.

Gear pumps depend upon the fluid being handled as the lubricants, and this is sometimes water. Also, water has been employed as lubricant in the case of some nonmetallic gears but has its limitations. First, the temperature range over which water can be used is limited. Next, it will contribute to rusting of the ferrous parts with which it comes in contact unless inhibitors are present. Further, water affords little protection against wear.

Selection of Automotive Gear and Transmission Lubricants

The  manufacturer  of  automotive  equipment  almost  universally  specifies  the  proper  gear  or  transmission  oil to  use in a  given  model. Few  users  deviate from  such  recommendations; and consequently, information relative  to suggested lubricants  is  widely  circularized, not  only  in  instruction  and  bulletins  for  customers  and dealers, but  also  in wall  charts  which  are  provided for  service  stations  and service  departments dealing with automotive  vehicles.

Such  charts  use  coding  with  notes starting  what  the  code  letters  signify. For example, the following are used in lubricating charts which have wide distribution:

AF         –   Automatic Transmission Fluid, Type A

AFA      -   Automatic Transmission Fluid, Type A, suffix A

EP          -    Extreme Pressure Gear Lubricant

GL         -    Straight Mineral Gear Lubricant

GL4       -   Multipurpose - Type Gear Lubricant

HF         -    Hydraulic Transmission Fluid, Type C

HP         -   Hypoid Gear Lubricant

MP        -   Multipurpose Gear Lubricant

WL        -    Worm Gear Lubricant, Heavy- Duty

In addition to the  above  coding, other  designations of gear and  transmission lubricants will be  found  in instruction manuals and in  lubricating charts, and it is wise  to  read  any  footnotes and  conform  to the  recommendations. Thus, a gear lubricant meeting the requirements of Military Specifications MIL-L-2105 or MIL-L- 2105B may be required. Many gear oils, offered by service stations will meet such requirements as well as   classify under EP, GL4, HP and MP. However , it might  be  noted  that GL4 is an  API designation  and that products so  designated  are  generally  accepted  as of  higher  performance  level than  those  qualifying  under MIL-L-2105. If  in doubt as to the  classification  of a gear  lubricant,  the oil  supplier will conform  the  fact that certain of  his  branded products  satisfy  stated requirements.

Since  all  gear  oils which  can  be  classified as EP, GL4, HP or MP will not  contain the  same  EP  additives, their  performance in service  may vary. Also  the period requiring  the most  effective  action from  EP additives is during  the  break  in of the gearing. Consequently,  a number  of  automotive manufacturers use a  gear  oil  containing  an SCL  additive  in the  initial  fill  of axles. This SCL indicates that the additives contain sulfur, chlorine and lead. Use  of this  particular combination in factory fill does not  mean  that some  other  combination of additives would not be  satisfactory  but does  indicate that  the  manufacturer is  reasonably satisfied  with  what  they use. Most  EP  lubricants  are  compatible  and  consequently  makeup  oil  or  refill  need  not  be of the  identical  composition as the  oil  in the  gear  casing. Also, after  the  break in it is possible  to  adds  a  small  proportion  of  straight  mineral oil, provided the  oil  level  is low  and no  other  oil  is  available, without  damage to the  gears. This is not  advisable, and  as  soon  as the  proper EP  oil is  available, the  diluted  oil  should be  removed  from  the  gear  box  and  a  refill made  with the  proper lubricant. It  might  be  kept  in mind  that  manually  operated  transmissions do not  require  an  EP oil. The latter type of lubricant is used largely as a matter of convenience and uniformity. More will be said about this subject when   consideration is given to choice of oils for trucks.

A cooperative  effort has been  underway  for  several  years to  reduce the  number  of  specific  axle  and transmission  lubricants so  that  a  service   station or service  department  would  have  to stock  only  one  truly  multipurpose  gear  lubricant. Gear  oil  conforming  to  specification  MIL-L-2105B   are  said  to  solve  this  problem  since  such  lubricants will  satisfy  vehicle   operation of gear  under  conditions of both  high  speed  and low  torque  and  low speed  and high  torque.  Several reasons preclude the adoption of a universal gear lubricant for automotive vehicles. Thus:  (a) the possible variation in automotive equipment may demand a like variation in lubricants. (b) no sooner is  a  composite  demand  for  such  a product  met  than  an  improvement  or  change  in  mechanisms  necessitates  a  corresponding  change  in  the  lubricant  and (c) all organizations  do not  obtain  identical  service  results  with  a given  product. Added  to this  is the  economic factor  in that  there  is no need  to  use  more  expensive  lubricants than  those  which  have been  found  satisfactory.

Since the  demands  required of gear  and transmission lubricants  are  not static, service people and  owners  of  vehicles  will do  well  to  conform  to the  latest recommendations of the  manufacturer of the  equipment. Many  vehicles  are  used  under  widely  varying  climatic conditions and  consequently  variations in  viscosity of gear oils are often desirable.

Oil compounds have so  improved the  viscosity temperature  characteristics of most  grades  of gear  oils that  it is only under  extreme  conditions  that  seasonable  changes of  automotive  gear lubricants are  necessary. Also multi grade gear oils are available from some sources. The  U.S.  Department of Defense provides a  sub zero  gear  oil,  equivalent  to about  an  SAE 75, which  is  designed for  Arctic  conditions. Under  such circumstances, civilian  automotive  operators  are prone  to dilute the  lubricant with a  little  5W  motor  oil or even  kerosene. If this is done, the gear case should   be drained and refilled with the proper grade of lubricant with the advent of warmer weather.

Where changes in grades of automotive gear oils are desirable, due to temperature changes, the practice is not uniform. Therefore, SAE^47  makes  the  following  suggestions: “In  recommending  gear  lubricants based  on lowest atmospheric temperatures the  following are suggested as a uniform practice in making such  recommendations: plus 32 degree F, plus 10 degree F  and  minus 10 degree F”.  

Rubber Swell due to Gear oils

Where gear  oils  are  in  contact  with  rubber gaskets, seals, etc., the  action of  the  fluid on the  rubber compositions is of  importance. Consequently a Federal Test Method, 3604-T, has been set up. Such a test  can  be  varied as to  temperature, time and the  polymer     composition tested  depending upon  the    actual  service conditions of the  gear oil.

Thus, for ATF the following procedure is used. Sections of piston seals made from Acadia 3612 rubber compound are subjected to hot transmission fluid. Measurements of volume and hardness are made before and after the test. Two  seals are  cut  into  eight  sections each  and  the  sections  are  suspended  in  test  tubes  containing the lubricant and  held in a bath  at  300 degree F for  70  hours. The  sections are  then  cooled for  thirty  minutes  to  one hour in a  fresh  portion of the  test  fluid maintained at room  temperature. After wiping dry, the final volume and durometer readings are taken.

Pour Point Depressants

Certain  polymers when  added to wax containing lubricating oils in  proportions of 0.1 to 5 per  cent, will  alter  the  crystal  structure  of the  wax  and  thus  permit  movement  of the  oil  at a much  lower  temperature  than  before  the addition. Some  agents  used  for  other  purposes, for  example V.I. improvers, may  have a similar  influence on  pour  points.

Most  industrial  gear  oils do not  require  this  type  of  additive but  pour  depressants are  often  included in  automotive  gear  lubricants. The agents  normally  used  include  “Acryloids”  which  are  methacrylate  polymers,  and  “Paraflow”  or “ Santopour”  which  are  wax condensation  products with either  naphthalene or  phenol. 

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.

Emulsion Tendencies of Lubricating Oils

Federal Test Method 3021.5 is intended for use in the determination of the emulsifying tendencies of lubricating oils. The test  consists  of  placing  40 ml each  of the  test  oil  and  of  either  distilled  water or  a one  per cent  salt solution in  a 100 ml graduated cylinder.  The  cylinder  is  then  placed in  a  bath  regulated to  either 130 0r  180 degree F and  the  contents  stirred with  a paddle  at  1500 rpm  for  5  minutes. Following this, while  still  maintaining  the  cylinder at  the  test  temperature , the time  required for  the  emulsion to  break  is  noted.

In  the  case  of  circulating  oils where  the  lubricant may  become  contaminated with   water, a firm emulsion  would  interfere  with  both  circulation and  lubrication. Therefore, under such  conditions  of  operation,  oils  which  show  little  tendency  to  emulsify  are  desirable.

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”.

Spray or Jet Application of Gear Oils

Fluid lubricants can be applied to gear sets by sprays, which are air borne, or by jets of the liquid alone. Such methods of application are employed when the peripheral speed exceeds the limits for splash lubrication or when the gears are not adapted to the dip method. The size and shape of spray patterns can be controlled by the use of different type of nozzles, the distance from the nozzle to the target, or the pressure and temperature of the lubricant. If the gearing has a wide face, two or more jets can be used on one gear set.

 Since one function of the gear oil is to cool as well as lubricate, a fan shaped spray or jet should spread the oil across the entire gear face. Adjustments of such sprays are a matter of experience, and firms supplying such equipment can give the best advice. For any cases, except exceedingly high speed, most authorities prefer to apply the oil spray on the incoming side of the gears so that the oil will be spread across the entire gear face. For moderate speeds the application should not be directly at the point of mesh, due to air turbulence. However, Dern^15 suggests that, for pitch line velocities above 16,000 to 18,000 feet per minute, the most satisfactory results are obtained  by spraying a solid stream of oil radially into the teeth of both gears at a points  as close as possible to the mesh. Further, in speaking of gears operating at pitch line velocities of 20,000 to 25,000 feet  per minute, Dern^15 agrees that  such  gears should be lubricated  on the leaving side of the mesh  where  the oil will  do the most  good as  a  coolant and not contribute to high dynamic loads.

In high speed operation of gears, lubricated by jets of oil, scavenging of the lubricant is sometimes a problem. Special  baffles or even a  change  in gear  case  design  may  be necessary  in order  to remove  considerable  volumes of oil  rapidly  enough  so  that  interference  with  gear  operation  does  not  result.

Automatic  spray  application  is used  on open gears where a controlled  amount  of  lubricant is  sprayed  over the gears at  desired  intervals. By heating  the oil, high  viscosity  lubricants can be  used  or  better  still,  cut  back oils  are applied. Gesdorf^21 offers details of such applications and the problems involved in the development and use of the same.

For  high speed  worm gears, which tend  to throw  oil  rather than  carry it to the  mesh, a  high velocity  jet stream  of  gear oil  is more  satisfactory  than  a  spray. In fact  two jets can be  used, one striking the worm  wheel  teeth  in a direction  parallel  to  the axis  and  the  other in a  direction  perpendicular  to  the  axis.

Viscosity Temperature Characteristics of Gear Lubricants

Since gear and transmission oils are subjected to widely varying temperatures, particularly in vehicles, it is desirable that such lubricants have as little change in viscosity with temperature changes as is possible. In other words, high V.I. oils are desirable in many gear oil applications and absolutely necessary in certain cases. An illustration of the latter is automatic transmission fluid. Base oils with V.I. values of 90 to 95 are readily available when needed. Where it is desired to increase the V.I. of oil, additives, known as V.I. improvers, are sometimes used. These agents are generally  polymers  which act by either coiling  up or  becoming less  soluble  at  low temperatures but reverse  this action as temperatures  increase,  so as to  contribute increasing body at higher temperatures. In normal gear  operation the shearing  effect  tends to break  down such polymers  into those  of lower molecular weight  which  are  less  effective  as V.I. improvers. However, these agents do have a place in services such as ATF where they maintain their effectiveness.

Certain  high molecular weight  petroleum resins  have been found to improve  the V.I. of some lubricating  oils  and  not  have  the defect  of  breakdown with  shear.

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.

Reduction of friction in gear operation

Bowden  and  Tabor^11  have shown  that  the coefficient  of  friction  for  one lubricated  surface  moving relative  to  another  will  depend  upon  the material  used  in the two  surfaces, the type  and  velocity  of  motion between  the  surfaces,  and  the composition  of  the lubricant. The  first  two factors  are fixed  by  machine  design, but the gear  lubricant  can be altered so  as to aid in providing  desirable  friction  characteristics. The  contribution  of  this  last factor  is  evident  if  we  consider  that the  coefficient  of  friction  of lubrication  metal  surfaces  is  about  1.0  while in  the  case of  such surfaces, lubricated  with  a  boundary  film,  the value  is  about 0.05  to 0.15.

Any explanation of friction is based on the fact that there are irregularities in surfaces. As one such surface moves relative to the other there is an interlocking of the asperities of the two areas. The role of the lubricant in overcoming boundary friction may be partly by filling in the low spots and in the case of a thick film, of lifting one surface over the other. However, under the best conditions, there will be some interlocking of the high spots. If, in so doing, some asperities plow through those of the other surface this will cause one type of friction. The remainder of  the friction  may be a result  of  welding  and  shearing  of  minute junctions between  the two  surfaces.

We are concerned, in the main, with two steel surfaces in rolling or sliding contact. While the thought has been advanced that rolling friction is essentially independent of the lubricant used, gears in operation, particularly hypoid or worm gears, have both a rolling and sliding motion. An example of the variation of friction with the type of motion concerns first a steel rider sliding across a lubricated plate of the same steel to give a coefficient of friction of about 0.14. With the same combination of materials, the rolling friction coefficient was about 0.00015.

Other operating variables to be considered are load, speed, and temperature. Rounds^45 using a test machine in which ball bearings had both a rolling and sliding action and, thus, simulated the action of gear  teeth, concluded that, in general, the coefficient of friction decreases as either the oil  temperature, the ball velocity, or the load  increases. However, the magnitude of the oil temperature and ball velocity effects tend to decrease as the load increases.

In choosing a gear oil one is faced with a compromise because the lowest viscosity oils have the least internal friction and, yet, high viscosity oils will maintain the most satisfactory film to prevent metal to metal contact, particularly at low speeds. This is true when hydrodynamic or thick film lubrication conditions prevail and where viscosity is the most important property affecting friction. While a condition of low friction is desirable in a lubricated gear set, it must be kept in mind that there is not necessarily a correlation between friction and wear. Beeck^3 has suggested that this may be due to the fact  that while wear takes place momentarily  at isolated spots, friction is ordinarily measured as an average of a large area and a longer time interval.

Irrespective of viscosity, various types of mineral lubricating oils may give different friction values. Rounds^45 found that naphthenic base oils gave higher friction values than paraffinic base oils, even after limited attempts to change the friction properties by fractionation or additional refining. In this investigation, the most commonly used synthetic fluids gave friction values similar to straight mineral oils. Since paraffinic oils show less increase in viscosity with pressure than do naphthenic type oils, this may account for the lower friction values of the former type of oil when used in lubricants. The discussion above has been concerned with kinetic friction of fluids containing no additives. When boundary lubrication conditions exist, nonreactive gear oils will not suffice; therefore, consideration must be given to lubricants containing additives, Rounds,^45 using a  naphthenic type oil as a base, found that different additives  would  lower kinetic friction and/or raise or lower static friction. Fatty acids and related compounds were the most effective of the agents investigated for lowering static friction. To lower kinetic friction at velocities above 100 fpm, reactive chlorine and sulfur compounds were most effective.

However, the type of service which demands the use of EP gear lubricants nullifies reduction in friction. For example, Bisson et al.,^5  when investigating the effect of  chemical reactivity of  lubricant additives on friction and surface welding at high sliding  velocities, found some decrease in friction up  to about 1300 fpm, when using  p-dichlorobenzene, followed by  a  very  abrupt increase in the coefficient  of  kinetic  friction. Thus it  was  found that :For all  additives, the existence  of  a  critical  sliding  velocity where  the friction  coefficient  increased  and surface welding occurred  was verified.

Kinetic friction in fluids causes heating and thus power losses. The  friction  due to gear oils  themselves  may be film friction, which  has  been considered, or churning  friction  which occurs  as  the  gear  teeth  rotate  in the oil bath. Other things being equal, gear oils which will have the lowest internal friction will be the most desirable from a power efficiency standpoint.

In connection with friction, consideration might also be given to the oiliness characteristics of gear lubricants. The following definition was adopted  by the SAE  Crankcase  oil  Oiliness  Committee  in  1937: Oiliness  is  a  term  signifying differences in friction greater  than can be  accounted  for  on a  basis   of  viscosity  when  comparing different lubricants  under identical  test conditions.

This quality is  desirable in  boundary lubrication  and  is  no doubt  due to adsorbed layers  on  the metal  contributed by  polar compounds  such as fatty acids, esters  of  fatty acids, metallic soaps, some  organic  compounds containing  chlorine, nitrogen, phosphorus, or sulfur, etc. Unfortunately there is  no  standard method   for  measurement of  oiliness.