Flour Milling and Gear Lubrication

Up to time grain enters the mills, conveyors are used for handling. These  may  take the  form  of screw,  bucket, ribbon or belt and  any  one of these  types can be  driven  by gear  reducers. A  turbine type of oil  having  a viscosity of 500 SUS  at 100 degree F,  that  is AGMA  No. 3 grade, can  be  used  throughout  these  gear  reducers. In  the  northern  states  this  oil  should  have  a pour point  of zero or lower. An  oil of this  viscosity  will not cause excessive  power   loss and  yet  it  will  protect the  moving  gear  teeth.

Gear motors may be used for some conveyors and blowers. The  same  type  and viscosity of oil  should  be  satisfactory  for the  bath  lubrication  of such  gears, especially  if bearings are  serviced from the same  source. Where the drives in gear motors run quite warm, an oil of about 750 viscosity SUS at 100 degree F or an AGMA No. 4, may be desired. If such motors have a plate showing the recommended viscosity of oil, this suggestion should be abided by.

A tight  housing  is  essential  in any  of the gear cases in flour mills, more  from  the  standpoint  of prevention of contamination  from dust  than  from  leakage. Since  the oil  level  in gear  cases  should  be  inspected  every  month  or  sixty days, care  should  be  exercised that  dust  does  not enter  when  the  filling  plug or cap is removed.

Open  gears  are not  used too  often  around  flour mills; but if  these  are  encountered, it  is wiser to use  a  light  oil  as the  lubricant  rather  than  a  residual type. This can be the same oil as suggested for use in conveyor gear reducers, that 500 viscosity SUS at 100 degree F. As  such an oil  becomes  mixed  with dust, the paste formed will  slump off rather  than pack  in  the  bottom  of gears; thus, misalignment should  not be a  problem.  

Flour mills  may  be  self contained, in that cleaning, tempering, grinding and  sifting may  all take  place  in one  enclosure, or the latter operation  may be  separated. Such machinery will vary, but often the rolls are driven by gears. Here again the turbine oil type 500 viscosity SUS at 100 degree F can be used. Machinery handling middlings or bran will be much the same as previously mentioned and if gearing is used, the same recommendation will hold.  

In the manufacture of corn meal or animal feeds the same type of processing and machinery will be found. Thus, conveyors  and screens  will have  similar  drives but the  crushing or milling  can be  by  rolls  or  discs. In any event the gear oil used can be the same type and grade as recommended for flour mills.

It will  be noted that  a simplified  lubrication  application for  reduction  gears  is suggested  in grain mills, that is, a  single  oil  throughout with one  exception. This  is in  gear motors and even here  the  sanction  of the motor manufacturer  might  be  obtained for use  of the  500 viscosity  oil.

High speeds

The term “high” of course is relative but with pitch line velocities of several thousand feet per minute the lubrication of gears is not simple. Naturally, a low  viscosity oil is  required and the problems are : to have  assurance that a film  of oil  is present when the gear teeth  mesh; to have  an  abundance of oil to remove  heat; and to be  sure that the leaving  oil  will  get out  of the way of fast moving gear teeth. If  a problem is  encountered  as to delivery of oil  to the  mating  surfaces  of high speed gears, the  experience of Dern^20 may  help. This  investigator found  that  when  gears  run at 16,000 to 18,000 feet  per minute, “more satisfactory  results may  be  obtained by   spraying oil radically into  the  teeth  of both gears  at a point as close  as  possible to the mash”. For the purpose, one or two jets of 0.040 to 0.060 inch in diameter, delivering a solid stream of oil, were used. Where the pitch line velocity was 20,000 to 25,000 feet per minute, jets on the leaving side of the gears removed most heat.

Trouble may be encountered with high speed gears churning the gear oil which in turn causes heating. This  is one  reason directional  baffles or  even  a shroud around  the gears  may be necessary in order to keep leaving oil away from the gears  and directed toward a gear case outlet. 

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.                              

Methods of Application of Gear oils to Heavy Duty Gears

Older equipment employing heavy duty gears may make use of bath systems   for application. Many  plants  have  converted  such  systems  to  circulating  application such as  is used  on most  modern  machinery. By  this  means, oil sprays  can  be  directed  to  the line  of  meshing  of the  gear  teeth and  if  necessary the  oil can be supplied  to  other  machine  components, such  as  bearings. By  this  means, when  accompanied with  proper auxiliary equipment, such as coolers, filters, settling tanks etc., the life of the oil is prolonged and contaminants removed.

Load Carrying Ability of Lubricating oils at 400 Degree F

Federal Test Method 6511 describes a procedure for determining load carrying ability of lubricating oils at 400 degree F with respect to gears. Using  a  modified  Ryder  Erdco  Test  Drive  System operating  at 10,000 rpm, the  lubricant  to a  series  of  400 degree F controlled tests  at increasing  gear tooth  loads. The  teeth  of one of the  gears  are   then  examined  to  determine  the  scuffing  area. The load carrying ability of the lubricant is rated in accordance with the per cent of the tooth working area scuffed.

Reserve Lubrication on Gear Teeth

The problem of providing reserve lubricant on gear teeth has been considered. A method suggested  for  ball  bearing  assemblies, to pack compositions containing  solid  lubricants  in depressions of  retainers, inner rings, etc., so that  reserve  lubricant  would  be available, seems out of  place in gears. The solids used were graphite or molybdenum disulfide or mixtures of the two. While solution of a problem of this nature is desirable, immediate results are not apparent.

                                                                  

                                                                       

Problems of Feed of Gear oil to Bearings

At  first  thought  the  oil  in gear  cases  is there  primarily  to  serve  gear  sets. However, lubrication of bearings is often of equal importance. In circulating systems an oil supply can be provided for bearings no matter what the location. Bath or splash systems pose a different problem. Here troughs may have to be provided to direct oil to bearings, or flingers dipping into the oil, in addition to the gears, may be necessary.

If  the gear  runs on a  bearing and oil  holes  are drilled from  the roots  of the gear teeth  with  the  thought that  this will provide oil, Merritt^34  mentions  that this  will be  quite  deceptive. Thus, the oil will flow outward due to centrifugal force and will flow inward only if the gear is stationary. Such holes, if they  do not  seriously  weaken  the gear  teeth, can  be used  to lead  oil  outward  from  a supply  at the  center  of a  hollow shaft.

Lubrication of starter Gears in vehicles

The perimeter of the flywheel has gear teeth which are engaged by the starter. Since the use of  this  gearing is very  intermittent, a little  multipurpose  lubricating  grease  at  long  intervals  will provide  sufficient  lubrication.

Motorcycle Gear Lubrication

While  some  models  of  motorcycles  recommend  special  motorcycle oil, the  transmission  of such  vehicles  often  use  the  heavier  grades  of  crankcase  oils. For low temperatures, SAE 30 or in some cases SAE 20W oil is suggested and for higher temperatures SAE 40 or 50. The recommendations of the  manufacturer  should  be  noted  and  followed  both as to  grade  and quantity  of transmission  oil. In at least one instance  the transmission is combined with the crankcase.

Diesel Locomotives –tractions Gear-Lubrication

Most traction gears on diesel locomotives are lubricated with high viscosity, adhesive gear oils. Residuum  type products  with  viscosities    as  high  as 500 Furol  at  210 degree F    have  been  used, but  in most  cases  a viscosity  of  2000 SUS at 210 degree F is  satisfactory. About  fifteen  pounds is  required  for an  initial  fill  and  since there  is  some  leakage,  the  gear  case  should  be  checked and refilled at  least  every  month. U.S. specification MIL-L-13914 (ORD) provides for two  grades of such oil, one  with  a viscosity  of 170 to 240  Say bolt  Furol  at 210 degree F and  the other  with a similar  viscosity of  480 to 580. A lubricating  greases is  recommended for the  purpose by  Morway  and  Ball^38  in which  the  thickener  is a  complex  sodium  soap  and the fluid is a mixture of  14.93 parts of a  low  cold  test oil  having  a  viscosity of 55  SUS at  210 degree F  and 75 parts of a  resin  from a Pennsylvania  oil  having a  viscosity  of  14,000 SUS at  210 degree F. When  such a  lubricant was used  in a  locomotive  in  freight  service  it did  not  leak  from  the gear  case, and  after  one  year  of  service there  was  only  slight  wear  on the  pinion  and driving  gear  teeth. 

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.

Bath or Splash Application of Gear Lubricants

Enclosed gear sets are often lubricated by bath or splash application. For this purpose, the gear oil is much more fluid than that used for open gears and hence not only lends itself to such methods of application but also acts as a coolant. Such systems are cleaner and more economical than many methods of application since only a tight housing is required.

These methods of lubrication are automatic since the oil feed starts with the start of the gears and ceases when they are shut down. These systems are efficient and reliable if the oil is kept at the correct level. Further, a minimum of attention is required.

Experience and location of the gear sets dictate the size of the gear case. Where the cases are small compared   to the size of the gears, as is the case in most vehicles, the amount of oil is also restricted with a consequent restriction in cooling effect. In addition, close fitting gear casings can cause excessive oil drag which in turn will increase heating.

Satisfactory lubrication is obtained by bath or splash methods of gearing up to peripheral speeds of 2500 to 3000 feet per minute and occasionally at higher speeds where the operating time is comparatively short.  The amount of oil carried to the point of mesh will depend upon the gear size, the speed, and the viscosity of the   lubricants and to some extent upon the construction of the gear case. As speeds increase, a point will be reached where centrifugal force will overcome surface tension and viscosity and most of the oil will be thrown off. This may finally reach a point where little lubricants will adhere to gear teeth at the point of mesh. In case an excessive amount of oil does reach the engaging teeth operating at high speeds, the lubricants will be displaced at considerable velocity and cause noise. According to Merritt^37 this condition is likely to occur with double helical gears with continuous teeth operating with the apex trailing.

Such methods of application may include the following:

(1)    By bath where the fluid is placed in a sump or bath in the bottom of the enclosed gear case. One of the gears dips into the bath and as this gear rotates it transfers the lubricant to the contacting teeth. Any excess is thrown against the housing and is guided, by means of troughs, either into the bearings or back to the sump. It is important that the gear case should not be too full and that the oil level be determined, if possible, when the gears are idle. Excess lubricant leads to foaming and causes excessive drag and heating. It is suggested that in industrial gearing the oil level should be such that the bottom gear dips into the oil about three times the depth of the tooth spaces. Automotive vehicles generally provide a filler opening and the gear oil should be up to the bottom of this opening; or, if a filler neck is used, the oil should be at the top of this filler neck. It is important that vehicle gear cases be filler when gears are stationary and also that the car or truck is on a level floor.

(2)    By splash, using either one of the gears or an auxiliary flinger which dips into the bath of gear oil and throws the lubricant toward the top of the gear case so that it drops back into both gearing and bearings. Merritt^37 states: “ All dip lubricated gears produce  different oil levels  at different points of running, and the running levels indicated by an oil level  gauge  may be either  higher or lower than the standing level.”

This again shows the necessity of still filling gear cases.

(3)    By idler gear. In some instances, particularly on slow moving gear sets, an idler gear dips into a sump and transfers oil to the contacting gears.

While mention is made of bearing lubrication, in bath lubrication the gear oil may or may not also serve the bearings. It is important to recognize this in determining what viscosity or grade of oil to use since bearings normally require lighter oils than do gears. A compromise as to viscosity may therefore be necessary if the lubricant serves more than one function.

                         The lower teeth of exposed gears are sometimes encased so as to permit bath lubrication. Such application is only practical when speeds are relatively slow, otherwise considerable oil would be thrown out of the gears

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.

Types of gears to be lubricated

Where   gears  are  on  parallel  axes, either  spur  or  helical  gears  are  generally  employed. Either type can be used as external or internal drives.   The  herringbone  gear  is  similar  to  two  helical   gears  having  reversed   directions   of   spiral, placed  side  by   side  so  that  the  teeth  come  together   to  form  a  chevron   pattern. The rack and pinion, used to convert rotary motion to reciprocating, generally   uses a spur gear.

For   intersecting   axes either straight   bevel    or   spiral   bevel   gears   are used   as a  rule. The   latter type may be used   on angle   drives where   the   shafts    do not    intersect at full   90 degrees. The contact   of the   teeth in such gears gives   a rolling motion. With  non intersecting  and   nonparallel  axes  the  types   of   gears  used  are  crossed   helical, single  enveloping  worm, double  enveloping    worm, or  hypoid. Here the  contact  of  the  teeth  gives  a  sliding  as  well  as  a  rolling   motion. In  most  cases  a  gear  set  will  be  used  to  change  speed, and  in  such  cases  the  smaller   gear  is  designated  as  the pinion. Both  the  number   of  teeth  on a  pinion  and  the  ratio    of  the  teeth  on  the  driving   and  driven   member   may  vary ,   but  with  bevel  gears  there  is  seldom  less  than  12 teeth  to  a  pinion.

While   some  spur  and  straight  bevel  gears  are  still   made  of  cast  iron, the  tendency  in  all   types   of  gearing  is  for  the  use of  steel. Exceptions   will  be found  to  such  practice, for  example,  in the   use  of  bronze  for   one  member   of  worm   gears. Some small gears and even   larger pinions are   made   of plastics, such as ‘Delrin,’’ ‘Nylon.’’ ‘Teflon,’’ etc. Pinions  have been and   may still be  made  of  rawhide, pressed  paper,  etc, but  our  concern  is  primarily   with   lubrication  of  metal  gears.

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.