Dynamic Loading of Automotive Hypoid Gears

While the  tests just described  permit  rating and screening  of  gear  oils  for  various purposes, an illustration of  further tests, their result  and  the  conclusions reached,  indicate  both  the  possibility of  extension of   full  scale and road  tests  and  also  the  extreme conditions to  which  gear  oils  can be subjected. Powell  and  Barton  reported  the  results  of an  investigation  relative  to the   magnitude of  tooth   loading  in  hypoid rear axle  gears  under  normal and  severe operating  conditions. For  the  purpose  they  used  a   6-cylinder  passenger  car  with  a  pre- war  torque drive; a drive   system   duplicating  that  of  this car  on a  “T”   dynamo meter  stand; and  Army  M-37  truck  with  a 4 ton  load  plus  a   1.75  ton  trailed  load; and  an  Army  M-211  truck with  a 12 ton  load  plus  a 4 ton  trailed  load. The conclusions reached were:

      (a)    With  heavily  loaded trucks  the  climbing  of  steep  grades  can  increase  the  ring  gear  torque         as  much  as  13 to  24  times  that  of  level  road  under  steady  speed.

      (b)   The addition of trailed loads causes large increases in gear torque.

      (c)    The  highest  gear  loads  produced   under  normal  operating   conditions  occur as  a  result  of     gear  shifting.

      (d)   Engine  misfiring  resulting  from  spark  plug  fouling  causes   repetitive  gear  loadings  equal  to  the  60 m.p.h. drive  side  shock  in  the  CRC  L-19  test.

      (e)   Changes  in  rate of  throttle  opening  result  in large   differences in  the  drive  side  shock  loading  of  the  gears. The  substitution    of  electric  solenoid  throttle  controls  for  hydraulic  controls in the  CRC L-19  test  nearly  doubled  the  drive  side  shock  torque.

      (f)     Rate  of  throttle  closing  appeared  to  have  little   effect  on  the  coast side  gear  shock  loads.

      (g)    Shocks  imposed  in gear  shifting  of  heavily  loaded  trucks   on  steep  grades  produce  severe  lubrication  requirements  by  imposing  large  stepwise  changes  of loading  on a   contact  point  in one   revolution  of the  pinion.

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.

Transmission and Axle lubrication

Since  the  lubrication of  transmissions  and axles  is  of  primary  importance  in  automotive vehicles, as gearing  is  concerned, this subject  will have first  consideration.  Mention of the lubrication of other gear sets in such vehicles   will then follow:

Also the major discussion will consider internal combustion engines as a source of power. Other  sources  of power  and methods other  than  gearing  for  driving  vehicles and the    consequent fluids  require  will also  receive  mention. Gearing  or  transmission  mechanisms  of automobiles, buses or  trucks  are  lubricated, in the  main , by  bath  or  splash  systems  with some   use of  forced  feed. The gear  cases are  generally  as small  as  possible  so that  consequently  the amount  of gear  oil  present  is  restricted. While  the fluids  present  in gear  boxes  may have  other uses, the primary  functions  are  to  prevent  wear  of gears  and to  act  as a  heat  transfer   medium. The  general  subject of wear  prevention  has been  discussed  previously  and  may  be  due  either  to  the  oil film  alone  or to  this fluid  supplemented by EP  films. The  oil  in gear  cases  of  vehicles takes  up heat  from  the  moving  gears  and transfers  it to the  inside  of the casing. Form here it is dissipated to the surrounding air. Such  radiation is not  always  what  is  desired  because  the gear  housing  becomes  coated  with  dust  or  dirt. Consequently, particularly  in  heavy  trucks  and  busses, the  temperature of the  lubricant may  reach  250  to 300  degree F  causing  excessive oil  deterioration.

 The  loads, speeds,  conditions of use and the  operators  of  vehicles  vary  so  widely  that  it is a wonder  that  automotive  gears  give such  long  and  trouble  free  operation as they  do in  most cases. The service  obtained  from such  gearing is  probably more of a  tribute  to the people who  formulated and  supplied   the  lubricants than  to the  designers  of  the  mechanisms. In fact Raymond, a few years  ago, stated: “ The  hypoid  axle  is easily  the hardest  working  and possibly  one  of the most  neglected   automotive  components”. Elsewhere  the  thought  was  advanced that gear  oil  manufacturers have been  asked to  perform  miracles, in order to lubricates  such  gears.

  

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”.  

Truck Transmission and Differential Lubricants

Truck gears, in the main, require higher viscosity oils than do the gears in passenger cars. However, it is difficult to generalize since any one manufacturer of trucks may employ a variety of methods of speed reduction as well as perhaps more than one type of differential. However, lubrication charts are kept up to date and such charts, as well as the recommendations of the truck manufacturer should be consulted as to the proper grade and type of gear lubricant to use. Lubricating charts should be read with care because blanket recommendations are not made for all trucks of a given make. Also footnotes  are  frequently used  to  provide additional  or  more  detailed  information about  truck gear  oils.

A number of truck manufacturers depend upon specialty companies to provide them with transmissions or axles. In this case the parts manufacturer may suggest the most satisfactory gear lubricant. These firms often issue. Field maintenance Manuals devoted to each type of speed reduction unit or each type of axle. Such manuals generally contain sections on lubrication.

Almost all truck distributors suggest some variation in the viscosity of gear oils used as the temperatures change. A blanket statement cannot be made, but sometimes above 32 degree F an  SAE 140  grade of  lubricant  will be  recommended and  below this  an SAE 90.

While it has  been pointed out that lubrication would be  simplified by the use of a common  lubricant  for  transmission and  axles of  trucks,  there is not  an  agreement on this. Thus, Nelson and  Valentine^40 mention that a rear axle  lubricant must be  capable  of  absorbing  greater  torque  or providing  better  load  carrying  qualities  than  a  transmission, particularly  in  the  first  three gears.

Fletcher^19  notes that  many fleets have  standardized on the use of  engine oils in transmissions and  cites  a large  bus line which has used  SAE 30  and  modified SAE 10 engine  oils in  transmissions  for  many  years. The 30  grade is used  in straight mechanical boxes  and  the 10  grade  when mechanical  transmissions are  combined with  a  converter or coupling  and  a common  oil  supply  is  required for  the  two  units. This author objects to the oxidation and sludging of EP oils compared to straight mineral oils. His suggestion is the use of SAE 50 oil for transmissions. Another argument in  favor of  the  engine oils  is that  in  corrosion tests, using bronze specimens, such oils showed  little corrosion whereas  both  HP and  MP  oils  showed excessive corrosion.

On the other hand, Calish^8 cites evidence of the practicality of the use of a common transmission and axle lubricant in heavy equipment. A specific additive oil composition was used in some 1420 vehicles representing sixteen makes of trucks. The inference is that such a uniform transmission and axle lubricant proved satisfactory to the thirty-four accounts operating the above trucks over a period of several years.

When transfer cases are used to drive various devices, the same grade and type of oil as is  used  in the  transmission case will be satisfactory. Transfer  case  is a term applied to gear assemblies which transfer  power from  the  main  drive  line to  auxiliary equipment such  as  front wheel  drives, pulleys, hydraulic pumps, windlasses, and other mechanisms. For example,  many heavy  duty utility  vehicles carry winches for  erecting  heavy poles, devices for  boring holes  in  the  earth, and  other tools which  must  be  driven from  the  same  power source.

Where trucks  are  equipped  with  automatic  transmissions, ATF is  satisfactory  for use  and is so  listed in most  lubrication charts. However,  as a  matter  of  economy, General Motors  suggests the  use  of a  Hydraulic transmission Fluid, type C for  certain heavy duty trucks, buses, and  earth- moving  vehicles. Type C does not  have all  the  characteristics of  ATF  but  does  have  to  pass a  Powerglide Transmission Test, have  low  varnish and  sludge  deposits, and  must have  a  minimum effect  on rubber  seals. This Type C should never be substituted for ATF where the latter is specified.

Truck  Axle Lubricants. There seems to be little question as to the necessity of EP gear oils in axles of heavy duty trucks. However, there is not  common agreement as to  the  proper  grade  or  viscosity  of  lubricant  for  such  applications. Johnson^27, who considered these subjects, states:

“ Our  field experience is quite  conclusive to  the  effect  that  an SAE-140  viscosity lubricant is  far  superior to an SAE-90 in  its  ability  to  prevent gear wear  and  related  problems. On  numerous occasions  we have been  able  to  overcome  gear  wear  problems by  merely  effecting  a change  from  an SAE-90 to an SAE-140 viscosity”.

Also, it was found  that, with the heavier gear oil, temperatures in the  gear  case  did not  increase  but actually were  5 to  10 degree F lower than when  operating  with  the  lower  viscosity oil. The use of SAE 250  gear  oil  was  investigated but  it  was  found  that  this did run hotter  than  the SAE 140 grade.

A  further  advantage  of  the  use  of the higher  viscosity oil was found  by  this  investigator^27 when  the two  grades of hypoid lubricants were tested  on  eighteen driven units in the laboratory, using hypoid gears with a  61/6 ratio. The results obtained were:

“At 3000 rpm, the SAE-140 lubricant supported a torque load 21% greater than the SAE-90. At 2500 rpm, the advantage was 23%, and at 2000 rpm, it was 20%”.

Another factor, indicating the desirability of higher viscosity oils in axles of trucks, is that pointed out by Blok^3. It  was  mentioned that,  under  conditions of  impact  load  on  gears, viscosity is of  prime importance and  that  this  property of the oil  cannot  be  replaced by  antiwelding  activity  if wear  is to be  prevented.

Gears in  heavy duty trucks are  subjected to  low  speeds  and  high  torque conditions during much  of  their operation. For example, Nelson and  Valentine^40 determined that  in  highway operation a truck having  a 5-speed transmission used first speed about 2 per cent of the time, second  speed about 3 per cent, and  third speed  about 10 per cent , or a total of  15 per cent. The resulting comment was: “It is in this interval that gear destruction is prevalent, if the lubricant does not have proper EP qualities”. Further, mention is  made  that  in  off -high way  service, such trucks  operate approximately 50 per cent of the  time  in  the  first  three transmission gears.

Not all EP gear oils will provide satisfactory protection to axle gears operating at low speeds, and high torque, as do those in trucks. Therefore,  assurance  should be  obtained that  lubricants satisfactory for  such service  are  used  for  truck  axles. Lead soap active sulfur EP lubricants have been favored by some for such application. Since the  prevalent operation of  passenger cars is  under  high  speed  and  low  torque conditions, it is  desirable that a single  type  gear  oil  satisfy these conditions as well as  those  of  operating trucks. With this in mind, what is considered by some as a truly multipurpose gear lubricant has been provided? Such oils are those  conforming  to  specification MIL-L-2105B  which  is accepted  for  the  requirements of  all  vehicles operated by  the  U.S Military and by  many  other  users.

Some axle manufacturers and truck producers prefer to  either issue specifications for  what  they  know to be  satisfactory  axle lubricants or  to  state the type of EP  gear oil  to  employ. Hence, it is well to abide by the opinion of the truck supplier when choosing lubricants for truck axles.

This is all the more true because the gearing in truck axles may vary. Most  of  such variations, with  the exception of worm  drives, will probably  require  the  same  type  and  grade of  gear  oil as will  straight  hypoid  gears.  Thus, Rockwell  standard for  either  Planetary Axles or Hypoid- Helical  Two- Speed Double-Reduction Drive Units, as  well as  Spiral Bevel Gears,  recommend oils conforming to  their  Standard 0-65, which  is  an  SAE 140  EP  product. Use of Standard 0-65, the SAE 90 grade is optional but is suggested for lower operating temperatures than the 0-65.

Lubrication of Worm Drives on Trucks. In the case of worm drives on trucks, recommendations for the lubricant vary. While some suppliers of EP gear lubricants claim such oils can be used safely in worm drives, Watson^51 states:

“Actually, in practice, an EP lubricant is usually unnecessary for a worm gear. When lubrication failure occurs in a  rear-axle worm gear, only  in  exceptional  circumstances can it be  attributed  to  lack  of load carrying capacity  of the  oil used”.

Certainly  if temperatures  exceed 200 degree F, as they  may  at  times in axles of any  large  vehicle, EP additives may  cause  excessive corrosion of  bronze. Since EP agents and combinations of the same cover such a wide range of chemical compounds, the above statement may require modification. Perhaps the worst offenders are sulfur compounds, even some not considered active at 200 degree F. The possible  explanation may  be  that since  copper has  considerable affinity  for  sulfur, temperatures at  contact  points  of  the  gear  and worm  will  be  high  enough  to  cause  reduction of the  metal and sulfur. That, is the action may be akin to temperature flashes which occur with hypoid gears.

However, substitutions  have been  made for  recommended worm gear  lubricants; and if this  is done, fleet operators  are in a  better position to  observe result  than are  individual vehicle operators. In  certain  busses of  the  London Transport Company  having worm  drives, it  was  reasoned that  a fuel saving would be possible if there was less  drag due  to  the  axle lubricant. An  exceptionally thin oil  gave  satisfactory lubrication, but  when  changing from  first  to second gear  on  a level road, the  lay shaft  spun so  freely  that  the  higher gear  could  not  be  engaged until the  bus  came  to rest. An inhibited castor oil was then tried and proved satisfactory. However, the operation of these  specific  vehicles with  considerable stop  and start does  not  result  in  heating of the axle  lubricant, with  a consequent  possibility of oxidation, as would be the  case  in over-the –road vehicles.

While possible variations in oils for servicing worm gears of trucks have been mentioned, this is for information only. The safest  course  for  the  owner or  user  of  such a truck  is to follow the  recommendations of  the  manufacturer  of the  equipment.

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.

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.

Conditions under which gear lubricants operate

With meshing gears, both rolling and sliding motions are present. However, the  two  types  of  contacts  vary  both  with  the  type  of  gears  and  the  speed  of  operation. The  sliding  component  is  of  greatest  importance  in  the  case  of  hypoid  or  worm  gears. The descriptions of the state of lubrication, which  immediately  follow, are concerned, to a large  extent, with the types of  gears  used  in  most  industrial  applications, transmissions  of  automotive  vehicles, etc. Some  of  the  thinking  and  speculation  can  also  be  applied  to  hypoid  and  worm  gear  operation.

Many  gear  lubricants  operate  under  very severe  conditions  and  yet  long  and trouble  free  lives  are  obtained  from  most  gear  sets. In spite  of  the  fact  that  the  action  of  such  lubricants  is not  completely explained, enough  information  is  available  to  permit  recommendations  of  gear  oils  which  will  perform  satisfactorily  provided  the  gear  design  and  installation  is  not  at  fault.

Borsoff^10  has  presented  the  mechanism  of  gear  lubrication  in  a  simple  descriptive  from  which  is essentially  as  follows. As  gears  rotate  in  the presence  of  a  lubricant, a hydrodynamic  wedge from, which tends  to separate  the  teeth  as  they  mesh  with a  thick  fluid  film  when  the  load  is  low. As  the load  increases, the  pressure  in  the  contact  zone  also  increases, causing  the  separating  Lubricating  film  to  decrease  in  thickness. Finally,  the  load  becomes great  enough  that  the fluid  film  fails  to  prevent  contact  of  the  high  spots  of  the  mating  surfaces, and  wear  results.

The  investigator^10   found  that  the  nature  of  the  were was dependent  upon  speed  and  that  gear  lubrication  can  be  divided,  with respect  to  speed, into  three  zones. First  is  the slow  speed zone, which  in  the  gears used  in  the investigation, extends  up  to  about 1000 rpm. Next, the medium  speed  zone  for  the  same gears  extends  approximately  from  1000  to  8000  rpm. Finally,  the  high  speed  zone  extends from  8000 to 30,000  rpm  in  the  case  of  the  investigation.

Three  regions  were  recognized  with  respect  to  type  of  wear  and  working  surface  conditions, namely: a region  of  thick  film  lubrication  where  there  was  absence  of  wear; a region  of  abrasive  wear; and a  region  of  scoring.

In  the slow  speed zone, the load  carrying  ability  of  a  given  lubricant  increases  with  decreasing  speed. Thus, using  an  oil  of  9.92 cs  at 100 degree F, no scoring  took  place  at  speeds  below  665 rpm. However, wear does  occur  at slow  speeds as loads  increase, and  under  these  conditions  it  was concluded  that  such  wear  was  due  to  abrasion.

Gears  operating  in  the medium  speed  zone  with light  loads  were  in the thick film region, and  hence wear was not  detectable. As  loads increased  in this  speed zone, heat  generated  by shear  caused  a  decrease  in  the viscosity  of  the  lubricant  followed  by a rupture  of  the fluid film with consequent  metal  contact  and scoring. It was found that this type of wear was not gradual  but increased  by jumps  at the beginning of each  load  period, after  which no additional  wear  was  observed  until  another load  increase .Under  these  conditions, the lowest viscosity  oil  permitted  the  greatest  wear, which also  started  at low  loads. The high  speed  zone was ‘characterized  by an increase in load  carrying  capacity  with  an increase  in  speed.’’ This was attributed   to two factors, one the relaxation phenomenon and the other the ‘squeeze’’ effect. Relaxation in this sense indicates that the liquid  lubricant  responds  as  an elastic  solid  when  subjected  to  high  deformation  rates. Since  a  definite  length  of  time  is  necessary  to squeeze all of  the  lubricant  out of  the contact  zone  between  gear teeth ,  at  high  speeds  the  contact  time  may be  too  short  to  eject  all  of  the oil. According  to Borsoff^10 “ high  speed  gear  operation  at  all  loads  below  the  score  load   is  in  the  thick  film  region.”The  above  mechanism  of  gear  lubrication  was  concerned  with  lubricants  consisting  of  unreactive  mineral   oils.   It  was  found  that the  higher  the  viscosity  of  the  lubricants, the greater  the  load carrying  capacities  and  wear  protecting  properties. However, other requirements often dictate the use of low viscosity oils. Since operation  of  gears  using  unreactive  mineral  oils  at  loads  above  their  first  score   load  may  lead  to  trouble, the  use  of  extreme   pressure  (EP)  lubricants  is  found  necessary   with  high  loads.

Borsoff^10  cautions  that  ,  when  using  EP   lubricants  at  loads  above    the  score  load  of  the base  oil, abrasive  wear  may  sometimes  be  present.  Likewise,  abrasive   wear  may  occur  with  heavy  loads  in  the  slow  speed  zone .  Such  action  and  the  degree  will  depend  upon   the  particular  EP  additive  as  well  as  the   concentration. In  order to  appreciate   more  thoroughly   the  problem  of   gear  lubrication,  the ideas  of   other  investigators  should  have  consideration. According to  MacConochie  and  Newman^37  contact  pressures  of  7000  to 10,000  kg/sq  cm  in  the  case  of  gears  compare  with 10 kg/sq  cm  for  journal  bearings . Also , instead  of  an  oil  film  thickness  of  about  0.001  cm , as  is present  in journal  bearings, the  lubricant  film  formed  between  meshing  gear  teeth  is  of  the  order  of  magnitude  of  the  surface  roughness  of  the  two  contacting  bodies  and  not  much  thicker  than  the  size  of  foreign  particles  in  a  highly  purified  oil  passing  through  the gap. The  function  of  the  oil  is  performed  in an  extremely  short  time  since, according  to  Smith^46,   the  residence  time  of  the  lubricant  in  the  contact  zone  may  be  as short  as 10^-6  seconds.

The  first  authors^37  visualize   the   conditions  under  which  gear  oil   films  operate  as  follows: ‘Sometimes  there  is  a  substantial   lubricant  film  between  the  surfaces, and  at  others  a  foreign  particle  wedges  its  way  through   between  two  surface  roughnesses , at other  times  two  or  more  asperities  come  into   physical  contact. Depending  upon  the  relative  waviness  of  the  surfaces  and  the  difference  in  particle  size,  contact  may  occur  only  at  one  point  along  the  contact  line  while  lubrication  is  hydrodynamic  in  other  regions. Another  factor  affecting  thickness  readings  is  the size  of  the  contact  zone  since  the  larger  the  contact  area  the  greater  the  chance  for  a  particle  of  large size  to be  in  the  gap.  To complicate the picture   further,  the  film formed   is  constantly  subjected  to  dynamic  load  on  the  gear  train, vibration  of   the  gear  teeth , and  shafts. Localized ‘conflagrations’ resulting in chemical reactions may occur at heavy loads.’’

Oil film thickness was measured by these investigators^37 using a continuous electrical arc to bridge the gap between gear teeth. A tracing  of the results indicates  that the lubricant  film  is  at  a  maximum  at  the  pitch  line  and at a minimum at  the roots  or tips  of  the gears,  see Fig.2.1.

Much remains to be explained regarding the lubrication of gear sets. For example, in a discussion  of  the report  of  “Instantaneous  Coefficients  of  Gear Tooth  Friction’’ Benedict  and  Kelley^4  state: “We   do  not know  from  our  results  the  true  state  of  lubrication, whether  it  be hydrodynamic, elasto-hydrodynamic, or partial hydrodynamic. In  our  analysis, we have chosen  partial  hydrodynamic  lubrication  as  a  simple  model  which   helped  visualize  the results. In order to determine precisely  the  state  of  lubrication  it  would appear necessary  to  determine  by  some  more  direct  observation  not  only  the thickness  of  the oil  film but  also  its  continuity’’.

Also  gears  are  used  under  such variable  conditions  that  a  blanket  statement  cannot  be  made  as  to  the  type  of  lubrication  which  prevails. This was recognized by the above investigators^4 as   follows :Lightly loaded high  speed gears  might  reach  full  fluid  film  lubrication, and  heavily  loaded  low  speed  gears show  signs  of  being  in the  boundary  region. These are  extremes, however, and in the results  discussed  here  and  in  gears  normally  used, mixed  lubrication  occurs.

In  view of  the fact  that most  gear  sets  operate  trouble  free, the  extreme conditions  indicated  by  MacConochie  and Newman^37 may  not  for average operations. Thus, Crook^19 who  used electrical  resistance  as  a  means  for measuring  the  thickness  of  oil  films  between  metal  dises, concludes  that  hydrodynamic  films  of  one  micron  in  thickness  result after  a  break  in  period. This author states: Furthermore, the existence of a hydrodynamic film is quite consistent with observations with gears themselves. Often signs of the original  machining  marks  can be seen even  after twenty  years  service  and such low  rates  of  wear  imply  hydrodynamic  rather  than boundary  lubrication.

Perhaps  a  new lubrication region  should  be  recognized  which Talley  and  Givens^47  have  defined  as ‘metadynamic’ and which  falls  between  the  hydrodynamic  and  boundary  regions. These  authors, in dealing  with  such  a  lubricating  film, have  measured  one  aspect  of  oiliness  and  derived equations describing  the  same. While  the  investigation in  question  was  concerned  with  journal  bearings, one  interested  in  the  theory  of  gear  lubrication  should  be  aware  of  this  thought.

Some of the complexities   of  boundary  lubrication  as  suggested  by Larsen  and  Perry^35a  may occur  during  gear  operation, particularly  if  high   temperatures  are  reached. Any  of  the  following  reactions  might  have  an  influence  on   gear  lubrication: oxidation  of  the  metal  surface; oxidation  of  the  lubricants  to  from  fatty  acids; chemical  or  physical  adsorption  of  polar  compounds, such  as  fatty  acids, on the  metal  surfaces; formation  of  multilayer  films  by  the  adsorption  of  the  above  fatty  acids, by  salts resulting  from reaction  of  the  acids  with  metal  oxides, or  by esters  present; oxidation  or polymerization  of  oils  or  unsaturated  constituents  to  from  resinous  films; orientation  of  these latter  films  due  to  pressure  and  shearing  stress; or  a  breakdown  of  any of  the  films  just  described.

While  the conditions  under  which  gear  lubricants  operate   seen  to  be quite  severe, it  should  be  realized  that  average  operating  conditions  are  less  serious. Producers  of  both gear sets and  gear  lubricants  provide enough  tolerance  in  their  products  so  that  difficulties  are  the  exception  rather  than  the rule. However, the possibility   of  conditions  such  as  those   cited  behooves  the  operator  of  gears to  provide  as  reasonable  service  conditions  as  possible  so as  to prevent  undue  wear  or  even  failure.

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.

Demands made on gear and transmission lubricants

Mention  of  some  of  the  demands  made  on  gear  and  transmission   lubricants  may  well  serve  as  a  further  justification  for  the  assembly   and   publication  of   the  information   to  follow. Reviewing the problems confronting the automotive industry, Raymond^9  considers  the  hypoid   axle  to  be  the  hardest  working   and  perhaps  the  most  neglected  automotive   component. It  is  said  to  be  far  easier  to  deliver  increased  horsepower  and  torque  to  a  rear  axle   than    to  build  satisfactory  durability  into  such  a   critical  unit  which  is  confined   by  size   and  weight  limitations. Since  higher  engine  horsepower  and  automatic   transmissions  have  greatly  increased   low  gear  loading  and  higher  pinion  offset   has  produced  more  sliding   motion  between   gears, Raymond^9  con-  clues: Lubricants   thus   have  become  a  limiting  factor    in  the  load  carrying   capacity  of  American  axles.

 Automatic   transmission  fluids  offer  another  challenge  to  the  oil  industry   in  that  a  complex  combination   of  requirements   exists   for  such  fluids, and  Raymond^9  believes  that  the  future  appears  to  offer  a  still  greater  challenge  in  severity  of  automotive   transmission  operation.

In  addition  there  is  a  tendency  for  the  use  of  a  single  gear  oil  to  per  from  several  functions. For  example , a  lubricant  may  be  called  upon  not  only  to  protect  bearings  and  gears  against  wear  and   corrosion, but  also  to  act  as  a  hydraulic  medium. Likewise, an  automatic  transmission  fluid, the  primary  purpose  of  which  is  that  of  a  torque  converter, may  also be  called  upon to  lubricate hypoid  axles   in  the  future.

The  severe  conditions  under  which  some  gear  oils  operate  is  stressed   by  Hundere^5   who   states: “ The  most  difficult  function  that  a  gear  lubricant   must   perform  is  that  of  preventing  excessive  metal  to  metal  contact  in  the  region  of  maximum  sliding   velocity. When  it  is  realized   that  the  unit  loading  at  the  point   of  contact  is  as  high  as  400,000  psi  and  the  sliding  velocity   is  as  high  as  6,000   fpm, it is  amazing   that  excessive    wear, let  alone  fusion,  can  be  avoided.’’

It  is  quite   evident  from  the  above  that  constant   improvement  in  qualities  of  gear  lubricants  is  necessary  to  meet  changing   demands. In  spite  of  the  upgrading  of  such  products, they  are  universally  available  at  reasonable  costs. It  is  well  to  keep  in  mind  that, from  an  economic  standpoint, the  benefits  of  correct   lubrication, including  that  of  gear  sets,  are  mainly  due  to  reduction   in  breakdown  or  maintenance   and  consequent   decrease   in  loss  due  to  downtime   or  curtailed   production. This  holds  true  for  both  transportation  and  manufacture. Saving s   in  power  and  frictional   energy,  while   often  attributed   to  correct  lubrication, are  small   compared   with  the  first  benefits   mentioned.

Introduction

Almost  all  modern  machinery  makes  some  use  of  gears  as  a  medium  for  transmitting  power. Such use  may  be  for  the  purpose  of  either  changing  the  speed  of  a  driven  member  or  changing  the  direction  of  movement. In  either  case  the  gears  act as a lever  so  that , in  most  cases, the  force  transmitted  is  multiplied  and  in  so  doing  speed  will  be  reduced. When  variations  in  the  torque  or  speed  transmitted  are  desired, a  series  of  gear  ratios  can  be  provided  in  the  same  mechanism.

Most  gear  sets  are  lubricated  with  fluid  products  or  gear  oils, and  consequently, such  compounds  will  receive  major  consideration . Since  plastic  or  semi-fluid  products, such  as  lubricating  greases, as  well  as  solids  are  sometimes  used  as  gear  lubricants, they  will  also  receive  mention.

It  is  also  the  purpose  to  describe  fluids  employed  in  certain  fluid  drives, particularly  automatic  transmissions  in  automotive  vehicles. Here  the  oils  in  question  serve  a  number  of  functions, and  it  is  necessary  that  a  single  fluid  satisfy  not  only  lubrication  of  different  mechanisms, but  also  act  as  a  torque  converter  and  hydraulic  agent.

This  is  simply  an  extension  of  the  functions  which  gear  oils  fill  in  many  case. Thus, gear lubricants   also take care of bearing lubrication in many   applications. It will  be  presumed  that  most  readers  are familiar  with  the  mechanisms  to  be  lubricated, such  as  automatic  transmissions, hypoid  gears, worm  gears, etc. Therefore, little description of mechanical details will be attempted.

The  primary  interest  will  be  in  lubricants  for  metal  gears, although  mention  will  be  made  of  the  requirements  for  gears  made  from  other  materials. An  attempt  will  be  made  to  thoroughly   explain  how  lubricants  for  gears  function, what  they  contain, how  they   are  compounded, and  the  limitations  of  their  use. In  all  of  this  it  should  be  kept   in  mind  that  to  be  most  effective   any  lubricant  should  be  applied  in  the  right  amount  at  the  right  place  and  the  right  time.

Fundamentally  the   types   of  motion  in  operating  gear  sets  are  those  occurring  in  other  machine  elements , namely, rolling  and  sliding  motion. The two types of movement   may occur simultaneously as in meshing   hypoid gears. This  is  pointed  out  so  that  it  will  be  realized  that, even  though  the  configuration  of  gears  differ   from  bearings, the  lubricating  problems  are similar  for  both  mechanisms. Another  point  to  keep  in  mind  is  that  while  the  title  of  the  mentions  both  gear  and  transmission  lubrication, the latter  mechanism  is  most  often  one  or   more  gear  sets  serving  a specific  purpose. Therefore, except  in  the  case  of  fluid  drives, the  problem  is  one  of  gear  lubrication. Finally , some of  the  possibilities  of  future  trends  and  developments  in  gear  lubricants  and  torque  fluids  will  be  mentioned.