When to Change Gear Oil

The  ideal method of lubricating  enclosed  gear sets would be to place the  proper  amount  and quality  of gear  lubricant  is a  sealed case and  make  no  renewal  of the oil  during  the life  of the  mechanism. Manufacturers  of  automobiles have  this  in  mind  and  approach  such  a  solution  of gear  lubrication by  recommending  no  drain  periods and prolonged  use of gear oils. Manufacturers of other equipment have the same thought in mind and the trend will no doubt increase. For illustration a side entering mixer description states: “Lubricant is sealed in the gear case at the factory and is designed to last five years.” Naturally, any statement as to the life of gear lubricants should be modified with an expression as to operating condition, environment, etc. However, it is questionable if the present gear oils are everlasting even under the best of conditions. Therefore, from  both a service and economy  standpoint, used gear  oils  should  be removed  from  the gear cases  and replaced with fresh  lubricant either  when the oil  has  deteriorated or become  contaminated  or at stated intervals.

AGMA, which is interested in insuring log and uninterrupted service from gear sets, recommends the following: “The oil in a new unit should be drained at the end of two weeks operation and the case thoroughly flushed with light flushing oil. After this, a change of oil every 2500 hours of operation or every six months, whichever occurs first, is recommended for the units operating under favorable conditions. Where  operating  conditions  are  severe  such  as  rapid rise  or fall  in temperature of the gear  case with  accompanied  sweating  of the inside  walls  resulting  in a formation of  sludge, or where  operation  is in  moist or dusty atmosphere or in the  presence of chemical fumes, it may be  necessary to change the oil at  intervals of one  to three  months.”   

It is also pointed out that gear sets are usually treated by the manufacturer with rust preventive s before shipment. Such materials may have an adverse effect upon the gear lubricant and, therefore, should be removed before gear oil is added. Petroleum solvents are best for this purpose, and such fluids will also help to remove contaminants, such as metal chips. Immediately  upon removal  of such solvents a low viscosity  flushing oil  should  be  sprayed  on the  gears; otherwise, the  metal  surfaces may  rust  in a few  minutes. Also the gears should not be operated, even for a short time, in a dry condition.

In enclosed  gear  cases  which do not  have  a  drain or a  circulating  system, the used gear oil or flushing oil  should  be removed by suction.  Use of pressure to  force  out  such oil may damage seals  and in the case  of  automotive  equipment will probably force oil  onto  the brake  bands. When  considering  oil changes  for reduction  gears, Forbes et al.^20  mention that  the first  month  of  operation is the most  critical in the life  of  gears; therefore, they  suggest  a change or  careful  filtration of the lubricants within two  weeks  after  the unit is put  in service. It is  pointed  out  that  fine metal  particles  resulting  from the run in period act  as catalysts  for oxidation if left  in the gear case. Rather than  setting  an arbitrary  period of change, the  above  authors^20  think  that  periodic oil samples should be taken  to determine if the oil  is in a  usable  condition. Such samples can be checked for presence of dirt, metals and water. Also the acid number, viscosity and interfacial tension can be determined. Certain fleet owners operating heavy trucks have a rule that the oil in transmissions or rear axles must be changed when the viscosity has increased by fifty per cent.

Where gears are lubricated by  circulating  systems, Forbes et al.^20 states: “ the  charge may often  be used for several  years without change,  particularly  when  adequate  filtering  equipment is employed”. However, cleaning such a system when changing oil requires more effort than in splash systems. Flushing  oils  are available  which, due to  either additives  or the  particular solvents used, will  remove most of the  deposits  from  the  oil reservoir and the piping as well as  the gear case. However, a final cleaning with dry rags free from lint may be necessary.

The greatest care should  be  used  in  cleaning  gear  cases and  auxiliary  equipment since  any used  gear  oil left  in a case and mixed with fresh  oil  will  tend to act as  a  catalyst to  promote deterioration of the new  lubricant. With the flushing oil on the gear set an inspection is possible. If this shows rust, the gear manufacturer should be consulted before operating. If the gears appear to be in satisfactory condition, they should be coated with the gear oil as soon as possible. In the  case  of a  circulating system this  can  be  done  without  operating  the gears. If a splash system is employed the gears can be sprayed or wiped with the lubricant before operating.

Where meshing gears are of dissimilar metals as in most worm gear sets, the importance of a change of oil and inspection of the mechanism is very necessary. As the gears seat themselves bronze particles may become attached to the worm. Such  particles  adhere  to the steel  and  cause  a rough  surface which  will  score the gear. Removal of the used lubricant and cleaning of the worm threads after a short period of use will often prevent further wear.

When oils are used on a once through basis flushing may not be possible. In the case of lubrication by oil fog, only fresh oil is supplied, and the tight system is under some pressure so that contaminants should not enter. However, in the case of open gearing, cleaning at intervals is advisable. Not only does the residuum  type  of gear oils pick  up  dirt  which  in turn  may act as abrasives, but  also some of the  heavy material  packs in the  roots  of the gear  teeth. If such build ups continue shafts may be thrown out of line. Such deposits can usually be softened by kerosene or some other solvent provided there is not a fire risk. If drip pans are provided for open gears these should also be cleaned at regular intervals.

Turbine oils for Automotive Vehicles

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

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

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

Transaxle Fluids

While a single lubricant for both transmissions and rear axles of certain automobiles is desirable, Hunsted et al.^28 ,when reviewing the situation with regard  to such fluids, concluded that as of January 1960 considerable  work  remained  to be done on transaxle fluids to achieve optimum results. Up to that  time  some  42 mineral  oil base and  synthetic base  fluids, with  various  additives, had  been  evaluated and only  three showed  any  promise  for  future  development. Since  it is  impractical  to  suggest even  tentative  formulations, all that  can be  done is to  indicate  the  requirements and  the handicaps.

A  transaxle  fluid  should  have  all  the  desirable  characteristics  of ATF and  in  addition  should  have  sufficient  EP value  to  lubricate  hypoid  gears. The above statement should be modified in that the desirable characteristics of ATF should also be maintained while in service. Thus, when  the  fluid is subjected to the  shear  effect of mating  gears, polymer type  additives may  lose  a large  part  of their  effectiveness  due  to  breakdown  of the  polymers to  lower  molecular weight  products. Therefore  base  fluids with  inherent  high  V.I. or  polymers of  initially  lower molecular  weight  are  dictated  in the  formulations.

EP agents  used  should  have no  adverse  effect  on any  of the  components of the  automatic transmission such  as  clutch  materials, various   metals, synthetic seals, etc. Of  course the most desirable  transaxle fluid  would  be  one  which  could  be  sealed  in a  case  and serve the  life  of the  car  but  first  a  fluid which  will perform  satisfactorily when  changed  periodically  will serve.

According to  Celanese  a  trimethylolpropane  ester  with  an EP additive  offers promise  for use  as a transaxle  fluid. Another suggestion for the purpose is methylchlorophenyl containing about 3 per cent chloride.

Reduction of noise,vibration and shock by gear lubricants

Gear lubricants, particularly those of high viscosity, will act as cushions and thus reduce noise, shock, and vibrations of meshing gears. However, it must be kept in mind that even the most viscous products are not correctives for poor mechanical conditions.

Mention might be made that Cardillo,^14 in an analysis of “Initial Axle Noise” in automobiles, concluded that it is a resonance problem and apparently lubricants or lubrication plays  no part in abating  this noise .A different noise effect has occurred in automatic transmission and limited slip differential mechanisms of vehicles and in both cases has been corrected by the use of oiliness additives. The degree of oiliness must be carefully controlled since, if deficient, the clutch operation will be rough and cause chattering or squawking but, if excessive, the clutch may slip. Mention will be made of some of the suggested additives for this purpose in a latter section

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.