Thermal Oxidation Stability of Gear Lubricants

This test is used to determine the deterioration of lubricants, particularly EP gear oils, under severe oxidation conditions. The sample  is  placed in a gear case in which a spur gear  set and a test  bearing  operate under load  while  heat  is applied and  air  is  bubbled  through the oil. The temperature of the  lubricant is  maintained at  325 degree F during  the test. The  above apparatus  and  method were  developed at the  Ordnance Fuels and Lubricants Research  Laboratory, Southwest  Research  Institute, and a report  by Meckel^10  summarizes the  contents as follows: “A description  is given of the  apparatus  recommended  and a list of Purchase Materials, Instructions for Cleaning, Assembling and  Disassembling the  Gearcase. Also  Test Procedure  and  Detail Drawings of the  Assembly are included.”

                                                 

Variations in test  conditions and  their effect  on  oxidation of gear  lubricants, using this spur gear thermal oxidation apparatus, were  reported by Meckel and  Quillian^9 and should  be  consulted  if a  program  of  tests on  the  subject are  to  be made. The test in question is particularly valuable  for  use  in developing  oil additive combinations  which  will  retard  oxidation in  service.

During this test, viscosity of the  oil are  determined at  intervals of  10  hours and  the test  is  concluded  when  the  increase  in  viscosity  reaches  a point  called for in the  specifications.

Specification MIL-L-2105B calls  for a test  time of 50  hours, at  which  time  the  increase  in  viscosity shall  be  a  maximum  of 100 per cent. Also, at the  end  of this  period, the  n-pentane  insolubles must  not  be  more  than  3 per cent  by  weight and  the  benzene insolubles  not  more  than  2 per cent  by weight. Further, the  test  method  requires  inspection of the  various  parts, that  is gears, bearings  and  catalyst,  for the  amount  and  type  of  deposits. The  gear  teeth are  inspected  and any  abnormalities of the  surfaces  noted. After  all  deposits are removed  from the  catalyst, this is  weighed  to  determine  the  copper activity  of the  lubricant. The  test  bearing wear is also  determined  with a  special  fixture and  dial  indicator.

                                                                            

Chemical Changes in steel surfaces During Extreme Pressure Lubrication

Analysis may aid in explaining the mechanisms by which gear lubricants, particularly EP oils, function. Which  it is  generally  accepted  that  iron sulfides, chlorides and  phosphates are  possible reaction products  of  EP additives  under extreme operating conditions, this  may not  be the  complete explanation. Godfrey^5 considered that there was a lack of understanding of the above actions. Consequently, EP films and  fragments  were  obtained  and  analyzed  by : electron  diffraction, X-ray diffraction, emission  spectrograph, proton  scattering, chemical spot tests, and  volumetric  analysis. For one wishing to study the chemical mechanisms of extreme pressure lubrication this article should provide suggestions as to analysis. Suggestions have also been made that changes in EP additives while in service may be measured by infra-red spectroscopic technique.

Zinc by paper Chromatography

The above methods are concerned primarily with unused lubricants and consequently a report by Goodwin and Begeman^6 are of interest. This  article  describes a methods  of  tracing  Zinc dialkyldithiophosphate  decomposition  in axle lubricants by  paper  chromatography. For  the  purpose, a  sample  of the oil  was  allowed to  soak  into  a sheet  of filter paper after  which  the  paper  was suspended  in  a  chromatogram   above  a  developing  solvent. About 1/8 inch of the lower edge of the paper was immersed in the solution until the solvent front had traveled to within 1 ½ inches from the top.  Following this treatment, the paper was dried. Since  the  zinc  compounds  in the developed chromatograms  are  colorless, a colored  zinc  derivative  was formed  by spraying  the paper with a 0.05  per cent  solution of  diphenylthiocarbozone  (dithiozone)  in  chloroform. This formed a purplish red zinc dithioozonate. Any  excess  dithiozone  was  removed  by  immersing  in a weak  ammonium  hydroxide  solution and then  washing  with water. The  red  color  fades over  a period of days  but may be  photographed  with  color  films to give  a permanent record. The intensity of the color indicates the relative amount of zinc present. Thus, the intensity of colors in tests decreased considerably in the interval of road mileage from 2,006 to 5,144.

Radioactive Tests for Metals in Oils

Tests in which the radioactivity of metals or their compounds is used as a means of determining their presence or concentration are largely confined to experimental investigations. This  is true  because, first  activated metals or  compounds must  be present  and  this is not true of  commercial  lubricants and second, the life  of some of the  isotopes  used  is short so that  after  prolonged service the activity would  almost  disappear.

Limitations on Heating of Lubricants for Application

Heavy bodied lubricants, particularly residual types used on exposed gears, are often heated in order to make application easier. If such lubricants are straight mineral oil products, the amount and intensity of heat should not harm them. However, if additives are included, only a very moderate heat should be used. Otherwise some change in the composition is possible. A supplier of the lubricant can advice the limitations on heating. A similar caution is necessary in case lubricating greases are used as gear lubricants. The thickeners for such products may be soaps which upon the application of considerable or prolonged heat will separate from the oil present. 

Processing Various Foodstuffs with Gear Driven Machinery

While  the listing of processes and  machinery used in handling and  preparing  foodstuffs  is not  complete, the examples  indicate  the gear lubricants which  are  suitable for such applications  and the precautions necessary in their use. However, a few  other  illustrations of such equipment  where gears are  the drives follow : cocoa and cocoa  butter are  handled  in gear  driven  mills; gelatin  is sometimes dried  in rotating  heated drums with connections to gear  reducers; spices  are milled in many  cases with the mill  being  gear  driven; yeast may be  concentrated  by centrifuging  with the rotation being  through  gear reducers; and  fish  flour in some  plants  makes use of  gear drives for both pulverizing and drying.

Industrial Application of Gear Lubricants

While  it  is  impossible  to give  specific  recommendations  for the  choice  and application of gear  lubricants for  every  type  of  mechanism  which  quently  of  lubrication is possible. Designations  of heavy  duty, medium  duty and  light  duty  gear sets  will  serve as a  basis  for  differentiation in  selection and  application  of  lubricants. Later, comments will be made on gear lubrication in certain industries. Unless otherwise mentioned, enclosed gears will be considered. 

  

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.

Wear Prevention Agents

A number of investigators have distinguished between wear prevention and EP agents in lubricants. The  former are  effective  by  a  chemical  polishing action, which  takes  place  at a  lower  temperature  than  does  the  formation of antiweld  films by EP agents. A very  extensive  investigation  of  wear  prevention  agents,  which are  effective  in  lubricants for  two  steel  surfaces was  reported by  Beeck, et  al^9. Calhoun and  Murphy^18, who  reported  on both  anti wear  and EP  additives  for  lubricants found that it  was  possible to  blend  two  or  more  types  of  additives  and  attain both  properties in  the same  composition.

Typical  of  anti wear  additives  are  tricresyl  phosphate  and  Zinc  dialkyl  dithiophosphate.  Such  agents  are  seldom  used  in  industrial  gear  oils  but  are  valuable  in  lubricants  such  as  those  for  jet  turbines  where  the  oil  serves  several  mechanisms  including  gearing.

Polymers as components of gear lubricants

Polymers find application in gear lubricants both as additives and as actual fractions of the base fluids. Specific types of polymers used as additives will be mentioned later. Since the fluids in gear lubricants consist  primarily of mineral  oils  or of  asphaltic materials, any polymers  added  should  be compatible  with  the  major  ingredients. Likewise, polymerization  of  the added compounds  should  be complete  so that  no  further  thickening  occurs  in  service  even  if  the  mixture  is  subjected  to  some  heat. Most  polymeric  materials will  break  down  under  shearing  action  of gear  to  products  of  lower  molecular   weight  and  hence  any  initial  bodying  effect  may  be  partly  lost  after  use.

Although the polymers may be miscible with lubricating oils, special procedures may be necessary to introduce them into gear oils. Thus the grade of  polyisobutylene  which  is used  to  give  tackiness  or stringiness to oils is about  the  nature  of, but tougher than, crepe  rubber. Consequently this polymer is dispersed by a rubber mill in lubricating oil and the solution known as paratac is available to compounders. Also  most  grades of  polyethylene  are  of  such  a  nature that  only  prolonged  heat and  agitation  will  disperse them  in  lubricating oils.

The following polymers have been  suggested or  actually  used as components of  fluids for  gear lubricants: polyisobutylene  of various molecular  weight; styrene-polyisobutylene;  polyethylene ;and resins recovered  from  mineral oils. With the continued  investigation and  production of new  or  modified polymers it  seems  reasonable that  increasing  use  of  such  materials can  be expected in gear  lubricants.  

Odor Control Agents

In general there has been no apparent  attempt  to cover up the odors  of gear  oils  which  arise  from EP  additives. Occasionally pine oil or pine tar will be detected in lubricants designed for exposed gears. Sulfur  compounds  perhaps  have  the most  disagreeable  odor  of any additives  and  consequently  Jonach^46  suggested  treating  such  agents  with  lead  peroxide  to  react  with  any  free hydrogen  sulfide  which might  be  present.

Gear Materials of the Future

The lubricants required in the future  for gear  service  will  probably be  tailored to the  mating  surfaces which are to  be  protected  against wear. Not only will improved ferrous alloys and other metals be used, but also there will be more nonmetallic gears in service. Whether these will be primarily plastic gears or simply improvements on such forms as laminated gears remain to be seen. Probably plastic gear use will increase, particularly in small mechanisms and where loads are light and there is no wide temperature variation.

Investigations are being made  to determine what  combinations of  mating  gears will  show  the  least  wear  when  no  lubricant is provided. This is a fertile field and the supplier of lubricants should keep advised of the advances.

Gas, Fluid, or Solid Lubricants and Their Future

Thought is being given to experiments and   perhaps usage of gear lubricants in each of the above states. Application will be found  for   both  gases  and  solids as gear lubricants, the  major  portion of operating  gears  in  the  future  will  continue to be lubricated with fluids.

Naturally these fluids will be improved for many applications. For such purpose, the first thought concerns synthetics. However, the abundance and relative cheapness of petroleum oils rules out synthetics for most application in the immediate future. It can  be expected  that  mineral  oils will continue to be  upgraded  by  greater  selectivity , more  chemical  processing, and use  of  improved additives.        

Food processing Industries and Gear lubrication

A variety of industries are grouped together so that prevention of contamination by gear or transmission lubricants can be stressed. While such possibility is remote, Government Inspectors are becoming more particular in this regard. Also, both  manufacturers  of food processing  machinery  and operators of plants are  aware  of the  importance of cleanliness  and lack  of contamination. Proper seals, well maintained, should prevent leakage from gear cases. As an aid in this direction, over lubrication should be avoided and when and if lubricating grease is applied, this should be at very low pressure. As a general rule, straight petroleum oils can be used in gear lubrication of the subjected equipment. Some operators of machinery handling foodstuffs prefer light colored lubricants. White oils are available in viscosities up to 300 or perhaps 500 SUS at 100 degree F. The  same  oils can  be  thickened  to  a  semi fluid or non  flowing  nature  by the  use of fine silica  or non toxic  soaps, such  as  aluminum  or calcium. Light  colored  fillers, such  as  magnesium  oxide  or zinc oxide, can be added to  lubricating greases, although fillers have little  if any  place  in gear  lubrication.

Caution  should  be  used  in supplying  gear  lubricants containing additives  to  food  processing  plants. Oils  containing  EP agents should  only  be  used  if the gear  cases  containing such  oils are  sufficiently  removed  from  the food  products so that  leakage  will  cause no  contamination. Foam inhibitors are permissible because the concentration is quite low. Stable oils are recommended because they will require a minimum of oxidation inhibitors. Such additives, as well as  rust  inhibitors, which  will  be  desirable  under  wet  conditions, should only  be used  with assurance  that  the compounds are not  harmful to  animals  or humans.

Simplification  of gear  lubrication  should  be  kept  in mind and  if  possible, only  one type  and grade  of gear  oil be used in a  specific  plant. Thus, in spite of the  recommendations above for the  use of such oils  with  very  judicious  inclusion  of  additives, the suggestion  has been made^31  that  a  premium  grade  rust  and oxidation  inhibited turbine and  hydraulic  oil  be  used in all  gear reducers  in dairies. Therefore, similar oils will also be recommended for other foodstuff handling equipment. Open gearing will be found in some food handling machinery or plants.  It  is  presumed  that such  gears  will  be so located  that  drips from  the  same  will  not  contaminate food. Therefore, a  general  recommendation  is to  apply  a  residual  type  of  gear  lubricant  very  sparingly. The viscosity will be dictated by the service but will probably be one of 1000 to 2000 SUS at 210 degree F.

In the  group  to  follow, equipment  used  to process some medicinal  items, alcohol  products etc., will be  included since  they  are  restricted   to the  same  limitations as far as contamination is  concerned. By  mentioning  some  of the unit  operations  which may be  encountered   in food  processing, the  variety  of machinery  involved and therefore,  the  possibility of the  use  of  gear  drives will   be  evident. Thus, cleaning,  coating, conveying, decorating,  disintegrating, drying, evaporating, forming, heating, mixing, packaging, pumping and  separating are a few  such  operations. 

Spray or Jet Application of Gear Oils

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

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

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

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

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

Desirable characteristics in gear and transmission lubricants

A number of characteristics, which were not evident in the discussion of functions of gear lubricants, contribute to satisfactory application of such products. After listing some of these qualities, consideration will be given as to what contributes to such characteristics. Such qualities include:

(a)    Adhesion

(b)   Compatibility

(c)    Freedomfrom separation

(d)   Minimumaction on other components of mechanisms, as seals, etc

(e)   Oiliness

(f)     Oxidation  stability

(g)    Satisfactory demulsibility

(h)   Satisfactory detergency

(i)    Satisfactory foam inhibition

(j)    Satisfactory viscosity temperature characteristics

(k)      Shear stability

(l)   Reasonable cost.

                                                                           

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.

Functions of gear lubricants

By stating the necessary functions of gear lubricants there will be a basis for consideration of how these purposes are accomplished and what characteristics are desirable in satisfactory gear oil. While the relative importance of the qualities of such lubricants may vary with the specific use, the listing below includes the most desirable functions of gear oils. Minor qualities will have consideration later.

(a)  Reduction of wear in gears and on adjacent moving parts.

(b)  Reduction of friction and consequently of power.

(c)   Dissipation of heat, that is, act as a coolant.

(d)  Prevention of corrosion ( this statement may have to be modified in the case of EP gear lubricants)

(e)   Reduction of noise, vibration, and shock between gear teeth.

(f)    Flush away contaminants.

(g)  Act as a carrier for additives.

(h)  Act as a structural material in that the lubricant is a factor in determining the strength of the gears against breakage, pitting, and welding.

                    

Total pounds of fluid gear lubricants produced

Period                           United  states                                   Canada

1957                             473,999,270                                         ----

1958                             495,298,609                                      31,963,603

1959                             507,667,875                                      24,864,899

1960                             543,857,266                                      42,664,849

1961                             524,432,779*                                    31,687,444

1962                             521,408,779                                      33,350,169