Moisture Corrosion Characteristics of Universal Gear Lubricants

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

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

Chemical Activity Toward Copper of Universal Gear Lubricants

This test, which provides for  immersion  of a clean  copper strip in a  sample  of the  lubricant for a  prescribed  time  and  at a given  temperature, rates the  strip at the  end  of the test  as follows: “ At the conclusion  of the  test  the copper  strip  is removed, rinsed with sulfur-free  acetone and  inspected. The  degree  of discoloration of the  strip  is expressed  in one  of the  following  terms:

a.       None

b.      Stained

c.       Light peacock

d.      Dark  peacock

e.      Black.”

Carbon Residue of Oils

These methods provide  some  indication of the  coke forming tendencies  of oils and  hence have  little  application  to gear  lubricants. Also oils  containing ash – forming  ingredients will have  erroneously  high  carbon residue. Values  obtained by  the  two  methods  noted  are not the same. Older  specifications for high  viscosity oils  often  give the Carbon  Residue by the Conradson Method, but the present thought is to  use the  Ramsbottom  Method

For the determination  of this  value a weighed  amount of oil is  subjected to heat so that  evaporation and  pyrolysis  takes  places under  conditions of  restricted  air  supply. In  fact  in the Ramsbottom  Method the  sample  is contained  in  a Coking  Bulb with  a very small  opening. While  the term  Carbon  Residue  is used, it is  recognized  that the  coke  remaining  after  such  tests is not  entirely  composed of carbon.

Sulfated Residue, Lead, Iron and Copper in Lubricating Oils

In this  methods, the sample  is completely  oxidized  by wet  ashing  with  concentrated  sulfuric acid, concentrated  nitric  acid and  hydrogen  peroxide, or  if  only iron and copper  are  to be  determined, by dry  ashing. To determine sulfated  residue, the  solution obtained by  wet  ashing  is  transferred  to a weighed  dish, evaporated  to  dryness, ignited at 775 degree C, cooled and  weighed. In determining lead, iron and  copper, the  solution resulting  from  the  wet  ashing is  mixed with  alcohol and  filtered  to give  a  precipitate  of  primarily  and  sulfate. This  precipitate  is  boiled  with  sodium carbonate and  then  precipitated  with  hydrogen  sulfide.

Iron  and  copper are  determined on  separate  aliquots  of the  filtrate  from  wet  ashing. The  iron is  separated  as the  hydroxide and  determined colorimetrically  as the  orange  ferrous  orthophenanthroline  complex,  while  copper  is  determined  colorimetrically  as the  yellow diethyldithiocarbamate  complex

Calcium in Lubricating Oil

While the ASTM Methods and Fed. Test Method use the same sample  for  determination  of  several  metals, the IP  Methods  is  specifically concerned with  calcium  analysis. This calls  for ashing  followed  by determination  of the  calcium  either  as oxide  or oxalate.

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.

                                                                            

Boron Compounds as EP Agents

Boron compounds have been  recommended as EP agents and also for other  purposes in fluid lubricants; hence, they may have a place in gear oils. Thus, organic borates  for  use in mineral oils to  prevent  excessive oxidation is  claimed. The use  of  compounds  such as  chlorophenylboric  acid  dibutyl  ester, as well as  organic fatty  materials reacted  with  boron trifluoride, have been  mentioned as EP  agents, finally boric  acid has been used for the purpose.

Identification of Gear oil Additives

Verification of the presence of  various additives  in gear oils is often of  interest and different methods have been  suggested for the purpose. To identify dialkyldithiophosphate additives in lubricating oils, lewkowitsch  isolated  the  compounds in the form of copper salts. Paper  chromatography  was applied to the  identification of certain  anti-oxidants by Delves. Among the compounds  distinguished  were  diphenylamine, phenothiazine, and  phenyl alpha and  phenyl  beta  naphthylamines.

Lubrication of Non-Reactive Surfaces at High Loads

By non-reactive is meant that the  surface will not react with  what  are  customarily  known  as EP elements such as  chlorine, phosphorus, or sulfur compounds. While there is little likelihood of extensive use of  some metal  combinations, it is well  to mention the possibilities. Thus,Antler suggests that 0.03 to 10  per  cent of trimeric  tin  sulfide compounds in  either  oils  or lubricating greases will  increase  the anti wear  qualities of the lubricant on surfaces such as  titanium-on-titanium, stainless steel-on-stainless steel, or gold-on-gold. The same  additives are also  said to be  effective on plastics, such as “nylon”, polyvinyl  chloride, polyethylene, etc.

Chromium is inert to most sulfur compounds but  will  react  with  most  chlorine compounds. Consequently, in  lubricating stainless steel  gears  under  heavy  loads, the  use  of chlorine compounds is  dictated.

Manufacturers  of gears  from  special  metals or  other  materials will no  doubt have  suggestions as to  the  proper  lubricants. Since EP agents are not  effective on  Babbitt, use of such  compounds in lubricants for  certain journal  bearings will  not  increase  the  lubricating  value.

Deodorizing Gear Lubricants

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

Self Repairing Surfaces on Gears

Some thought has been given to addition of material to lubricants which would aid in providing a more uniform surface on gear faces as  they operated. Thus,  a report was  made of the use  of molybdenum disulfide particles, mixed with the lubricant, which were  presumed to pack in  irregularities of the teeth  on a large gear. Where other solids are  used in  gear  lubricants, they may  act as mild abrasives and, thus, produce a  polished surface after  running. Where gear  faces are  smoothed out  during operation, the  action  is  probably  due  to  attrition. Therefore, any  self healing  procedure  for such  surfaces  remains  to be  devised.