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.

                                                                            

Malfunctioning of Other Mechanical Elements

Misalignment may be due to wear of bearings and in extreme cases of failures it may be difficult to determine whether a bearing or a gear failed first. In case of bearing failures, when lubrication was at fault, either oil did not get to the bearing. Or the proper oil was not used. Where  a common  oil is used for bearings and gearing, the  viscosity of the oil is  generally  a compromise  since  the bearings operate best with  a lower  viscosity oil than  is dictated for the gear sets. Therefore, when bearing trouble is encountered, thought might be given to use of a somewhat lower viscosity lubricant.

Malfunctioning of flexible couplings may also contribute to trouble with gear sets. Such couplings are often used to connect reduction gears with a different piece of apparatus. These  couplings may not only  break but  also  become  jammed, so that  they  are no longer flexible; thus the  gear  shaft will  be thrown out of line. Another possible source of trouble might be a break in a gear case or supports which would permit a shaft carrying gears to be misaligned.

Characteristics of lubricating oils

Specifications for lubricating oils commonly include all or some of the following values:

        (1)   Gravity

        (2)   Flash Point

        (3)  Fire Point

        (4)    Viscosity

        (5)   Viscosity Index (V.I)

        (6)    Pour Point

        (7)    Carbon Residue

         (8)    Color

        (9)    Neutralization Number

In  addition  certain other  characteristics  or  tests  will  be mentioned as  these  are  sometimes called for  and   hence  may  me  be of  value in  determining the  suitability of an oil  for  certain purposes. Some such qualities are:  Aniline Point; Ash; Corrosion Characteristics or Protection; Precipitation Number; and Steam Emulsion Number.

Methods  for  conducting  tests to  determine  values for  oil  characteristics  have been  standardized  by  American  Society  for  Testing  and  Materials  ( ASTM)  and  in many  cases  the  methods  are  in general  agreement with  Institute  of  Petroleum (IP)  and  U.S. Government  Methods as   shown  in  Federal  Test  Method  Standard  No. 791.

In  the  details to follow ASTM and  IP numbers for  methods of  test will  be given followed by  a short description  of what  the  test consists of. Also,  where  possible  the  significance of  such  qualities or  tests, as  they influence the  selection and use  of an oil for gear  lubricants, will be  noted.  

Minimum action of gear oils on components of mechanisms

Well refined mineral lubricating oils have little if any action on most metals, particularly ferrous metals. It is only upon prolonged use at elevated temperatures that such oils may from compounds which will act upon metals. Since such oil changes can be retarded or almost completely arrested by the use of oxidation inhibitors and also metal deactivators or pacifiers can be included, there should be little concern about the action of straight mineral oils upon the metal components with which they come in contact.

What we are concerned with here is the action on parts made from other materials, such as paper, plastics, rubber, etc. Seals are likely to be made from rubber, either artificial or natural, and any deterioration of the compositions due to the lubricant should be at a minimum. Many seals consist of compounded materials, such as “neoprene,” and it is found that oils high in aniline points, as are most high V.I. oils, will have little effect on this compound.

Automatic transmission mechanisms may be found to contain paper and “Nylon” parts. In future devices which will require transmission fluids, a greater variety of components may be used. The safest course when supplying oils for contact with unusual materials is to have the fluids pretested to determine their suitability.