Deposit Forming Tendencies of Aircraft Turbine Lubricants

This test method describes  a procedure for  determining the  deposit  and  sludge  forming  tendencies  of  aircraft gas  turbine  lubricants when  a sample  of the  oil is circulated  under  controlled conditions for a  prescribed  period of time  through an  aerated test  chamber  containing  an  aluminum  tube  held at a constant  temperature.

The coking tube is held at 590 degree F while oil  heated  to  300 degree F  is circulated by  a pump  from  the  chamber through  a  cooler and a line  filter and  back  into  the chamber. The oil flow is regulated to 300 ml per minute while air flow is the same amount.

At  the  end  of the test, the  weight of solid  decomposition  products  on  the  heated tube  and  in the  line after  are  determined. Also,  changes  in the  viscosity and  neutralization  number  of the oil can be  determined  if  desired.

Corrosion fog cabinet

A number of test  methods, of which  this is one, make  use  of  bench  tests to  indicate  how  well  a lubricant will  protect steel  from rusting. These  methods  are  most  often  used  for  comparatively  low viscosity oils,  such  as  turbine or  aircraft oils.

Cold - rolled sheet steel panels 2 x 4 x 1/8 inch, surface ground to a finish of approximately 20 micro –inches as measured by  a  Brush analyzer, are  used  as  specimens. These are  coated with  the  oil by  dipping  after  which  they  are  held  in a  rotating  table in a  cabinet  into  which  water  is  atomized. The  cabinet  is capable  of  regulation  from  110  to 160  degree F, but  most  tests are  run  at the lower  temperature  for a given  number  of days  or  hours.

The  specimens  are  observed  through  a window  at the  end of the  first  24  hours  and  each  subsequent  24  hours  increment of  exposure. The  time  of  failure  of a  specimen  is  recorded  as  the  day  on which  at least  3 rust  sports 1 millimeter  in  diameter  forms  on the  front  surface  of  the  specimen  in the  central  area  which  excludes  ¼ -inch zones  adjacent  to  the  top  and  sides  and a ½ - inch zone  at the  bottom. Three specimens are coated with particular oil. At  least  two  of  these  must  last  the  specified time  without  failure.

Chlorine in lubricating oils

In the first  methods the  sample  is oxidized  by  combustion  in a  bomb  containing  oxygen  under  pressure. The  chlorine  compounds thus  liberated  are  absorbed  in a  sodium carbonate  solution and  the amount  of  chlorine  present is determined gravimetrically  by  precipitation  as  silver  chloride.

With the second method, the sample, dissolved in a low boiling hydrocarbon mixture, is boiled under reflux with metallic sodium and n-butyl   alcohol. Under  these  conditions the  chlorine  is converted  to sodium  chloride which  is then  extracted  with  water. The  chloride in  the  extract is  determined  volumetrically  by  titration  with silver in the  presence of  thiocyanate.

  

Chemical Analysis for Metals in Lubricating oils

These methods of chemical analysis are intended  for the  determination of barium, tin, silica, zinc, aluminum, calcium, magnesium, sodium and potassium in  new  and used  lubricating  oils. Other metallic elements, sulfur, phosphorus and  chlorine in  amounts  commonly  found  in lubricating  oils  do not  interfere in this  method.

Essentially,  the  method  consists  of  igniting  the sample, dissolving  the  residue  in  mineral acid and  then  separating  the various  metals by  conventional  methods.  

Sulfur in Gear Oils

In this method  the sample is vaporized  and  burned in a  stream of air, and  the  oxidation completed  by passing  over  quartz particles  maintained  at a temperature  of 950 to 1000 degree C. The combustion  products are then  passed  through hydrogen  peroxide  which  absorbs  the  sulfur  as sulfuric  acid  and  the  chlorine  as hydrochloric  acid. The  absorbent  is then  analyzed  for total  acidity  and  for chloride ion. The  procedure for  the  determination  of  chlorine  is  included  only  for  correcting   the  sulfur  content  and  the  method is not  recommended  for the  determination  of chlorine  alone. 

Pentane and Benzene Insolubles in Used Lubricating Oils

Normals  pentane insolubles are the  insoluble  matter  which can be separated from a solution of oil  in n-pentane and  in addition  to benzene  insolubles, may  include  insoluble resinous  bitumens produced from the  oxidation of oil. While  this  test  was  designed  primarily  for  use with used  engine oils, it does  have  some  value   in determining that  a breakdown has  occurred  in gear  oils. Any  appreciable  amount  of pentane  insolubles  in used  gear  lubricants indicates that  the  gear  box  has been  quite hot, usually  above 300 degree F.

Benzene  insolubles  are  that  portion of n-pentane  insolubles  not  soluble  in benzene and  may  include  insoluble  matter  produced by  oxidation and  thermal  decomposition  of the  oil  and  oil  additives. Any  suspended  particles  from  metal  wear  or from  external  contamination  will  also be  present  in the  benzene insoluble  fraction.

Aniline Point of Petroleum Products

Aniline point is the minimum equilibrium  solution temperature, generally given  in F although C is also used, for  equal  volumes  of  aniline  and  oil. Aromatic  compounds in oils  contribute to a low  aniline point  which  in  turn  indicates that  such  an  oil will  soften or  swell  both  natural  and  most  synthetic  rubbers. Therefore  to  insure  minimum  action  on rubber  gaskets, seals, etc., oils  with high  aniline  points  should  be  selected. Solvent refined oils  as a  rule  satisfy  such  requirements. 

Missile and Space Vehicles Gear Mechanisms and their Lubrication

While details of space vehicles are not publicized, it can be expected that gears may enter, even if only for small instruments. These will no doubt be of a nature which will not require fluid lubrication. However, Hartman^24 mentions that there may be a gear drive between the turbine and the shaft on certain liquid rocket engines. Where kerosene is the fuel used, this also provides lubrication for the gears. However, kerosene alone allowed scoring of gears and consequently additives were included. Use of 2 per cent by volume of zinc dialkyldithiophosphate  in the fuel, decreased gear wear. This combination also improved the rust resistance of gears. Such a kerosene additive mixture is suggested as a break in lubricant no matter what type of lubricant may be used in service. In this connection, an article by Watson^51 entitled “Materials and Ratings for Dry Running Gears” should be of interest. After  experimenting  with  various  materials for  gears, it  was  found  that  under  light  loads, spur  gears, made of case  hardened  En  steel, Phosphate  prior  to  coating  the  flanks with  molybdenum  disulfide, would run  continuously  in a dry state  without measurable wear.