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.

Automatic Transmission Fluid proving Ground Tests

Satisfactory operation of such  fluid is  observed  during  a 2000 mile  schedule  on proving  Ground Automatic   Transmission  Performance Test  Schedule. This requires a new or rebuilt Hydro Matic transmission   in stalled in a car. Characteristics of the fluid, such as viscosity at 200 degree F before and after the test are observed. Tendency to squawk is noted and also smoothness of operation.

Type Designations for Specific Applications

Viscosity or grade designations may be used by other groups or organizations. AGMA are  the  best  example of  such a group, but  the  American Society of  Refrigeration Engineers have set up  viscosity standards and  other  organizations are  contemplating  such  action.

The  manufacturer  of  machinery  often  finds it  necessary  to  provide a  designation of a  lubricant which  has been  found  satisfactory for a specific mechanism. At times  such a  designation may  include  the name  of the  device as  is  the case  with  automatic  transmission fluid, known as ATF. Other  such  types  of  lubricants may be  evident  when  specific  applications  are  considered.  

Rubber Swell due to Gear oils

Where gear  oils  are  in  contact  with  rubber gaskets, seals, etc., the  action of  the  fluid on the  rubber compositions is of  importance. Consequently a Federal Test Method, 3604-T, has been set up. Such a test  can  be  varied as to  temperature, time and the  polymer     composition tested  depending upon  the    actual  service conditions of the  gear oil.

Thus, for ATF the following procedure is used. Sections of piston seals made from Acadia 3612 rubber compound are subjected to hot transmission fluid. Measurements of volume and hardness are made before and after the test. Two  seals are  cut  into  eight  sections each  and  the  sections  are  suspended  in  test  tubes  containing the lubricant and  held in a bath  at  300 degree F for  70  hours. The  sections are  then  cooled for  thirty  minutes  to  one hour in a  fresh  portion of the  test  fluid maintained at room  temperature. After wiping dry, the final volume and durometer readings are taken.

Hydraulic Transmission Fluid, Type C-1

In 1995 Allison  Division  of  GMC  provided a  specification  for a  fluid  for  use  in  heavy  duty  truck, bus  and  earth  moving  vehicle  hydraulic  transmission, which  they  manufactured. In  these  application, Type  A  and Type A, Suffix A,  ATF  products,  described  later, showed  exceptionally  satisfactory   performance. However, the service in question did not demand all of the characteristics   of ATF. Therefore, by reducing the requirements of the fluid, less costly ingredients could be used.

The Allison specification was designated Hydraulic Transmission Fluid, Type C. Many heavy duty motor oils in the SAE 10W, SAE 20, and 20W viscosity classes in addition to some special fluids, adequately met the specification. Subsequent service showed that fluids in the SAE 10W class were most desirable. Consequently the specification was modified and updated as of 1997. A designation Hydraulic Fluid Type C-1 was   assigned to the new product. It  will  be  found  that other  manufacturers also  recommend  fluids of this type  for their  heavy  duty  torque  converters.