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.

Mixers and Gear Lubrication

Mixers include a wide variety of speeds, size and types of equipment; consequently, the lubrication of gear drives will be equally varied. Also, both enclosed and open gears will be encountered in such machines. Lacking  recommendations by the equipment manufacturer, one should  rely upon  the fact  that  high speeds in the  gears  demand low  viscosity oils and  low speeds  require high  viscosity lubricants. If the service is severe, use EP oils.

High speeds

The term “high” of course is relative but with pitch line velocities of several thousand feet per minute the lubrication of gears is not simple. Naturally, a low  viscosity oil is  required and the problems are : to have  assurance that a film  of oil  is present when the gear teeth  mesh; to have  an  abundance of oil to remove  heat; and to be  sure that the leaving  oil  will  get out  of the way of fast moving gear teeth. If  a problem is  encountered  as to delivery of oil  to the  mating  surfaces  of high speed gears, the  experience of Dern^20 may  help. This  investigator found  that  when  gears  run at 16,000 to 18,000 feet  per minute, “more satisfactory  results may  be  obtained by   spraying oil radically into  the  teeth  of both gears  at a point as close  as  possible to the mash”. For the purpose, one or two jets of 0.040 to 0.060 inch in diameter, delivering a solid stream of oil, were used. Where the pitch line velocity was 20,000 to 25,000 feet per minute, jets on the leaving side of the gears removed most heat.

Trouble may be encountered with high speed gears churning the gear oil which in turn causes heating. This  is one  reason directional  baffles or  even  a shroud around  the gears  may be necessary in order to keep leaving oil away from the gears  and directed toward a gear case outlet. 

Power Take off Tractor Lubrication

Farm  tractors  from  different  sources  employ  various  types  of power  take off  devices. Therefore, a common recommendation cannot be given for lubrication of such equipment. In  order to  regulate  the  speed  of the  PTO  independently  of the  speed  of the  tractor, clutches  or a  different  set  of gears  may be   used. With  gears  alone, the grade  and type  of gear oil used  for the tractor  transmission may  be  encountered  and in this  case  ATF  may  be  used. The best  procedure is not  depend  upon  the  recommendations  of the  supplier  of  the  machine.

Garden Tractors and Gear Lubrication

The gears in most garden tractors can be lubricated with engine oil. This should preferably be at least an SAE 30 in weight. Any classification of automotive oil available will be satisfactory. At the end of the season the gear case should be drained and refilled with fresh oil.

  

Problems of Feed of Gear oil to Bearings

At  first  thought  the  oil  in gear  cases  is there  primarily  to  serve  gear  sets. However, lubrication of bearings is often of equal importance. In circulating systems an oil supply can be provided for bearings no matter what the location. Bath or splash systems pose a different problem. Here troughs may have to be provided to direct oil to bearings, or flingers dipping into the oil, in addition to the gears, may be necessary.

If  the gear  runs on a  bearing and oil  holes  are drilled from  the roots  of the gear teeth  with  the  thought that  this will provide oil, Merritt^34  mentions  that this  will be  quite  deceptive. Thus, the oil will flow outward due to centrifugal force and will flow inward only if the gear is stationary. Such holes, if they  do not  seriously  weaken  the gear  teeth, can  be used  to lead  oil  outward  from  a supply  at the  center  of a  hollow shaft.

Nonferrous Metal Rolling Gear Lubrication

Metals such  as aluminum, brass, copper etc., are  rolled  into  sheets or  other  shapes in  continuous rolling  mills designed  somewhat like those  found in steel  mills. Modern mills use circulating oils to lubricate the gear drives, pinion stands, and journal bearings. Either mild EP or MP gear oils of a noncorrosive nature can   be employed in such service. The  grade  of  oil  will depend  upon the speeds  of the gears  and  may  vary  from an  SAE  80  to an SAE  140. In  screw down  equipment  either of the above  type  of oils  or a straight  mineral  oil  can  be  used. Where  any  of the above  drives  are  through open gears, a residual  type  of gear  oil  containing  a  rust  inhibitor  and having  a viscosity  of 1000 to  2000 SUS at 210  degree F  should  prove  satisfactory. The  best  practice  is to  apply  such a lubricant automatically so as  to  insure a coating on  the  gears  at all  times. 

Environment and its Effect on Gear Lubrication

The possibility that industry  will provide a different atmospheric  environment or that  the  mechanisms will have  to operate  in space  are  other trends which necessitate  modification in   thinking  on gear lubrication.

For example, Baber, et al.^2  found   that some oils: “ exhibited a decided  increase  in load carrying capacity when the gears were  operated in an  atmosphere of nitrogen  or  argon  instead  of  air”. Also it has been found that controlled corrosion can lubricate metals at high temperatures. This  study  was  concerned  with  oxide  and  halide  films  which  formed on metals  up to 1500 degree F. Here the action was due to the atmospheres surrounding    the metals.

Korp^4  in a  discussion of “Fluid Gear Lubricants—Their  Future” cited  various  environmental  conditions which  gear  lubricants for  space  vehicles  radiation, nuclear radiation, and acceleration and gravity. To meet all these conditions and still provide lubrication affords thoughts for the future.

Armour  Research and Colorado Dynamics Corporation report development of a new  fluid  that  can be changed  from free flowing  to almost  solid by  impressing an electric  field. Viscosity can be controlled through a range of more than 200 to 1. This control is said to offer possibilities for use in mechanical transmissions.

Deep Freezing of Foods Gear Lubrication

The  primary  mechanical  device  concerned  with  freezing  of  foodstuffs  is a  compressor. Many  compressors, used in  refrigeration, depend upon  forced  feed  lubrication and this  feed  is provided in many  cases by  either  gear  pumps or gear  driven  pumps. In both instances, the  oil  being  circulated  provides lubrication for the  gears and is  selected on the basis of that  needed for  the  compressor cylinders and  the bearings. While  deep  freezing is followed in most cases by  conveyor handling, either of packages  or of  containers, the lubrication of  conveyor drives  follows  conventional  practice and therefore, will not  be  detailed. If  reduction gears for conveyors  should  operate in cold rooms, an oil of about  100 viscosity SUS  at 100 degree F and with a pour  point  no  higher than  -40 degree F  should  be used  in  the gear case.

Compressors and Gear Lubrication

Gearing in connection with compressors, whether for air, gas or refrigeration, is almost entirely confined to auxiliary functions. Some  air  conditioning units  have  gear  pumps to provide  forced  feed  lubrication  for  the  equipment. Also, some portable air compressors have gear driven rotary oil pumps. In either case lubrication of the gears may be by the crank case oil.

Shear Stability of Gear and Transmission Lubricants

While most mechanisms containing gears will tolerate a considerable variation in viscosity of the gear oil used for lubrication, a radical change in viscosity at a stated temperature, while in use is not desirable. Further, if such a change is due to a partial breakdown of an additive the purpose of the agent may be defeated. Such changes may occur due to shear while in service. The components most often affected are polymers such as V.I. improvers.

At present, products used in vehicles are the lubricants most often influenced by shear. Such changes will become increasingly important as a single fluid is used for several purposes such as a hydraulic fluid, for ATF, and perhaps as an axle lubricant. Further, a multirange gear and transmission oil has advantage in cars and trucks. That is, oil which will cover two or three SAE viscosity ranges.

Under present formulations some of the lubricants offered for the above services contain polymers as V.I. improvers. However, the action of gears or even pumps tends to change the polymers by shear. The shearing action causes either a chemical or mechanical breakdown of the large polymer molecules so that their value is largely lost in the oil. In some cases a viscosity decrease in service is temporary and in such instances there may be an alignment of the polymer molecules at high rates of shear. Of course some polymers or additives are more resistant to break down with shear than others but unfortunately those of high molecular weight, which prove the most effective V.I. improvers, are also most susceptible to loss of viscosity with shear.

Where high V.I. is necessary or desirable in gear oils, tests should be made to determine the viscosity after shear tests. This is most often done by using a test where a pump forces the lubricant through a sharp edge orifice for a stated time at a given temperature. A similar breakdown of polymers occurs with sonic shear, and a method using this procedure has also been used to evaluate the shear resistance of V.I. improvers.

Klaus and Fenske^34 tested fluids containing about 7 per cent of polymer for their permanent decrease in viscosity due to shear. After 5000 cycles in a pump at 100 degree (F) and a pressure drop of 800 psi, decreases of 25.5 to 30.5 per cent occurred. At a pressure drop of 1500 psi, the decreases were 38.5 and 40.5 per cent. The time required to stabilize viscosity will vary both with the mechanism and the fluid used. 

History related to gear lubrication

We  are  little  concerned  with  the  first  gears, which   were   said  to  consist    of  wooden  wheels   with  wood  pegs   for   teeth, since  speeds   and  pressures   were   low  and  lubrication was  not  much  of  a   problem    at  that  time. However metal    gears of cast iron required   a lubricant   to   reduce   both noise   and wear. For the purpose, animal fats were   used, followed   by   petroleum fractions when   the latter   became   available. The  first  mineral  gear  lubricants  were residua  which  were  quite    sticky  and  therefore   resisted  displacement  by   tooth   pressure.  While   such products still   have some usage, high speeds   and closer   tolerance   led   to the use   of   lower    viscosity   gear   oils.

In  factories  the  transition   from    steam   drives, with   line   shafts,  pulleys, and  belts, to the  use  of  electric    motors    for   specific   apparatus   led   to    the   use   of  gearing   to  reduce  or  change   the  direction   of    drive.  Further   changes  in  industrial   gear  sets   has   been   largely   due   to  both  increased  power    and  speed  of   the  driven   units.  This trend has increased   to the point where   5500 hp   and   higher   rolling   mill   drives have   been    installed   in   steel mills. On  the other  hand , gears  in  watches  and , no  doubt, in  some   instruments   have  decreased  in  size.  Therefore,  when  we  speak  of  gear  lubrication  we  think  in  terms   of  power  delivery   varying  from  a  fraction  of  a  hp  to  several  hundred  hp.

The  wide  use  of  automobiles   and  the  development  of  gearing   for  all  automotive  vehicles   has  been  responsible  for  the  greatest   changes   in   gear   lubricants   over  the  last thirty  or  forty   years. The  Society  of  Automotive   Engineers (SAE)   has  been  a  large  factor  in  improvement  of  automotive  gear  oils. The  SAE   fuels  and  lubricants   committee, which  consists  of  technical  men  from  both  the  motor  car  manufacturers  and  the  suppliers  of  lubricants,  has  been  a  meeting   ground  for  ironing  out  differences   and  arriving  at  a  solution  of  many  technical  problems. While  people  from  governmental  departments  entered   the  picture  a  little  later   than   the  above  two  groups, their   suggestions  and  help  has  aided  in  standardizing   gear  and  transmission   lubricants.    

One  cannot  discount  the  efforts   of   the  American  Gear  Manufacturers  Association  (AGMA)  who  have  suggested  and  tabulated  standard   oils  for  use  in  industrial  gearing  under  various    operation  condition . AGMA   was founded in  1917   and  consists  of  a  group    supplying  about  75  per cent  of  the  cut  gears  marketed   in  the  United  States  and  Canada.

Since  that time this  organization  has issued  certain  engineering   standards  and such  specifications, relative  to  gear  lubricants  and   gear  lubrication, have  been  an  aid  to  the  lubricants  industry  and, therefore, will  receive  further  reference. One of  the first  steps  of  the  SAE   group  was  to  establish  viscosity  ranges  for  transmission  and  rear  axle  lubricants  so  that  the  consumer  would  secure  a  material    within  the  same  viscosity  range  no  matter  who  the  supplier  might  be. The designations  were  in  terms  of   the  approximate viscosity SUS at  210 degree F, thus  No.90, No. 110, and No. 160.Naturally  , a certain  range  was  permitted  in  each  grade, and other grades  have  been in use at various time, such as SAE 80,SAE 250, etc. An  SAE  report , adopted  in  February  1924, indicated  that  at  that  time  transmission  and  rear  axle  lubricants  were  made  from  mineral  oil  with  or  without  the  addition  of  animal  or  vegetable  oils, soaps, etc. The purpose  of  the  soaps  was  to  decrease  the  tendency of  the  lubricant  to  leak   from  the  housings. Such  addition  was  said  to  have  little  or  no  effect  on  the  load  carrying  property, nor  did  it  prevent  ease  of shifting  of  gears . The introduction  of the hypoid  differential drive  changed  the requirements  for  gear  lubricants  for  automobiles  and  led  to the  use  of what  are called extreme  pressure (EP)  gear oils. This change started in 1925 when  the Gleason Gear Works perfected gear generating  machines  which  would  produce  gears  of  the hypoid  type  with  improved  standards of accuracy, strength, and quietness  of  operation. The Packard  Motor Car  Company adopted these  gears  for  final  drives  in  their  1926  models. Other  motor car manufacturers  started  to  consider  the  use  of  hypoid  gears  and  to  change  over to such use  until, by 1937,  practically  the  entire  U.S. passenger automobile  industry  had  adopted  the  hypoid rear axle. A number of truck manufacturers in this country likewise converted to this type of differential. The change in the type of gears in the final drives of automobiles abroad was more gradual. Thus, Towle^10  mentions that the  first use of hypoid  gears  in production cars  in England was  in 1929  and that  it was not until 1934  that further models appeared using  this type of  gear. In the 1951 Motor show in the United Kingdom

                                                                                             

Ninety nine models were equipped with the hypoid axles as compared with forty one with spiral bevel gears. On the continent, the change to hypoid   gears has been even more gradual.

Since  such gears subject  two metal surfaces to a sliding  action  as  well  as  to a rolling one, the problem  of  lubrication  is  more  severe  than  with  involute  gear types and, yet, is as important as  the production  of the gears. Experience quickly demonstrated that hypoid gears could not be lubricated with straight mineral oil particularly under severe operating conditions. However, as early as 1869 a “plumboleum’’ lubricant consisting  of  lead soap and sulfur^4  had been found  satisfactory in one model of  spiral  bevel  gears  where all  other  lubricants failed. Gear  oils  containing  lead  soaps  were being  used  in  industrial  applications  at  the  time  hypoid  gears were introduced  in automobiles. It also  happened  that  the oils used  with such lead  soaps  contained  sulfur  compounds  which  became active  at relatively  low  temperatures. Consequently, such gear  lubricants  were  tried  in the  differentials  of vehicles  equipped  with  hypoid  gears and found useful.   

This  type  of  gear compound  was  used  for  hypoid axles  from  1925 to 1932, but all  such compositions  did  not  prove  satisfactory. At  about  this time  it was found that other compounds might  be  desirable  in  hypoid  lubricants  and Wolf  and Mougey^11 listed  three  general  types  of  gear  oils for  the purpose, namely:

               (a) Sulfur chlorine treated saponifiable oil base with petroleum oil or sulfur petroleum oil;

                (b) Sulfur treated saponifiable oil base with mineral oil or sulfur treated petroleum oil;  

                (c) Lubricants containing lead soap and sulfur.

At this period the motor car manufacturers were appealing to the distributers of lubricants to provide the necessary EP gear compounds. Thus, Wolf and Mougey^11 stated: advances in gear design were urgently awaiting the development of satisfactory extreme pressure lubricants. In1933 Mougey^7 said:  EP lubricants are at the cross roads. Many  of the refiners  are  assuming  the  attitude that EP lubricants are not needed  at the present  time, and  if and  when  required, they will  produce  them, while the automotive  manufacturers  are  hesitating  to introduce gear designs which require satisfactory performance in service  until these lubricants  are universally  distributed  and are available at all filling stations.

 During this development  period  in perfecting  satisfactory  hypoid gear  lubricants the problem  was not only availability  and  composition  but also methods  of evaluation of EP  oils. For this purpose thought was given to testing machines which, by bench tests, would determine the quality of the lubricant quickly. Unfortunate of the value of  EP gear oils did not prove simple.

While  several  EP test  machines  have been  proposed  and  are  still  in use, none  of  these  give sufficient  information  or correlation  to  permit  approval  of  EP  gear  formulations  based  on  such  tests  alone. Initially the Gleason Gear Works set up a testing procedure using hypoid gears, and lubricants were  approved  on the basis  of  this “Four –Square Test.’’ Later, any laboratory  tests, even if on full  scale  axles, were  supplemented  by  use  in  cars  on  the  proving  grounds  of  automobile  manufacturers.

Specifications  under  which  hypoid  gear  lubricants  have  been  manufactured  and  sold  have  changed  frequently  over  the  period  from  the  introduction  of  such  gears  up  until  the  present. Using  the  experience  of  motor  car  manufacturers  and  of  oil  companies, the  Federal  Government  set  up  such  specifications  in  1942.Since  products  meeting  these  requirements   did  not  prove  entirely  satisfactory  for  high  torque  low  speed  performance  of  heavily  loaded  axles, a  Coordinating  Lubricants   Group, under  the  Coordinating  Research  Council  was  formed. Under  their  direction  further  standardization  of  test  methods  was  arrived  at   and  some  suggested   changes  in  government  specifications  for  EP  gear  oil  could  be  produced  which  would satisfy  all  automotive  vehicle  requirements, whether  the  operating  conditions  be  one  of  high  speed  and  low  torque  or  low speed  and  high  torque. At the time of  writing, formulations  are  available  which  satisfy  both  conditions, but a  few  consumers  are  somewhat  dubious. 

Automatic  Transmission  Fluids (ATF)  have  somewhat  the  same  history  and  resulting  solution  as  in  the  case  of  hypoid  lubricants  at   an  earlier  date. Since  the  type  of  fluid  used  is  rather  critical  for  proper  performance  and  there  was  no  wide  distribution  of  a  suitable  fluid, the  motor  car  manufacturers  at  first  provided  the  lubricant  under  a parts  number. Within  a  matter  of  a  couple  of  years  after  the  introduction  of   automatic   transmissions  on  various  cars, the  oil  companies  were  able  to  offer  approved  ATF  quite  generally.