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.

Transaxle Fluids

While a single lubricant for both transmissions and rear axles of certain automobiles is desirable, Hunsted et al.^28 ,when reviewing the situation with regard  to such fluids, concluded that as of January 1960 considerable  work  remained  to be done on transaxle fluids to achieve optimum results. Up to that  time  some  42 mineral  oil base and  synthetic base  fluids, with  various  additives, had  been  evaluated and only  three showed  any  promise  for  future  development. Since  it is  impractical  to  suggest even  tentative  formulations, all that  can be  done is to  indicate  the  requirements and  the handicaps.

A  transaxle  fluid  should  have  all  the  desirable  characteristics  of ATF and  in  addition  should  have  sufficient  EP value  to  lubricate  hypoid  gears. The above statement should be modified in that the desirable characteristics of ATF should also be maintained while in service. Thus, when  the  fluid is subjected to the  shear  effect of mating  gears, polymer type  additives may  lose  a large  part  of their  effectiveness  due  to  breakdown  of the  polymers to  lower  molecular weight  products. Therefore  base  fluids with  inherent  high  V.I. or  polymers of  initially  lower molecular  weight  are  dictated  in the  formulations.

EP agents  used  should  have no  adverse  effect  on any  of the  components of the  automatic transmission such  as  clutch  materials, various   metals, synthetic seals, etc. Of  course the most desirable  transaxle fluid  would  be  one  which  could  be  sealed  in a  case  and serve the  life  of the  car  but  first  a  fluid which  will perform  satisfactorily when  changed  periodically  will serve.

According to  Celanese  a  trimethylolpropane  ester  with  an EP additive  offers promise  for use  as a transaxle  fluid. Another suggestion for the purpose is methylchlorophenyl containing about 3 per cent chloride.

Power Steering Gear Fluids

As a base for such a fluids, a solvent refined oil of about 250 viscosity SUS at 100 degree F may be used. While , as Johnson and Mortensen state, the major requirements are shear  stability  and high  V.I., the oil should  also  be  resistant  to oxidation and  have no  adverse effects on both metals  and  seal and  hose  materials. Likewise the blended oil should have a low pour point. All of the above point to a solvent refined oil which will have the desired high V.I. and aniline number.

Chrysler MS 3590 “Hydraulic Steering Fluid with Rust Preventive”, summarizes the composition requirements as follows:

“Well refined oil with admixture of proper rust preventive additives. Small amounts of oxidation inhibitor and pour depressant may also be used to meet the details requirements. The use of   detergents and V.I. improvers is prohibited. The additives small be selected to have a minimum effect on the seal and hose materials”.

Therefore, additives may  include up to 0.5 per cent  of an  oxidation inhibitor, such as  can be  recommended  by  an  additive supplier, or  may  be  phenyl  alpha napththylamine , 2,6-ditertbutyl – 4- methyl  phenol, mixtures  of these  two  types, or  an oil-soluble  organic thiophosphate salt of certain  metals, such  as zinc  methylphenyl  dithiophosphate.

If the pour point of the oil is above -20 degree F, from 0.2 to 1 per cent of a pour depressant, such as “Acryloid  150”, “Paraflow”, or “ Santopour C”, can be added.

Suitable rust inhibitors, used in concentrations of 0.5 to 2 per cent, include petroleum or synthetic sulfonates, such as “Petronate” or barium dinonyl naphthalene sulfonate. A combination oxidation and rust inhibitor, which is usually used in a concentration of 1 per cent or less, is “Vanlube 76”. 

Nonmetallic Gears and Their Lubrication

A variety of materials has been and will be used for the manufacture of special gears and thus a blanket recommendation cannot be made for the lubrication of all nonmetallic gears. While the concern  here is  lubrication, it is of  interest  that these materials  tend  to  have less  mass  per  unit  volume than  the  ferrous metals and thus the resulting  gearing  has  less inertia of  rotating parts. Further, injection molded gears are about as low in cost as any gears. AGMA 250.02, Dec. 1955, makes the following statement:

“The gears should be lightly loaded if lubrication is not permissible.”

Certain small gears, particularly if made from “Teflon” or “Nylon,” are run without lubrication. However, Carlyon^9, when considering plastics for bearings, found that: “Even small amounts of oil present during typical manufacturing processes often will improve” friction and PV ratings. It was also shown that the continuous presence of oil raises the PV values of “Nylon” and “Delrin”. Graphite is sometimes used as filler in plastics and may thus aid in lubrication.

Medium sized nonmetallic gears are generally used in mesh with metal gears. Thus, “Bakelite” spur gears, which are made by impregnating fabric with “Bakelite” resin and then applying pressure and heat, are used with metal gears. Laminates may also be made of paper impregnated with phenolic resins. Heavy oil or lubricating grease is used as the lubricant on such pairs and the amount and frequency of application depends upon the severity   of service.

Rawhide pinions  disintegrate  when  in contact  with  mineral oil and therefore, a mixture of tallow and  graphite can be used  for lubrication of  such  gears in mesh with metal  gearing. Such pinions furnish  noiseless,  resilient  service with  little  vibration  and  prolong  the life  of the metal  mate.

Pinions  can  be made  from most  materials which  can  be molded or cast, often  in  conjunction with  other material. Thus, “Fabroil” pinions  are  stated to  be  made of  pressed  cotton  held  under  compression by  steel  shrouds. The manufacturer of nonmetallic gears should be in position to recommend suitable lubricants for the same.

Plastic gears are sometimes used in small gear pumps, and here the material being handled, such as water, petroleum products or solvents, may serve as a lubricant.

Undoubtedly the use of gears made from plastics will increase both as to number and types. While it is not  the  function  here to  treat of  their  characteristics, some  of  the  qualities  should  be recognized in  applying  lubricants. The  fact  that these  synthetic  materials  have lower specific  gravities  and consequently  less  inertia  than  metals indicates  less  impact   when  gear  teeth mesh. Also plastic gears  tend  to  have greater  elasticity  than  metal; therefore, they  should  conform to a  mating  gear without  the necessity  of a thick lubricating  film. Since  some  of  the  non-metallic  gears are  thermoplastic they  will  not  be used  under  conditions  of  elevated  temperatures; consequently, high temperature  lubricants will  not  be required.

The solubility characteristics of plastics will vary with the type and a lot of gears made from such materials can advise as to any restrictions in fluids to be used with specific plastics. Bueche  and  Flom^7  who investigated “Surface Friction  and  Dynamic  Mechanical properties  of  Polymers” found  that  water solutions of  sodium  stearate  were satisfactory lubricants for either “Neoprene” or branched  or unbranched  polyethylene  when in contact with steel. While  “Delrin” can be  operated  in a bone-dry  state, pretreatment  with lubricating  oil  has  been  found  to  reduce its  erratic  operations and  improve  the  abrasion  resistance. The recommended  pretreatment  consists  of  complete  immersion in a low  viscosity  oil for one  hour at 250 degree (F) after  which the part is wiped  dry. Gears  of  this  materials  have been  operated  in  contact  with  both  oil and  lubricating  grease with no  deformation or  wear. Since the variety  of  types of plastics  used  in gears will  increase,  the  safest procedure in  lubricating  such  parts is  to  consult the  supplier.

Gear lubricants as structural materials

Since gear sets would not function without a proper lubricant, it has been suggested that gear oils be given the status of structural materials. As early as 1942 Almen^1 stated that a gear lubricant in addition to being: “A lubricant in the usual accepted  sense of a liquid film separating two rubbing surfaces,” should also be considered as “a structural material in the sense that it is an important factor in determining the size of gears.”

More recently, Blok^7a devoted an entire paper to “Gear Lubricants as Constructural Elements”. This author suggests that the designers of gear sets keep in mind that:

“The lubricant is to be conceived as a constructional material, and thus its constructional properties, such as viscosity and antiscuffing properties, well deserve to be accounted for even in an early design state”.

Consequently, Blok^7a plotted curves connecting power transmitted with speed, showing the barriers which must be raised either by improvement in materials or by the use of special lubricants if load capacity of gears is to be increased

Introduction

Almost  all  modern  machinery  makes  some  use  of  gears  as  a  medium  for  transmitting  power. Such use  may  be  for  the  purpose  of  either  changing  the  speed  of  a  driven  member  or  changing  the  direction  of  movement. In  either  case  the  gears  act as a lever  so  that , in  most  cases, the  force  transmitted  is  multiplied  and  in  so  doing  speed  will  be  reduced. When  variations  in  the  torque  or  speed  transmitted  are  desired, a  series  of  gear  ratios  can  be  provided  in  the  same  mechanism.

Most  gear  sets  are  lubricated  with  fluid  products  or  gear  oils, and  consequently, such  compounds  will  receive  major  consideration . Since  plastic  or  semi-fluid  products, such  as  lubricating  greases, as  well  as  solids  are  sometimes  used  as  gear  lubricants, they  will  also  receive  mention.

It  is  also  the  purpose  to  describe  fluids  employed  in  certain  fluid  drives, particularly  automatic  transmissions  in  automotive  vehicles. Here  the  oils  in  question  serve  a  number  of  functions, and  it  is  necessary  that  a  single  fluid  satisfy  not  only  lubrication  of  different  mechanisms, but  also  act  as  a  torque  converter  and  hydraulic  agent.

This  is  simply  an  extension  of  the  functions  which  gear  oils  fill  in  many  case. Thus, gear lubricants   also take care of bearing lubrication in many   applications. It will  be  presumed  that  most  readers  are familiar  with  the  mechanisms  to  be  lubricated, such  as  automatic  transmissions, hypoid  gears, worm  gears, etc. Therefore, little description of mechanical details will be attempted.

The  primary  interest  will  be  in  lubricants  for  metal  gears, although  mention  will  be  made  of  the  requirements  for  gears  made  from  other  materials. An  attempt  will  be  made  to  thoroughly   explain  how  lubricants  for  gears  function, what  they  contain, how  they   are  compounded, and  the  limitations  of  their  use. In  all  of  this  it  should  be  kept   in  mind  that  to  be  most  effective   any  lubricant  should  be  applied  in  the  right  amount  at  the  right  place  and  the  right  time.

Fundamentally  the   types   of  motion  in  operating  gear  sets  are  those  occurring  in  other  machine  elements , namely, rolling  and  sliding  motion. The two types of movement   may occur simultaneously as in meshing   hypoid gears. This  is  pointed  out  so  that  it  will  be  realized  that, even  though  the  configuration  of  gears  differ   from  bearings, the  lubricating  problems  are similar  for  both  mechanisms. Another  point  to  keep  in  mind  is  that  while  the  title  of  the  mentions  both  gear  and  transmission  lubrication, the latter  mechanism  is  most  often  one  or   more  gear  sets  serving  a specific  purpose. Therefore, except  in  the  case  of  fluid  drives, the  problem  is  one  of  gear  lubrication. Finally , some of  the  possibilities  of  future  trends  and  developments  in  gear  lubricants  and  torque  fluids  will  be  mentioned.