Lubrication of Gear Sets on Agricultural Equipment

While some open gears are still encountered on farm machinery, modern equipment, for the most part, has precision cut gear sets operating in enclosed gear cases. Therefore, most gearing on farm machinery can be satisfactorily lubricated with two types of gear oils.

First , for  open gears  a  residual  type  of  adhesive  gear lubricant, which can  also  be used on  wire ropes  and  some  chains, is  recommended. Such  a  lubricant is available as a material  which will  hardly  pour at ordinary temperatures  and  must  sometimes be  heated in  order to apply. Much more convenient is a cut back from of such lubricants which can be applied by pouring or dipping. Likewise, aerosol containers of the same type of material permit spray application.

The enclosed gearing on farm equipment is bath lubricated so that the partially submerged gears, when in operation, pick up the gear oil and where necessary, transfer a portion to bearings. While  it is  not  possible  to  list every  variation  in the  mechanisms of  gearing  on  agricultural  machinery, the  rules for  lubrication of such expensive  and  often  intricate  equipment follow  general  pattern. That is, the use of the proper gear lubricant in the proper amount and the regular replacement of such oils as required.

While the  recommendations  of  the  manufacturer  are a  guide  as to the proper  gear  oil to use  in each  piece of  equipment, there  is a  tendency  to  simplify the  number of  lubricants  required  on a farm. With this in mind, multipurpose gear oil will serve most enclosed gear sets. This  can be  a  mild  or  regular EP  type with  the  use of an SAE  140  grade for   hot weather  and  an SAE 90 grade  for  winter. In very cold weather an SAE 75 grade of gear oil can be used.  The  matter  of  reduction in the number  of  lubricants  necessary  for  farm  equipment  will have  further mention in  the next section. It  will  then be  evident that  manufacturers   of  such  machinery also  have  this  in  mind.     

High and Low Temperature Stability

While  separation  of  additives at  low  temperatures  might  influence  either  pour  points of low  temperature  fluidity, fed. Test Method 3460 is primarily concerned with the stability of blends. The method is used  to  determine  the  tendency  of the  components of  blended oils to  be  incompatible  when  subjected to  temperature cycling. It  consists  of heating  the  sample  of  oil to  205 degree F for  24 hours, cooling it  to  0 degree F for  16  hours,  storing  it at  room  temperature for  10  days and  examining  it  for  changes in appearance.

Nonferrous Gears and Their Lubrication

While the tendency is to think only of gears made of ferrous metals, we find in Dudley^16 the statement: “A wide variety of bronzes, aluminum alloys, zinc alloys, and nonmetallic plastics and laminates are used to make gears.”

Fortunately, well refined mineral oils have little action on most of the combinations. We say  combinations  because in  many  cases the nonferrous  gear is driven by a  steel pinion, but when the loads are light and the  gears are small, both members  may be of the nonferrous  material.

Nonreactive oils should be used with nonferrous gears unless specific recommendations state otherwise. This is illustrated in the case of worm gears where the gear is normally made of bronze. The  general  recommendation for  such units is a mineral  oil  containing  tallow, although  often  such  oils  contain  lead  soaps  and  occasionally certain EP agents.

Synthetic fluids, both diesters and “Ucon fluids, have been used as lubricants with nonferrous or ferrous and nonferrous combinations of gears. Where a problem of lubricating  an  unusual combination of  gear  materials is  encountered, the manufacturer of  the gear set should  be  able to  make a safe  recommendation. However, lacking a  suggestion, a well  refined  lubricating  oil  with no reactive additives  present  can  generally be used with safety in the  case of  nonferrous  gear sets.

Detergent qualities of gear and transmission lubricants

Detergent additives are not required or included in most gear oils. However, in mechanisms where the lubricant serves both gears and control devices, such as clutches, some of the moving parts will not tolerate deposits. In such cases detergent additives are included in the lubricants. A good example of such usage is in ATF.

The term detergent dose not properly describes the function of such compounds which are really dispersants or peptizers for materials that would otherwise appear as deposits on parts of mechanisms. Antioxidants which may be present in the gear oils are not completely effective in preventing formation of gum and varnish constituents, hence, the need for dispersants.

Detergents now used consist largely of phenates or sulfonates with a lesser amount of polymers. Either the phenates or sulfonates are added as metal salts, most often of barium or calcium. While neutral salts are satisfactory dispersants, the tendency is to use basic compounds since they will neutralize any acidic products formed during oxidation. A further advance is the use of non-ashing detergent additives, most of which are specific polymers, with the suggestion also of nitrogen containing soaps

Demulsibility of gear oils

Lubricating oils vary in their tendency to emulsify with water; therefore, if emulsification of gear oils is likely to be a problem, the base oil should be investigated. Any polar compounds remaining in the oil after refining, such as sulfonates, petroleum acids, and even asphaltic bodies, may help to stabilize emulsions. Well refined oils of low viscosity  will  have  the  least  tendency  to  from  permanent  emulsions  with  water.

High interfacial tension will tend to cause emulsions to break. Many oil field emulsions are broken by causing the emulsion to revert from water in oil type to oil in water type. However, the best solution for this type of trouble is to choose oil additive combinations which do not promote emulsification.

Corrosion prevention by gear lubricants

Ellis et al.^22 consider that staining, tarnishing, and rusting are all  indications of corrosion. The thought is that light stain or tarnish represents the early stages of corrosion since, unfortunately these  changes do not proceed very far before pitting starts. Unreactive gear oils, which have not been subjected to excessive high temperature oxidation, have no tendency  to corrode  metals but, under moist or humid conditions or in the presence of  most salts or acids, do not offer proper protection against  rusting of ferrous  metal  surfaces such as gears. However, additives can be included in gear compounds which will provide rust prevention. Where conditions of incipient rusting prevail, the gears and other metal parts even to the inside of the gear case may require protection, particularly when idle. In such cases not only will the presence of a rust inhibitor but also the viscosity of the base oil be factors. Thus, the higher the viscosity of the gear lubricant, the slower this will drain from the metal surfaces and consequently the greater the rust prevention. Rusting may occur in different environments and various theories are offered as to the mechanisms of corrosion, but normally moisture and oxygen are the offenders

Most rust preventives are polar substances, such as long chain fatty acids, fatty amines, metal sulfonates, certain esters, oxidized petroleum fractions, etc. Such materials wet a metal surface preferentially and displace any water which may come in contact with the steel. The coating of polar substance then acts as a barrier against water reaching the metal surface.

As previously mentioned, controlled  corrosion due  to  EP  additives  is generally beneficial in  that  it  corrodes  away high  spots   on the gear  teeth after  which corrosion may decrease. With the proper selection of the chemical agents, these are not activated except under extreme conditions of load and /or temperature. Further, most of the EP additives which are  used  in  gear  lubricants will have little effect  upon  other  metals such  as bronze, copper, etc; at the bulk oil temperatures maintained  in normal  gear operation .

Types of gears to be lubricated

Where   gears  are  on  parallel  axes, either  spur  or  helical  gears  are  generally  employed. Either type can be used as external or internal drives.   The  herringbone  gear  is  similar  to  two  helical   gears  having  reversed   directions   of   spiral, placed  side  by   side  so  that  the  teeth  come  together   to  form  a  chevron   pattern. The rack and pinion, used to convert rotary motion to reciprocating, generally   uses a spur gear.

For   intersecting   axes either straight   bevel    or   spiral   bevel   gears   are used   as a  rule. The   latter type may be used   on angle   drives where   the   shafts    do not    intersect at full   90 degrees. The contact   of the   teeth in such gears gives   a rolling motion. With  non intersecting  and   nonparallel  axes  the  types   of   gears  used  are  crossed   helical, single  enveloping  worm, double  enveloping    worm, or  hypoid. Here the  contact  of  the  teeth  gives  a  sliding  as  well  as  a  rolling   motion. In  most  cases  a  gear  set  will  be  used  to  change  speed, and  in  such  cases  the  smaller   gear  is  designated  as  the pinion. Both  the  number   of  teeth  on a  pinion  and  the  ratio    of  the  teeth  on  the  driving   and  driven   member   may  vary ,   but  with  bevel  gears  there  is  seldom  less  than  12 teeth  to  a  pinion.

While   some  spur  and  straight  bevel  gears  are  still   made  of  cast  iron, the  tendency  in  all   types   of  gearing  is  for  the  use of  steel. Exceptions   will  be found  to  such  practice, for  example,  in the   use  of  bronze  for   one  member   of  worm   gears. Some small gears and even   larger pinions are   made   of plastics, such as ‘Delrin,’’ ‘Nylon.’’ ‘Teflon,’’ etc. Pinions  have been and   may still be  made  of  rawhide, pressed  paper,  etc, but  our  concern  is  primarily   with   lubrication  of  metal  gears.