Foam inhibition in gear oils

The rapid movements of gears tend to aerate oils and produce foam. This tendency may be aggravated by the presence of some additives, such as EP agents. Also, the higher the viscosity of the oil the more permanent the foam as a rule. Cases have been noted where foam became so great that it filled the gear case to overflow and long before this the gear teeth failed to obtain sufficient lubricant. While  foaming of gear oils might  result  from vaporization of entrained  water or driving of air  out  of solution, the general  cause in gear sets is churning  of air  into  the oil  by  agitation.

Little trouble is experienced from foaming of gear oils in service because the use of foam inhibitors in such lubricants is almost universal. It has been suggested that there is a difference between a foam inhibitor and a foam depressant, the latter being an insoluble material. Robinson and Woods^44 use the term “antifoaming agent” to embrace all aspects of the destruction, elimination, or prevention of foams. These investigators state that a foam inhibitor may act:

“ (1) by causing  coalescence of smaller bubbles into large bubbles at or below the surface, (2) by causing the rupture  of  individual bubbles at the surface, (3) by destroying the inherent stability of the liquid  films, or (4) by causing any or all of  these actions simultaneously.”

McBain et al.^39 found that the most complete defoamers for oils are generally, but not always, insoluble. This is true of silicone fluids which have wide usage for this purpose. There is an optimum amount of antifoam agent required which is quite low. Consequently most suppliers furnish defoamers as dispersions or solutions so that low dosages will be more accurate. While Woods and Robinson^50, in testing varying proportions of  DC 200 fluid in two oils, found that 0.01 per cent gave  the greatest  foam  inhibition, actual  usage  in most  gear oils is only a fraction  of this proportion. Thus, Klaus  and Fenske^34, using two oils which foamed badly with no additive, found  that both responded to silicone  antifoam additives at concentrations as low as 0.00001 weight per cent and that maximum effectiveness  was achieved with 0.00005 per cent  or greater.

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.

Oxidation stability of gear lubricants

Once a proper gear lubricant is selected for a given application it should suffer a minimum chemical and physical change during use. One of the changes most likely to occur is oxidation  of  the oil  which ultimately  will  lead  to the formation of undesirable  products  and  changes in the characteristics  of the oil. Such changes may result in the formation of acidic  products which  may corrode  the metal  surfaces, in an increase  in viscosity  of  the  oil, or in production  of  insoluble  materials. Oxidation  of  lubricants  is  accelerated  by  high  temperatures or  by  the  presence of  certain catalysts, particularly  soluble  metals. The  immediate  effects  of  oxidation  may appear  beneficial  in  that  petroleum  acids formed  function  as  oiliness  agents, perhaps by  the formation  of  monolayers  of metallic  soaps. Ultimately, as oxidation of oil proceeds, the harmful effects become evident. The degradation of the oil by oxidation may result in not only the formation of acidic products but also asphaltenes, resins, or other polymers. Changes in the lubricant will  probably  be  accompanied  by  increase  in  viscosity , darkening  in  color, and  the  formation  of  sludge. Cases have been noted where gear oils became almost solid due to oxidation.

However, oxidation of gear lubricants can be retarded by addition of antioxidants or oxidation inhibitors. The use of such agents in most gear oils is wise since the environment for the lubricants is favorable for oxidation in that both air and heat are present and thin films of the oil are in contact with the air.

The mechanism of the action of antioxidants is generally considered to be that of chain breaking as the additive reacts with a “hot” molecule, thus being itself oxidized. In this process the oxidant molecule is destroyed, but with dissipation of the energy possessed by the “hot” molecule, so that the chain reaction is broken. Thus, the oxidation of hundreds or perhaps thousands of molecules of hydrocarbons is prevented, since the energy would be passed on from one molecule to the next in the normal chain reaction.

The suggestion was made by Larsen and Diamond^35 that antioxidants may be either inhibitors or retardants, the former acting to break reaction chains and the latter being converted into an inhibitor during the oxidation process. Three possibilities were given by Murphy et al.^42  for the possible disposition of such inhibitors after they had reacted: (a) the inhibitor is oxidized to a compound which is incapable of further antioxidant action; (b) the inhibitor is oxidized to a compound which still exhibits antioxidant action, but generally to a reduced extent; (c)  the inhibitor is capable of regeneration. The latter type of additive is the most desirable, provided the rate and degree of regeneration are high.

Specific compounds suitable as antioxidants will be suggested in a later section, but most of these agents fall in the following bellows: (a) various types of phenols, (b) certain sulfur bearing compounds, (c) numerous organic phosphites, and (d) certain of the amines. A number of additives function as dual purpose agents and thus, in some cases, a specific antioxidant may not be required.

Oiliness of gear lubricants

As lubricating conditions in gear sets change from that of thick film to boundary lubrication, the oil benefits by the presence of additives. For conditions with spur gear lubrication, some agent which will provide increased lubricity or oiliness may prevent film rupture and thus maintain a low friction. Oiliness additives consist of polar materials such as fatty acids or even animal or vegetable oils. One end of such fatty acid molecules will adhere to the metal surface and resist removal by shear of the gear teeth.

Sulfurized fatty oils have also been used for oiliness additives but have not always prevented a stick slip condition in automatic transmissions. By proper choice of materials and also of the sulfurizing methods, oiliness additives are provided which are said to satisfy the requirements of automatic transmissions and yet prevent “squawking”. Also, certain Phosphorus compounds have found application in ATF as lubricity agents.

Minimum action of gear oils on components of mechanisms

Well refined mineral lubricating oils have little if any action on most metals, particularly ferrous metals. It is only upon prolonged use at elevated temperatures that such oils may from compounds which will act upon metals. Since such oil changes can be retarded or almost completely arrested by the use of oxidation inhibitors and also metal deactivators or pacifiers can be included, there should be little concern about the action of straight mineral oils upon the metal components with which they come in contact.

What we are concerned with here is the action on parts made from other materials, such as paper, plastics, rubber, etc. Seals are likely to be made from rubber, either artificial or natural, and any deterioration of the compositions due to the lubricant should be at a minimum. Many seals consist of compounded materials, such as “neoprene,” and it is found that oils high in aniline points, as are most high V.I. oils, will have little effect on this compound.

Automatic transmission mechanisms may be found to contain paper and “Nylon” parts. In future devices which will require transmission fluids, a greater variety of components may be used. The safest course when supplying oils for contact with unusual materials is to have the fluids pretested to determine their suitability.

Freedom from separation in gear oils

Precipitation or settling of some components in gear oils is sometimes noted. This most often occurs in mixtures containing EP additives. The separation may be due to lack of solubility or to reaction of ingredients resulting in formation of sludge. Since such additives are often present in concentrations of 9 per cent or more, the base oil must keep a high proportion of heavy chemical compounds in dispersion or suspension. Both additive manufacturers and oil blenders select ingredients which will keep any separation of such agents at a minimum.  

Fletcher^23 selected three SAE 90 hypoid gear lubricants and three multipurpose SAE 90 oils meeting MIL-L-002105A specification. By precipitation tests, the first three oils showed some sludge in the unused oil which increased after a service test in two of the lubricants. In the multi-purpose oils there was only a trace of sludge before use but measurable amounts up to 7 per cent after the tests.

Of course, settling or sludge formation in gear lubricants results in loss from the action zone of valuable active ingredients, but the greatest concern is the effect sludge may have on operating mechanisms. Thus, Fletcher^23 mentions that precipitation of sludge out of oil due to centrifuging in transmission cases may result in carbon like deposits in pocket bearing positions, internal clutch teeth, and in some cases in oil grooves  and synchronizer grooves. It is conceivable that such deposits could adversely affect the operation of the unit. This fact was probably recognized by one tractor manufacturer who specified that oils used in their equipment should be filterable, thus, indicating that sludge should not separate during  normal operation.

Where sludge is formed in EP gear oils the action is accelerated by increasing temperature. It is therefore probably a result of reaction of the chemical compounds which constitute the EP additives. Detergent agents do not seem to be a correction for such sludge separation, and any improvement in the condition probably lies in selection of the EP additives.

Compatibility of gear oils

Gear lubricants consisting of straight mineral oils will mix in all proportions at normal ambient temperatures. The resulting physical characteristics of the mixtures will not be an exact proportional average but will approximate this.   

On the other hand, gear lubricants containing additives may be changed either chemically or physically by mixing those of different compositions. For example, some EP additives have limited solubility in high V.I. oils. If then such additives are blended with naphthenic oils which will hold them in suspension and such blends in turn are mixed with high V.I. oils, a portion of the additive might settle out of the mixture. Also, if gear oil containing lead soap were mixed with such oil containing an active sulfur compound, a precipitate of lead sulfide might be formed. Compatibility of gear lubricants is of little concern in industrial service where either the life of the lubricant is long or, if additional oil is required in a gear case, it will likely be from the same source. However, in automotive equipment the necessity of compatibility of gear oils is important because vehicles may be serviced by distributers handling different brands of oil than that originally used in the gear cases. Recognizing the possibility of mixtures of gear oils from various sources U.S. Government agencies have the following requirement in most gear lubricant specifications: “the lubricant shall be compatible with each of the other lubricants qualified under this specification.”Commercial multi-purpose gear oils are almost universally compatible with each other.

Adhesion of gear and transmission lubricants

Adhesion is a comparative term, and what is in mind in this case is stickiness which will insure that the lubricant will resist the action of centrifugal force and thus remain on the gear teeth. This quality is most important in the case of open gears. Desirable adhesion is obtained by the use of high molecular weight and high softening point compounds. That is, asphaltic residua, resins, and polymers are largely used for the purpose. Which this type of product is desirable for open gears, it is not suitable for most gear sets because of its high fluid friction and poor cooling effect.

Desirable characteristics in gear and transmission lubricants

A number of characteristics, which were not evident in the discussion of functions of gear lubricants, contribute to satisfactory application of such products. After listing some of these qualities, consideration will be given as to what contributes to such characteristics. Such qualities include:

(a)    Adhesion

(b)   Compatibility

(c)    Freedomfrom separation

(d)   Minimumaction on other components of mechanisms, as seals, etc

(e)   Oiliness

(f)     Oxidation  stability

(g)    Satisfactory demulsibility

(h)   Satisfactory detergency

(i)    Satisfactory foam inhibition

(j)    Satisfactory viscosity temperature characteristics

(k)      Shear stability

(l)   Reasonable cost.

                                                                           

Dissipation of heat by gear lubricants

Under the most ideal conditions of lubrication of two moving metal surfaces heat is developed. In fact Bowden and Tabor^11 found that, even though lubricating films are present, surface temperatures of metals  may exceed several hundred degrees Centigrade  at  relatively small loads and sliding speeds. Blok^6, 7 first postulated and then verified conditions of “temperature flashes” between operating gear teeth. The temperature at the points of contact was shown to be proportional to CfP^3V, where C is a constant, f is the coefficient of friction, P the mean pressure, and V the gear engaging speed. This formula holds for both spur and hypoid gears, but the action of the latter type develops the greater amount of frictional heat. Since the contact points are small with respect to the overall dimensions of the gears, this heat is conducted into the two moving metal surfaces. A lesser amount of heat may also be developed by churning friction where gears are bath lubricated. 

Gear oils are an aid in dissipating this frictional heat. How effective this action is depends upon the amount of fluid coming in contact with the gears as well as the temperature and viscosity of the oil and the manner in which such oil is flushed over the gear teeth. Oils are not the ideal coolants since the specific heat of petroleum products is about half that of water.

Design and application influences heat dissipation in that the size of the gear case determines the total gear oil present and radiation from the fluid and the metal depends upon the surface exposed. If the oil application is by spray, the jets can directed at the points where the greatest heat is present, perhaps on the leaving side of the gear teeth. Circulating systems permit not only placement of oil streams but also adjustment of quantity. In case heat dissipation is not rapid enough, additional oil storage or settling tanks can be used to provide more radiation.

The lower the viscosity of  the lubricant the more effective it is in transferring heat from the tooth surfaces to the bulk oil and then to the gear housing and thence to the atmosphere. The value of low viscosity gear oil in dissipating heat was shown in certain truck operations. Here the differential oil ran about 35degree ( F) lower  in  temperature  when an SAE 90  lubricant was  substituted for an SAE 140 gear oil.

Abrasive or cutting wear

Gear oils are not correctives for abrasive wear because here the action is due to hard particles between the gear face as they mesh. If the abrasive is due to loose metal, sand, etc; gear oils may wash the foreign particles from the moving areas, but unless the abrasives settle out they will continue to act as lapping compounds. However, if the viscosity of the gear oil is low, the large foreign particles may be deposited in areas where the velocity of the oil is low, and thus they will be harmless.

The best corrective for abrasive wear of gears is to drain and flush out the gear case and refill with clean oil. Circulating oil systems used for gear oils can be equipped with filters or strainers. Likewise, a settling period can be provided in the storage system for the fluid. Some gear cases in automotive  vehicles  have magnetized  drain plug  so that  most  iron or steel  particles will  become attached  as  the  gear oil  circulates. Where vehicles operate under conditions promoting  dust, as do many  tractors, it is wise to  drain  gear  cases  frequently  so  that  abrasives  filtering  into  the  gear  oil  will be removed.

Fatigue wear

Fatigue wear may take place even where gear oil provides a satisfactory lubricating film and there seems to be some question as to what quality of a gear lubricant is responsible for decreasing fatigue wear of gear sets. Hutt^30 presented data from an IAE machine which showed that a 190 per cent increase in fatigue life resulted from an increase in viscosity from 90 to 140 cs at 158 degree (F) . On the other hand, Hundere^29 is of the opinion that it is not viscosity as measured by standard viscometers that controls the effect on surface fatigue. Davidson and KU^20 likewise state: It was found that lubricant viscosity, at least as measured by the conventional viscometric  method, did not have as predominant an effect on  gear tooth surface  fatigue as some other undetermined  lubricant characteristics.

In tests conducted by these investigators the lubricant having  the highest rating  on a Ryder Gear Test ran three to four times as long before pitting occurred as did the other oils examined.

Corrosive wear

Corrosive   wear  in  the  presence of a gear  lubricant may  be  due to the  environment if air, water , or electrolytes  are  present. If gear cases are not tight and high humidity prevails, rusting may occur, not only on idle gears above the oil line, but also on the walls of the gear case. If necessary, rust preventive compounds can be added to gear oils to counteract the action of moisture. Such additives may be polar compounds, often containing long chains, which will be adsorbed at the metal oil interface to form hydrophobic films. Prevention of corrosion due to electrolytes may be more difficult than prevention of rusting. However, if the contaminant is salt, the same types of additives as mentioned above will aid in corrosion prevention. If water soluble acids entering the gear case cannot be prevented, ordinary gear oils will not serve to prevent corrosion. In this case it may be necessary to use gears of different composition. Stainless steel will resist most acids and some electrolytes. High silica irons, while somewhat brittle, also have this faculty.

The corrosive wear most apt to occur in gear operations is that due to chemical additives, known as EP agents. The secret of a satisfactory EP gear oil is to obtain controlled  corrosion so that welding  of  the  metal  surfaces  will not take place  and  yet  asperities will  be  reduced. In  the  case of most EP gear oil  compositions  corrosive  wear  should  not  be excessive and  is actually  beneficial in  that it extends the life of the gears under extreme operating  conditions

Functions of gear lubricants

By stating the necessary functions of gear lubricants there will be a basis for consideration of how these purposes are accomplished and what characteristics are desirable in satisfactory gear oil. While the relative importance of the qualities of such lubricants may vary with the specific use, the listing below includes the most desirable functions of gear oils. Minor qualities will have consideration later.

(a)  Reduction of wear in gears and on adjacent moving parts.

(b)  Reduction of friction and consequently of power.

(c)   Dissipation of heat, that is, act as a coolant.

(d)  Prevention of corrosion ( this statement may have to be modified in the case of EP gear lubricants)

(e)   Reduction of noise, vibration, and shock between gear teeth.

(f)    Flush away contaminants.

(g)  Act as a carrier for additives.

(h)  Act as a structural material in that the lubricant is a factor in determining the strength of the gears against breakage, pitting, and welding.

                    

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 .

Reduction of wear in gear sets and transmissions

Wear has been defined by a Committee of the Institute  of  Mechanical Engineers as: “Progressive loss  of substance  from the surface of a body  brought  about by mechanical  action ( usually it  reduces the serviceability of  a  body  but  can  be  beneficial  in its  initial  stages  in  running in)”.

It is evident from  this definition  that  what  is desired  in a gear lubricant  is  prevention  of  continuous  wear. With highly loaded moving parts, which include gears, the following types of wear may occur:

                            

                            (a) Abrasive and cutting wear.

                            (b) Corrosive wear           

                            (c) Fatigue wear.

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

Reduction of noise,vibration and shock by gear lubricants

Gear lubricants, particularly those of high viscosity, will act as cushions and thus reduce noise, shock, and vibrations of meshing gears. However, it must be kept in mind that even the most viscous products are not correctives for poor mechanical conditions.

Mention might be made that Cardillo,^14 in an analysis of “Initial Axle Noise” in automobiles, concluded that it is a resonance problem and apparently lubricants or lubrication plays  no part in abating  this noise .A different noise effect has occurred in automatic transmission and limited slip differential mechanisms of vehicles and in both cases has been corrected by the use of oiliness additives. The degree of oiliness must be carefully controlled since, if deficient, the clutch operation will be rough and cause chattering or squawking but, if excessive, the clutch may slip. Mention will be made of some of the suggested additives for this purpose in a latter section

Reduction of friction in gear operation

Bowden  and  Tabor^11  have shown  that  the coefficient  of  friction  for  one lubricated  surface  moving relative  to  another  will  depend  upon  the material  used  in the two  surfaces, the type  and  velocity  of  motion between  the  surfaces,  and  the composition  of  the lubricant. The  first  two factors  are fixed  by  machine  design, but the gear  lubricant  can be altered so  as to aid in providing  desirable  friction  characteristics. The  contribution  of  this  last factor  is  evident  if  we  consider  that the  coefficient  of  friction  of lubrication  metal  surfaces  is  about  1.0  while in  the  case of  such surfaces, lubricated  with  a  boundary  film,  the value  is  about 0.05  to 0.15.

Any explanation of friction is based on the fact that there are irregularities in surfaces. As one such surface moves relative to the other there is an interlocking of the asperities of the two areas. The role of the lubricant in overcoming boundary friction may be partly by filling in the low spots and in the case of a thick film, of lifting one surface over the other. However, under the best conditions, there will be some interlocking of the high spots. If, in so doing, some asperities plow through those of the other surface this will cause one type of friction. The remainder of  the friction  may be a result  of  welding  and  shearing  of  minute junctions between  the two  surfaces.

We are concerned, in the main, with two steel surfaces in rolling or sliding contact. While the thought has been advanced that rolling friction is essentially independent of the lubricant used, gears in operation, particularly hypoid or worm gears, have both a rolling and sliding motion. An example of the variation of friction with the type of motion concerns first a steel rider sliding across a lubricated plate of the same steel to give a coefficient of friction of about 0.14. With the same combination of materials, the rolling friction coefficient was about 0.00015.

Other operating variables to be considered are load, speed, and temperature. Rounds^45 using a test machine in which ball bearings had both a rolling and sliding action and, thus, simulated the action of gear  teeth, concluded that, in general, the coefficient of friction decreases as either the oil  temperature, the ball velocity, or the load  increases. However, the magnitude of the oil temperature and ball velocity effects tend to decrease as the load increases.

In choosing a gear oil one is faced with a compromise because the lowest viscosity oils have the least internal friction and, yet, high viscosity oils will maintain the most satisfactory film to prevent metal to metal contact, particularly at low speeds. This is true when hydrodynamic or thick film lubrication conditions prevail and where viscosity is the most important property affecting friction. While a condition of low friction is desirable in a lubricated gear set, it must be kept in mind that there is not necessarily a correlation between friction and wear. Beeck^3 has suggested that this may be due to the fact  that while wear takes place momentarily  at isolated spots, friction is ordinarily measured as an average of a large area and a longer time interval.

Irrespective of viscosity, various types of mineral lubricating oils may give different friction values. Rounds^45 found that naphthenic base oils gave higher friction values than paraffinic base oils, even after limited attempts to change the friction properties by fractionation or additional refining. In this investigation, the most commonly used synthetic fluids gave friction values similar to straight mineral oils. Since paraffinic oils show less increase in viscosity with pressure than do naphthenic type oils, this may account for the lower friction values of the former type of oil when used in lubricants. The discussion above has been concerned with kinetic friction of fluids containing no additives. When boundary lubrication conditions exist, nonreactive gear oils will not suffice; therefore, consideration must be given to lubricants containing additives, Rounds,^45 using a  naphthenic type oil as a base, found that different additives  would  lower kinetic friction and/or raise or lower static friction. Fatty acids and related compounds were the most effective of the agents investigated for lowering static friction. To lower kinetic friction at velocities above 100 fpm, reactive chlorine and sulfur compounds were most effective.

However, the type of service which demands the use of EP gear lubricants nullifies reduction in friction. For example, Bisson et al.,^5  when investigating the effect of  chemical reactivity of  lubricant additives on friction and surface welding at high sliding  velocities, found some decrease in friction up  to about 1300 fpm, when using  p-dichlorobenzene, followed by  a  very  abrupt increase in the coefficient  of  kinetic  friction. Thus it  was  found that :For all  additives, the existence  of  a  critical  sliding  velocity where  the friction  coefficient  increased  and surface welding occurred  was verified.

Kinetic friction in fluids causes heating and thus power losses. The  friction  due to gear oils  themselves  may be film friction, which  has  been considered, or churning  friction  which occurs  as  the  gear  teeth  rotate  in the oil bath. Other things being equal, gear oils which will have the lowest internal friction will be the most desirable from a power efficiency standpoint.

In connection with friction, consideration might also be given to the oiliness characteristics of gear lubricants. The following definition was adopted  by the SAE  Crankcase  oil  Oiliness  Committee  in  1937: Oiliness  is  a  term  signifying differences in friction greater  than can be  accounted  for  on a  basis   of  viscosity  when  comparing different lubricants  under identical  test conditions.

This quality is  desirable in  boundary lubrication  and  is  no doubt  due to adsorbed layers  on  the metal  contributed by  polar compounds  such as fatty acids, esters  of  fatty acids, metallic soaps, some  organic  compounds containing  chlorine, nitrogen, phosphorus, or sulfur, etc. Unfortunately there is  no  standard method   for  measurement of  oiliness.

Conditions under which gear lubricants operate

With meshing gears, both rolling and sliding motions are present. However, the  two  types  of  contacts  vary  both  with  the  type  of  gears  and  the  speed  of  operation. The  sliding  component  is  of  greatest  importance  in  the  case  of  hypoid  or  worm  gears. The descriptions of the state of lubrication, which  immediately  follow, are concerned, to a large  extent, with the types of  gears  used  in  most  industrial  applications, transmissions  of  automotive  vehicles, etc. Some  of  the  thinking  and  speculation  can  also  be  applied  to  hypoid  and  worm  gear  operation.

Many  gear  lubricants  operate  under  very severe  conditions  and  yet  long  and trouble  free  lives  are  obtained  from  most  gear  sets. In spite  of  the  fact  that  the  action  of  such  lubricants  is not  completely explained, enough  information  is  available  to  permit  recommendations  of  gear  oils  which  will  perform  satisfactorily  provided  the  gear  design  and  installation  is  not  at  fault.

Borsoff^10  has  presented  the  mechanism  of  gear  lubrication  in  a  simple  descriptive  from  which  is essentially  as  follows. As  gears  rotate  in  the presence  of  a  lubricant, a hydrodynamic  wedge from, which tends  to separate  the  teeth  as  they  mesh  with a  thick  fluid  film  when  the  load  is  low. As  the load  increases, the  pressure  in  the  contact  zone  also  increases, causing  the  separating  Lubricating  film  to  decrease  in  thickness. Finally,  the  load  becomes great  enough  that  the fluid  film  fails  to  prevent  contact  of  the  high  spots  of  the  mating  surfaces, and  wear  results.

The  investigator^10   found  that  the  nature  of  the  were was dependent  upon  speed  and  that  gear  lubrication  can  be  divided,  with respect  to  speed, into  three  zones. First  is  the slow  speed zone, which  in  the  gears used  in  the investigation, extends  up  to  about 1000 rpm. Next, the medium  speed  zone  for  the  same gears  extends  approximately  from  1000  to  8000  rpm. Finally,  the  high  speed  zone  extends from  8000 to 30,000  rpm  in  the  case  of  the  investigation.

Three  regions  were  recognized  with  respect  to  type  of  wear  and  working  surface  conditions, namely: a region  of  thick  film  lubrication  where  there  was  absence  of  wear; a region  of  abrasive  wear; and a  region  of  scoring.

In  the slow  speed zone, the load  carrying  ability  of  a  given  lubricant  increases  with  decreasing  speed. Thus, using  an  oil  of  9.92 cs  at 100 degree F, no scoring  took  place  at  speeds  below  665 rpm. However, wear does  occur  at slow  speeds as loads  increase, and  under  these  conditions  it  was concluded  that  such  wear  was  due  to  abrasion.

Gears  operating  in  the medium  speed  zone  with light  loads  were  in the thick film region, and  hence wear was not  detectable. As  loads increased  in this  speed zone, heat  generated  by shear  caused  a  decrease  in  the viscosity  of  the  lubricant  followed  by a rupture  of  the fluid film with consequent  metal  contact  and scoring. It was found that this type of wear was not gradual  but increased  by jumps  at the beginning of each  load  period, after  which no additional  wear  was  observed  until  another load  increase .Under  these  conditions, the lowest viscosity  oil  permitted  the  greatest  wear, which also  started  at low  loads. The high  speed  zone was ‘characterized  by an increase in load  carrying  capacity  with  an increase  in  speed.’’ This was attributed   to two factors, one the relaxation phenomenon and the other the ‘squeeze’’ effect. Relaxation in this sense indicates that the liquid  lubricant  responds  as  an elastic  solid  when  subjected  to  high  deformation  rates. Since  a  definite  length  of  time  is  necessary  to squeeze all of  the  lubricant  out of  the contact  zone  between  gear teeth ,  at  high  speeds  the  contact  time  may be  too  short  to  eject  all  of  the oil. According  to Borsoff^10 “ high  speed  gear  operation  at  all  loads  below  the  score  load   is  in  the  thick  film  region.”The  above  mechanism  of  gear  lubrication  was  concerned  with  lubricants  consisting  of  unreactive  mineral   oils.   It  was  found  that the  higher  the  viscosity  of  the  lubricants, the greater  the  load carrying  capacities  and  wear  protecting  properties. However, other requirements often dictate the use of low viscosity oils. Since operation  of  gears  using  unreactive  mineral  oils  at  loads  above  their  first  score   load  may  lead  to  trouble, the  use  of  extreme   pressure  (EP)  lubricants  is  found  necessary   with  high  loads.

Borsoff^10  cautions  that  ,  when  using  EP   lubricants  at  loads  above    the  score  load  of  the base  oil, abrasive  wear  may  sometimes  be  present.  Likewise,  abrasive   wear  may  occur  with  heavy  loads  in  the  slow  speed  zone .  Such  action  and  the  degree  will  depend  upon   the  particular  EP  additive  as  well  as  the   concentration. In  order to  appreciate   more  thoroughly   the  problem  of   gear  lubrication,  the ideas  of   other  investigators  should  have  consideration. According to  MacConochie  and  Newman^37  contact  pressures  of  7000  to 10,000  kg/sq  cm  in  the  case  of  gears  compare  with 10 kg/sq  cm  for  journal  bearings . Also , instead  of  an  oil  film  thickness  of  about  0.001  cm , as  is present  in journal  bearings, the  lubricant  film  formed  between  meshing  gear  teeth  is  of  the  order  of  magnitude  of  the  surface  roughness  of  the  two  contacting  bodies  and  not  much  thicker  than  the  size  of  foreign  particles  in  a  highly  purified  oil  passing  through  the gap. The  function  of  the  oil  is  performed  in an  extremely  short  time  since, according  to  Smith^46,   the  residence  time  of  the  lubricant  in  the  contact  zone  may  be  as short  as 10^-6  seconds.

The  first  authors^37  visualize   the   conditions  under  which  gear  oil   films  operate  as  follows: ‘Sometimes  there  is  a  substantial   lubricant  film  between  the  surfaces, and  at  others  a  foreign  particle  wedges  its  way  through   between  two  surface  roughnesses , at other  times  two  or  more  asperities  come  into   physical  contact. Depending  upon  the  relative  waviness  of  the  surfaces  and  the  difference  in  particle  size,  contact  may  occur  only  at  one  point  along  the  contact  line  while  lubrication  is  hydrodynamic  in  other  regions. Another  factor  affecting  thickness  readings  is  the size  of  the  contact  zone  since  the  larger  the  contact  area  the  greater  the  chance  for  a  particle  of  large size  to be  in  the  gap.  To complicate the picture   further,  the  film formed   is  constantly  subjected  to  dynamic  load  on  the  gear  train, vibration  of   the  gear  teeth , and  shafts. Localized ‘conflagrations’ resulting in chemical reactions may occur at heavy loads.’’

Oil film thickness was measured by these investigators^37 using a continuous electrical arc to bridge the gap between gear teeth. A tracing  of the results indicates  that the lubricant  film  is  at  a  maximum  at  the  pitch  line  and at a minimum at  the roots  or tips  of  the gears,  see Fig.2.1.

Much remains to be explained regarding the lubrication of gear sets. For example, in a discussion  of  the report  of  “Instantaneous  Coefficients  of  Gear Tooth  Friction’’ Benedict  and  Kelley^4  state: “We   do  not know  from  our  results  the  true  state  of  lubrication, whether  it  be hydrodynamic, elasto-hydrodynamic, or partial hydrodynamic. In  our  analysis, we have chosen  partial  hydrodynamic  lubrication  as  a  simple  model  which   helped  visualize  the results. In order to determine precisely  the  state  of  lubrication  it  would appear necessary  to  determine  by  some  more  direct  observation  not  only  the thickness  of  the oil  film but  also  its  continuity’’.

Also  gears  are  used  under  such variable  conditions  that  a  blanket  statement  cannot  be  made  as  to  the  type  of  lubrication  which  prevails. This was recognized by the above investigators^4 as   follows :Lightly loaded high  speed gears  might  reach  full  fluid  film  lubrication, and  heavily  loaded  low  speed  gears show  signs  of  being  in the  boundary  region. These are  extremes, however, and in the results  discussed  here  and  in  gears  normally  used, mixed  lubrication  occurs.

In  view of  the fact  that most  gear  sets  operate  trouble  free, the  extreme conditions  indicated  by  MacConochie  and Newman^37 may  not  for average operations. Thus, Crook^19 who  used electrical  resistance  as  a  means  for measuring  the  thickness  of  oil  films  between  metal  dises, concludes  that  hydrodynamic  films  of  one  micron  in  thickness  result after  a  break  in  period. This author states: Furthermore, the existence of a hydrodynamic film is quite consistent with observations with gears themselves. Often signs of the original  machining  marks  can be seen even  after twenty  years  service  and such low  rates  of  wear  imply  hydrodynamic  rather  than boundary  lubrication.

Perhaps  a  new lubrication region  should  be  recognized  which Talley  and  Givens^47  have  defined  as ‘metadynamic’ and which  falls  between  the  hydrodynamic  and  boundary  regions. These  authors, in dealing  with  such  a  lubricating  film, have  measured  one  aspect  of  oiliness  and  derived equations describing  the  same. While  the  investigation in  question  was  concerned  with  journal  bearings, one  interested  in  the  theory  of  gear  lubrication  should  be  aware  of  this  thought.

Some of the complexities   of  boundary  lubrication  as  suggested  by Larsen  and  Perry^35a  may occur  during  gear  operation, particularly  if  high   temperatures  are  reached. Any  of  the  following  reactions  might  have  an  influence  on   gear  lubrication: oxidation  of  the  metal  surface; oxidation  of  the  lubricants  to  from  fatty  acids; chemical  or  physical  adsorption  of  polar  compounds, such  as  fatty  acids, on the  metal  surfaces; formation  of  multilayer  films  by  the  adsorption  of  the  above  fatty  acids, by  salts resulting  from reaction  of  the  acids  with  metal  oxides, or  by esters  present; oxidation  or polymerization  of  oils  or  unsaturated  constituents  to  from  resinous  films; orientation  of  these latter  films  due  to  pressure  and  shearing  stress; or  a  breakdown  of  any of  the  films  just  described.

While  the conditions  under  which  gear  lubricants  operate   seen  to  be quite  severe, it  should  be  realized  that  average  operating  conditions  are  less  serious. Producers  of  both gear sets and  gear  lubricants  provide enough  tolerance  in  their  products  so  that  difficulties  are  the  exception  rather  than  the rule. However, the possibility   of  conditions  such  as  those   cited  behooves  the  operator  of  gears to  provide  as  reasonable  service  conditions  as  possible  so as  to prevent  undue  wear  or  even  failure.

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.

Designation and recommendation of gear lubricants

While  industrial  gear  lubricants  are  often  marketed  under  trade  names, information  is  always  available  as  to  how  these  products  conform  to  AGMA  numbers . Therefore, little   difficulty occurs   with the designation of this time of gear oils.

However, in  the  case  of automotive  transmission  and  axle  types  of  oils the  marketing  designation  has  not  always  been  clear  and is, in fact, sometimes  misleading. The SAE  Handbook, in  about  1924, started  to  include SAE  viscosity  numbers  for  such  lubricants, and when , in 1951, API approved  definitions  for ‘Regular-Type Gear  Lubricant,’’ ‘Mild  Type  EP  Gear  Lubricant,’’ and  ‘Multipurpose Type  Gear  Lubricant’’ these  terms  were  suggested  for  use  by  SAE. With the  availability  of   improved  EP  gear  oils, as  mentioned  above,  the  API   Lubricating  Committee, in 1957,agreed  that  such  products  should  be  known  as  ‘Multipurpose  Type  Gear  Lubricants (API  Service  GL 4).” Such oils were described as follows:

“This  term  designates  Lubricants  which  have  adequate  load  carrying  ability  and  other   required  properties  to protect  hypoid  gears  in  sustained  high   speed  and /or  high  torque  service  in  modern  high  powered  passenger  cars  and  trucks.  Also  suitable   for  use  in  spiral   bevel  gears, many  transmissions, and  worm  gears  in  some  types  of  service.  Such Lubricants are identified as meeting ‘A.P.I. Service GL4’.’’

The  consumer   of  gear  lubricants  should  bear  in  mind  that   such  products  are  like  other  commodities, in  that , while  a  conscious  organization  may  set  tolerances  for  material   to  be  supplied  under  a  given  designation, some  products  offered  will  barely  meet  the  required  specifications. Therefore, the  purchaser  or  user  of  gear  oils  should   deal  only  with  reputable  firms.

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. 

Demands made on gear and transmission lubricants

Mention  of  some  of  the  demands  made  on  gear  and  transmission   lubricants  may  well  serve  as  a  further  justification  for  the  assembly   and   publication  of   the  information   to  follow. Reviewing the problems confronting the automotive industry, Raymond^9  considers  the  hypoid   axle  to  be  the  hardest  working   and  perhaps  the  most  neglected  automotive   component. It  is  said  to  be  far  easier  to  deliver  increased  horsepower  and  torque  to  a  rear  axle   than    to  build  satisfactory  durability  into  such  a   critical  unit  which  is  confined   by  size   and  weight  limitations. Since  higher  engine  horsepower  and  automatic   transmissions  have  greatly  increased   low  gear  loading  and  higher  pinion  offset   has  produced  more  sliding   motion  between   gears, Raymond^9  con-  clues: Lubricants   thus   have  become  a  limiting  factor    in  the  load  carrying   capacity  of  American  axles.

 Automatic   transmission  fluids  offer  another  challenge  to  the  oil  industry   in  that  a  complex  combination   of  requirements   exists   for  such  fluids, and  Raymond^9  believes  that  the  future  appears  to  offer  a  still  greater  challenge  in  severity  of  automotive   transmission  operation.

In  addition  there  is  a  tendency  for  the  use  of  a  single  gear  oil  to  per  from  several  functions. For  example , a  lubricant  may  be  called  upon  not  only  to  protect  bearings  and  gears  against  wear  and   corrosion, but  also  to  act  as  a  hydraulic  medium. Likewise, an  automatic  transmission  fluid, the  primary  purpose  of  which  is  that  of  a  torque  converter, may  also be  called  upon to  lubricate hypoid  axles   in  the  future.

The  severe  conditions  under  which  some  gear  oils  operate  is  stressed   by  Hundere^5   who   states: “ The  most  difficult  function  that  a  gear  lubricant   must   perform  is  that  of  preventing  excessive  metal  to  metal  contact  in  the  region  of  maximum  sliding   velocity. When  it  is  realized   that  the  unit  loading  at  the  point   of  contact  is  as  high  as  400,000  psi  and  the  sliding  velocity   is  as  high  as  6,000   fpm, it is  amazing   that  excessive    wear, let  alone  fusion,  can  be  avoided.’’

It  is  quite   evident  from  the  above  that  constant   improvement  in  qualities  of  gear  lubricants  is  necessary  to  meet  changing   demands. In  spite  of  the  upgrading  of  such  products, they  are  universally  available  at  reasonable  costs. It  is  well  to  keep  in  mind  that, from  an  economic  standpoint, the  benefits  of  correct   lubrication, including  that  of  gear  sets,  are  mainly  due  to  reduction   in  breakdown  or  maintenance   and  consequent   decrease   in  loss  due  to  downtime   or  curtailed   production. This  holds  true  for  both  transportation  and  manufacture. Saving s   in  power  and  frictional   energy,  while   often  attributed   to  correct  lubrication, are  small   compared   with  the  first  benefits   mentioned.