Thickeners for gear oils

The use of thickeners in gear lubricants is not common:  hence, little space need be devoted to the subject. Where lubricating  greases  are  used  in gear sets,  soap  is  normally  the thickener  and  hence will  be  present. In  gear  housings  which  are not  tight, grease will  stay  in  place better  than  will  oil. Later  some  specific  applications  will be  mentioned  where  lubricating  greases are  employed, and  in  such  cases  the  type  of thickener   present will  be  evident.

Resins, both natural and synthetic, are occasionally recommended as thickeners in gear lubricants. Thus, resins separated from Pennsylvania residua are so used. Likewise, certain  grades of  polyethylene  are  thought  to  have not  only  thickening  power  but  also  to  contribute  film  strength  to  lubricants.

Inorganic solids, which  were  previously  mentioned as  being  used  in  gear oils  because of their EP  characteristics, have some  thickening  power  but  are  normally  used  in such  low  proportions that  bodying  is  not  evident. However, fine  silica  is a  component  of  some  semi fluid lubricating  greases  used  in   gear  cases  which  are not  tight and thus would  show  abnormal  leakage  with  fluids. 

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.

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.