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.
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