Fog or Mist Application of Gear Oils

Airborne oil mist or fog is being used to advantage in the lubrication of gear sets. The fog, consisting of oil atomized by compressed air into particles in the order of 2 microns in size, can be carried in pipes for long distances. At the point of application these will have to be reclassified to larger particles so that they will wet a metal surface. The air carrier must be vented from the lubricated part and is said to carry away heat. However, perhaps the heat dissipated will not be as great as in cases where gear oils splash or flow over gear sets.

The oil is used on a once through basis; consequently, clean, unoxidized oil is applied continuously. The amount of lubricant used is quite low, and the gears expend no energy in overcoming fluid friction of gear oils. With such a system, gear boxes are under some pressure; hence, no contaminants, such as dust, will enter.

Fog lubrication is only practical where sufficient points for lubrication justify a centralized system or where compressed air is available for use with a few points. The latter reason, therefore, probably rules out such systems in the case of aircraft and vehicles. If properly  adjusted, definite amounts  of oil  can be applied by  fog, and the system  requires  little  attention,  provided  there  are no air failures.

Air-Oil Devices for Lubricating Gears

By injecting or pumping oil drop-by drop into an air stream the air will carry the lubricant to the points of application, such as gears. A similar mixture can be obtained by the aspiratory action of compressed air. With such systems, provision can be made to start the oil flow with the start of the machine and stop it when the gears stop. The delivery is positive and there is little chance for contamination because the gear box is under some pressure which prevents entry of dust. Also,rate of oil feed can be regulated.

On the negative side, it is necessary to provide compressed air and a considerable flow of air escapes through the vent of the gear case. While this air will have some cooling value, it may carry some oil mist. The oil is used on a once through basis and provision has to be made to drain the sump of the gear case at times.

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