While Forbes et al.^20 mention that gear housings should be vented so that oil vapors may escape, such openings may tend to permit entrance of humid air. In fact water has been drawn off from gear cases which could only have arisen from condensation. If such conditions generally prevail, the gear oil should contain a rust preventive additive. Cartridges or filters containing dehydrating agents and attached to the vent of gear cases are not too practical because of the necessity of frequent renewal. Removal of condensed water from circulating systems is simpler than where the gears are bath lubricated. Entrance of contaminants other than water through gear housing vents is possible and often problem. The location and atmosphere surrounding the gear set will determine such possibilities and also what corrective measures need be taken. Thus, gear cases of large tractors, such as are used on road and construction work, have a breather with an air filter which should be renewed at 250 hours operation intervals. One manufacturer reports some trouble with dirt in gear cases probably due to the filter not being changed at the required interval. Farm tractors also have the same problem and a proper air filter and frequent renewal is desirable on vents from gear cases.
Automobile and truck gear cases are subjected to similar contamination. Thus, a large car manufacturer mentioned in a service bulletin that two major contributing factors to a considerable number of ring and pinion failures in axles was moisture and foreign materials entering the axle and contaminating the lubricant. The corrective action was to install a new vent system. The vent was fitted with a hose and hose clamp. The hose, 19 inches long , goes up under the frame and body of the car and is clamped so that the open end faces toward the side of the vehicle.
Industrial gear set installations may require similar filters or extensions of gear case vent opening to points where the concentration of contaminants is at a minimum.
