Troubles Connected with venting of gear cases

Vents are necessary on practically all gear cases. However, such openings may lead to trouble. Thus, with  a  humid  atmosphere and  considerable  change  in  temperatures  of  gear  boxes, moisture will  enter and  condense, thereby  increasing  the possibility of rusting. Also dirt may enter through such openings. On  automobile  manufacturer actually  experienced  the  entrance of  both  dirt  and moisture  through the vents  in  differential  housings. The  correction was  to  extend  a house  from  the  vent  forward  with  the open  end  placed  on the frame, under  the  body and  facing  toward  the side  of  the car. A location for  the  opening  was chosen where  the  least  throw off from  the  tires  would  the present. If  the  intake  of  dust  or  dirt  is  material, the  use of air  filters is  suggested. Such precautions are taken in the case of machines operating in very dusty atmospheres. Thus, most tractors use filters on vents of gear cases. However, this adds another problem in that such filters must be kept clean. Suggestions have been  made  that  gear  cases  be kept  under  slight  pressure, thus  preventing  entrance of  contaminants. However, this is impractical in most cases. Installation of  filters  containing  silica  gel  on  vents  from  gear boxes has  been  suggested. While this would remove moisture, the value would depend upon the frequency of renewal of the absorbent. 

   

Venting Gear Housings

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.

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 .