When to Change Gear Oil

The  ideal method of lubricating  enclosed  gear sets would be to place the  proper  amount  and quality  of gear  lubricant  is a  sealed case and  make  no  renewal  of the oil  during  the life  of the  mechanism. Manufacturers  of  automobiles have  this  in  mind  and  approach  such  a  solution  of gear  lubrication by  recommending  no  drain  periods and prolonged  use of gear oils. Manufacturers of other equipment have the same thought in mind and the trend will no doubt increase. For illustration a side entering mixer description states: “Lubricant is sealed in the gear case at the factory and is designed to last five years.” Naturally, any statement as to the life of gear lubricants should be modified with an expression as to operating condition, environment, etc. However, it is questionable if the present gear oils are everlasting even under the best of conditions. Therefore, from  both a service and economy  standpoint, used gear  oils  should  be removed  from  the gear cases  and replaced with fresh  lubricant either  when the oil  has  deteriorated or become  contaminated  or at stated intervals.

AGMA, which is interested in insuring log and uninterrupted service from gear sets, recommends the following: “The oil in a new unit should be drained at the end of two weeks operation and the case thoroughly flushed with light flushing oil. After this, a change of oil every 2500 hours of operation or every six months, whichever occurs first, is recommended for the units operating under favorable conditions. Where  operating  conditions  are  severe  such  as  rapid rise  or fall  in temperature of the gear  case with  accompanied  sweating  of the inside  walls  resulting  in a formation of  sludge, or where  operation  is in  moist or dusty atmosphere or in the  presence of chemical fumes, it may be  necessary to change the oil at  intervals of one  to three  months.”   

It is also pointed out that gear sets are usually treated by the manufacturer with rust preventive s before shipment. Such materials may have an adverse effect upon the gear lubricant and, therefore, should be removed before gear oil is added. Petroleum solvents are best for this purpose, and such fluids will also help to remove contaminants, such as metal chips. Immediately  upon removal  of such solvents a low viscosity  flushing oil  should  be  sprayed  on the  gears; otherwise, the  metal  surfaces may  rust  in a few  minutes. Also the gears should not be operated, even for a short time, in a dry condition.

In enclosed  gear  cases  which do not  have  a  drain or a  circulating  system, the used gear oil or flushing oil  should  be removed by suction.  Use of pressure to  force  out  such oil may damage seals  and in the case  of  automotive  equipment will probably force oil  onto  the brake  bands. When  considering  oil changes  for reduction  gears, Forbes et al.^20  mention that  the first  month  of  operation is the most  critical in the life  of  gears; therefore, they  suggest  a change or  careful  filtration of the lubricants within two  weeks  after  the unit is put  in service. It is  pointed  out  that  fine metal  particles  resulting  from the run in period act  as catalysts  for oxidation if left  in the gear case. Rather than  setting  an arbitrary  period of change, the  above  authors^20  think  that  periodic oil samples should be taken  to determine if the oil  is in a  usable  condition. Such samples can be checked for presence of dirt, metals and water. Also the acid number, viscosity and interfacial tension can be determined. Certain fleet owners operating heavy trucks have a rule that the oil in transmissions or rear axles must be changed when the viscosity has increased by fifty per cent.

Where gears are lubricated by  circulating  systems, Forbes et al.^20 states: “ the  charge may often  be used for several  years without change,  particularly  when  adequate  filtering  equipment is employed”. However, cleaning such a system when changing oil requires more effort than in splash systems. Flushing  oils  are available  which, due to  either additives  or the  particular solvents used, will  remove most of the  deposits  from  the  oil reservoir and the piping as well as  the gear case. However, a final cleaning with dry rags free from lint may be necessary.

The greatest care should  be  used  in  cleaning  gear  cases and  auxiliary  equipment since  any used  gear  oil left  in a case and mixed with fresh  oil  will  tend to act as  a  catalyst to  promote deterioration of the new  lubricant. With the flushing oil on the gear set an inspection is possible. If this shows rust, the gear manufacturer should be consulted before operating. If the gears appear to be in satisfactory condition, they should be coated with the gear oil as soon as possible. In the  case  of a  circulating system this  can  be  done  without  operating  the gears. If a splash system is employed the gears can be sprayed or wiped with the lubricant before operating.

Where meshing gears are of dissimilar metals as in most worm gear sets, the importance of a change of oil and inspection of the mechanism is very necessary. As the gears seat themselves bronze particles may become attached to the worm. Such  particles  adhere  to the steel  and  cause  a rough  surface which  will  score the gear. Removal of the used lubricant and cleaning of the worm threads after a short period of use will often prevent further wear.

When oils are used on a once through basis flushing may not be possible. In the case of lubrication by oil fog, only fresh oil is supplied, and the tight system is under some pressure so that contaminants should not enter. However, in the case of open gearing, cleaning at intervals is advisable. Not only does the residuum  type  of gear oils pick  up  dirt  which  in turn  may act as abrasives, but  also some of the  heavy material  packs in the  roots  of the gear  teeth. If such build ups continue shafts may be thrown out of line. Such deposits can usually be softened by kerosene or some other solvent provided there is not a fire risk. If drip pans are provided for open gears these should also be cleaned at regular intervals.

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.