Moisture Corrosion Characteristics of Universal Gear Lubricants

A full  scale axle  gear  unit is used  for this test. Before the test , the  parts  are cleaned as follows: immerse   in a 1 per cent solution of Sodium Hydroxide  for  1 hour. Rinse  the  parts with water  until free  of caustic. Dry  the parts  by  rinsing  with  methanol, followed by  air-blowing to  remove  the  methanol. After  the assembly, 28 ml of  distilled  water  is put in the axle housing followed by  filling with  the gear lubricant to the  level plug. The  propeller  shaft is  rotated  at a speed of 2,400 rpm  for  4 hours while  the  oil  is maintained  at a temperature  of 180 degree F. At the  end of the  4 hour period, the  drive is stopped and  the  axle  test  unit  is placed on the  storage  rack  without  draining  the  lubricant. There  it  is allowed to remain for a  10 day period at room  temperature.

At the  end of the 10 day period, the lubricant is  drained and  the  axle  disassembled  for  inspection. The  rating  of the  lubricant shall  be based  upon  visual  inspection for  rusting  of the  gears, the  pinion-gear  bearing,  the  differential-carrier  bearings and  the  differential-gear  thrust surfaces.     

Meat Packing and Gear Lubrication

Most of the machinery in meat packing plants is subjected to moisture and thus the conditions are such that rusting occurs. For this  reason  gear  cases  should  be  tight  to  prevent  entry of water. Even so, the gear oils used should contain a rust inhibitor. Another  factor  to  consider  is that  the  temperatures  to  which  the  lubricant  is  exposed  will  be  below  normal. Therefore, low cold test oils should be employed. Gear reduction units found in most packing plants may consist of spur or worm gears. A number  of these  will be on  conveyors and in  general  an oil  of 300 to 500  viscosity  SUS  at 100 degree F  and  containing  both  oxidation and rust  inhibitors  should  be  used. For the worm drives a compounded oil of 125 to 150 SUS at 210 degree F is best. When making frankfurters or sausages, grinders and mixers are used. The grinder will probably be driven by a motor coupled to a herringbone gear. The  mixer may  consist  of a hopper  with  paddle  agitators which  are  driven  through a reduction  gear. In either case the 300 to 500 oil noted above may be used on the enclosed gears. A reservation should  be made  that if gear  reducers operate in a cold  room  where  the temperature  is near  or below  zero, it  will be best  to use  a low  pour  point oil  of 150 to 200 viscosity SUS  at 100 degree  F. Occasionally open  gears  will be  found  in packing  plants. If this is true, residual type oil having a viscosity of 500 to 1000 SUS at 210 degree F and containing a rust inhibitor should be used as needed.

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. 

   

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