Influence of Additives on Viscosity Tests

Erratic viscosity results can be obtained in some cases with oils containing additives. If the gear oil contains diluents, some of this fraction might escape before a test is completed, particularly if the determination is conducted at 210 degree F.

Some  additives  which  are sensitive  to  heat  may change  under similar  conditions  or two agents  may  react and thus change  the final viscosity. Also, in case  the  lubricant contains a polymers, such as “Paratac”, heat  may tend  to  reduce its  effectiveness  and  in turn  the  viscosity of the  mixture.

Dispersion of Silicone Antifoam Agents

Most often  the  additives  employed  in   compounding  gear  oils  are  readily  soluble  in the  base  oil; therefore , simple  mixing, as described  above, is  sufficient  to obtain a  satisfactory  blend. On the other hand,  since  silicone fluids which are  frequently  used  as  antifoam  agents have  limited  solubility in oils, they  must  be  thoroughly  dispersed, or they  will settle  out. In view  of this  Beerbower  and  Barnum  made  a study  of  dispersions  of  silicone  defoamers  in oils  using  radioactive tracers  to  determine  dispersion  and  separation of these  additives. They  concluded  that  to  form  stable  dispersions  of  silicones  in oil  the  droplet  size  of the  additive  should  be  in the  10  micron  range. This  could  be arrived at by  heat  and the use  of  high  intensity  mixing  approaching colloid  milling. In laboratory mixers, a speed of 4800 rpm and a temperature of 200 degree F “gave blends in which the silicone was uniformly dispersed after 70 days of storage”.

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.