Mixers and Gear Lubrication

Mixers include a wide variety of speeds, size and types of equipment; consequently, the lubrication of gear drives will be equally varied. Also, both enclosed and open gears will be encountered in such machines. Lacking  recommendations by the equipment manufacturer, one should  rely upon  the fact  that  high speeds in the  gears  demand low  viscosity oils and  low speeds  require high  viscosity lubricants. If the service is severe, use EP oils.

High speeds

The term “high” of course is relative but with pitch line velocities of several thousand feet per minute the lubrication of gears is not simple. Naturally, a low  viscosity oil is  required and the problems are : to have  assurance that a film  of oil  is present when the gear teeth  mesh; to have  an  abundance of oil to remove  heat; and to be  sure that the leaving  oil  will  get out  of the way of fast moving gear teeth. If  a problem is  encountered  as to delivery of oil  to the  mating  surfaces  of high speed gears, the  experience of Dern^20 may  help. This  investigator found  that  when  gears  run at 16,000 to 18,000 feet  per minute, “more satisfactory  results may  be  obtained by   spraying oil radically into  the  teeth  of both gears  at a point as close  as  possible to the mash”. For the purpose, one or two jets of 0.040 to 0.060 inch in diameter, delivering a solid stream of oil, were used. Where the pitch line velocity was 20,000 to 25,000 feet per minute, jets on the leaving side of the gears removed most heat.

Trouble may be encountered with high speed gears churning the gear oil which in turn causes heating. This  is one  reason directional  baffles or  even  a shroud around  the gears  may be necessary in order to keep leaving oil away from the gears  and directed toward a gear case outlet. 

Prevention of Contamination of Gear Lubricants

Gear sets operate under such varied conditions that it is difficult to give general suggestions for prevention of contamination of the gear lubricant. The two contaminants most often encountered are water and dust. In the latter are included large particles such as scale in steel mills. Less trouble is encountered where a circulating  system  provides  the  lubricant  rather  than  a  splash system, since the former  can  be  supplied  with settling  tanks  or filters. Even here regular inspection is necessary and accumulated water should be drawn off whenever noted. Such accumulations may  be quite  large  in the case of  gear  oils  for  ship  propellers or in oils servicing  paper  mill  machinery  or steel  mill  gears.

Where  gear cases  are  vented, the location  of the vent or a pipe  connection to the  same  should  be so  located  that  the least  dirt  possible  can enter. One automobile  manufacturer reduced  contamination in  differential cases by  extending  a  line  from  the  vent  and  securing  it  forward  under  the  car  frame  with  the  opening  toward  the  side  of  the  vehicle.

Prevention of contamination of lubricants by chemicals must have individual consideration. In  extreme  cases it  may  be  necessary  to  provide  pressure  on  gear  cases to  prevent  entry  of contaminants.