Flour Milling and Gear Lubrication

Up to time grain enters the mills, conveyors are used for handling. These  may  take the  form  of screw,  bucket, ribbon or belt and  any  one of these  types can be  driven  by gear  reducers. A  turbine type of oil  having  a viscosity of 500 SUS  at 100 degree F,  that  is AGMA  No. 3 grade, can  be  used  throughout  these  gear  reducers. In  the  northern  states  this  oil  should  have  a pour point  of zero or lower. An  oil of this  viscosity  will not cause excessive  power   loss and  yet  it  will  protect the  moving  gear  teeth.

Gear motors may be used for some conveyors and blowers. The  same  type  and viscosity of oil  should  be  satisfactory  for the  bath  lubrication  of such  gears, especially  if bearings are  serviced from the same  source. Where the drives in gear motors run quite warm, an oil of about 750 viscosity SUS at 100 degree F or an AGMA No. 4, may be desired. If such motors have a plate showing the recommended viscosity of oil, this suggestion should be abided by.

A tight  housing  is  essential  in any  of the gear cases in flour mills, more  from  the  standpoint  of prevention of contamination  from dust  than  from  leakage. Since  the oil  level  in gear  cases  should  be  inspected  every  month  or  sixty days, care  should  be  exercised that  dust  does  not enter  when  the  filling  plug or cap is removed.

Open  gears  are not  used too  often  around  flour mills; but if  these  are  encountered, it  is wiser to use  a  light  oil  as the  lubricant  rather  than  a  residual type. This can be the same oil as suggested for use in conveyor gear reducers, that 500 viscosity SUS at 100 degree F. As  such an oil  becomes  mixed  with dust, the paste formed will  slump off rather  than pack  in  the  bottom  of gears; thus, misalignment should  not be a  problem.  

Flour mills  may  be  self contained, in that cleaning, tempering, grinding and  sifting may  all take  place  in one  enclosure, or the latter operation  may be  separated. Such machinery will vary, but often the rolls are driven by gears. Here again the turbine oil type 500 viscosity SUS at 100 degree F can be used. Machinery handling middlings or bran will be much the same as previously mentioned and if gearing is used, the same recommendation will hold.  

In the manufacture of corn meal or animal feeds the same type of processing and machinery will be found. Thus, conveyors  and screens  will have  similar  drives but the  crushing or milling  can be  by  rolls  or  discs. In any event the gear oil used can be the same type and grade as recommended for flour mills.

It will  be noted that  a simplified  lubrication  application for  reduction  gears  is suggested  in grain mills, that is, a  single  oil  throughout with one  exception. This  is in  gear motors and even here  the  sanction  of the motor manufacturer  might  be  obtained for use  of the  500 viscosity  oil.

Nuclear Power plants Gear Lubrication

Present indications are that the principal gearing in nuclear power plants will be in connection with turbines. Such  reduction units should be  far  enough removed  from the area of high  radiation that no degradation  of the oil  from such  a source  will occur.

Cox et al.^14 who have considered this matter state: “Irradiation tests of turbine oils show that maximum expected radiation doses in  current and projected power plants over a twenty year period  does not change the  physical  properties of the oil. Oxidation stability and other properties are, therefore, still of most importance in turbine oil selection”.

Also  Okrent^43  in treating  design considerations of nuclear powered  surface vessels concludes that the  reduction gearing  in connection with  the  turbine presents no problems  from a radiation standpoint. In spite of the  above thoughts, Watson^51  mentions  that in  nuclear  power  generating  stations, a  number of applications  will be found where  gears  should  be run  without  lubrication. Experiments were, therefore, made with various materials, run in a dry state, with latter wear. As a  result, it is suggested that if loads are not heavy, spur gears, made of  case hardened  En steel, phosphate prior to coating  the flanks with  molybdenum  disulfide, can  be run  continuously, in a dry state, without  measurable  wear. Also  a worm wheel , made  from  woven  asbestos base  with  a case  hardened  steel worm, is promising for  operation in a dry  state.

No doubt, when and if gear oils with radiation resistance are necessary, suitable fluids will have been developed. Thus, polybenzenoid   compounds containing short alkyd groups show promise in such applications.

One  interested  in this subject might  avail himself  of a series of  eight  papers devoted to “Non-conventional  Lubricants  and Bearing  Materials such  as Are Used  in Nuclear  Engineering”. These were presented at the Manchester college of Science and Technology on April 12, 1962 by the lubrication and Wear Group of The Institute of Mechanical Engineers (British).   

Problems in Connection with Flushing of Gear Cases

 Different problems may arise in connection with flushing gear cases. First the used oil should be removed from the case as  completely as  possible so that  contaminants  are also  removed  and none  of the Oxidized  oil  remains to act  as a catalyst  for the  fresh oil. In so doing, if a volatile solvent is used, this should also be removed completely. Very little of a solvent is required to reduce the viscosity of gear oil, and naturally this is undesirable. Further the  gears should not  be left  without  a coating  of oil  for even a short  period because  rusting  will take place  under  such  conditions. Also  the gears should  not  be  operated  without  a coating  of oil,  even though there  is no load. Troubles have been encountered when gear cases were washed out with chlorinated solvents. 

                                                                 

After fresh oil was added, corrosion developed because some of the solvent was trapped in the case. The best correction for this problem is not to use such a solvent. In fact the safest course in flushing gear cases is to use prepared flushing oil. 

The Navy Gear Wear Tester

The Navy Gear Wear Tester is described in Federal Methods 791, Methods 335. The  equipment  makes  use  of  small  brass and  steel  gears,  but  Ninos  has  also  used  mating  gears  of other metals, such  as  brass on  stainless  steel, Phosphor  Bronze, and  ST Aluminum   on  SAE     4130 steel  and  SAE B-1112  steel  against  stainless  steel.

In  the  test  two  helical  gears  of  dissimilar  metals, each  approximately  one half inch  in  diameter are  rotated  together  as the  driving  motor  delivers a simple  harmonic motion     of 4.0  inches  amplitude  and  40 cycles per minute, through a  crack  to  the  upper  brass gear. This gear  oscillates  approximately  one  revolution while  a torque load of about  three  and  one  half inch pounds  is applied to  the  test  gears by  means  of a seven  pound  weight. The  gears  are  oscillated  for the  desired  number  of  cycles, or until gear  tooth  failure  due  to  excessive  wear  occurs. At the completion of the test, the gears are removed from the fixture, cleaned as before, and reweighed to    determine weight loss. The wear rate in mg for 10,000 cycles is then calculated permitting a comparison of different lubricants. New  test  gears  are  used  for  each  run even  though  there  is  virtually  no wear  of the  steel  gear  as  compared  to the  brass  gear.

Both fluid products and lubricating greases can be tested as gear lubricants on this apparatus. Indications  are  that  with  increase  in viscosity  of  gear  oils  the  gear wear   decreases. No  speculations are  given  as to how  much  of    the  wear  might  be  due  to abrasion which  is due  to particles  from  the gears.