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.

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.

Corrosion prevention by gear lubricants

Ellis et al.^22 consider that staining, tarnishing, and rusting are all  indications of corrosion. The thought is that light stain or tarnish represents the early stages of corrosion since, unfortunately these  changes do not proceed very far before pitting starts. Unreactive gear oils, which have not been subjected to excessive high temperature oxidation, have no tendency  to corrode  metals but, under moist or humid conditions or in the presence of  most salts or acids, do not offer proper protection against  rusting of ferrous  metal  surfaces such as gears. However, additives can be included in gear compounds which will provide rust prevention. Where conditions of incipient rusting prevail, the gears and other metal parts even to the inside of the gear case may require protection, particularly when idle. In such cases not only will the presence of a rust inhibitor but also the viscosity of the base oil be factors. Thus, the higher the viscosity of the gear lubricant, the slower this will drain from the metal surfaces and consequently the greater the rust prevention. Rusting may occur in different environments and various theories are offered as to the mechanisms of corrosion, but normally moisture and oxygen are the offenders

Most rust preventives are polar substances, such as long chain fatty acids, fatty amines, metal sulfonates, certain esters, oxidized petroleum fractions, etc. Such materials wet a metal surface preferentially and displace any water which may come in contact with the steel. The coating of polar substance then acts as a barrier against water reaching the metal surface.

As previously mentioned, controlled  corrosion due  to  EP  additives  is generally beneficial in  that  it  corrodes  away high  spots   on the gear  teeth after  which corrosion may decrease. With the proper selection of the chemical agents, these are not activated except under extreme conditions of load and /or temperature. Further, most of the EP additives which are  used  in  gear  lubricants will have little effect  upon  other  metals such  as bronze, copper, etc; at the bulk oil temperatures maintained  in normal  gear operation .

Reduction of noise,vibration and shock by gear lubricants

Gear lubricants, particularly those of high viscosity, will act as cushions and thus reduce noise, shock, and vibrations of meshing gears. However, it must be kept in mind that even the most viscous products are not correctives for poor mechanical conditions.

Mention might be made that Cardillo,^14 in an analysis of “Initial Axle Noise” in automobiles, concluded that it is a resonance problem and apparently lubricants or lubrication plays  no part in abating  this noise .A different noise effect has occurred in automatic transmission and limited slip differential mechanisms of vehicles and in both cases has been corrected by the use of oiliness additives. The degree of oiliness must be carefully controlled since, if deficient, the clutch operation will be rough and cause chattering or squawking but, if excessive, the clutch may slip. Mention will be made of some of the suggested additives for this purpose in a latter section

Demands made on gear and transmission lubricants

Mention  of  some  of  the  demands  made  on  gear  and  transmission   lubricants  may  well  serve  as  a  further  justification  for  the  assembly   and   publication  of   the  information   to  follow. Reviewing the problems confronting the automotive industry, Raymond^9  considers  the  hypoid   axle  to  be  the  hardest  working   and  perhaps  the  most  neglected  automotive   component. It  is  said  to  be  far  easier  to  deliver  increased  horsepower  and  torque  to  a  rear  axle   than    to  build  satisfactory  durability  into  such  a   critical  unit  which  is  confined   by  size   and  weight  limitations. Since  higher  engine  horsepower  and  automatic   transmissions  have  greatly  increased   low  gear  loading  and  higher  pinion  offset   has  produced  more  sliding   motion  between   gears, Raymond^9  con-  clues: Lubricants   thus   have  become  a  limiting  factor    in  the  load  carrying   capacity  of  American  axles.

 Automatic   transmission  fluids  offer  another  challenge  to  the  oil  industry   in  that  a  complex  combination   of  requirements   exists   for  such  fluids, and  Raymond^9  believes  that  the  future  appears  to  offer  a  still  greater  challenge  in  severity  of  automotive   transmission  operation.

In  addition  there  is  a  tendency  for  the  use  of  a  single  gear  oil  to  per  from  several  functions. For  example , a  lubricant  may  be  called  upon  not  only  to  protect  bearings  and  gears  against  wear  and   corrosion, but  also  to  act  as  a  hydraulic  medium. Likewise, an  automatic  transmission  fluid, the  primary  purpose  of  which  is  that  of  a  torque  converter, may  also be  called  upon to  lubricate hypoid  axles   in  the  future.

The  severe  conditions  under  which  some  gear  oils  operate  is  stressed   by  Hundere^5   who   states: “ The  most  difficult  function  that  a  gear  lubricant   must   perform  is  that  of  preventing  excessive  metal  to  metal  contact  in  the  region  of  maximum  sliding   velocity. When  it  is  realized   that  the  unit  loading  at  the  point   of  contact  is  as  high  as  400,000  psi  and  the  sliding  velocity   is  as  high  as  6,000   fpm, it is  amazing   that  excessive    wear, let  alone  fusion,  can  be  avoided.’’

It  is  quite   evident  from  the  above  that  constant   improvement  in  qualities  of  gear  lubricants  is  necessary  to  meet  changing   demands. In  spite  of  the  upgrading  of  such  products, they  are  universally  available  at  reasonable  costs. It  is  well  to  keep  in  mind  that, from  an  economic  standpoint, the  benefits  of  correct   lubrication, including  that  of  gear  sets,  are  mainly  due  to  reduction   in  breakdown  or  maintenance   and  consequent   decrease   in  loss  due  to  downtime   or  curtailed   production. This  holds  true  for  both  transportation  and  manufacture. Saving s   in  power  and  frictional   energy,  while   often  attributed   to  correct  lubrication, are  small   compared   with  the  first  benefits   mentioned.