Malfunctioning of Other Mechanical Elements

Misalignment may be due to wear of bearings and in extreme cases of failures it may be difficult to determine whether a bearing or a gear failed first. In case of bearing failures, when lubrication was at fault, either oil did not get to the bearing. Or the proper oil was not used. Where  a common  oil is used for bearings and gearing, the  viscosity of the oil is  generally  a compromise  since  the bearings operate best with  a lower  viscosity oil than  is dictated for the gear sets. Therefore, when bearing trouble is encountered, thought might be given to use of a somewhat lower viscosity lubricant.

Malfunctioning of flexible couplings may also contribute to trouble with gear sets. Such couplings are often used to connect reduction gears with a different piece of apparatus. These  couplings may not only  break but  also  become  jammed, so that  they  are no longer flexible; thus the  gear  shaft will  be thrown out of line. Another possible source of trouble might be a break in a gear case or supports which would permit a shaft carrying gears to be misaligned.

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.

Wear Prevention Agents

A number of investigators have distinguished between wear prevention and EP agents in lubricants. The  former are  effective  by  a  chemical  polishing action, which  takes  place  at a  lower  temperature  than  does  the  formation of antiweld  films by EP agents. A very  extensive  investigation  of  wear  prevention  agents,  which are  effective  in  lubricants for  two  steel  surfaces was  reported by  Beeck, et  al^9. Calhoun and  Murphy^18, who  reported  on both  anti wear  and EP  additives  for  lubricants found that it  was  possible to  blend  two  or  more  types  of  additives  and  attain both  properties in  the same  composition.

Typical  of  anti wear  additives  are  tricresyl  phosphate  and  Zinc  dialkyl  dithiophosphate.  Such  agents  are  seldom  used  in  industrial  gear  oils  but  are  valuable  in  lubricants  such  as  those  for  jet  turbines  where  the  oil  serves  several  mechanisms  including  gearing.

Abrasive or cutting wear

Gear oils are not correctives for abrasive wear because here the action is due to hard particles between the gear face as they mesh. If the abrasive is due to loose metal, sand, etc; gear oils may wash the foreign particles from the moving areas, but unless the abrasives settle out they will continue to act as lapping compounds. However, if the viscosity of the gear oil is low, the large foreign particles may be deposited in areas where the velocity of the oil is low, and thus they will be harmless.

The best corrective for abrasive wear of gears is to drain and flush out the gear case and refill with clean oil. Circulating oil systems used for gear oils can be equipped with filters or strainers. Likewise, a settling period can be provided in the storage system for the fluid. Some gear cases in automotive  vehicles  have magnetized  drain plug  so that  most  iron or steel  particles will  become attached  as  the  gear oil  circulates. Where vehicles operate under conditions promoting  dust, as do many  tractors, it is wise to  drain  gear  cases  frequently  so  that  abrasives  filtering  into  the  gear  oil  will be removed.

Fatigue wear

Fatigue wear may take place even where gear oil provides a satisfactory lubricating film and there seems to be some question as to what quality of a gear lubricant is responsible for decreasing fatigue wear of gear sets. Hutt^30 presented data from an IAE machine which showed that a 190 per cent increase in fatigue life resulted from an increase in viscosity from 90 to 140 cs at 158 degree (F) . On the other hand, Hundere^29 is of the opinion that it is not viscosity as measured by standard viscometers that controls the effect on surface fatigue. Davidson and KU^20 likewise state: It was found that lubricant viscosity, at least as measured by the conventional viscometric  method, did not have as predominant an effect on  gear tooth surface  fatigue as some other undetermined  lubricant characteristics.

In tests conducted by these investigators the lubricant having  the highest rating  on a Ryder Gear Test ran three to four times as long before pitting occurred as did the other oils examined.

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

Reduction of wear in gear sets and transmissions

Wear has been defined by a Committee of the Institute  of  Mechanical Engineers as: “Progressive loss  of substance  from the surface of a body  brought  about by mechanical  action ( usually it  reduces the serviceability of  a  body  but  can  be  beneficial  in its  initial  stages  in  running in)”.

It is evident from  this definition  that  what  is desired  in a gear lubricant  is  prevention  of  continuous  wear. With highly loaded moving parts, which include gears, the following types of wear may occur:

                            

                            (a) Abrasive and cutting wear.

                            (b) Corrosive wear           

                            (c) Fatigue wear.