Radioactive Tests for Metals in Oils

Tests in which the radioactivity of metals or their compounds is used as a means of determining their presence or concentration are largely confined to experimental investigations. This  is true  because, first  activated metals or  compounds must  be present  and  this is not true of  commercial  lubricants and second, the life  of some of the  isotopes  used  is short so that  after  prolonged service the activity would  almost  disappear.

Limitations on Heating of Lubricants for Application

Heavy bodied lubricants, particularly residual types used on exposed gears, are often heated in order to make application easier. If such lubricants are straight mineral oil products, the amount and intensity of heat should not harm them. However, if additives are included, only a very moderate heat should be used. Otherwise some change in the composition is possible. A supplier of the lubricant can advice the limitations on heating. A similar caution is necessary in case lubricating greases are used as gear lubricants. The thickeners for such products may be soaps which upon the application of considerable or prolonged heat will separate from the oil present. 

When to Change Gear Oil

The  ideal method of lubricating  enclosed  gear sets would be to place the  proper  amount  and quality  of gear  lubricant  is a  sealed case and  make  no  renewal  of the oil  during  the life  of the  mechanism. Manufacturers  of  automobiles have  this  in  mind  and  approach  such  a  solution  of gear  lubrication by  recommending  no  drain  periods and prolonged  use of gear oils. Manufacturers of other equipment have the same thought in mind and the trend will no doubt increase. For illustration a side entering mixer description states: “Lubricant is sealed in the gear case at the factory and is designed to last five years.” Naturally, any statement as to the life of gear lubricants should be modified with an expression as to operating condition, environment, etc. However, it is questionable if the present gear oils are everlasting even under the best of conditions. Therefore, from  both a service and economy  standpoint, used gear  oils  should  be removed  from  the gear cases  and replaced with fresh  lubricant either  when the oil  has  deteriorated or become  contaminated  or at stated intervals.

AGMA, which is interested in insuring log and uninterrupted service from gear sets, recommends the following: “The oil in a new unit should be drained at the end of two weeks operation and the case thoroughly flushed with light flushing oil. After this, a change of oil every 2500 hours of operation or every six months, whichever occurs first, is recommended for the units operating under favorable conditions. Where  operating  conditions  are  severe  such  as  rapid rise  or fall  in temperature of the gear  case with  accompanied  sweating  of the inside  walls  resulting  in a formation of  sludge, or where  operation  is in  moist or dusty atmosphere or in the  presence of chemical fumes, it may be  necessary to change the oil at  intervals of one  to three  months.”   

It is also pointed out that gear sets are usually treated by the manufacturer with rust preventive s before shipment. Such materials may have an adverse effect upon the gear lubricant and, therefore, should be removed before gear oil is added. Petroleum solvents are best for this purpose, and such fluids will also help to remove contaminants, such as metal chips. Immediately  upon removal  of such solvents a low viscosity  flushing oil  should  be  sprayed  on the  gears; otherwise, the  metal  surfaces may  rust  in a few  minutes. Also the gears should not be operated, even for a short time, in a dry condition.

In enclosed  gear  cases  which do not  have  a  drain or a  circulating  system, the used gear oil or flushing oil  should  be removed by suction.  Use of pressure to  force  out  such oil may damage seals  and in the case  of  automotive  equipment will probably force oil  onto  the brake  bands. When  considering  oil changes  for reduction  gears, Forbes et al.^20  mention that  the first  month  of  operation is the most  critical in the life  of  gears; therefore, they  suggest  a change or  careful  filtration of the lubricants within two  weeks  after  the unit is put  in service. It is  pointed  out  that  fine metal  particles  resulting  from the run in period act  as catalysts  for oxidation if left  in the gear case. Rather than  setting  an arbitrary  period of change, the  above  authors^20  think  that  periodic oil samples should be taken  to determine if the oil  is in a  usable  condition. Such samples can be checked for presence of dirt, metals and water. Also the acid number, viscosity and interfacial tension can be determined. Certain fleet owners operating heavy trucks have a rule that the oil in transmissions or rear axles must be changed when the viscosity has increased by fifty per cent.

Where gears are lubricated by  circulating  systems, Forbes et al.^20 states: “ the  charge may often  be used for several  years without change,  particularly  when  adequate  filtering  equipment is employed”. However, cleaning such a system when changing oil requires more effort than in splash systems. Flushing  oils  are available  which, due to  either additives  or the  particular solvents used, will  remove most of the  deposits  from  the  oil reservoir and the piping as well as  the gear case. However, a final cleaning with dry rags free from lint may be necessary.

The greatest care should  be  used  in  cleaning  gear  cases and  auxiliary  equipment since  any used  gear  oil left  in a case and mixed with fresh  oil  will  tend to act as  a  catalyst to  promote deterioration of the new  lubricant. With the flushing oil on the gear set an inspection is possible. If this shows rust, the gear manufacturer should be consulted before operating. If the gears appear to be in satisfactory condition, they should be coated with the gear oil as soon as possible. In the  case  of a  circulating system this  can  be  done  without  operating  the gears. If a splash system is employed the gears can be sprayed or wiped with the lubricant before operating.

Where meshing gears are of dissimilar metals as in most worm gear sets, the importance of a change of oil and inspection of the mechanism is very necessary. As the gears seat themselves bronze particles may become attached to the worm. Such  particles  adhere  to the steel  and  cause  a rough  surface which  will  score the gear. Removal of the used lubricant and cleaning of the worm threads after a short period of use will often prevent further wear.

When oils are used on a once through basis flushing may not be possible. In the case of lubrication by oil fog, only fresh oil is supplied, and the tight system is under some pressure so that contaminants should not enter. However, in the case of open gearing, cleaning at intervals is advisable. Not only does the residuum  type  of gear oils pick  up  dirt  which  in turn  may act as abrasives, but  also some of the  heavy material  packs in the  roots  of the gear  teeth. If such build ups continue shafts may be thrown out of line. Such deposits can usually be softened by kerosene or some other solvent provided there is not a fire risk. If drip pans are provided for open gears these should also be cleaned at regular intervals.

Methods of Application of Gear oils to Heavy Duty Gears

Older equipment employing heavy duty gears may make use of bath systems   for application. Many  plants  have  converted  such  systems  to  circulating  application such as  is used  on most  modern  machinery. By  this  means, oil sprays  can  be  directed  to  the line  of  meshing  of the  gear  teeth and  if  necessary the  oil can be supplied  to  other  machine  components, such  as  bearings. By  this  means, when  accompanied with  proper auxiliary equipment, such as coolers, filters, settling tanks etc., the life of the oil is prolonged and contaminants removed.

Desirable low and high temperature qualities of gear oils

Transmission and gear oils which are used either in vehicles or in outside locations in cold climates should remain fluid at low temperatures. Low temperature is a relative term and an extreme case is that of aircraft flying at high altitudes which, therefore, encounter temperatures approaching – 70 degree (F). The gear oils used in this case are compounded from synthetic fluids.

Disregarding synthetic lubricating fluids, two general types of mineral oils are available. These are naphthenic and paraffin. The former, as a rule has the best cold test because no wax is present, but as temperatures are reduced such oils may tend to become quite viscous. This tendency will vary both with the viscosity and the source of the base oil. Most paraffin type oils contain wax which crystallizes at low temperatures and may therefore form a solid mass which will not flow or pump and may even channel in a gear case so that the gear teeth would lack a lubricant. By the addition of pour depressants, generally certain  types of  polymers, the wax crystals are coated  so  that in place  of growing to long  needles  thy deposit as fine crystals which produce a slurry  which moves at a lower temperature  than the untreated oil.

A more positive approach to securing low cold test gear oil is to choose lubricating oil which has been refined so as to have a low pour point. Low viscosity oils generally have lower cold tests than high viscosity oils of the same type. Thus, lubricating oils are available with viscosity of 70 to 80 SUS at 100 degree (F) which have pour points of -60 to -70 degree (F). Proper dewaxing will also lower pour points. In extreme cases gear oils are offered which consist of lubricating oils diluted with fractions boiling below the lubricating fractions. Likewise, in the arctic regions Kerosene has been used to dilute gear oils. This poses a  hazard to the equipment  if the diluted  lubricant  is not removed  from  the  gear  case  with the advent  of normal temperatures.

High temperature limitations of most gear oils will be due to deterioration of the oil and additives. Oils should be chosen having high flash points if operating temperatures are excessive. The flash point of oil is generally indicative of the temperature at which volatility starts. In most industrial gear applications, the bulk oil temperature seldom exceeds 125 to 150 degree (F). In vehicles such temperatures seldom exceed 300 degree (F). Therefore, for  what considered normal  gear lubrication, additives are  selected  with  the  thought  that  temperatures of  use  will  not exceed 300 degree(F)

However, Borsoff et al ^8 describes tests run at both 400 degree (F) and 600 degree(F) on a  gear test  machine in which the gear compartment and the oil were  heated while  the machine  was in operation. Conclusions drawn from this investigation were:

                                   “Due to the thermal instability of the lubricants, the operation of gears for any prolonged period of time at 600 degree (F) or higher is inadvisable; (2) no new or unusual types of failures were observed during the operations of gears at 400degree (F) and 600degree (F). Just as at normal operating temperatures, scoring and abrasion were the two primary destructive failures; (3) the load carrying capacities of oils decreased with the increase in temperature. However, at temperatures over 400degree (F) gum deposits are formed by oils. These deposits serve as a protective coating and improve scoring performance of gears and gear lubricants”. For this reason SAE 20 and SAE 30 mineral oils and “Plexol 201” showed about the same load carrying capacity at 400degree (F) as at 100 degree (F). However, 1010 grade and SAE 60 mineral oils, “Ucons LB-170” and 50-HB-170 and “DC-200” silicone fluid all scored at lower loads at 400degree (F) than at 100 degree (F).

Bowden and Tabor^11 investigated the frictional behavior of oil films on steel surfaces as they were heated and as oxidation took place. At 302 degree (F) after 30 minutes heating the friction became low. At 392 degree (F) the reduction in friction occurred after 15 minutes heating and at 572 degree (F) low friction was observed after less than 2 minutes heating. In the latter case if heating was continued for 20 minutes a thick gum was formed and  friction increased to a high value. The above action was no doubt due to the formation of acidic products due to oxidation. Such products, in time, might lead to corrosive wear. Next the oil would increase in viscosity and compounds would from which are insoluble in the oil. When and if acidic products result from heating of gear oils, metal compounds will eventually from and these in turn will act as catalysts to promote further oxidation.

In view of the fact that high temperatures cause rapid deterioration of most gear lubricants, equipment should be so designed that operating gears are subjected to only moderate temperatures if long life of gear oils is expected.

However, since the military  desire gear lubricants for  jet aircraft  and  other agencies for space vehicles which  will stand  very  high  temperatures, it can be  expected that synthetic fluids or solids will  be  available  which will withstand 700 degree (F) or even 1000 degree(F) for a period.

Minimum action of gear oils on components of mechanisms

Well refined mineral lubricating oils have little if any action on most metals, particularly ferrous metals. It is only upon prolonged use at elevated temperatures that such oils may from compounds which will act upon metals. Since such oil changes can be retarded or almost completely arrested by the use of oxidation inhibitors and also metal deactivators or pacifiers can be included, there should be little concern about the action of straight mineral oils upon the metal components with which they come in contact.

What we are concerned with here is the action on parts made from other materials, such as paper, plastics, rubber, etc. Seals are likely to be made from rubber, either artificial or natural, and any deterioration of the compositions due to the lubricant should be at a minimum. Many seals consist of compounded materials, such as “neoprene,” and it is found that oils high in aniline points, as are most high V.I. oils, will have little effect on this compound.

Automatic transmission mechanisms may be found to contain paper and “Nylon” parts. In future devices which will require transmission fluids, a greater variety of components may be used. The safest course when supplying oils for contact with unusual materials is to have the fluids pretested to determine their suitability.