Deposit Forming Tendencies of Aircraft Turbine Lubricants

This test method describes  a procedure for  determining the  deposit  and  sludge  forming  tendencies  of  aircraft gas  turbine  lubricants when  a sample  of the  oil is circulated  under  controlled conditions for a  prescribed  period of time  through an  aerated test  chamber  containing  an  aluminum  tube  held at a constant  temperature.

The coking tube is held at 590 degree F while oil  heated  to  300 degree F  is circulated by  a pump  from  the  chamber through  a  cooler and a line  filter and  back  into  the chamber. The oil flow is regulated to 300 ml per minute while air flow is the same amount.

At  the  end  of the test, the  weight of solid  decomposition  products  on  the  heated tube  and  in the  line after  are  determined. Also,  changes  in the  viscosity and  neutralization  number  of the oil can be  determined  if  desired.

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.

Oxidation stability of gear lubricants

Once a proper gear lubricant is selected for a given application it should suffer a minimum chemical and physical change during use. One of the changes most likely to occur is oxidation  of  the oil  which ultimately  will  lead  to the formation of undesirable  products  and  changes in the characteristics  of the oil. Such changes may result in the formation of acidic  products which  may corrode  the metal  surfaces, in an increase  in viscosity  of  the  oil, or in production  of  insoluble  materials. Oxidation  of  lubricants  is  accelerated  by  high  temperatures or  by  the  presence of  certain catalysts, particularly  soluble  metals. The  immediate  effects  of  oxidation  may appear  beneficial  in  that  petroleum  acids formed  function  as  oiliness  agents, perhaps by  the formation  of  monolayers  of metallic  soaps. Ultimately, as oxidation of oil proceeds, the harmful effects become evident. The degradation of the oil by oxidation may result in not only the formation of acidic products but also asphaltenes, resins, or other polymers. Changes in the lubricant will  probably  be  accompanied  by  increase  in  viscosity , darkening  in  color, and  the  formation  of  sludge. Cases have been noted where gear oils became almost solid due to oxidation.

However, oxidation of gear lubricants can be retarded by addition of antioxidants or oxidation inhibitors. The use of such agents in most gear oils is wise since the environment for the lubricants is favorable for oxidation in that both air and heat are present and thin films of the oil are in contact with the air.

The mechanism of the action of antioxidants is generally considered to be that of chain breaking as the additive reacts with a “hot” molecule, thus being itself oxidized. In this process the oxidant molecule is destroyed, but with dissipation of the energy possessed by the “hot” molecule, so that the chain reaction is broken. Thus, the oxidation of hundreds or perhaps thousands of molecules of hydrocarbons is prevented, since the energy would be passed on from one molecule to the next in the normal chain reaction.

The suggestion was made by Larsen and Diamond^35 that antioxidants may be either inhibitors or retardants, the former acting to break reaction chains and the latter being converted into an inhibitor during the oxidation process. Three possibilities were given by Murphy et al.^42  for the possible disposition of such inhibitors after they had reacted: (a) the inhibitor is oxidized to a compound which is incapable of further antioxidant action; (b) the inhibitor is oxidized to a compound which still exhibits antioxidant action, but generally to a reduced extent; (c)  the inhibitor is capable of regeneration. The latter type of additive is the most desirable, provided the rate and degree of regeneration are high.

Specific compounds suitable as antioxidants will be suggested in a later section, but most of these agents fall in the following bellows: (a) various types of phenols, (b) certain sulfur bearing compounds, (c) numerous organic phosphites, and (d) certain of the amines. A number of additives function as dual purpose agents and thus, in some cases, a specific antioxidant may not be required.

Freedom from separation in gear oils

Precipitation or settling of some components in gear oils is sometimes noted. This most often occurs in mixtures containing EP additives. The separation may be due to lack of solubility or to reaction of ingredients resulting in formation of sludge. Since such additives are often present in concentrations of 9 per cent or more, the base oil must keep a high proportion of heavy chemical compounds in dispersion or suspension. Both additive manufacturers and oil blenders select ingredients which will keep any separation of such agents at a minimum.  

Fletcher^23 selected three SAE 90 hypoid gear lubricants and three multipurpose SAE 90 oils meeting MIL-L-002105A specification. By precipitation tests, the first three oils showed some sludge in the unused oil which increased after a service test in two of the lubricants. In the multi-purpose oils there was only a trace of sludge before use but measurable amounts up to 7 per cent after the tests.

Of course, settling or sludge formation in gear lubricants results in loss from the action zone of valuable active ingredients, but the greatest concern is the effect sludge may have on operating mechanisms. Thus, Fletcher^23 mentions that precipitation of sludge out of oil due to centrifuging in transmission cases may result in carbon like deposits in pocket bearing positions, internal clutch teeth, and in some cases in oil grooves  and synchronizer grooves. It is conceivable that such deposits could adversely affect the operation of the unit. This fact was probably recognized by one tractor manufacturer who specified that oils used in their equipment should be filterable, thus, indicating that sludge should not separate during  normal operation.

Where sludge is formed in EP gear oils the action is accelerated by increasing temperature. It is therefore probably a result of reaction of the chemical compounds which constitute the EP additives. Detergent agents do not seem to be a correction for such sludge separation, and any improvement in the condition probably lies in selection of the EP additives.