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22.3.1 Problematic properties of lubrication oils.

 Problems of lubrication oils

Oil does not only serve as lubrication. It must also dissipate /remove heat, to avoid overheating. These requirements must be guaranteed with a sufficient long usability. For this lubrication oils in turbo aeroengines must fulfill many demands.

  • Good lubrication properties.
  • Heat transport/remove from the lubricated components.
  • Good Pumpability.
  • Bad inflammability/ignitability.
  • Thermal and mechanical stability.
  • Passive and aktive compatibility with wetted materials.
  • Little tendency for the absorption of water and air.
  • Storability.
  • Little formation of foam.
  • Low evaporation rate.
  • Good filterability.
  • Environment frendly, as possible non toxic.
  • Low-cost.
  • Available.

Tese properties must guaranteed at its best from the application specified chemical and physical data. Depending of the design of the oil system, the components which must be lubricated and the operation temperatures, this is not always fully satisfactory possible. So the short haul and long haul use of airliners or in fighters can markedly differ. It is quite possible that problems arise first after longer operation times, caused by changes of the oil properties during operation. To these count aging effects, formation of coke and the absorption of water.

 Lubrication oils

Fig. "Lubrication oils" (Lit. 22.3.1-2 and Lit. 22.3.1-4): Norms (egineer standards) respectively spezifications only define the frame respectively the limits of the oil properties. These can be fulfilled from different product specific „compositions“. This is the reason why oils of a single specification experience different operation caused changes (aging, Fig. "Causes of oil discolorations").
The most important requirements at the operation behaviour of the aeroengine oils are:

  • Sufficient low viscosity respectively fluidity at low temperatures. The viscosity lies with a kinematic viscosity of 2
  • 9 mm2s-1 at 100 °C markedly below this ot piston motor oils with more than 20 mm2s -1.
  • Enough load bearing capacity at high temperatures and
  • high thermal stability with oxidation resistance.


Mineral oils have been used in aeroengine types of the 50s. Its viscosity above 10 mm2s-1 at 40 °C was relatively high and was required for the, at this time, frequent highly loaded propeller gears. Anyway especially for gears of helicopters additional additives (extreme pressure = EP additive) are used to guarantee the demanded high load bearing capacity.
The two oils at the beginning of the chart (oil 2 and oil 3) are mineral. Its viscosity at room temperature lies at about 10-12 mm2s-1 and 100°C in the region of 2 mm2s-1 respectively 3 mm2s-1.
With the development of aeroengines the demands at the oil rose. Stability and volatileness of the mineral oils is now no more sufficient. Therefore it was passed to synthetic oils (dark field). The production used natural oils. Its basis are organic compounds, so called esters. Those develop during the reaction with acids and consist from chains of hydrocarbons which are connected with oxygen atoms by double bonds. The viscosity is about 3 mm2s-1 at 100°C and with this relatively low (Ill. 22.3-5). The viscosity at 100 °C serves the grading to a oil type/class. To improve the load bearing capacity thickeners, also esters have been added. Addtionally stabilisers are necessary to improve the thermal stability.
It was succeeded to reach viscosities at 100 °C of about 7 mm2s-1. These have been preferred especially in England (DEF-STAN 91-96). At the beginning in the USA synthetic oils, accordant the viscosity type of 3 mm2s-1(MIL-7808), have been used, those generally prevail. For an increased lifetime of the oil, DEF-STAN 91-94 was developed.
For a further improvement of thermal stability and load bearing capacity '5 mm2s-1-oils', according to MIL-PRF-23699, have been developed. For the use in supersonic aeroplanes the oil according DEF-STAN-91-100/1 (DOD-L-65734) has further improved values.
Although the 5 mm2s-1-oils established itself in the main aeroengines of airliners, `3 mm2s-1-oils' are increased used in APUs. Cause is the relative low viscosity at very low temperatures between -20 and -40°C. APUs are normally shut down during cruise in great heights and then cool down to the extreme surrounding temperatures. A high oil viscosity increases the forces which must be overcome. This leads to a high starter power and a drop in rotation speed during the starting process. This can cause a „hang up” (volume 3, chapter 11.2.1.1) of the aeroengine compressor with dangerous damages (overheating, blade vibrations). Those problems intensify with the bad ignitiability of cold fuels (Fig. "Combustion process influenced by change of fuel properties" and Fig. "Properties of fuels 3").
Basically it must be said, that the ester compounds in the synthetic oils can attack and deteriorate plastics like elastomeres (seals, rub in coatings), resins (paints and coatings, insulations, plastic materials). This is also true for some metals (Fig. "Metallic contaminations in oil show problems" and Fig. "Compatibility of oils with materials"). This demands the selection of suitable materials in the oil system. It must be considered, that also regions of the aeroengine, outside the oil system can be wetted by leakage oil during filling or disassemblies (maintenance). To those belong rub in coatings of elastomeres and plastics in the fan. If necessary, for all potential concerned materials during the development phase of the aeroengine or durig changes, a proof of uncritical behaviour is needed (Lit. 2.3.1-8). This is applied for oil wetted spline toothings of couplings (Fig. "When leakage oil can not be missed"). Thereby also changed oil properties like aging (Fig. "Influences at oil consumption") must be considered. Samples for the proof must include this possibility with the suitable sufficient realistic sampling.

Notes about the oils, specified in the table (labeling, marking by NATO-Code or GB-Code):

O-133: Mineral oil of the type 2 mm2s-1 with oxidation retardant additive (anti-oxidant). Is also used as corrosion protection oil in fuel store systems, especially in tanks. It correlates the russian oil type MK-8.
O-135:Mineral oil of the type 3 mm2s-1 with corrosion retardant additive (anti-corrosion). Was developed for former single spool axial flow aeroengines and certified for certain helicopter aeroengines. Used as corrosion protection oil in stored fuel systems, especially in tanks. It correlates the russian oil type MK-8.
O-148: Synthetic oil of the type 3 mm2s-1 with additives for the improvement of oxidation, corrosion and wear.
DEF STAN 91-94: Synthetic oil of the type 3 mm2s-1 with additives for the improvement of the oxidation resistance and the load bearing capacity. Because of its relatively low viscosity at low temperatures (Fig. "Oil viscosity and lubrication by temperature") it lends itself especially for APUs to minimise the consequences of the so called „cold soak“. Standard oils have at -40°C a viscosity of 10 000 mm2s-1, compared with 2000 mm2s-1 of the DEF STAN 91-94.
O-149/O-159: Synthetic oil of the 71/2 mm2s-1 type with viscosity increasing additives as well as improved oxidation resistance, thermal stability and load bearing capcity. Especial suitable for für turboprop aeroengines because of the propeller gear.
O-154: Synthetic oil of the „third generation” of the type 5 mm2s-1 with additives for the suppression of the coke formation by increasing the oxidation stability and thermal stability. Offers therefore advantages for modern civil aeroengines with typical high pressure ratio like:

  • Clean oil system,
  • Increased load bearing capacity enables longerlifetimes.
  • Lower maintenance costs.

To those belong the military versions „high thermal stability“ (HTS), „standard” (STD) and „corrosion protection oil“ type (C/L).
O-160: Synthetic oil of the „third generation” of the 5 mm2s-1 type with additives for the oxidation resistance and thermal stability as well as the load bearing capacity. It was conceived with regard to projected supersonic airliners. The application now occurs at a progressive rate in transfer gears of helicopters.

Mixing of oils among each other in the same viscosity group is possible for synthetic oils. Does this happen by repeated filling up in the oil tank during successive manitenances (top-off mixing) the change („shock“) for the aeroengine is not so abrupt. Because of this reason, aeroengines usually will be only filled up. So an oil change is avoided. Fig. "Oil coaking endangers main bearings" shows the case where failures caused by oil `coal' (coke) have been markedly influenced by mixing of different oils. Therefore, procedures have been specified in detail. However basically apply the instructions of the OEM. Additionally a frequent inspection of the oil filters is recommended.

Unacceptable and dangerous is the mixing of mineral oils with synthetic oils!

Concluding the health hazards by handling lubrication oils should be discussed (Lit. 22.3.1-6).
The most probable risk with all oils is an irritation of the eyes, skin and respiratory system. This can occur with frequent direct contact of the skin with oil, oil vapour and/or oil mist. During increased oil temperatures and badly vented areas/chambers a danger through oil vapour and/or oil mist exists. Therefore safety instructions (e.g., safety data sheet = MSDS) must be met. To avoid a contact with oil during handling prescribed eye protection, gloves and clothing must be used.
In no case oil may be suctioned with the mouth (e,g., by means of a pipe). This is not so alarming for the stomach. But the danger of a temporal pneumonia exists.

Gets oil into the mouth, the doctor must be consulted at once.

Burning oil can discharge toxic gases and combustion products like aldehydes and carbon monoxide. Therefore here especial attention must be applied. If necessary oxygen must be inhaled. In this connection also oil contaminations of the cabin air must be considered (Ill. 19.2-9 and Fig. "Survey of cockpit crew").

 Compatibility of oils with materials

Fig. "Flight time dictated by oil shortage" (Lit. 22.3.1-2 und Lit. 22.3.1-4): The desining engineer had already to select suitable materials for the oil system. For this he has to consider the compatibility with the used oil, also after the sheduled operation times. During operation, assembly and maintenance, the prescribed/certified/approved parts/components must be used. In this connection consumables (e.g. sliding ring seals) with elastomeres and auxilary materials (e.g., films, sealing compounds) can be especially problematic. Here also suspected unapproved parts (SUPs, chapter 20.2.1) can play a role. In the following, the behaviour of some materials against synthetic oils will be considered in detail:
Paints: The only not attacked respectively softened paints are epoxides. They merely discolour up to slight shades. This may be connectred with oxidation inhibiting additives.
Plastics: The best durability show polytetrafluorethylene (= PTFE, „Teflon”® ). That these materials are not fully unproblematic shows Fig. "Negative influence in operation by oil". Especially PTFE filled with bronce as sliding/sleeve bearing materials have a lowered chemical durability what primarily can be traced back to the filler material. Also light swelling can be extremely alarming for bearings with tight tolerances. An example is the jamming of the actuation of variable compressor guide vanes.
PVC is more sensitive and softens fast in hot oil. Glazes based on compounds (varnishes) like phenols and silicons can get soft in hot oil. In contrast hardened alkyd basis shows a good durability but is sensitive for water. In this case additionally a waterproof sealing must be applied. Mineral oils and edible oils are incompatible with synthetic oils. In no case they may be mixed. All traces of the other oil must be removed before if the possibility of a contact exists.
Pure copper has in mineral oils and synthetic oils at high oil temperatures a pronounced catalytic effect. Acids will be developed. Such a deterioration will not be triggered from copper alloys like brass and bronzes.
Aluminium alloys and steels are resistant.
Magnesium alloys will be only attacked if water is present. For this a decomposition of the esters from the synthetic oil by a reaction with water (hydrolysis) is necessary. As protection epoxid resin is suitable (volume 1, Ill. 5.4.1.2-9).
As a degree for the acid formation the so called TAN (Fig. "Formation of depositions in hot oil systems") is used. It depends from the aging of the oil.
Zinc as a galvanic applied corrosion protection develops with synthetic oils zinc placers (soaps). Therfore for the storage and the handling of synthetic oils containers made from tinned sheet, steel (tin is not attacked) or stainless steel (type CrNi18/8) should be used.
Silver and silver alloys can discoulour dark in synthetic oils with certain additives (for the increase of the load bearing capacity). In an extreme case this layers can dissolve.
Cadmium serves in elder aeroengine types as corrosion protection for steels. It dissolves in synthetic oil at higher temperatures. Indeed oil properties will not be critical changed, however the corrosion protection will be degraded.
Lead and lead alloys are attacked by synthetic oils and therefore must be avoided in such oil systems (e.g. as solder in filters and sieves/screens).
Chromium and nickel as well as its alloys are not attacked.

Note: Also the compatibility with possible/by mistake with aeroengine oil wetted components must be checked . This applies especially for the introduction of an other oil sort.

Fig. "Negative influence in operation by oil" (Lit. 22.3.1-8): The swelling of plastics in aeroengine oil can be reversely. In this case oil is again driven out without the lasting change of the properties from the plasic. Anyway swelling can cause dangerous problems. This applies especially for the jamming/sticking of sliding surfaces in tight gaps. In the shown case, which was described in a company magazine, obvious after the introduction of a new oil sort an impotant shaft in the control system jammed .

 Negative influence in operation by oil This was traced at a swelled PTFE bearing bushing. Short time remedy was the increase of the clearance, longtime the change of the bushing material offered itself. However this may be difficult. The here used PTFE of the bushing is still one of the most important plasticy, which normally tolerate synthetic oils (Fig. "Compatibility of oils with materials"). Possibly the bushing contained a filler material which increases the pressure bearing capacity and promoted the change of the volume.

Generally it can be said about the swelling of plastics in aeroengine oil:
The highest risk exists in the swelling of elastomere seals of the oil system. An oil leakage as consequence can at least demand extensive maintenance work. In aeroengines at important points bushings of sleeve bearings with close tolerances are used To these belong also the actuations of variable compressor guide vanes. Here indeed usually an oil wetting as lubrication is not planned. However, gets oil by mistake (as lekage oil) or knowigly (lubrication with not specified oil) during maintenance work in contact with bearing sleeves/bushings from plastics swelling can cause jamming.
The introcuction of a new oil sort/type therefore must prove besides the materials of the oil system also always such parts outside the oil system for compatibility, which can come in contact with the oil. Especially during maintenance and overhaul the probability of a wetting by a mistake exists (Fig. "Problems due to change of the oil type", volume 2, Ill. 7.1.3-22). Critical are rub in coatings in the fan region made from filled elastomeres (Ill. 23.3.1-5 and volume 2, Ill. 7.1.3-21).

 Problems due to change of the oil type

Fig. "Problems due to change of the oil type" (Lit. 22.3.1-9): Elastomers are especially deterioration ebdangered from synthetic oils respectively its additives (Fig. "Compatibility of oils with materials"). These influences can be intensified by aging processes in the oil. Thereby different changes can occur oil specific and material specific (Fig. "Influences of O-ring materials"):

  • Embrittlement with crack formation (sketches below),
  • Swelling with a drop in strength anddebonding (Fig. "Operation behaviour of elastomers by swelling") or pinchings with the extrusion from gaps.
  • Plastic (permanent) deformations like flattenings (sketch below, Fig. "Aged O-rings") or elongations (sketch above).
  • Altered friction properties/sliding properties.
  • Deteriorated thermal durability.

In the case shown above, with the introduction of a new oil type, plastic deformations of the suction snorkel to plastically deformations. This consisted a corrugated hose from rubber(?), stiffened from the inside with separate wire rings. The plastically elongation triggered a displacement of the stiffening rings and with this to kinking of the hose. So the function was no more guaranteed.
The sketches below show O-rings from an elastomere, not resistant against synthetic oil. Depending from the type of the change, caused by the oil, different failure modes can occur (chapter 23.4.2).

 Operation behaviour of elastomers by swelling

Fig. "Operation behaviour of elastomers by swelling" (Lit. 22.3.1-9): The swelling of elastomere seals can cause leakages by plastic deformation and lifting. This can occur even at seals which are supported by a metallic frame. Usually such seals are sufficient tested and apprroved by the OEM. However during unexpected changes of the oil properties or the use of not approved seals (SUPs, chapter 20.2.1) such problems are possible.

Fig. "Causes of oil discolorations" (Lit. 22.3.1-10 and Lit. 22.3.1-11): Unusual discolourations of the oil can be an (early) indicator of failures in the oil system. Therefore they must taken seriously and its cause, respectively developing mechanism must be clarified.
In the shown case the operators of an airliner type found black carbon accumulations in the oilpipes to the mainbearings and the oil filter of the aeroengines. In some cases it came to fluctuations of the oil pressure and/or the blocking of the intake filter to the main scavenge pump.
Detailed investigations showed, that the cause was no oil coking. Instead the cause have been carbon particles of a decomposed filled rubber (nitril butadien rubber = NBR) from the pipes/hoses of the oil cooler. Thereupon those have been exchanged by hoses made of PTFE (`teflon').
The responsible authority recommended the following procedure:

 Causes of oil discolorations

  • Pressure test of the oil filter. Visual testing of an oil sample for suspended carbon particles in the oil.
  • Inspection of the intake seive to the scavenge pump of the accessory gear for carbon deposits.
  • Information of a „positive“ finding to the responsible authority.

In an informal writing (service information letter), the operators of the concerned aeroengines have been informed about the background. Concened was a synthetic oil af a certain producer according to the spec. MIL-L-23699F STD (Fig. "Lubrication oils"). The dark discolouration („black oil”) has no direct effects at the properties of lubrication and cooling. Dark oil must not necessarily contain free carbon. Its colour can vary from orange up to black. The discolouration develops in the case on hand during expecially long accumulated operation time periods (several 1000 hours). It is than merely a sign, that the oxidation inhibiting additives are effective and must so evaluated as normal. Fine didstributed carbon discolours the oil dark with a green tint. The best to decide if the dark discolouration derives from free carbon is an oil sample in a glass test tube (below 2 cm diameter). Contaminated aeroengines showed in the oil system high accumulations of zinc oxides and black carbon particles/soot (carbon black). The small heat exchange tubes in the oil cooler at the side of the fuselage (detail above right) consist of rubber (NBR). They had inside axial cracks and cross-cracks which could not be seen from the outside. The rubber was filled with zinkc oxide and soot as a component of the rubber. During the degradation both are released into the oil (chart below) and get so into the oil system. Zinkc oxide settles as sludgy compounds Verbindung, soot remains fine dispersed in the oil.

 Toxic TCP getting into the the bleed air

Fig. "Toxic TCP getting into the the bleed air" (Lit. 22.3.1-12, -13, -14 and Ill. 22.3.1-13): Synthetic lubrication oils for aeroengines consist of esters with additives (Fig. "Lubrication oils"). Concerned are organic phosphorous compounds. Its behaviour during unnormal conditions in bearings like increased pressures and temperatures is often not sufficient known. TCP is an additive with pronounced wear inhibiting properties. It is a so called EP additive (extreme pressure-additive = high pressure additive). Its wear retarding effect is caused from the formation of a ductile reaction film which serves as solid lubricant with dry-running properties (Lit. 22.3.1-15 and Lit. 22.3.1-16). It develops due to a reaction under high temperatures and pressures in the lubrication gap. This film prevents during metallic contact (e.g., mixed friction) seizing/galling of the contact surfaces. With this TCP is of high importance for the lifetime but also for the safety of anti friction bearings and gears. Therefore obviously it is not easy to find a non health hazardous alternative or to abandon TCP at all.
TCP has especially harming properties to the environment. It is tricresylphosphate (= TCP), a nerve poison. The content of TCP in oil is about 3 % up to 5 %. This seems little, however it can influence the cockpit crew temporary up to a blackout. Because of this, in many cases oxygen masks had been used. Now there are instructions to this behaviour from the responsible aviation authorities.
Obviously over a long time, dangerous symptoms like constricted reactions, harmstrung of the pilots, heavy headaches and indisposition have not been traced back at TCP. Because of this reason, no proofs in the cabin air took place.
A further problem are possible permanent injuries to health by frrequent inhalation in especially concerned airplane types.
TCP gets into the compressor air, because sealing air is contaminated from oil leaks caused by sealing failures and bearing failures or short-term leaking due to operation (e.g., too little sealing air pressure in the compressor air at low rotor speeds). So aeroengine oil (oil vapour) gets with the bleed air for the ventilation into cabin and cockpit. This is also possible in certain airplanes through the APU.

References

22.3.1-1 I.E.Traeger, „Aircraft Gas Turbine Engine Technology, Second Edition“, Verlag : Glencoe/McGraw-Hill 1994, ISBN 0-07-065158-2., page 204, 209, 245.

22.3.1-2 The AeroShell Book, Edition 18, 2003, „Aeroshell Turbine Oils”, www.shell.com/aviation, page 1-90.

22.3.1-3 M.J.Kroes, T.W.Wild, „Aircraft Powerplants, Seventh Edition“, Verlag : Glencoe/McGraw-Hill 1990, ISBN 0-02-801874-5, page 343-356, 470.

22.3.1-4 M.Duncanson, „Detecting and Controlling Water in Oil”, Exxon Mobil, www.practicingoilanalysis.com, 17.02.2006, Seite 1-9.

22.3.1-5 W.R.Herguth, „New Lubricant Evaluation and Acceptance - A Laboratory's Perspective“, Proceedings des „Annual Meeting der Society of Tribologists and Lubticant Engineers” Cincinnati Ohio 1996 page 1-5.

22.3.1-6 „Turbo Oil for Industrial & Marine Gas Turbines“, Air BP, www.airbp.com, 17.02.2006, page 1-20.

22.3.1-7 F.T.Barcroft, „Lubricants for the Aircraft Gas Turbine”, Zeitschrift „Aircraft Engineering“, Okt. 1979, page 7-9.

22.3.1-8 W.Uedelhoven, „Testing of Elastomer Compatibility of Turbine Aviation Oils”, Wehrwissenschaftliches Institut für Materialuntersuchungen (WIM), Erding, BRD Okt. 1979, page 7-9.

22.3.1-9 K.Meier, „In-service Engine Oil Problems“, MTU-München, page 4.1-4.10.

22.3.1-10 Transport Canada, Service Difficulty Advisory AV 2005-02 vom 12 April 2005, „Possible Oil Contamination”, Pratt & Whitney Canada, PT6A-67D Series Engines, www.tc.gc.ca/, page 1 and 2.

22.3.1-11 Pratt & Whitney Canada, Service Information Letter, S.I.L No. PT6A-112R1, „PT6A-67D, Oil Discoloration“, 29.Oct 2004, page 1-3.

22.3.1-12 O.Lundström, H.Elinder, Statens haverikommission (SHK), Report RL 2001:41e (ISSN 1400-5719) „Incident onboard aircraft SE-DRE during flight between Stockholm and Malmö, M county, Sweden, on 12 November 1999”, 2001-11-23, page 1-34.

22.3.1-13 „Trikresylphosphate“, de.wikipedia.org/wiki, aktualisiert 24. März 2009, page 1-3.

22.3.1-14 Air Accident Investigation Branch, (AAIB) Air Accident Report 1/2004, „Repoert on the incident to BAe 146, G-JEAK, durimng the descent into Birminghham Airport on November 2000”, January 2004, page -61.

22.3.1-15 „EP-Additiv“, http://de.wikipedia.org/wiki/EP-Additiv, Stand 26.Sept.2012, page 1-3.

22.3.1-16 G. Niemann,H. Winter,B.-R. Höhn, „Maschinenelemente Band 1”, 4. Auflage, Springer-Verlag Berlin Heidelberg New York, 2005, ISBN 3-540-25125-1, page 721 and 722.

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