Cursos Prácticos Trimestrales

For lubrication of a high-speed turbine shaft running in contact bearings, an oil with good boundary lubrication properties and low viscosity is required. Because of the small amount of oil in circulation and the high bearing temperatures, good resistance to oxidation is essential.

The earliest gas turbine engines were developed using straight mineral oils, but the operational requirements for low temperatures either on the ground or at a high altitude, led to the development of a range of straight mineral oils with viscosity’s far lower than those of conventional aircraft engine oil of that time. Mineral turbine oils are very rarely used now.

JAR 66 CATEGORY B1

9.6.1 FIRST GENERATION SYNTHETIC OILS

With the progressive development of the gas turbine engine to provide a higher thrust and compression ratio, mineral oils were found to lack stability and to suffer from excessive volatility and thermal degradation at the higher temperatures to which they were subjected. At this stage, a revolutionary rather than evolutionary oil development took place concurrently with engine development; lubricating oils derived by synthesis from naturally occurring organic products found an application in gas turbine engines. The first generation of synthetic oils were based on the esters of sebacic acid, principally dioctyl sebacate. As a class these materials exhibited outstanding properties which made them very suitable as the basis for gas turbine lubricants.

Unlike straight mineral oils, the synthetic oils relied on additives (and in later formulations on multi-component additive packages) to raise their performance.

This was particularly necessary to improve resistance to oxidation and thermal degradation (important properties which govern long term engine cleanliness).

9.6.2 SECOND GENERATION SYNTHETIC OILS

The introduction of the by-pass or turbo-fan engine raised further problems; in this engine the by-pass air acts as an insulating blanket and increases heat rejection to the lubricant. Therefore the requirement arose for an oil with an even greater resistance to thermal and oxidative stress. Several synthetic oils which meet this requirement have been developed. Known as Type 2 lubricants, they are blended from more complex esters and an additive package consisting of anti-oxidants, load-carrying additives, corrosion inhibitors, metal deactivators and foam inhibitors.

9.6.3 THIRD GENERATION SYNTHETIC OILS

Sustained flight at speeds in excess of Mach 1 aggravates the lubricant problem still further as the kinetic heating of the fuel reduces the effectiveness of fuel-cooled oil coolers. At Mach 2, oil temperatures may reach 260 - 316C, at which level standard ester-based oils degrade rapidly. In some military aircraft, Type 1 and Type 2 ester oils are still used under these conditions, but at greatly increased oil change frequencies. This procedure is expensive to operate as ideally the oil should remain in the engine for full engine life, with only periodic replenishment.

More complex chemicals have been discovered which are more thermally stable than esters. However, they have various deficiencies such as poor low temperature properties or poor steel-on-steel lubricity. All are more expensive than esters.

High temperature lubricants blended from specially developed ester oils, with new additives to limit oxidation degradation and corrosiveness and of increased load carrying ability, appear to offer the most practical solution for lubricating the jet engines in commercial supersonic transport (SST) aircraft. Many firms have been active in developing lubricants of this type and, after many submissions, two lubricants have been adopted for the Olympus 593 engines which power the BAC-Aerospatiale Concorde.

JAR 66 CATEGORY B1 MODULE 15 GAS TURBINE

ENGINES

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9.6.4 SAFETY PRECAUTIONS

There is much less risk of fire with oil, however it will burn if the conditions are right.

The main risk with oil is to the body; prolonged contact with oil can cause dermatitis and/or cancer. The use of barrier cream and gloves cannot be overstated. Washing of hands before going to the toilet or eating is important, as is the reapplication of protection afterwards.

Oil spills should be cleaned up as soon as possible and waste disposed of in accordance with company procedures.

JAR 66 CATEGORY B1 MODULE 15 GAS TURBINE

ENGINES

engineering uk

Intentionally Blank

JAR 66 CATEGORY B1 apparently smooth surfaces have small undulations, minute projections and depressions and actually touch at only a comparatively few points. Motion makes the small projections catch on each other and, even at low speeds when the surface as a whole is cool, intense local heat may develop leading to localised welding and subsequent damage as the two surfaces are torn apart. At higher speeds and over longer periods, intense heat may develop and cause expansion and subsequent deformation of the entire surface; in extreme cases large areas may be melted by the heat, causing the metal surfaces to weld together.

The gas turbine engine is designed to function over a wider environment and under different operating conditions from its piston engine equivalent and therefore special lubricants have been developed to cope with the following main problems:

a. High rpm compared with piston engines.

b. Cold starting in winter can mean initial bearing temperatures of -54C which rapidly increases after starting to 232C. Therefore a good viscosity index and adequate cooling are required.

On the other hand, the following advantages can be claimed for the gas turbine:

a. There are fewer bearings and gear trains.

b. Oil does not lubricate any parts directly heated by combustion and therefore oil consumption is low.

c. There are no reciprocating loads.

d. Bearings are generally of the rolling contact type and therefore only low oil pressures are needed (40 psi is normal).

Turbo-prop engine lubrication requirements are more severe than those of a turbo-jet engine because of the heavily loaded reduction gears and the need for a high-pressure oil supply to operate the propeller pitch control mechanisms. (For example, a twin relief valve in the Dart provides 35 psi for engine lubrication and 70 psi, which is fed to the propeller controller and boosted by a further pump to a pressure of 600 psi).

10.2 BEARINGS

The early gas turbines employed pressure lubricated plain bearings but it was soon realised that friction losses were too high and that the provision of adequate lubrication of these bearings over the wide range of temperatures and loads encountered was more difficult than for piston engine bearings.

As a result, plain bearings were abandoned in favour of the rolling contact type as the latter offered the following advantages: