Chapter 4

Bearings and Lubrication

Bearings
Nearly all gas turbine engines employ ball or needle roller bearings to support the main rotating shaft and other rotating components. Plain bearings are found in experimental turbo-charger based units. Small gas turbine engines rotate at extremely high speeds at up to 85,000 rpm, to cope with these speeds special high speed bearings are used. The way in which the bearings in small gas turbines support the main shaft can be characterised in two ways.-

Preloaded angular contact bearings.
Here two identical ball races are mounted apart on the engine main shaft and are preloaded with a spring. The races are a special type where the regions of contact between the balls and the races are inclined an angle. The bearings prevent both radial movement and axial movement of the shaft, the axial retention is in one direction only per bearing. The two bearings face each other and so prevent axial movement in either direction. The bearing outer races are allowed to move axially and are preloaded with a spring, the whole assembly is then contained in a sleeve. The spring loading has the added advantage that the pre-load is unaffected by temperature. The design of the whole assembly will be carefully chosen so that the bearing system dampens any rotational resonances. On certain designs of engine the whole assembly will move very slightly back and fourth in the sleeve to allow for manufacturing tolerances and wear.

Angular contact ball races will normally tolerate axial forces in one direction only so care must be exercised when removing them from an engine so that the pair of bearings are not mixed up and are refitted with the same orientation.

Spring loaded angular contact bearings can be found in some Garrett engines, the Microturbo Saphir and the Plessey Solent GTS. Most miniature gas turbines intended for model aircraft propulsion also adopt this bearing configuration.

Ball and Needle Bearing systems.
A second bearing system commonly used in small gas turbines consists of a ball race and a needle roller bearing. A ball race capable of accepting axial loads is placed at one end of the main shaft (usually the compressor end) and a needle roller bearing is fitted the other end. The ball bearing maintains the axial position of the shaft and the roller bearings allows for a small axial movement which will be created by differential thermal expansion between the shaft and the engine. The carriers for the bearings may be allowed to "Float" or may be "Sprung" in such a way as to dampen various resonances, the oil supply is also used to dampen some bearing systems.

The Rover 1S60 gas turbine engine adopts a ball race and needle roller configuration. In larger gas turbines where the main shaft is carried in three or more bearings, the engines adopt a combination of ball and roller bearings to allow for thermal expansion.

Lubrication
Most small gas turbines lubricate themselves with a circulating oil system, a few designs notably the Saurer GT15 actually use the fuel as a lubricant. Gas turbine lubricating oil is generally of a thinner viscosity to that of automotive types, automotive types should never be put into gas turbines. A number of specifications are laid down for gas turbine oils, most modern oils are synthetic but mineral based oils are also used. When choosing an oil for a particular engine, the manufactures recommendations should be checked as to which is most suitable. The ambient operating temperature can have an effect on which oil should be used, small gas turbines are often specified able to accept more than one viscosity of oil.

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Oil systems
The main shaft bearings in a small gas turbine engine run at very high speeds, for this reason lubrication is very important to ensure a reasonable life expectancy. The high speed bearings in a small gas turbine are normally lubricated by jets or orifices which meter quantities of oil directly on to the bearings as they rotate. The jets can be quite fine and so effective filtration of the oil is important, the bearings themselves are also be relatively small and sensitive to particles foreign matter.

The quantity of oil fed to the high speed bearings must be kept within certain limits, if excessive oil is supplied to them, it has a detrimental effect on the bearing performance. The bearing is rotating at very high speed, the bearing cage will also be turning very fast, too much oil can restrict its movement and cause the balls to "Skid" instead of "Roll" between the races.

Complete gas turbine oil systems fall into two basic categories, a dry sump system and a wet sump system. The wet and dry sump oil systems are similar to that which can be applied to piston engines.

Typical Wet Sump Oil System
Oil is stored in a cavity which forms part of the main engine gearbox casing. The oil is drawn through a scavenge filter and into a pump. Oil pumps normally consist of a gear pumps or a xxxxxxxxx type, in most cases the pump is driven at a relatively slow speed (3000-4000 rpm) via a train of gears from the engine compressor shaft. The oil system is fitted with a pressure relief valve which controls the oil pressure within prescribed limits and protects the pump form excessive pressures. A pressure switch is often fitted to warn of low oil pressure or a transmitter and electrical indicator combination can also be used.

The oil from the pump is then fed to a filter. Oil filters may consist of fine mesh types or a removable paper element type similar to automotive types. A loss of oil pressure in a gas turbine engine is very critical and so some systems employ a bypass valve which opens should the filter become blocked. Oil from the filter is then distributed to the main engine high speed components such as bearings and the high speed gears. The slower rotating components inside the gearbox are splash lubricated by oil emanating from the high speed components. The oil eventually drains under gravity into the sump cavity ready for re-circulation.

The advantage of a wet sump gas turbine is simplicity, a scavenge pump or pumps are not normally required, the oil simply drains in to the sump at the bottom of the engine. The Rover 1S60 gas turbine engine employs this principal and so does the Garrett GTP 30 engine. An oil cooler is sometimes fitted to the oil pressure feed or the sump may be finned like a heat sink to aid the heat dissipation from the oil.

Dry Sump Oil System
The dry sump oil system is the most common gas turbine oil system. Oil is stored in a separate oil tank or reservoir which is mounted close to the engine. Oil is drawn from the tank through a suction filter and into a pump which pressurises it and then via a filter the oil is fed to the engine. A second pump of greater capacity to that of the pressure pump scavenges oil back from the engine bearings and gearbox, the oil is then returned to the tank. The increased return capacity is because the oil becomes foamy and aerated and so occupies a greater volume. An oil cooler radiator may also be fitted in the return or supply line from the tank.

Many small gas turbine engines including the Microturbo Saphir, Lucas GTS/APU,
Man Turbo 6012, Artouste and Palouste all employ a dry sump oil system. In a Palouste engine, two oil tanks are cast into the air intake assembly which helps cool the oil. Warm oil contained in the tanks placed near air intake region, have the added advantage of reducing the likelihood of ice formation in the air intakes.

Oil Coolers
A great deal of heat is generated in small gas turbine engines, a portion of this heat is picked up by the circulating oil and needs to be dissipated. Oil coolers normally consist of a radiator type device which is cooled by airflow blown over it. The airflow can be provided in a number of ways. A motor driven or engine driven blower is used to cool the radiator or the radiator is placed in the air intake of the engine where it is cooled by the ingested air. Another technique is to use compressor air bled from the engine. A jet of high pressure air is ejected into a venturi which draws additional ambient air through it. The total air flow from the venturi is then passed through the oil cooler radiator.

Rover 1S60 models which are used to drive a fire pump make use of a water/oil inter-cooler to reduce the temperature of the circulating oil.

Microturbo Saphir
In the case of the many Micortubo Saphir models, the oil system is circulated by an external electrically driven pump. The pump shares the same motor with the fuel pump, and so