This invention relates to a gas turbine engine and is particularly concerned with a bearing arrangement for supporting the main shaft or shafts of such an engine.
Gas turbine engines typically comprise gas compression and expansion sections that are spaced apart by a combustion section. Usually, the gas compression section comprises two or more compressor portions that are drivingly interconnected with a corresponding number of turbine portions in the gas expansion section. An appropriate number of hollow coaxial shafts extend along the longitudinal axis of the engine interconnecting the compressor and turbine portions.
In operation, air entering the compression section is progressively compressed thereby before being mixed with fuel and the mixture combusted in the combustion section. The resultant hot gases then expand through, and thereby drive, the turbine portions of the expansion section.
Conventionally, the shaft or shafts interconnecting the compressor and turbine portions are supported by rolling element bearings that are strategically positioned along the lengths of the shafts. The bearings are designed to carry the radial and axial loads expected to be imposed upon them and are lubricated by a fluid lubricant, usually an appropriate oil. The lubricant flows between the various engine bearings and storage reservoirs through passages located within and around the engine. This obviously presents difficulties if the lubricant is required to flow through areas of the engine (such as the turbine portions) that operate at high temperature. Thus the lubricant is rendered vulnerable to thermal degradation or even combustion. Thermal degradation can lead to the formation of particulate material within the lubricant. This is clearly undesirable in view of the filter blockage that can result and the prejudicial effect that it can have on the bearings.
One way in which the problem of lubricant thermal degradation could be addressed is by increasing the lubricant flow rate. However, this requires the use of more powerful pumps and the increased pressure of the liquid lubricant could give rise to cold combustion of the lubricant, bringing about in turn carbon formation and filter blockage.
It has been proposed, in for instance EP0816654, to support a gas turbine engine shaft by electromagnetic bearings. This is clearly desirable in terms of the elimination of lubricant. However, such use of electromagnetic bearings can add complexity to an engine. Moreover, some bearings within gas turbine engines can be subject to very high and variable loading. This would in many cases dictate the use of electromagnetic bearings of impractical size and power consumption. Moreover, electromagnetic bearings in some applications within a gas turbine engine would, as result of varying loads and other parameters, present difficulties in the maintenance of appropriate engine running clearances.
It is an object of the present invention to provide a gas turbine engine having a bearing system that substantially avoids the aforementioned difficulties.
According to the present invention, a gas turbine engine comprises at least one shaft having an axis of rotation providing a driving interconnection between two portions of said engine, said at least one shaft being primarily supported from the remainder of said engine during normal engine operation by at least one rolling element bearing and at least one electromagnetic bearing, said bearings being axially spaced apart from each other so that said at least one electromagnetic bearing is or are positioned in a portion of said engine that is operationally at a higher temperature than the portion of said engine in which said at least one rolling element bearing is or are positioned.
Said portions of said engine are preferably a compressor portion and a turbine portion, said at least one electromagnetic bearing being situated in said turbine portion.
Preferably said at least one electromagnetic bearing is or are actively controlled.
Preferably said at least one electromagnetic bearing is or are journal bearings.
Said at least one shaft may drive an electrical generator, said electrical generator constituting the primary source of electrical power for said at least one electromagnetic bearing.
Said at least one rolling element bearing may be lubricated by a liquid lubricant.
Said at least one electromagnetic bearing may be gas or liquid cooled.
Said gas turbine engine may be a ducted fan gas turbine engine having at least two shafts, each of said shafts being primarily supported from the remainder of said engine by at least one rolling element bearing and at least one electromagnetic bearing, said electromagnetic bearings being situated in the turbine section of said engine.