The present invention relates to shaft coupling and is particularly concerned with the temporary coupling of shafts using fluid coupling. It finds particular applications in gas turbine engines where shafts may be rotating at very different speeds but may equally find utility in other applications.
It is commonplace in gas turbine engines used for aircraft propulsion to remove power from a shaft of an engine during flight to drive aircraft systems and accessories including cabin systems, in-flight entertainment systems and cabin air pressurisation systems. In a three-shaft gas turbine engine it is known to take power from the high pressure (HP) shaft. Since more power is usually available from the intermediate pressure (IP) shaft than the HP shaft, it is beneficial to take power from the IP shaft, particularly during engine descent, and hence reduce fuel consumption compared to the condition if power is taken from the HP shaft. There is a consequent reduction in the amount of fuel required for a flight and thus the cost of that flight. However, there is a requirement to drive the high pressure compressor during engine starting meaning it may be desirable to transfer starting torque from the IP to the HP shaft. Furthermore, under high power off-take conditions when the engine is at idle it may be necessary to transfer power from the HP to the IP shaft. Therefore, there may be a need to couple the high pressure and intermediate pressure shafts for some periods in the engine cycle.
A conventional method of temporarily coupling two shafts is to use fluid coupling. A typical arrangement is shown in FIG. 1 in which a first, intermediate pressure shaft 10 and a second, high pressure shaft 12 are coupled by fluid couplings 14. Typically the fluid couplings 14 comprise two or more ball chambers annularly arrayed around the ends of shafts 10, 12. The chambers 14 are linked by fluid passages 16 so that the working fluid, for example oil, can be distributed into and between the chambers 14 or removed from them. Each chamber 14 and the associated passages 16 is formed in two sections, connected to the IP and HP shafts 10, 12 respectively. When the chambers 14 and passages 16 are filled with oil the two sections of the chambers 14a, 14b are compelled to rotate in approximate synchronicity and thus the shafts 10, 12 are coupled. When decoupling is required, the oil is drained from the chambers 14 and passages 16 so that the two sections are no longer constrained to rotate together but are free to rotate in synchronicity with their respective shafts, which may be rotating at different speeds. Hence the two sections of each chamber 14 rotate independently at different speeds to each other.