GB790550A - Improvements relating to aircraft gas-turbine engines - Google Patents

Abstract

790,550. Gas turbine plant. ROLLS-ROYCE, Ltd. Jan. 18, 1956 [Jan. 25, 1955], No. 2259/55. Class 110 (3). [Also in Group XXIV] In a compound gas turbine engine comprising independently rotatable low-pressure and highpressure rotor systems each comprising a compressor and compressor-driving turbine in which the low-pressure compressor delivers at least in part to the high-pressure compressor and the low-pressure turbine is connected to drive an airscrew, helicopter rotor or ducted fan, the low-pressure turbine is connected to the low-pressure compressor through a freewheel or equivalent device to permit the compressor to over-run the turbine, the airscrew, helicopter rotor or ducted fan is permanently connected to the low-pressure turbine and clutching means are provided for drivingly inter-connecting the high and low-pressure compressors. A lowpressure compressor 10, Fig. 1, feeding a highpressure compressor 11 is driven by a lowpressure turbine 13 through a shaft 15 and freewheel device 14. The shaft 15 is arranged coaxially with a shaft 16 connecting the highpressure turbine 12 to the compressor 11. The shaft 15 drives an airscrew 18 through a reduction gear 17. The airscrew 18 is of the variable pitch type and has the pitch governed by a constant speed unit 18a having a datum speed selecting device (not shown). A drive connection including clutching means to establish a connection between the low and high-pressure compressor rotors is shown diagrammatically at 22. The shaft 15, Fig. 1a, has ramp formations 25 which co-operate with roller elements 26 housed in a cylindrical extension 27 of the lowpressure compressor 10. The arrangement is such that the low-pressure compressor rotor can over-run the shaft 15 if it is driven at a higher speed than the shaft. Figs. 2, 2a show one form of the driving connection 22. Extensions 30, 31, Fig. 2, of the high and low-pressure compressor rotors are supported in stationary structure by bearings 32, 33. The extension 31 is formed internally with ratchet-teeth 34 arranged to be engaged by pawls 35 pivoted on flanges 36 on a sleeve 37. The sleeve 37 extends into the extension 30 and is provided with helical splines 39 which engage internal splines 38 on the extension 30. The sleeve 37 is also provided with a flange having teeth 40 adapted to co-operate with the teeth 41 of the extension 31. If the high-pressure compressor rotor is rotating faster than the low-pressure compressor rotor, the pawls 35 ride over the teeth 34. If, however, the low-pressure rotor speed tends to exceed that of the high-pressure rotor the pawls 35 will be engaged by the teeth 34. This will cause relative rotation between the sleeve 37 and extension 30 and the sleeve 37 will move to the right due to the action of the helical splines 38, 39. When the sleeve 37 and extension 30 are locked with respect to the extension 31 by the pawl-and-ratchet mechanism, the teeth 40, 41 are aligned so that axial movement of the sleeve 37 causes these teeth to move into engagement. The axial movement of the sleeve 37 to the right is limited by the flange 36 on the righthand side of the pawls coming into abutment with the teeth 41. A driving connection between the high and low-pressure compressor rotor is established when the teeth 40, 41 are engaged. This connection, however, will be disengaged if the low-pressure compressor rotor tends to rotate at a speed lower than that of the high-pressure compressor rotor. To maintain a positive drive between the compressor rotors under these conditions, a second selectivelyoperable clutch is provided which consists of a sleeve 50 having at one end splines 51 which co-operate with internal splines 52 on the extension 30 and at the other end dog-teeth 53 adapted to engage the teeth 41. Axial movement of the sleeve 50 is controlled by one or more hydraulic piston-and-cylinder devices 54. To start the engine, the clutch 22 is disengaged. The engine starts as a normal compound engine in which the high-pressure turbine and compressor accelerates faster than the low-pressure turbine which drives the low-pressure compressor and airscrew. After starting and during subsequent acceleration, the speed of the lowpressure compressor can be caused to reach that of the high-pressure compressor. The mechanism described in Figs. 2, 2a may then be engaged to establish a driving connection between the two compressors. Thereafter the speed of the low-pressure turbine can be controlled by the variable-pitch airscrew speed-setting mechanism to rotate at a speed less than that of the high-pressure turbine and the two compressors driven by it. The freewheel device 14 allows the low-pressure compressor rotor to over-run the shaft 15. Axial movement of the sleeve 50 to effect positive engagement of the two compressor rotors may be controlled by the operator. Disengagement may be effected by means of an electrical circuit energized when the undercarriage is loaded. Alternatively, the sleeve 50 may be moved to effect engagement and disengagement at preselected rotational speeds of the combined rotor system. The driving connection 22 may be adapted to operate, as shown, when the two compressors rotate as one at the same speed or when the compressors rotate at speeds having a predetermined ratio to one another.