GB834833A - Improvements in or relating to air turbines - Google Patents

Abstract

834,833. Air turbines. ROTOL Ltd. April 2, 1958 [April 5, 1957], No.11248/57. Class 110(3). [Also in Group XXIX] Air from a common pressure source is fed independently to one or other of two air turbines 11, 18, having different optimum operating pressure ratios through valves 53, 54, which are controlled by means responsive to changes in the operating pressure ratio so as to allow air to pass to the turbine 18 when the ratio exceeds a predetermined value and to the turbine 11 when the ratio falls below a second predetermined value not greater than the first value. The air is tapped from the compressor of a gas turbine aircraft engine. The turbines are connected through free-wheel devices 14, 21 to gears 15, 22, respectively, meshed with a gear 16 on a common output shaft 17 in such a ratio that the speed of the output shaft is substantially constant whichever turbine is in operation. A speed governor 51 operates a rod 31 through an hydraulic linkage to adjust simultaneously the nozzle inlet guide vanes of both turbines. The valves 53, 54 are coupled so that one is closed when the other is open, and are operated by a spring-loaded piston 60 slidable in a cylinder 61 and actuated by a control valve 64 comprising a member 71 with slotted lands 72, 73 and a casing which communicates with the air supply duct 23 through an aperture 68 and with the atmosphere through an aperture 69. A rod 74 extends from the member 71 and projects with clearance through the aperture 69 to engage a lever 76 loaded by an evacuated capsule 79 subject to atmospheric pressure. Initially, when the aircraft is on the ground with the engine idling, the capsule 79 is at its shortest length and holds the member 71 so that it closes the aperture 68 and connects the cylinder 61 to the atmosphere through the conduit 63 and aperture 69. The piston 60 is then displaced to the right in the cylinder 61 and air is fed to the turbine 11. When the engine speed increases the air pressure in the duct 23 increases and at cruising speed this pressure acting on the member 71 is sufficient to overcome the force of the capsule 79 and displace the member to close the aperture 69 and connect the cylinder 61 to the duct 23. The piston 60 is then displaced against the action of the spring 62 to open the valve 54 and bring the turbine 18 into operation. Changeover to the turbine 11 occurs when the critical pressure ratio is again reached through an increase in the atmospheric pressure or a decrease in the pressure in duct 23. To enable the changeover to the turbine 18 to occur at a higher operating pressure ratio than the changeover to the turbine 11, a control valve (Fig. 2) is used in which the conduit 63 is connected to the atmosphere or to the duct 23 by a valve 129 linked through a lost-motion device 128 to a piston 122 having a unique position for each value of the ratio. The piston rod 126 carries a fulcrum roller 127 for a beam 111 balanced between a bellows 110 connected to the duct 23 and an evacuated capsule 114. Air tapped from the duct 23 is fed to both sides of the piston 122 and passes to opposed bleeds 117, 118, playing on a reed 116 carried by the beam. Any variation in the pressure ratio unbalances the beam so that the reed obstructs one of the bleeds and increases the pressure on one side of the piston, which then moves until the roller reaches a new position of balance.