Abstract:
A shaft coupling device to couple first and second shafts, said device comprising a fluid coupling having a first portion attached to the first shaft and a second portion attached to the second shaft wherein the device further comprises engagement means operable to switch the device between an engaged position and a disengaged position, the engaged position having each of the first and second shafts engaged with its respective portion of the fluid coupling and the disengaged position having at least one of the first and second shafts disengaged from its respective portion of the fluid coupling.

Description:
BACKGROUND 
   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  14   a ,  14   b  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. 
   SUMMARY 
   One disadvantage of this method is that the air that fills the chambers to replace the oil causes frictional drag between the two sections of each chamber  14   a ,  14   b  and thereby generates waste heat energy. The amount of waste heat generated is proportional to the difference in shaft speeds and, therefore, at large shaft speed differentials such as those experienced in gas turbine engine applications, the heat losses are very large. Hence, the chambers  14  may have undesirable life limitations and the oil be degraded as a result of this waste heat. This has a consequent effect on safety and reliability, and on engine maintenance timescales. 
   The present invention seeks to provide an improved shaft coupling device that seeks to address the above mentioned problems. 
   Accordingly the present invention provides a shaft coupling device to couple first and second shafts, said device comprising a fluid coupling having a first portion attached to the first shaft and a second portion attached to the second shaft wherein the device further comprises engagement means operable to switch the device between an engaged position and a disengaged position, the engaged position having each of the first and second shafts engaged with its respective portion of the fluid coupling and the disengaged position having at least one of the first and second shafts disengaged from its respective portion of the fluid coupling. 
   Preferably at least one of the first and second shafts is engaged by a gear arrangement. Preferably the gear arrangement is a synchromesh gear arrangement. 
   Preferably the engagement means is a hydraulic piston. Preferably the hydraulic piston switches the shaft coupling device to the engaged position by fluid pressure. 
   Preferably the shaft coupling device is biased to the disengaged position by at least one tension spring. Alternatively it may be biased to the disengaged position by fluid pressure. 
   Preferably the first and second shafts are shafts of a gas turbine engine. More preferably the first shaft is an intermediate pressure shaft of a three-shaft gas turbine engine and the second shaft is a high pressure shaft of a three-shaft gas turbine engine. 
   The present invention also provides a gas turbine engine provided with a shaft coupling device as described in any of the previous five paragraphs. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully described by way of example with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic drawing of a related art coupling device. 
       FIG. 2  is a schematic sectional side view of a gas turbine engine that incorporates a shaft coupling device in accordance with the present invention. 
       FIG. 3  is a schematic drawing of a shaft coupling device according to the present invention. 
       FIG. 4  is a more detailed schematic view of the shaft coupling device of  FIG. 3 . 
       FIG. 5  is a schematic drawing of a step-aside gear box of a gas turbine engine incorporating a shaft coupling device according to the present invention. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   A gas turbine engine  18  is shown in  FIG. 2  and comprises an air intake  20  and a propulsive fan  22  that generates two coaxial airflows A and B. The gas turbine engine  18  comprises, in radially inner axial flow A, an intermediate pressure compressor  24 , a high pressure compressor  26 , a combustor  28 , a high pressure turbine  30 , an intermediate pressure turbine  32 , a low pressure turbine  34  and an exhaust nozzle  36 . A nacelle  38  surrounds the gas turbine engine  18  and defines, in radially outer axial flow B, a bypass duct  40 . A step-aside gear box  42  comprising shaft coupling for starting and power off-take is located between the intermediate pressure compressor  24  and the high pressure compressor  26  and may comprise an accessory drive shaft output for power off-take and engine starting components (not shown). 
   An exemplary embodiment of the present invention is shown in  FIG. 3  in which the intermediate pressure shaft  10  and the high pressure shaft  12  are coupled by a shaft coupling device according to the present invention. The IP shaft  10  terminates in a section defining part of fluid passages  16  leading to a first half of fluid coupling  14   a . The HP shaft  12  is selectively engaged with a section defining a second part of the fluid passages  16  and a second half of the fluid coupling  14   b . Engagement means  44  are provided to switch the shaft coupling device between engaged and disengaged positions. The engagement means  44  comprises a synchromesh gear arrangement  46 , provided to connect the IP shaft to the second half of the fluid coupling  14   b , and a hydraulic piston  48  that engages the gear arrangement  46  by overcoming a biasing force that biases the gear arrangement  46  towards the disengaged position. In operation working fluid, oil, is pumped into or against the hydraulic piston  48  from a source, through ducts and a pump that are external to the present invention and are omitted from the figure for clarity. 
   The hydraulic piston  48  acts to bring the synchromesh gear  46  into engagement and thereby engage the HP shaft  12  to the section defining the fluid passages  16  and therefore the second half of the fluid coupling  14   b . As can be seen in the more detailed view of  FIG. 4 , a fluid flow valve  52  provides fluid communication between the hydraulic piston  48  and the fluid passages  16  and fluid coupling  14 . Once the synchromesh gear  46  is engaged, the fluid flow valve  52  is opened to allow the oil to flow into the fluid coupling  14  and thus couple the IP and HP shafts  10 ,  12 . 
   To decouple the IP and HP shafts  10 ,  12  it is necessary to decouple the fluid coupling  14  by draining the oil from them. This may be via drain means (not shown) or natural spillage resulting from the pressure loss experienced when the oil supply is removed. The flow valve  52  is subsequently shut. The present invention benefits from a substantial reduction in waste heat caused by air drag between the two shafts  10 ,  12  when in the decoupled position by also disengaging the HP shaft  12  from the fluid passages  16  and fluid coupling  14 . Tension springs  50 , shown in  FIG. 4 , act to return the hydraulic piston  48  and synchromesh gear  46  to the disengaged position thereby disengaging the HP shaft  12  from the fluid coupling  14 . 
   In the decoupled position the first half of the fluid coupling  14   a  rotates with the IP shaft  10  and the second half of the fluid coupling  14   b  is free to rotate as it is disengaged from both shafts. Air, which fills the fluid coupling  14  when the oil is removed, provides a frictional force on the second half of the fluid coupling  14   b  caused by the movement of the first half  14   a . Since the second half  14   b  is disengaged from the HP shaft  12 , the frictional force acting upon it drags it to rotate in approximate synchronicity with the first half  14   a . Thus there is little or no speed differential between the two halves of the fluid coupling  14  and consequently little waste heat generated. 
     FIG. 5  shows a step-aside gearbox, such as may be found in a gas turbine engine, including a shaft coupling device according to the present invention that couples the intermediate pressure shaft  10  and the high pressure shaft  12 . The IP shaft  10  comprises a gear  54  perpendicularly at its end that meshes with another gear  56  perpendicularly at one end of a shaft  58 , the shaft  58  being perpendicular to the IP shaft  10 . Similarly, the HP shaft  12  comprises a gear  60  perpendicularly at its end that meshes with another gear  62  perpendicularly at one end of a shaft  64 , the shaft  64  being coaxial with shaft  58 . Shaft  58  has, at its opposite end to gear  56 , a gear  66  that meshes with a gear  68  at one end of a shaft  70  such that shaft  70  is in the same alignment as shaft  58  but is offset by the radius of gears  64  and  68 . Similarly, shaft  64  has, at its opposite end to gear  62 , a gear  72  that meshes with a gear  74  at one end of a shaft  76 . Shaft  76  is coaxial with shaft  70 . Shaft  70  is connected to a section defining part of the fluid passages  16  and thence to the first half of the fluid coupling  14   a . Shaft  76  is connected via engagement means  44 , comprising the synchromesh gear  46  and hydraulic piston  48 , to a section defining part of the fluid passages  16  and thence to the second half of the fluid coupling  14   b . Hence the IP shaft  10  is connected, via gears  54 ,  56 ,  66 ,  68  and shafts  58 ,  70 , to the first half of the fluid coupling  14   a  and the HP shaft  12  is connected, via gears  60 ,  62 ,  72 ,  74 , shafts  64 ,  76  and the engagement means  44 , to the second half of the fluid coupling  14   b.    
   The gear arrangement  46  has been described as a synchromesh gear. However, any suitable alternative may be used to derive the benefits of the present invention without prejudice. Although the apparatus for engaging and disengaging the gear  46  has been described as a hydraulic piston  48  and two tension springs  50  other arrangements may be envisaged by the skilled reader within the scope of the invention claimed. For example, the hydraulic piston may be a dual-state piston which is hydraulically activated to either of its two extents. Alternatively, the engagement means may comprise, for example, a ratchet clutch. A single tension spring could be used or more than two tension springs. Alternatively another method of biasing the piston to the disengaged position could be used. 
   The working fluid has been described as oil. However, any suitable working fluid can be substituted. 
   The fluid flow valve for directing oil into the fluid coupling  14  and passages  16  has been described as a separate valve  52 . However, other suitable arrangements are possible. For example, the piston may cover one or more apertures in the shaft wall in the disengaged position but leave it uncovered when it moves to the engaged position to allow fluid to flow into the coupling. Alternatively, a sliding plate may cover one or more apertures in the shaft wall and be moved by the movement of the piston, an independent control signal or a combination of these. 
   The engagement means  44  has been described in relation to the HP shaft  12 . However, it is equally appropriate to provide the engagement means  44  on the IP shaft  10  and keep the HP shaft  12  permanently connected to the fluid coupling  14 . Alternatively, both shafts  10 ,  12  may be disengaged from their respective halves of the fluid coupling  14 . Although the present invention has been described to couple the IP and HP shafts, alternatively the low pressure shaft could be coupled to the intermediate or high pressure shafts. Further, although the present invention has been described with reference to a three-shaft gas turbine engine it is equally applicable to a two-shaft engine. 
   The present invention could equally be used to temporarily couple shafts in fields other than gas turbine engines, including coaxial shafts.