Patent Publication Number: US-2004055850-A1

Title: Drive disconnect device

Description:
[0001] The present invention relates to a drive disconnect device. Such a device is suitable for controllably disconnecting the drive between a drive shaft and a driven shaft. The drive disconnect device is especially suited for controlled disconnection of a generator from a prime mover.  
       [0002] U.S. Pat. No. 4,997,072 discloses a disconnect device in which a solenoid holds a spring operated annular cam assembly in a retained position. The retraction of the solenoid allows the cam to slide into a position where it engages with a rotating pin on the shaft of the drive assembly and moves the pin against a compression spring to a position where it allows a driving element to move to a position where it drivingly disconnects input and output shafts. Such an arrangement is difficult to test as the annular cam relies on the shafts to be rotating in order to give rise to the disconnect operation.  
       [0003] U.S. Pat. No. 4,685,550 discloses an arrangement in which coaxial input and output shafts are coupled together by an axially slidable drive connection element which is normally held in position by a detent mechanism. A magnetic coil encircles the output shaft and can be energised to attract an end plate which in turn is coupled to a cylindrical element disposed within one of the shafts to displace it axially such that a detent ball can fall into a pocket, thereby allowing the drive to become disconnected. The magnetic coil must provide the entirety of the force required to overcome any friction and the spring biasing of the detent mechanism, and the disengagement of the driving and driven elements depends upon there being sufficient torque transmitted therebetween to generate displacement forces acting between inclined jaws within a clutch mechanism. Such a mechanism may not always operate to disconnect a load. For example, in the context of an avionics generator, suspected failure of the generator may result in an immediate electrical disconnection of the device. Thus, apart from internal frictional losses, it will present no load to its drive device. Nevertheless, if failure is suspected it will still be desirable to drivingly disconnect the generator from the prime mover. The arrangement described in U.S. Pat. No. 4,685,550 may not be able to perform disconnect under these conditions since very little load is transmitted through disconnect device and hence very little axial force will occur between the inclined teeth.  
       [0004] According to a first aspect of the present invention, there is provided a drive disconnect device for releasably connecting an output shaft to a drive shaft, comprising a drive transfer element movable between a first position where it drivingly connects the drive shaft to the output shaft, and a second position where there is no driving connection between the drive shaft and the output shaft, and wherein at least one actuator is arranged to act against a first region of the drive transfer element so as to urge it to the second position.  
       [0005] It is thus possible to provide an arrangement in which at least one actuator acts directly on the drive transfer element. This ensures that the element can be moved irrespective of the load or torque being transferred through the drive disconnect device (provided this is less than a maximum torque for which the device is rated).  
       [0006] Preferably the at least one actuator is a fluid operated actuator.  
       [0007] The at least one fluid operated actuator is advantageously in the form of a piston and cylinder with the piston biased to a predetermined position, for example a retracted position, where the piston does not engage or substantially engage the drive transfer element.  
       [0008] Preferably the or each piston is selectively connectable to a source of pressurised fluid via an electrically operable valve which has a bleed path to a low pressure region. This is advantageous as it stops leakage through the valve from triggering an unexpected or unscheduled operation of the disconnect device. Alternatively an arrangement can be used where the piston is held in the first position by the provision of pressurised fluid action on a face of the piston. Thus the piston can be moved to the second position by allowing fluid to bleed to low pressure.  
       [0009] Preferably an accumulator is provided to maintain a sufficient store of pressurised fluid to operate the disconnect device. Thus the device can still be operated to perform a disconnect function or otherwise maintain normal operation of the device even in the event of a loss of a source of pressurised fluid or insufficient pressure therefrom.  
       [0010] The use of pressurised fluid means that high mechanical loadings can be achieved from relatively small and light actuators. Furthermore, fluid flow can be controlled by electrically operated valves which themselves are small and light, and certainly much lighter than would be required to obtain the same forces from a solenoid alone or through magnetic attraction.  
       [0011] Preferably a lock mechanism is provided to lock the drive transfer element in the second position. The lock may act directly on the drive transfer element or may act on one or more of the pistons in order to hold them in an extended or operated position.  
       [0012] Preferably the lock is a spring loaded pin or other detent that engages with the drive transfer element when it is at the second position. Advantageously the lock mechanism is external to the drive transfer element such that it can be reached and operated manually, for example by service technicians. This facilitates testing of the drive disconnect device by maintenance staff.  
       [0013] Preferably, the drive transfer element is biased, for example by a spring, towards the first position. This ensures that inadvertent disconnects do not occur as a result of vibration or dynamic loading.  
       [0014] Advantageously the drive transfer element is coaxially disposed with either or both of the drive shaft and the output shaft. Engagement between the drive transfer element and the drive shaft or the output shaft may be via drive dogs. The dogs may have inclined teeth giving rise to an axial load. However, the invention works equally well where the teeth are not so inclined.  
       [0015] Preferably the drive transfer element is in splined engagement with the output shaft. Thus the drive transfer element is axially slidable with respect to the output shaft and tends to stop rotating once the drive disconnect device has been operated. This in turn reduces the wear that may be experienced by a locking mechanism acting directly on the drive disconnect device to hold it in the disconnected position.  
       [0016] Preferably the fluid is a liquid, although pneumatic operation is also possible. In the context of an avionics environment supplies of pressurised liquid and pressurised air are routinely available.  
       [0017] Advantageously the drive transfer element includes a flange against which the at least one actuator and a spring bias element can act. The at least one actuator may be in the form of an annular piston encircling one of the drive shaft and the output shaft.  
       [0018] During the disconnect operation the actuator, which is held on a stator, comes into contact with the flange of the drive transfer element, which is rotating. The actuator does not have to endure the sliding contact with the flange for long. However it is advantageous for the contacting region of the actuator to be lubricated and/or formed or coated with a low friction material.  
       [0019] According to a second aspect of the present invention, there is provided a generator in combination with a drive disconnect device according to the first aspect of the present invention. 
     
    
    
     [0020] The present invention will further be described, by way of example only, with reference to the accompanying figures in which:  
     [0021]FIG. 1 is a schematic diagram of a drive disconnect device constituting an embodiment of the present invention;  
     [0022]FIG. 2 shows a further embodiment in the driving position; and  
     [0023]FIG. 3 shows the embodiment of FIG. 2 in the disconnect position. 
    
    
     [0024]FIG. 1 illustrates an arrangement in which a quill shaft  2  receives power, either directly or indirectly, from a prime mover (not shown) such as a gas turbine jet engine. The quill shaft  2  is in splined engagement with a annular element  4  which functions as a drive shaft. The drive shaft  4  has a first region  6  which engages with bearings  8  which act to support the shaft, and a second region  10  which has a castellated end section which forms drive dogs for drivingly engaging with a drive transfer element  12 . The drive transfer element  12  has a similar castellated end region  14  which forms drive dogs which cooperate with the drive dogs  10  of the annular element  4 . The drive transfer element  12  is coaxially disposed with respect to a rotor shaft  16  and is drivingly coupled thereto via splines  18 . Thus the drive transfer element  12  does not undergo rotational movement with respect to the rotor shaft  16  but can slide axially with respect to the shaft  16 . Line  20  in the figure represents an axis of rotational symmetry for the quill shaft  2 , the drive shaft  4 , the transfer element  12  and the rotor shaft  16 .  
     [0025] The drive transfer element  12  has a circularly symmetric flange  22  disposed thereon, advantageously towards the end opposite to the drive dogs. The flange  22  has a first surface  24  which opposes a similar surface  26  on the rotor shaft and between which a compression spring  28  is held. The action of the compression spring is to urge the drive transfer element to move away from the surface  26  such that its drive dogs  14  interengage with the drive dogs  10  of the drive shaft  4 . Stepped portions  30  and  32  of the flange  22  and on the rotor shaft  16  serve to hold the compression spring  28  in a position such that it is coaxially disposed about the rotor shaft  16 . Thus, in use, rotation of the quill shaft  2  is transmitted to the drive shaft  4  via coupling elements therebetween, for example splines. Rotation of the drive shaft  4  is transmitted via the dogs  10  and  14  to the drive transfer element  12  which in turn transfers drive to the rotor shaft  16  via the splines  18 . The drive transfer element  12  is held coupled to the drive shaft  4  via the action of the compression spring  28 .  
     [0026] The annular flange  22  also defines a second surface  40  against which a piston  42  (which may for example be an annular piston or be a member of a group of one or more cylindrical pistons) can be urged to bear against by a virtue of pressurised fluid being admitted into a cylinder  44  within which the piston  42  is in substantially fluid sealed sliding engagement. The piston  42  is biased to a predetermined position, for example by a compression spring  46  which serves to hold the piston in position against possible movement that may be induced by vibration. The or each cylinder  44  is connected to a reservoir of pressurised fluid  48  via an electrically operable valve  50  and interconnecting ducts  52 , which may for example be in the form of drillings. The reservoir of pressurised fluid  48  may receive fluid from a pump supply  54  via a one way valve  56 . The electrically operable valve  50  may comprise a valve element  58 , for example in the form of a spherical member, which bears against a cooperating valve seat under action of a biasing spring (not shown). The valve may include an internal duct  60  venting to low pressure such that leakage of fluid through the valve when it is in a position which notionally prevents the supply of fluid from the reservoir  48  to the cylinder  44  can be drained away thereby ensuring that pressure does not build within the cylinder under these circumstances.  
     [0027] A resetable detent or lock mechanism  70 , for example in the form of a plunger  72  biased to move to an extended position by a compression spring  74  may be provided in order to hold the drive transfer element  12  in a disconnected position once drive disconnection has occurred.  
     [0028] In order to invoke disconnection of the rotor shaft  16  from the drive shaft  4 , an electrical signal is supplied to the valve  50  so as to energise a solenoid therein to move the valve element  58  away from sealing engagement with the valve seat, and simultaneously to close or at least obstruct the passageway  60  that vents to low pressure. Alternatively, the passageway  60  may include a constriction therein such that even though it does not become obstructed by the valve element  58 , the rate of fluid passage therethrough is maintained at acceptable levels. This second configuration is in fact preferable as it prevents a pressure differential across the valve element  58  from occurring during operation which may serve to prevent movement of the valve element  58  back to the position where it sealingly engages with the valve seat.  
     [0029] Following actuation of the valve, high pressure fluid from the reservoir  48  flows along the duct  52  and into the cylinder  44  where the increase in pressure urges the piston  42  to bear against the surface  40  and then to push the drive transfer element against the urging of the compression spring  28  to a disconnect position where the drive dogs  10  and  14  no longer interengage. As this occurs, an annular recess  80  formed in the drive transfer element  12  moves to a position whereby the end of the element  72  can engage the annular recess  80  thereby preventing further axial motion of the drive transfer element  12 . Thus, the drive transfer element  12  becomes locked in the disengaged position and will remain there even when the supply of pressurised fluid to the piston  44  is removed.  
     [0030] It will be understood by the person skilled in the art that relatively small piston and cylinder arrangements fed from high pressure sources of fluid can produce large forces. Thus it is possible to provide a compact and lightweight actuation system which acts directly on the drive transfer element  12  to move it to a disengaged position. Furthermore, it is possible to provide a fully testable system as the operation of the piston and the resetable disconnect lock  70  can be initiated and observed by maintenance personnel whilst the quill shaft  2  and hence the rotor is not rotating. This confers a significant advantage over prior art systems which are either difficult to test, difficult to reset, or require a torque to be transferred in order to initiate the disconnection process.  
     [0031]FIG. 2 shows a further embodiment of the present invention in the driving configuration where torque is transmitted from a prime mover to a load. For convenience, only a portion of the disconnect mechanism is shown, although it will be appreciated that most of the components of the disconnect mechanism are circularly symmetric and encircle input and output shafts  100  and  102 . In FIG. 2, line  95  represents the axis of rotation of the shafts  100  and  102 . Typically the input shaft  100  is driven from an aircraft gearbox via a quill shaft  104  incorporating a shear neck  106 . The input shaft  100  is supported within a housing, typically a generator housing  108 , by bearings  110 . The bearings  110  may, for example, be a single deep groove ball bearing configuration. The input shaft  100  also carries a plurality of dogs  112  which face towards and engage with corresponding dogs on a drive sleeve  114 . The drive sleeve  114  drivingly engages with the output shaft  102  via splines  116 , thereby allowing the drive sleeve  114  to move axially with respect to the output shaft  102 . The drive sleeve  114  is biased into engagement with the input shaft via a compression spring  118  acting between an end face  122  of the drive transfer sleeve  114  and a face  124  formed by a flange on the output shaft. The output shaft  102  is supported on bearings  120  which may advantageously be similar to the bearings  110 .  
     [0032] The housing  108  is profiled so as to form the radially outermost surfaces of hydraulic chambers  130  and  132 . The radially innermost surface of the hydraulic chamber  130  is defined by element  134 . The element  134  is circularly symmetric around the longitudinal axis of the input and output shaft and hence takes the form of a cylindrical element having an enlarged end  136  distal from the input shaft  100 . The enlarged end  136  is in fluid sealed sliding engagement with a radially inward extending portion  138  of the housing  108 . An accumulator piston  140  is provided within the chamber  130 , and is spring biased by a compression spring  142  so as to move away from the bearing  110 , that is to the right as shown in FIG. 2. A volume V1 defined by the co-operation of the housing  108 , the accumulator piston  142  and the element  134  is connected to an oil supply via a duct  144 . An end surface  146  of the element  134  may abut against the body of a disconnect piston  150 . However in a preferred arrangement the element  134  and the disconnect piston  150  are formed as a unitary component  153  as shown in FIG. 3. The disconnect piston  150  has an enlarged head  152  which engages the wall of the second hydraulic chamber  132 . A volume V2 within the second hydraulic chamber is connected to a duct  154 . The enlarged head  150  in co-operation with the region  136  of the element  134  serves to form a differential piston having surfaces exposed to chambers V1 and V2 respectively.  
     [0033] The duct  154  can be selectively connected to the duct  144  or to a low pressure vent via an electrically operated valve  156 .  
     [0034] The disconnect piston  150  is provided with a recess  160  which can engage with a locking pin  162  to lock the disconnect piston in an operated position so as to cause permanent disengagement of the input shaft and the drive sleeve  114 .  
     [0035] In use, oil under pressure, P1, is provided to the duct  144  via a check valve  180 . With the solenoid operated valve  156  kept in the de-energised position, as shown in FIG. 2, the inlet oil pressure, which is typically in the region of 415 kPa (60 psi) acts on the accumulator piston  142  via the duct  144  overcoming the spring load exerted by the compression spring  142 . Thus a volume of oil is accumulated under pressure within chamber V1. This oil pressure also acts on both faces of the disconnect piston, i.e. face  182  (shown in FIG. 3) which faces the volume V1, and face  184 , also shown in FIG. 3, which faces the volume V2. The area of the disconnect piston in chamber  132  is greater than the area of the disconnect piston  150  in chamber  130  and consequently the disconnect piston is held in the stowed position as shown in FIG. 1.  
     [0036] When the solenoid valve is energised, as shown in FIG. 3, the connection between duct  154  and duct  144  is closed and instead duct  154  is vented to low pressure. Thus the oil pressure in the volume V2 is also vented to the case. However the inlet oil pressure V1 is maintained. Consequently the pressure in volume V1 acts upon the disconnect piston via face  182  to move it axially from the stowed position to an extended position. This motion causes the disconnect piston to abut a flange  190  of the drive sleeve thereby moving the drive sleeve against the urging of the compression spring  118  and also causing the dogs inter-engaging the drive sleeve and the input shaft to disengage thereby removing driving connection between the input shaft and the output shaft.  
     [0037] The pin  162  is, as noted before, located in a recess in the disconnect piston  150  and consequently reacts the frictional torsional forces acting between the drive sleeve and the disconnect piston. The motion of the disconnect piston  150  is limited by the pin  162  which drops into a radial channel  161  under the load exerted by compression spring  200 . This action causes the disconnect piston  150  to become locked in the extended position.  
     [0038] If the disconnect mechanism is provided within an aeronautical generator, then the disconnect operation would cause the generator&#39;s pumps to be disconnected. Consequently the oil pressure P1 would drop and eventually the pressure P2 would exceed P1 and the check valve  180  would close. When the solenoid valve  156  is de-energised the pressure in volume V1 is dissipated and the path the volume V2 is reopened. The accumulator piston then moves axially under the load of its compression spring  142 , which typically will be a wave washer.  
     [0039] In the event of cessation of the oil supply during normal use, the pressure P2 will exceed P1 and the check valve  180  would close. Typically the check valve operates once the pressure P2 exceeds P1 by 7 kPa (1 psi). Closure of the check valve maintains the 415 kPa (60 psi) pressure within the chambers V1 and V2. Thus disconnect is not automatically actioned by a pressure drop occurring in the oil pressure feed to the disconnect device. However, if it is desired to operate the disconnect mechanism then, once again, the solenoid valve  156  is energised thereby causing the chamber V2 to be vented to low pressure. Also, as shown in FIG. 3, the valve closes the vent path from the duct  144  to the case or to V2 when it is energised. Under these circumstances the compression spring  142  acts upon the accumulator piston  140  and, through the hydraulic pressure within the volume V1, acts upon the disconnect piston  150  moving it from the stowed position to the operated position thereby causing the drive sleeve  114  to move thereby disengaging the drive dogs.  
     [0040] Once operated, the disconnect piston remains in the extended/disconnect position by virtue of the pin  162 . In order to restore driving connection between the input shaft and the output shaft the pin  162  has to be reset. This is done by manually grasping a pin extension  202  and pulling against the action of the compression spring  200  so as to release the pin  162  from the hole  161 . Under these conditions the compression spring  118  urges the drive sleeve  114  to move back to the driving engaged position. Upon commencement of drive, either from the starter generator or an aircraft gearbox rotational misalignment between the dogs is initially removed and the dogs re-engage. This therefore also re-engages drive to the oil pump allowing oil pressure on line P1 to be restored. This in turn causes oil pressure to be admitted into the chambers V1 and V2 allowing the accumulator piston to return to its correct position.