Abstract:
A drive assembly for providing two outputs from a motor to actuators or the like has a first drive shaft connected with the motor via a torque limiter and a reduction gear. A high efficiency spur gear also connects the motor with a second, parallel drive shaft via a second torque limiter and a reduction gear. The two drive shafts are also interconnected by a low efficiency gear provided by worm pinions on the shafts and a worm gear wheel engaging both pinions, which only transfers load between the drive outputs if there is uneven torsional loading in excess of the settings of the torque limiters.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to drive assemblies.  
         [0002]     The invention is more particularly concerned with drive assemblies for providing two or more outputs to mechanical actuators or the like.  
         [0003]     In various aircraft and industrial applications there is a need to drive two or more mechanical actuators or other structures in a synchronized fashion. These applications include engine nacelle thrust reversers, aircraft flap and slat systems, cargo or passenger doors, radar antenna or other large surface area panels that are required to move in a synchronized fashion. Drive assemblies used in such applications should operate in such a way that there is no structural damage in the event of a jam in any part of the guide or drive system. Examples of arrangements providing synchronized outputs are described in U.S. Pat. No. 6,598,386 and U.S. Pat. No. 6,443,034. Conventional arrangements suffer from various disadvantages and, in particular, the drive transmission components tend to be heavy and require a relatively high power motor to drive them.  
       BRIEF SUMMARY OF THE INVENTION  
       [0004]     It is an object of the present invention to provide an alternative drive assembly.  
         [0005]     According to one aspect of the present invention there is provided a drive assembly including a drive input arranged to be rotated by a motor, a first drive output driven by the drive input via a first torque limiter, a second drive output driven by the drive input via a high efficiency gear arrangement and a second torque limiter, and a low efficiency gear arrangement interconnecting the first and second drive outputs and operable to transfer load between the first and second drive outputs only if there is uneven torsional loading between the two drive outputs above the torque limiter setting.  
         [0006]     The first and second drive outputs preferably have parallel shafts. The high efficiency gear arrangement preferably includes a spur gear and the low efficiency gear arrangement preferably includes a worm pinion and worm gear arrangement. The first and second drive outputs are preferably driven by the drive input via respective reduction gears, such as including planetary gears.  
         [0007]     According to another aspect of the present invention there is provided a drive assembly including first and second parallel output shafts each provided with a worm pinion towards one end, a common worm gear engaging the respective worm pinions to provide a low efficiency synchronizing interconnection between the two shafts, a rotary drive input connected with the first output shaft via a first torque limiter and connected with the second output shaft via a transfer gear and a second torque limiter, such that the rotary drive input rotates both output shafts synchronously.  
         [0008]     The drive input may include an electric motor or other rotary drive.  
         [0009]     A drive assembly according to the present invention will now be described, by way of example, with reference to the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a perspective plan view of the assembly with the upper part of its housing removed; and  
         [0011]      FIG. 2  is a cross-sectional plan view of the assembly. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]     The drive assembly includes an outer metal housing  1  of generally rectangular section and provided with four mounting feet  2  by which it can be secured in position on an airframe structure. The housing  1  has two parallel opposite end faces  3  and  4  through which drive connection is made to and from the assembly. In the example shown, there are two drive output  5  and  6  at opposite end faces  3  and  4  respectively, which are driven together synchronously by an electric motor  7  mounted of the right-hand end face  4 .  
         [0013]     Within the housing  1 , the two drive outputs  5  and  6  are driven by respective drive shafts  15  and  16  arranged parallel with and spaced laterally from one another. The first, primary drive shaft  15  is connected to be rotated by the motor  7  in the following manner. The drive shaft  70  of the motor  7  is connected with the boss  8  of a spur gear wheel  9  arranged axially and to the right of the output drive shaft  15 . The spur gear wheel  9  forms the outer part of a first torque limiter  10  of conventional construction having a tubular, axial output or torque shaft  11 . The torque shaft  11  extends forwardly axially and provides the inner part of a planetary reduction gear assembly  12 . The gear assembly  12  steps down the rotation of the torque shaft  11  by a factor of 4:1 before it is communicated to the output shaft  15  and thereby increases the motor torque available. Thus, the drive from the motor  7  passes first through the torque limiter  10  and then through the reduction gear  12  before driving the primary output shaft  15  and hence the output  5 . By locating the torque limiter  10  close to the motor  7  and before the reduction gear  12 , the size and weight of the torque limiter can be kept small.  
         [0014]     The second, parallel output shaft  16  is connected with the motor  7  by means of a second spur gear wheel  19  arranged axially of the output shaft and alongside the first spur gear wheel  9 , in meshing engagement. The two spur gear wheels  9  and  19  act as a transfer gear assembly to transfer torque with high efficiency and low losses, ensuring equal input rotation of the parallel drive paths. The second spur gear wheel  19  forms the outer part of a second torque limiter  20  having a tubular torque shaft  21 , which drives a second planetary reduction gear  22  of the same construction and ratio as the first gear  12 . The second reduction gear  22  drives the output shaft  16  to rotate at the same rate as the primary output shaft  15 . The teeth  34  and  36  on the end of the planetary ring gears  35  and  37  of the two reduction gears  12  and  22  do not mesh with one another but can be used to connect with a common manual drive (not shown). The common manual drive ensures that the planetary ring gears  35  and  37  do not rotate when the device is driven with the motor  7 .  
         [0015]     The two output shafts  15  and  16  are interconnected by a synchronization gearing set indicated generally by the numeral  30  towards the forward, left-hand end of the shafts  15  and  16 , adjacent the end plate  3 . The gearing set  30  comprises a worm pinion  31  and  32 , formed on the respective shafts  15  and  16 , and a worm gear  33  mounted with the housing  1  between the two shafts and rotatable about an axis orthogonal to the axes of the shafts. The worm gear  33  engages the worm pinions  31  and  32  on the two shafts  15  and  16  so that it is rotated by the shafts. The worm gear  33  ensures that the rotational position and speed of both output shafts  15  and  16  are positively matched with one another; neither output shaft can rotate without a corresponding rotation of the other output shaft. The worm and pinion synchronization gearing set  30  is specifically chosen to have a low efficiency due to the double mesh friction losses. In this way, if there is a jam load on one of the output paths, only a relatively small part of the torque available in the other path will be delivered across the double worm mesh synchronization gear set  30  to the output  5  or  6 , the remainder of the load being transferred to the housing  1 .  
         [0016]     In normal operation, the motor  7  provides two reduced speed output drives at the outputs  5  and  6 , which rotate synchronously. As long as the load is within predetermined limits, there is no slipping of either torque limiter  10  or  20 . The output shafts  15  and  16  rotate together and no torque is transmitted across the synchronization gearing  30 .  
         [0017]     If the load at the two outputs  5  and  6  should become unequal but both are still within the torque limits, there will still be no slippage of the torque limiters  10  or  20 . There will be negligible load or torque transmitted across the synchronization gearing set  30  because the two output shafts  15  and  16  are still driven directly with one another via the transfer gear assembly  9  and  19 .  
         [0018]     If the load at one of the outputs  5  or  6  should rise sufficiently to jam the output, the torque limiter  10  or  20  driving that output would reach its slip value but would not slip until additional torque was applied by the motor  7  along the parallel path across the synchronization gearing set  30  and the total torque limiter setting was reached. The torque from the motor  7  is, therefore, summed into the common output that is experiencing the jam condition. Both torque limiters  10  and  20  slip when the load on them exceeds the slip limit and both slip in unison to dissipate the kinetic energy. The motor  7  is in its stall condition and, when motor current exceeds a preset limit, a breaker (not shown) opens to disconnect the motor from its electrical drive and thereby terminate drive to the assembly. Synchronization between the two output shafts  15  and  16  is maintained by the synchronization gearing set  30  while the torque limiters  10  and  20  are functioning.  
         [0019]     Similarly, if there should be a jam load at both outlets  5  and  6 , both torque limiters  10  and  20  would slip to dissipate the kinetic energy. It is possible that there may be some minor load/torque sharing across the synchronization gearing set  30  under this condition but the effect will be negligible because both sides are in an overload condition.  
         [0020]     In all cases, the torque delivered through the torque limiters  10  and  20  and the synchronization gearing set  30  does not exceed a predetermined value and both output drives  5  and  6  are always synchronized.  
         [0021]     The drive path through the assembly is highly efficient, thereby enabling the power, size and weight of the motor  7  to be kept to a minimum. The inefficient nature of the synchronization gearing set  30  together with the torque limiters  10  and  20  enable the jam load torque transmitted from the primary and parallel drive paths to the jam point to be reduced considerably. This is an advantage because it enables the structure and the drive path driven by the outputs to be made less robust and, therefore, enables weight to be reduced. In conventional drive assemblies producing synchronized drive of two outputs, when the torque limiter slips, torque in excess of twice the maximum operational requirement is delivered to the jammed-side components. It will be appreciated that this requires the structure being driven to be made much heavier so that it is sufficiently robust that it is not damaged by these torques. In the present arrangement, by contrast, the load at which the torque limiters  10  and  20  slip is only slightly greater than the maximum operational load requirement. The material and geometry of the worm pinion and worm gear synchronization gearing set  30  can be altered to increase or reduce the transfer efficiency as necessary.  
         [0022]     It is not essential for the assembly to be driven by an electric motor since various other motors or combinations of motors could be used, including hydraulic or pneumatic motors. Multiple motors could be used if these were advantageous, such as, for example, if the space available did not permit the use of one large motor but did enable two smaller motors to be used.  
         [0023]     The configuration of the parallel output shafts of the present arrangement enables increased flexibility in the location of the outputs. For example, the parallel output shaft  16  could have an output  6 ′ provided at the same end as the primary output  5 , simply by removing a cover plate  60 ′ on the end plate  3  and making connection with the output shaft  16 .  
         [0024]     The drive assembly could be provided with additional outputs by arranging one or more additional output shafts (complete with torque limiter and reduction gear) parallel with the primary shaft and coupling them either directly with the primary shaft, or indirectly through a parallel shaft, via a high efficiency transfer gear and a low efficiency synchronizing gear.