Abstract:
A pawl and ratchet assembly has a rotatable pawl, a spring for urging the pawl towards the ratchet, and a member for interfering with travel of the pawl such that force of the spring upon the pawl is reduced.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to pawl and ratchet clutches for use on unidirectional drive systems, and more specifically to a pawl holdback means for limiting pawl travel. The pawl and ratchet clutch of the present invention is particularly suitable for use in starters for starting engines, such as aircraft turbine engines. 
         [0002]    Pawl and ratchet clutches are often utilized in unidirectional drive systems for transmitting drive torque from a drive shaft to a driven shaft. For example, starters of the type commonly used to start engines, in particular the turbine engines of modern gas turbine powered aircraft often employ a pawl and ratchet type clutch which functions to transmit rotational drive torque from a drive shaft of the starter to drive the engine being started to starting speed. One of type of starter often employing a pawl and ratchet clutch is the pneumatic starter, also known as an air turbine starter, such as disclosed, for example, in U.S. Pat. Nos. 3,727,733; 4,899,534; 4,914,906; and 4,926,631. 
         [0003]    A pawl and ratchet clutch of a type commonly used in such pneumatic starters includes a toothed ratchet member mounted on a central drive shaft and a plurality of pivotal pawls supported from and rotating with a driven output shaft disposed coaxially about the drive shaft. The pawls are operatively disposed at circumferentially spaced intervals about the ratchet member in cooperative relationship therewith. Each pawl is biased to pivot radially inwardly by a leaf spring operatively associated therewith to engage a tooth of the ratchet member thereby coupling the drive shaft in driving relationship to the driven output shaft so long as the pawls remain engaged with the teeth of the ratchet member. The drive shaft is connected, either directly or through suitable reduction gearing as desired, to the shaft of the pneumatic starter turbine, which is powered by extracting energy from a flow of pressurized fluid passed through the turbine of the starter. 
         [0004]    To start the turbine engine, the output end of the driven output shaft of the starter is connected, for example by mating splines, to an engine shaft operatively connected to the main engine shaft through a gear box, and pressurized fluid, typically compressed air, is passed through the turbine of the pneumatic starter. As the starter turbine extracts energy from the compressed air passing therethrough, the drive shaft of the starter turbine is rotated to in turn rotatably drive the output shaft of the starter, and consequently the turbine engine shaft connected thereto, through the engagement of the pawls pivotally mounted to the output shaft with the ratchet member mounted to the drive shaft. Typically, the starter is designed to accelerate the engine shaft from zero to a predetermined cut-off speed, typically of about 5000 revolutions per minute, in about one minute or less. 
         [0005]    Once engine light-off has occurred and the engine shaft is rotating at the desired cut-off speed, the flow of pressurized air to the starter turbine is terminated, when this happens, torque transfer from the ratchet into the pawl ceases. With the flow of pressurized air to the starter turbine shut-off, the drive shaft of the starter rapidly slows down. Consequently, the ratchet member mounted to the starter drive shaft also rapidly slows down, while the pawls supported from the starter output shaft continue to rotate with the engine of the operating turbine engine at the relatively high cut-off speed. The pawls become disengaged from the ratchet member when the rotational speed of the output shaft exceeds a threshold speed whereat the pawls lift-off of the ratchet member (the lift off speed is a design requirement), that is pivot radially outwardly out of contact with the teeth of the ratchet member, under the influence of the centrifugal forces acting thereon due to the continued rotation of the pawls at the relatively high speed of the engine shaft and remain disengaged from the ratchet member so long as the rotational speed of the engine shaft remains high enough that the centrifugal forces acting on the pawls exceed the opposing moment imposed on the pawls by the force of their associated bias springs. 
         [0006]    When the turbine engine is later shut-down, the operating speed of the engine shaft of the turbine engine to which the output shaft of the starter is connected rapidly decreases as the turbine engine spools down. As the starter shaft slows down, the centrifugal force on the pawls consequently decreases and the force of each bias spring progressively pivots its associated pawl radially inwardly again toward the ratchet member until each pawl reengages a ratchet tooth on the non-rotating ratchet member so as to reengage the clutch. The speed at which the reengagement of the pawls with the ratchet member occurs, commonly referred to as the reengagement speed, is less than the pawl lift-off speed by an amount that is a function of the clutch hysteresis. 
       SUMMARY OF THE INVENTION 
       [0007]    According to an example disclosed herein, a pawl and ratchet assembly has a rotatable pawl, a spring for urging the pawl towards the ratchet, and a member for interfering with travel of the pawl such that force of the spring upon the pawl is reduced. 
         [0008]    According to a further example disclosed herein, a method for reducing motion of a rotatable pawl in a pawl and ratchet assembly that has a spring urging the pawl towards a ratchet includes mounting a stop on the assembly for interfering with motion of the pawl. This reduces the spring deflection, which reduces the force acting at the contact face, which reduces contact stress. 
         [0009]    According to a still further example disclosed herein, an air turbine starter includes a clutch carrier and a pawl and ratchet assembly mounted on the clutch carrier. The pawl and ratchet assembly includes a spring for urging the pawl towards a ratchet and a member for interfering with travel of the pawl during operation of the assembly so that force of the spring upon the pawl is reduced. 
         [0010]    These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a prior art view of a pneumatic starter having a pawl and ratchet assembly. 
           [0012]      FIG. 2  is a sectional side view of the pawl and ratchet assembly of the pneumatic starter of  FIG. 1  including a first embodiment of a stop. 
           [0013]      FIG. 3  is a sectional side view of the pawl and ratchet assembly of the pneumatic starter of  FIG. 1  including a second embodiment of a stop. 
           [0014]      FIG. 4  is a sectional side view of the pawl and ratchet assembly of the pneumatic starter of  FIG. 1  including a third embodiment of a stop. 
           [0015]      FIG. 5  is a sectional side view of the pawl and ratchet assembly of the pneumatic starter of  FIG. 1  including a fourth embodiment of a stop. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0016]    Referring now to  FIG. 1 , there is depicted a prior art pneumatic starter  10  of the general type often utilized to start gas turbine engines, such as for example aircraft turbine engines (not shown). The pneumatic starter  10 , also known as an air turbine starter, includes a turbine wheel (not shown) which is driven by a compressed gas, most commonly compressed air from an external supply, passing therethrough so as to extract energy from the gas and convert the extracted energy to mechanical energy through a gear train (not shown) in a manner well known in the art. Although the present invention is described herein with reference to an air turbine starter, it is to be understood that the pawl and ratchet assembly of the present invention has application on any unidirectional drive system wherein a pawl and ratchet clutch is utilized to transmit rotation from a driving shaft to a driven shaft. 
         [0017]    Extending axially outwardly from the transmission (not shown) is an output shaft  60  which has means, such as for example splines  62 , for engaging an engine shaft (not shown) in the gear box of the turbine engine (not shown) on which the starter  10  is utilized to start the turbine engine. The output shaft  60  is rotatably supported on bearing means  66 . The aforementioned pawls  48 , of which there are typically three, are supported on a clutch carrier  70  of the pneumatic starter  10  that extends radially outward from the output shaft  60 . Each of the pawls  48  is pivotally supported on a shaft  52  mounted to and extending axially from the clutch carrier  70  such that the pawls  48  are disposed in cooperation with the ratchet member  46  at equally spaced intervals about the circumference of and in radially spaced relationship from the ratchet member  46 . 
         [0018]    Additionally, each of the pawls  48  is biased to pivot about its support shaft  52  to rotate the toe end  47  thereof radially inwardly towards the ratchet member  46  under the force of a bias spring  54  mounted to an axial flange portion  72  of the clutch carrier  70 . Each bias spring  54  may comprise a leaf spring operatively bearing against the heel end  49  of its associated pawl  48  so as to, in a manner well known in the art, function during operation of the starter  10  to load the toe end  47  of its associated pawl  48  into engagement with the teeth  45  of the ratchet member  46  of the pawl and ratchet clutch to ensure transmission of torque from a ring gear (not shown) to the output shaft  60  until the output shaft  60  has reached a relatively high desired cut-off speed, for example about 5000 rpm, at which the pawls  48  pivot away from the ratchet member  46  under the influence of centrifugal force after disengagement from the teeth  45  of the ratchet member  46  upon slowing down of the ratchet member  46  after termination of the flow of compressed air through the starter turbine (not shown). 
         [0019]    When the pawls  48  are engaged with the teeth  45  of the ratchet  46  during operation of the starter  10 , the output shaft  60 , and the engine shaft of the turbine engine (not shown) connected therewith, are driven in rotation through the engaged pawl  48  and ratchet member  46 . 
         [0020]    After the shaft of the turbine engine (not shown) engages with the output shaft  60  of the starter  10  reaches the preselected desired engine speed, the flow of compressed air to and through the turbine wheel (not shown) is shut off. As a result, drive power is no longer being delivered to the ratchet member  46 . Consequently, the ratchet member  46  slows down and its rotational speed rapidly decreases, while the output shaft  60  of the starter  10  and the pawls  48  mounted thereto continue to rotate at the higher engine speed, thereby causing the pawls  48  to become disengaged from the ratchet member  46  and pivot away from the ratchet member  46 . Centrifugal force acting on the pawls  48  overcomes the engagement force provided by spring  54  to cause the pawls  48  to rotate away from and disengage the ratchet member  46 . The contact force at the tip of the pawl that results from torque transfer from the ratchet into the pawl is what keeps the pawl in contact with the ratchet during starts. When the air is turned off and torque transfer ceases, the centrifugal force exceeds the spring force and the pawl lifts off the ratchet. 
         [0021]    When the turbine engine is later shut down, the starter output shaft  60  begins to slow down and its rotational speed steadily decreases as the turbine engine spools down. As the output shaft  60  coasts down, the centrifugal force acting on the pawls  48  so as to urge the toe ends  47  of the pawls  48  radially outwardly steadily decreases and the toe end  47  of each pawl  48  begins to pivot radially inwardly toward the ratchet member  46  under the bias force applied by the springs  54  on the heel  49  of the pawls  48  until the toe ends  47  of the pawls  48  re-engage with the teeth  45  of the ratchet member  46 . 
         [0022]    A spring clamp  75  is attached by bolts  80  to the axial flange portion  72  of the clutch carrier  70  to hold the spring  54 . 
         [0023]    Existing ratchet and pawl clutch designs experience spring failures and pawl heel wear. A spring fracture and pawl heel wear may alter the performance of the unit in a negative manner. A source of spring fractures and pawl heel wear may include contact forces experienced at the spring tip  85  and pawl heel  49  during over-running conditions. During over-running conditions centrifugal force rotates the pawl  48  radially outward which increases spring deflection. The force resulting from the deflected spring is non-linear and subsequently a greater deflection results in a significantly greater force. The greater force accelerates wear between the pawl heel  49  and the spring tip  85 . 
         [0024]    Referring now to  FIGS. 2-5 , a pawl stop member  90   a - d  may be included in the ratchet and pawl clutch assembly and can be located on several components, including to but not limited to the spring clamp  75  ( FIG. 2 ), the bias spring  54  ( FIG. 3 ), the pawl  48  ( FIG. 4 ), or the axial flange portion  72  ( FIG. 5 ). The pawl stop  90   a - d  limits the radial rotation of the pawl  48  about the shaft  52  to decrease the spring deflection to minimize damage to the spring  54  and to the pawl heel  49 . The pawl stop  90   a - d  may be constructed of a steel or other metal that is added for its longevity or an elastomeric or the like to damp vibrations of the pawl  48  as the clutch carrier  70  rotates. The pawl stops  90   a - d , if metal, are attached to their respective places by welding or gluing or the like, and, if elastomeric, by gluing or the like. 
         [0025]    Referring now to  FIG. 2 , pawl stop  90   a  is added to spring clamp  75  to minimize the force provided by the spring  54 . The pawl stop  90   a  may have a flat back  95 , a notch  100  for receiving an angled rear portion  105  of the spring clamp  75 , an inner portion  110  conforming to and in register with an inner surface  115  of the spring clamp  75 , and an angled portion  120  angling away from said inner portion  110  for contacting the pawl  48 . Rotation of the pawl  48  away from the ratchet  46  is minimized as the pawl stop  90   a  is contacted by the pawl  48 . 
         [0026]    Referring now to  FIG. 3 , pawl stop  90   b  is added to the spring  54  to minimize the force provided by the spring  54 . The pawl stop  90   b  may have a flat back  125  and a semicircular body  130 . The pawl stop  90   b  engages an extension  135  attaching to a rear end  140  of the spring clamp  75 . Rotation of the pawl  48  away from the ratchet  46  causes the spring  54  to react towards the spring clamp  75 . Motion of the spring  54  is limited by contact of the pawl stop  90   b  with the extension  135 . Alternatively, the pawl stop  90   b  may attach to the extension  135  as opposed to the spring  54  to achieve a similar result. 
         [0027]    Referring now to  FIG. 4 , pawl stop  90   c  is added to the pawl  48  on a back surface  145  thereof to reduce the force provided by the spring  54 . The pawl stop  90   c  may have a relatively flat back  150  and a semicircular body  155 , where the relatively flat back  150  is configured to interface with an upper surface of the pawl  48 . Rotation of the pawl  48  away from the ratchet  46  is minimized as the pawl stop  90   c  contacts the axial flange portion  72  of the clutch carrier  70  which is in registration with the pawl  48  to allow contact therebetween. 
         [0028]    Referring now to  FIG. 5 , pawl stop  90   d  is added to the axial flange portion  72  of the clutch carrier  70  to minimize the force provided by the spring  54 . The pawl stop  90   d  may have a curved back  160  and a semicircular body  165 , where the curved back  160  is aligned with curvature of the axial flange portion  72  of the clutch carrier  70 . Rotation of the pawl  48  away from the ratchet  46  is minimized as the pawl stop  90   d  is contacted by the rotating pawl  48 . 
         [0029]    Although the invention has been shown and described with respect to a best mode embodiment exemplary thereof, it should be understood by those skilled in the art that various modifications, changes, omissions and additions in the form and detail thereof may be made without departing from the spirit and scope of the invention. For example, although bias spring  54  of  FIGS. 2-5  is depicted as a leaf spring, the pawl stops  90   a - 90   d  may be used in conjunction with pawl  48  and any spring means suitably configured and disposed for biasing the toe portion  47  of the pawl  48  radially inwardly toward the ratchet member  46 . Additionally, pawl stop backs  125 ,  150 , and  160  can be shaped to align with surfaces to which they are coupled. 
         [0030]    Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments. 
         [0031]    The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.