Abstract:
A pawl and ratchet clutch assembly for use in transmitting rotational drive torque has a rotatable pawl; a spring for urging the pawl towards the ratchet; and, a member rotatably mounted between the pawl and the spring for rotating along the spring as the pawl moves relative to the spring.

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 roller for minimizing wear of the pawl. The pawl and ratchet 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 that is powered by extracting energy from a flow of pressurized fluid passed through a starter turbine. 
         [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 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. 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 if the rotational speed of the output shaft exceeds a threshold speed whereat the pawls lift-off of the ratchet member, 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 commonly referred to as the clutch hysteresis. 
       SUMMARY OF THE INVENTION 
       [0007]    According to an embodiment shown herein a pawl and ratchet clutch assembly for use in transmitting rotational drive torque has a rotatable pawl; a spring for urging the pawl towards the ratchet; and, a member rotatably mounted between the pawl and the spring for rotating along the spring as the pawl moves relative to the spring. 
         [0008]    According to a further embodiment shown herein, a pawl for engaging a ratchet has a body having an axis for rotation thereabout, a spring for urging the body about the axis towards the ratchet; and, a member rotatably mounted upon the body for rotating along the spring as the pawl moves relative to the spring. 
         [0009]    According to a still further embodiment shown herein, a pawl and ratchet clutch assembly for use in transmitting rotational drive torque to start a jet engine has a rotatable pawl; a spring for urging the pawl towards the ratchet; and, a member rotatably mounted between the pawl and the spring for rotating along the spring as the pawl moves relative to the spring. 
         [0010]    According to a still further embodiment shown herein, a method for inserting a roller pawl in a clutch of an air turbine starter includes the steps of creating an opening in a heel portion of a pawl, placing a rotating member in the opening in the heel portion for rotating therein, and attaching the pawl to a clutch carrier. 
         [0011]    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 
         [0012]      FIG. 1  is a partially-sectioned side view of a prior art starter having a pawl and ratchet assembly. 
           [0013]      FIG. 2  is a sectional side view of the pawl and ratchet assembly incorporating a rolling member. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0014]    Referring now to the prior art shown in  FIG. 1 , there is depicted therein a 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. 
         [0015]    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  (or “body”), 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  of the pneumatic starter  10  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 . 
         [0016]    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  49  of its associated pawl  48  so as to function during operation of the starter  10  to load the toe end  47  of its associated pawl  48  into engagement with the teeth of the ratchet member  46  of the pawl and ratchet clutch to ensure transmission of torque from a ring gear (not shown) in the pneumatic starter  10  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). 
         [0017]    When the pawls  48  are engaged with the teeth 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 . 
         [0018]    After the shaft of the turbine engine engaged with the output shaft  60  of the starter  10  has reached 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 force provided by spring  54  to cause the pawls  48  to rotate away from and disengage the ratchet member  46 . 
         [0019]    If 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 of the ratchet member  46 . 
         [0020]    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 . 
         [0021]    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. The source of spring fractures and pawl heel wear is the contact forces experienced between the spring end  90  and pawl heel  49  during rotation of the pawl  48 . During faster rotation, centrifugal force may rotate the pawl  48  radially outward, here in a counter-clockwise direction against the force and sliding/rubbing of the spring  54  against the pawl heel  49 . As rotation slows, the spring forces the pawl to rotate in a clockwise direction causing the spring to slide/rub against the heel  49 . Further, as rotational speed fluctuates during operation, centrifugal force changes, as does the deflection of the spring, and subsequent contact and/or sliding forces wear the parts. 
         [0022]    Referring now to  FIG. 2 , a rotating member  81 , such as a wheel, pin or ball or the like, may be attached by a pin  85  to the heel  49  of the pawl  48 . The rotating member  81  rides on an unsupported spring end  90  of the spring  54 . As the pawl  48  is influenced by centrifugal force as the assembly  10  rotates, the rotating member  81  engages and rolls along the spring end  90  as the pawl  48  rotates clockwise and counter-clockwise. The rotating member  81  rotating along the spring end  90  minimizes any friction and/or sliding forces that was caused by the relative movement on the heel of the spring end  90  and the heel  49  as the pawl  48  rotate in a clockwise and counterclockwise direction. The wheel  81  extends beyond dimensions of the heel  49  and into a discontinuity  83  along the axial flange portion  72  that also receives the pawl  48 . 
         [0023]    The rotating member  81  reduces sliding/rubbing between the spring end  90  and the heel  49  thereby minimizing wear caused by such sliding/rubbing. 
         [0024]    To assemble a pawl for use herein, a user evacuates enough material in the heel  49  of the pawl  48  and creates a hole  51  in which the shaft  52  is fitted for rotation thereabout. Similarly, a hole  87  is drilled in the heel  49  to insert the pin  85  therein about which the rotating member  81  rotates. Alternatively, the heel  49  is cast, or the like, with an opening  95  for receiving the rotating member  81 . A pawl  48  without a rotating member (see  FIG. 1 ) may be replaced with a pawl  48  with a rotating member  81  by separating the pawl without a rotating member from the shaft on inserting the pawl  81  with a rotating member therein while deflecting the spring end  90 . 
         [0025]    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 spring end  90  of  FIG. 2  is depicted as a leaf spring, the pawl  48  with rotating member  81  can be used in conjunction with any spring means suitably configured and disposed for biasing the toe portion  47  of the pawl  48  radially inwardly toward the ratchet member  46 . 
         [0026]    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. 
         [0027]    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.