Pawl stop for use in ratchet and pawl style clutch

A pawl and ratchet assembly including a rotatable pawl, a spring, a ratchet, and a member. The rotatable pawl is configured to rotate about a fixed axis during operation. The spring urges the pawl towards the ratchet. The member interferes with the travel of the pawl during operation. The member is non-magnetized.

BACKGROUND OF THE INVENTION

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.

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 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.

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.

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.

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.

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

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.

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.

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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now toFIG. 1, there is depicted a prior art pneumatic starter10of the general type often utilized to start gas turbine engines, such as for example aircraft turbine engines (not shown). The pneumatic starter10, 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.

Extending axially outwardly from the transmission (not shown) is an output shaft60which has means, such as for example splines62, for engaging an engine shaft (not shown) in the gear box of the turbine engine (not shown) on which the starter10is utilized to start the turbine engine. The output shaft60is rotatably supported on bearing means66. The aforementioned pawls48, of which there are typically three, are supported on a clutch carrier70of the pneumatic starter10that extends radially outward from the output shaft60. Each of the pawls48is pivotally supported on a shaft52mounted to and extending axially from the clutch carrier70such that the pawls48are disposed in cooperation with the ratchet member46at equally spaced intervals about the circumference of and in radially spaced relationship from the ratchet member46.

Additionally, each of the pawls48is biased to pivot about its support shaft52to rotate the toe end47thereof radially inwardly towards the ratchet member46under the force of a bias spring54mounted to an axial flange portion72of the clutch carrier70. Each bias spring54may comprise a leaf spring operatively bearing against the heel end49of its associated pawl48so as to, in a manner well known in the art, function during operation of the starter10to load the toe end47of its associated pawl48into engagement with the teeth45of the ratchet member46of the pawl and ratchet clutch to ensure transmission of torque from a ring gear (not shown) to the output shaft60until the output shaft60has reached a relatively high desired cut-off speed, for example about 5000 rpm, at which the pawls48pivot away from the ratchet member46under the influence of centrifugal force after disengagement from the teeth45of the ratchet member46upon slowing down of the ratchet member46after termination of the flow of compressed air through the starter turbine (not shown).

When the pawls48are engaged with the teeth45of the ratchet46during operation of the starter10, the output shaft60, and the engine shaft of the turbine engine (not shown) connected therewith, are driven in rotation through the engaged pawl48and ratchet member46.

After the shaft of the turbine engine (not shown) engages with the output shaft60of the starter10reaches 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 member46. Consequently, the ratchet member46slows down and its rotational speed rapidly decreases, while the output shaft60of the starter10and the pawls48mounted thereto continue to rotate at the higher engine speed, thereby causing the pawls48to become disengaged from the ratchet member46and pivot away from the ratchet member46. Centrifugal force acting on the pawls48overcomes the engagement force provided by spring54to cause the pawls48to rotate away from and disengage the ratchet member46. 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.

When the turbine engine is later shut down, the starter output shaft60begins to slow down and its rotational speed steadily decreases as the turbine engine spools down. As the output shaft60coasts down, the centrifugal force acting on the pawls48so as to urge the toe ends47of the pawls48radially outwardly steadily decreases and the toe end47of each pawl48begins to pivot radially inwardly toward the ratchet member46under the bias force applied by the springs54on the heel49of the pawls48until the toe ends47of the pawls48re-engage with the teeth45of the ratchet member46.

A spring clamp75is attached by bolts80to the axial flange portion72of the clutch carrier70to hold the spring54.

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 tip85and pawl heel49during over-running conditions. During over-running conditions centrifugal force rotates the pawl48radially 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 heel49and the spring tip85.

Referring now toFIGS. 2-5, a pawl stop member90a-dmay be included in the ratchet and pawl clutch assembly and can be located on several components, including to but not limited to the spring clamp75(FIG. 2), the bias spring54(FIG. 3), the pawl48(FIG. 4), or the axial flange portion72(FIG. 5). The pawl stop90a-dlimits the radial rotation of the pawl48about the shaft52to decrease the spring deflection to minimize damage to the spring54and to the pawl heel49. The pawl stop90a-dmay 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 pawl48as the clutch carrier70rotates. The pawl stops90a-d, if metal, are attached to their respective places by welding or gluing or the like, and, if elastomeric, by gluing or the like.

Referring now toFIG. 2, pawl stop90ais added to spring clamp75to minimize the force provided by the spring54. The pawl stop90amay have a flat back95, a notch100for receiving an angled rear portion105of the spring clamp75, an inner portion110conforming to and in register with an inner surface115of the spring clamp75, and an angled portion120angling away from said inner portion110for contacting the pawl48. Rotation of the pawl48away from the ratchet46is minimized as the pawl stop90ais contacted by the pawl48.

Referring now toFIG. 3, pawl stop90bis added to the spring54to minimize the force provided by the spring54. The pawl stop90bmay have a flat back125and a semicircular body130. The pawl stop90bengages an extension135attaching to a rear end140of the spring clamp75. Rotation of the pawl48away from the ratchet46causes the spring54to react towards the spring clamp75. Motion of the spring54is limited by contact of the pawl stop90bwith the extension135. Alternatively, the pawl stop90bmay attach to the extension135as opposed to the spring54to achieve a similar result.

Referring now toFIG. 4, pawl stop90cis added to the pawl48on a back surface145thereof to reduce the force provided by the spring54. The pawl stop90cmay have a relatively flat back150and a semicircular body155, where the relatively flat back150is configured to interface with an upper surface of the pawl48. Rotation of the pawl48away from the ratchet46is minimized as the pawl stop90ccontacts the axial flange portion72of the clutch carrier70which is in registration with the pawl48to allow contact therebetween.

Referring now toFIG. 5, pawl stop90dis added to the axial flange portion72of the clutch carrier70to minimize the force provided by the spring54. The pawl stop90dmay have a curved back160and a semicircular body165, where the curved back160is aligned with curvature of the axial flange portion72of the clutch carrier70. Rotation of the pawl48away from the ratchet46is minimized as the pawl stop90dis contacted by the rotating pawl48.

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 spring54ofFIGS. 2-5is depicted as a leaf spring, the pawl stops90a-90dmay be used in conjunction with pawl48and any spring means suitably configured and disposed for biasing the toe portion47of the pawl48radially inwardly toward the ratchet member46. Additionally, pawl stop backs125,150, and160can be shaped to align with surfaces to which they are coupled.