Aircraft starter motor assembly

An aircraft starter motor assembly for starting an aircraft engine is provided, and includes a DC motor, a planetary gearset, a ring gear clutch, and a starter adapter. The sun gear from the gearset is mounted to the motor output shaft. The ring gear clutch is used to rotationally fix the ring gear or to permit the ring gear to freewheel. The starter adapter including a worm, a spring, a worm gear, a wrap spring clutch and an output shaft. The worm drives the worm gear, which drives the output shaft via the wrap spring clutch. By using the ring gear clutch to let the ring gear freewheel, the wrap spring clutch can more easily unwind and disengage from the output shaft after the engine has been started.

FIELD

The disclosure generally relates to the art of aircraft starter motor assemblies.

BACKGROUND

In some propeller driven aircraft, a starter motor assembly is used to turn over the piston engine that drives the propeller. The starter motor assembly is generally constructed to provide a certain torque and to operate at a certain RPM in order to successfully start the engine. However, it has been found for many years that certain starter motor assemblies have suffered from a relatively short operating life. This has resulted in relatively high maintenance costs for such aircraft, unforeseen periods of inoperability due to an unexpected failure of the starter motor and drive, and the potential to damage other parts of the aircraft in certain situations. It would be desirable to at least partially resolve these problems.

SUMMARY

In one aspect an aircraft starter motor assembly for starting an aircraft engine is provided. The assembly includes a DC electric motor, a planetary gearset, a ring gear clutch, and a starter adapter. The DC electric motor has a rotor and a stator. A brush electrically connects a power source to the rotor. The rotor drives a motor output shaft. The planetary gearset includes a ring gear, a planet carrier that holds a plurality of planet gears and a sun gear. The sun gear is mounted to the motor output shaft. The ring gear clutch is controllable to be in an engagement state to fix the angular position of the ring gear and a disengagement state to permit the ring gear to freewheel. The starter adapter including a worm, a worm biasing member, a worm gear, a wrap spring clutch and an adapter output shaft. The worm is driven by the planet carrier and drives the worm gear. The worm is movable linearly between a home position and a torque transfer position and is biased towards the home position by the worm biasing member. Torque transfer to the worm gear caused by rotation of the worm in a first worm direction drives the worm to the torque transfer position against a biasing force of the worm biasing member. The worm gear is drivable in a first rotational direction to bring the wrap spring clutch to a driving state in which the wrap spring clutch is operatively engaged the wrap spring clutch with the adapter output shaft. The adapter output shaft is operatively connected to the aircraft engine and is rotatable by the worm gear for cranking the aircraft engine. The aircraft starter motor assembly is operable in a first mode wherein the ring gear clutch is in the engagement state and the electric motor drives the rotation of the planet carrier. The planet carrier rotates the worm in the first worm direction to transfer torque from the motor to the worm gear, so as to drive the worm gear in the first rotational direction to bring the wrap spring clutch to the driving state so as to cause rotation of the adapter output shaft, so as to crank the aircraft engine. Bringing the ring gear clutch to the disengagement state disengages the motor from the planet carrier, which in turn permits the worm biasing member to drive the worm towards the home position, which in turn drives rotation of the worm gear in a second direction that is opposite the first direction, which in turn brings the wrap spring clutch to a non-driving state in which the wrap spring clutch is at least partially disengaged from the adapter output shaft.

DETAILED DESCRIPTION

FIG. 1is a perspective view of an aircraft starter motor assembly10that can be used to start an aircraft engine shown at12. The engine12may be a multicylinder (e.g. six-cylinder) piston engine that is used to drive a propeller, shown at14. Referring toFIG. 2, the starter motor assembly10includes an electric motor16, a planetary gearset18, a ring gear clutch20and a starter adapter22. The electric motor16has a rotor24and a stator26. The electric motor16may be called upon to transmit a high amount of torque and may be a permanent magnet, brushed DC motor. In an embodiment, the motor16makes about 3.3 hp at 24VDC and 2.7 hp at 12VDC. A set of brushes28electrically connects a power source (e.g. an on-board battery30) to the rotor24. During operation of the motor16, the rotor24drives a motor output shaft32.

The planetary gearset18includes a ring gear34, a planet carrier36that holds a plurality of planet gears38and a sun gear40. The sun gear40is mounted to and driven by the motor output shaft32. The planet carrier36may end in a gearset output shaft41with a drive feature42thereon.

The ring gear clutch20is controllable via a controller43to be in an engagement state (FIG. 2) to fix the angular position of the ring gear34and a disengagement state (FIG. 3) to permit the ring gear34to freewheel. The planetary gearset18has an associated longitudinal axis A.

The ring gear clutch20may include, for example, an electromagnetic coil assembly44including an electromagnetic coil46and an electromagnetic coil housing48, a first friction plate50that has thereon a first friction surface52, and a second friction plate54that has thereon a second friction surface56. The first friction plate50is rotationally fixed, and may be stationary both axially and rotationally. In the embodiment shown the first friction plate50forms part of the electromagnetic coil housing48. The second friction plate54may be movable (e.g. slidable) axially but may be rotationally connected to the ring gear34through a friction plate biasing member58, fasteners57aand57band a support member112as described further below.

Energization of the electromagnetic coil46(by the controller43) causes axial movement of at least one of the first and second friction surfaces52and56into engagement with the other of the first and second friction surfaces52and56so as to rotationally fix the ring gear34. In the particular embodiment shown, wherein the first friction plate50is stationary axially and rotationally, energization of the electromagnetic coil46causes axial movement of the second friction surface56into engagement with the first friction surface52so as to rotationally fix the ring gear34.

In some embodiments, energization of the electromagnetic coil46brings the ring gear clutch20to the engagement state, and deenergization of the electromagnetic coil46brings the ring gear clutch20to the disengagement state. A friction plate biasing member shown at58inFIGS. 2, 5 and 6may optionally be provided to urge the second friction plate54out of engagement with the first friction plate50. The friction plate biasing member58is connected to the support member112at first selected points (e.g. three points 120 degrees apart (FIG. 6) at a selected radius from the axis A (FIG. 2)) by first fasteners, shown at57a. Additionally, the friction plate biasing member58is connected to the second friction plate50at second selected points (e.g. 120 degrees apart (FIG. 6)) by second fasteners57b. The second selected points may be 120 degrees apart at the same radius as the first selected points such that the second fasteners57balternate with the first fasteners57a. The fasteners57aand57bcould be any suitable type of fasteners, such as rivets, screws, or any combination thereof. The friction plate biasing member58biases the friction plate54away from the friction plate50(i.e. out of engagement with the friction plate50), so that deenergization of the electromagnetic coil46brings the ring gear clutch20to the disengagement state. By way of the friction plate biasing member58, the fasteners57aand57band the support member112, the friction plate54is connected rotationally to the ring gear34.

The friction plate54and the friction plate biasing member58are shown inFIG. 6. The friction plate biasing member58is shown with exaggerated warpage to illustrate the flexing it goes through when the friction plate54has pulled away from the support member112and is engaged with friction plate50. As can be seen, the three fasteners57a(which hold the friction plate biasing member58to the support member112, not shown inFIG. 6) alternate with the three fasteners57b(which hold the friction plate biasing member58to the friction plate54).

As a result of the illustrated structure, no energy is required to hold the ring gear clutch20in the disengagement state, which is the state it will be in all conditions except when the starter motor assembly10is being used to start the engine12. Thus, there is little or no lost energy associated with the ring gear clutch20when the clutch20is not in use. In some embodiments the friction plate54is spaced from the friction plate50by about 0.008 to about 0.010 inches. In some embodiments it has been found that about 1.4 Amps are needed to bring the plates50and54together. The close spacing of the friction plates50and54is in part responsible for the low amperage needed to bring them together and to hold them together. The close spacing is possible in part because the biasing member58is positioned on the side shown at59of the friction plate54facing away from the friction plate50instead of being positioned between the friction plates50and54.

It will be noted that the friction force that holds the ring gear34against rotation is dependent on the normal force (i.e. the force of engagement between the surfaces52and56), the friction coefficient between the surfaces52and56and the area over which they are engaged. By selecting materials and surface finishes appropriately, a relatively low normal force will be needed in order to satisfactorily hold the ring gear34. Thus, a relatively low current and power draw may be associated with this arrangement. It has been found that, in use, as little as 8 pounds of force have been needed to hold the ring gear34stationary.

In an alternative embodiment the second plate54may simply be left to float axially when the electromagnetic coil46is not energized, so that it will find an equilibrium position that is just slightly spaced from the first plate50on its own without the need for a biasing member.

The gearset18may provide any suitable amount of gear reduction. For example, it may provide a gear reduction of about 3.8:1 (e.g. 3.785:1). Any other suitable gear reduction may be used. It will be noted that the force needed to hold the ring gear34is relatively low in a planetary gearset, which is advantageous in the assembly10since a lower required force reduces even further the normal force (and therefore the current and power draw) needed to hold the ring gear34.

Referring toFIGS. 2 and 5, the starter adapter22includes a worm60, a worm biasing member62(e.g. a compression spring), a worm gear64, a wrap spring clutch66and an adapter output shaft68with a final gear70thereon. The worm60is mounted on a worm shaft72that has an input feature74that mates with the drive feature42on the gearset output shaft41. The worm60is fixed rotationally with the worm shaft72, but is slidable axially (linearly) along a worm axis Aw on the worm shaft72. The worm60is biased by the worm biasing member62between a home position (FIG. 3) and a torque transfer position (FIG. 2). When the worm60is driven rotationally by the planet carrier36, the worm60applies a force to cause rotation of the worm gear64. Resistance to rotation by the engine12(FIG. 1) may be sufficiently high such that rotation of the worm60does not drive rotation of the worm gear64and instead drives movement of the worm60linearly away from the home position (to the right in the view shown inFIG. 2), thereby flexing (i.e. compressing) the worm biasing member62. As the worm60moves further from the home position (and towards the torque transfer position) the resistive force of the worm biasing member62increases. At some point, either the resistive force of the worm biasing member62matches the force urging the worm60to move away from the home position, or the worm60reaches the end of its travel. Regardless of whichever it is, once this occurs, the worm60begins to drive rotation of the worm gear64. At this point the worm60may be said to be in the torque transfer position. Put another way, during operation of the assembly10to start the engine12, torque transfer to the worm gear64caused by rotation of the worm60in a first rotational direction drives the worm60to the torque transfer position against a biasing force of the worm biasing member62.

Referring toFIG. 4, the worm60may be axially slidable on the worm shaft72by way of engagement between a Woodruff key76that mounts in a shaft slot78in the worm shaft72and that engages a worm slot80in a radially inner surface82of the worm60.

The worm60in the embodiment shown may have any suitable number of starts. For example, it may be a three-start worm. In some embodiments it may instead be a two-start worm. The worm60may be backdrivable in some embodiments. In other embodiments it may be substantially not back drivable.

The wrap spring clutch66has a first end83, a second end84, a radially inner surface85and a radially outer surface86. At the first end83, the wrap spring clutch66includes a hook feature87(FIG. 4) thereon that connects the first end83to the worm gear64. Referring toFIG. 2, a first portion88of the wrap spring66surrounds a worm gear drum90which holds the worm gear64, and a second portion91surrounds the adapter output shaft68, and more particularly a wrap spring engagement surface92on the adapter output shaft68.

The wrap spring clutch66is positionable in a driving state (FIG. 2) in which it has coiled inwardly to grip the worm gear drum90and the wrap spring engagement surface92so that torque transfer takes place from the worm gear64to the adapter output shaft68. The wrap spring clutch66is also positionable in a non-driving state (FIG. 3) in which it has coiled outwardly so that it is at least partially disengaged from the adapter output shaft68sufficiently to permit relative movement between the worm gear64and the adapter output shaft68without generating excessive heat buildup. It is preferable, however, for the wrap spring clutch66to be fully disengaged from the output shaft68, so that there is substantially free movement between the worm gear64and the adapter output shaft68.

A friction ring98on a housing100(FIG. 4) for the starter adapter22is positioned to engage the radially outer surface86of the wrap spring clutch66proximate the second end86and to hold the second end86substantially rotationally stationary when the wrap spring clutch66is in the non-driving state.

Holding of the second end86of the wrap spring clutch66by the friction ring98permits the worm gear64to be drivable in a first rotational direction (shown at D inFIG. 4) to cause inward coiling of the wrap spring clutch66so that its inner surface85grips the worm gear drum90and the wrap spring engagement surface92on the adapter output shaft68, thereby bringing the wrap spring clutch66to the driving state. At this point the wrap spring clutch66is operatively engaged with the adapter output shaft68. Continued rotation of the worm gear64transfers torque from the worm gear64to the adapter output shaft68.

The wrap spring clutch66may be biased towards the non-driving state. This permits the wrap spring clutch66to return to the non-driving state upon a release of the torque that drives the worm gear64.

The wrap spring engagement surface92may have a plurality of axially extending grooves102thereon that serve to remove debris that may accumulate between the radially inner surface85of the wrap spring clutch66and the wrap spring engagement surface92. Additionally, the edges of the grooves94assist in cleaning debris from the inner surface96of the wrap spring clutch66. Additionally, the grooves94assist in gripping the wrap spring clutch66during operation of the starter motor assembly10in the first mode.

The adapter output shaft68is operatively connected to the aircraft engine, e.g. via engagement of the final gear70on the adapter output shaft68with an input gear104that is connected to the crankshaft (not shown) on the engine12. By way of the wrap spring clutch66, the adapter output shaft68is rotatable by the worm gear64for cranking the aircraft engine12in order to start the aircraft engine.

To start the aircraft engine12, the aircraft starter motor assembly10is operable in a first mode (shown inFIG. 2) wherein the ring gear clutch20is brought to the engagement state and the electric motor16is energized and drives the rotation of the planet carrier36via the sun gear40. The planet carrier36in turn rotates the worm60in a first worm direction Dw (FIG. 4) to transfer torque from the motor16to the worm gear64so as to drive the worm gear64in the first rotational direction D to bring the wrap spring clutch66to the driving state. This in turn causes rotation of the adapter output shaft68, so as to crank the aircraft engine12.

Once the engine12has started, power is cut to the motor16and the ring gear clutch20is brought to the disengagement state, which disengages the motor16from the planet carrier36. This, in turn, releases any torque from the motor16on the worm60, which permits the worm biasing member62to drive the worm60towards the home position. This movement of the worm60towards the home position drives rotation of the worm gear64in a second direction that is opposite the first direction D. This in turn brings the wrap spring clutch66to the non-driving state.

To assist the components that make up the adapter22in operating and releasing when desired, a flow of lubricant (i.e. oil) may be provided into and through the adapter housing100. Accordingly, the adapter housing100may be sealed as necessary to prevent leakage of oil therefrom.

Referring toFIG. 2, bushings and bearings generally shown at108may be provided between the motor output shaft32and the gearset output shaft41, between the gearset output shaft41and a stationary support110for the electromagnetic coil assembly48, and between the stationary support110and the support member112that holds the ring gear34and the second friction plate54.

The spacings between components shown inFIGS. 2 and 3may be exaggerated for the purpose of clarity. As will be understood these drawings are not to scale.

By providing the ring gear clutch20, it has been found that the amount of resistance in the starter motor assembly10to unwinding of the wrap spring clutch66is relatively small. By contrast, it has been found that, in a prior art configuration that does not include the ring gear clutch20, significant resistance was encountered to the unwinding of the associated wrap spring clutch. In some instances this resistive torque can be as high as 15 inch pounds or more at the motor output shaft. When this resistive torque is combined with the gear reduction that takes place with a planetary gearset, the actual resistance to turning of the worm gear in a second direction can be significant. In such cases even if the wrap spring clutch is made to disengage from the adapter output shaft, it does so after significant stresses are incurred, thereby reducing the effective life of the motor starter assembly. In some instances the wrap spring clutch has been found not to disengage completely, which results in significant heat buildup in the adapter. Thus, the ring gear clutch20has been found to greatly reduce stresses and heat buildup in the adapter22and to extend the effective life of the motor starter assembly10as compared to some prior art assemblies. Furthermore, these advantages are provided while consuming very little power. It is also noted that, depending on the position of the pistons in the aircraft's engine when the engine is stopped, there may be some back-rotation of the engine (due to residual pressure in some of the cylinders). In such cases, a prior art starter motor assembly can incur significant stresses if the wrap spring clutch has not properly disengaged with the adapter output shaft.

It has been found that the problem with prior art starter motor assemblies has existed for many years without an effective solution, and has resulted in many premature failure of components such as the wrap spring clutch and the worm gear.

While the above describes one or more particular embodiments, it will be appreciated that modifications and variations may be made to the embodiments described herein without departing from the proper scope of the claims appended hereto.