Patent Description:
In a typical rotary wing aircraft, such as a helicopter for example, a tail rotor system converts tail driveshaft rotary power into the aerodynamic forces necessary to control the direction of flight and to counteract main rotor torque. However, component failures can cause additional torque to be introduced into the helicopter's mechanical system. The added torque can damage system components at unpredictable points in the system.

<CIT> shows a tie-back device restrains full extension of an evacuation slide until a predetermined inflation is attained. Inflation of the slide causes tension to be applied to the device. The device comprises two attachment rings and two components which are coupled by a shear pin. A transparent cover is permanently attached to one component. This cover makes the device tamperproof and nonreusable. The cover, in conjunction with the attachment rings and the two components, allows tensile forces to be applied to the shear pin from only one direction.

<CIT> shows an attachment assembly for coupling a rotary blade to a hub includes a hollow shear pin, a nut cap, and a tensioning fastener. The hollow shear pin defines an attachment axis. The nut cap abuts the shear pin along the attachment axis. The tensioning fastener is seated within the shear pin, is threadably engaged to the nut cap, and has an axial length that is smaller than an axial length of the shear pin to fix a rotary blade to a rotor assembly within a profile of the rotary blade.

The invention relates to a clevis assembly for a tail rotor system of a rotary wing aircraft and to a rotary wing aircraft according to the appended claims.

In addition to one or more of the features described above or below, a proximal end of the shearing device extends outward from an aperture of an end of the two shackle ends and a distal end of the shearing device is arranged within an aperture of an outer raceway of the bearing.

In addition to one or more of the features described above or below, or as an alternative, the clevis assembly further includes a threaded insert arranged between an inner surface of the structure and an outer surface of the shearing device.

In addition to one or more of the features described above or below, or as an alternative, the piston comprises a plurality of splines such that coupling the piston with the bearing creates a spline joint.

In addition to one or more of the features described above or below, or as an alternative, the shearing device is a cylindrical rod, and wherein the frangible point is a portion of the cylindrical rod having a diameter smaller than another portion of the cylindrical rod.

In addition to one or more of the features described above or below, or as an alternative, the frangible point is a portion of the shearing device comprised of a material having a lesser tensile strength than another portion of the shearing device.

In addition to one or more of the features described above or below, or as an alternative, the frangible point is a hollow portion of the otherwise solid shearing device.

In addition to one or more of the features described above or below, or as an alternative, the frangible point is a portion of the shearing device having divots along a surface of the portion.

In addition to one or more of the features described above or below, or as an alternative, the shearing device is attached to the clevis by a fastener.

In addition to one or more of the features described above or below, or as an alternative, the bearing is a journal bearing.

In addition to one or more of the features described above or below, the shearing device is operable to allow the piston to spin with a tail rotor of the rotary wing aircraft.

In addition to one or more of the features described above or below, or as an alternative, the piston is in operable communication with a pitch change shaft of the tail rotor system.

In addition to one or more of the features described above or below, or as an alternative, the frangible point is configured to break upon application of a pressure greater than a threshold amount.

In addition to one or more of the features described above or below, or as an alternative, the pressure is received by the frangible point from an outer raceway of the bearing.

A detailed description of one or more embodiments of the disclosed apparatuses are presented herein by way of exemplification and not limitation with reference to the Figures.

With reference now to <FIG>, an example of a vertical takeoff and landing (VTOL) aircraft is schematically illustrated. The aircraft <NUM> in the disclosed, non-limiting embodiment includes a main rotor assembly <NUM> supported by an airframe <NUM> having an extending tail <NUM> which mounts an anti-torque system/tail rotor (TR) system <NUM>. The main rotor assembly <NUM> is driven about an axis of rotation A through a main rotor gearbox (MGB) <NUM> by one or more engines <NUM>. The engines <NUM> generate the power available for flight operations and couple such power to the main rotor assembly <NUM> and the TR system <NUM> through the MGB <NUM>. The main rotor assembly <NUM> includes a multiple of rotor blades <NUM> mounted to a rotor hub <NUM>. Although a particular helicopter configuration is illustrated and described in the disclosed embodiment, other configurations and/or machines, such as high speed compound rotary-wing aircraft with supplemental translational thrust systems, dual contra-rotating, coaxial rotor system aircraft, turbo-props, tilt-rotors tilt-wing aircraft with a critical bearing of the configuration described herein will also benefit here from.

With reference to <FIG>, <FIG>, and <FIG>, an aircraft TR system <NUM> that converts tail driveshaft rotary power into aerodynamic forces necessary to control the direction of flight of the aircraft <NUM> and to counteract the main rotor torque. <FIG> and <FIG> are a cross-sectional view of the clevis assembly across line BB of <FIG> <FIG> illustrates the clevis assembly <NUM> prior to any shearing of a mechanical fuse <NUM>. <FIG> illustrates the clevis assembly <NUM> with a sheared mechanical fuse <NUM>. The TR system <NUM> provides a mounting point for tail rotor blades (TRBs) and for a blade pitch change mechanism. The pitch of the TRBs is controlled by the position of the tail rotor pitch change shaft (PCS) <NUM>. The positioning of the PCS <NUM> is controlled by a pitch change servo (hereinafter referred to as a "translating element <NUM>"). When the translating element <NUM> pulls the PCS <NUM> inboard, a pitch walking beam (not shown) and pitch change control links (not shown) associated with each of the TRBs twist the TRBs about internal elastomeric bearings (not shown) to increase blade pitch. Conversely, when the translating element <NUM> permits the PCS <NUM> to move outboard, the pitch walking beam and pitch change control links associated with each of the TRBs twist the TRBs about the internal elastomeric bearings to decrease blade pitch. An increase in blade pitch will turn the aircraft <NUM> to the left and a decrease in pitch will turn the aircraft <NUM> to the right.

The PCS <NUM> rotates with and moves linearly within a rotating tail rotor shaft (hereinafter referred to as a "rotating element <NUM>"). A PCS bearing structure <NUM> supports the PCS <NUM> within the rotating element <NUM> and allows the PCS <NUM> and the rotating element <NUM> to rotate independently of the non-rotating translating element <NUM>. As will be described below, an outer raceway of the PCS bearing structure <NUM> rotates with the PCS <NUM> and the rotating element <NUM>, while an inner raceway is non-rotating and coupled to the translating element <NUM> by way of a translating element's clevis <NUM> and thereby moves linearly with the translating element <NUM>.

The translating element <NUM> is connected to a tail rotor of the TR system <NUM> through a PCS bearing structure <NUM> that allows the tail rotor to spin while the translating element <NUM> translates only and does not spin. The translating element <NUM> has an anti-rotation tab <NUM> and an anti-rotation plate <NUM> to react to normal drag loads from the PCS bearing structure <NUM>. The anti-rotation tab <NUM> and the anti-rotation plate <NUM> (collectively known as the "anti-rotation feature") hinder binding in the translating element's mechanical input controls, which are connected to the translating element <NUM> output (not shown).

One or more embodiments of the present invention include a fail-safe feature (shearing device <NUM>) that disconnects only the translating element <NUM> output from the anti-rotation feature while maintaining the connection to the mechanical input controls. Under certain conditions, high torque levels are transmitted by the TR system <NUM> leading to the shearing of components in the translating element <NUM>. For example, a failure of the tail rotor pitch shaft bearing can cause an increase in the torque transmitted by the TR system <NUM>.

In the event of the PCS bearing structure <NUM> seizing, high torque is transmitted to the translating element <NUM> leading to the failure of the piston <NUM> is restrained from rotation as in a conventional system. The shearing device <NUM> shears under high torque and allows the piston <NUM> to spin with the tail rotor drive as opposed to reacting to the tail rotor torque as in a conventional system. This protects the structural integrity of the translating element <NUM> and allows for continued although limited operation with a temporarily spinning translating element <NUM> output. According to one or more embodiments of the present invention, the shearing device <NUM> includes, but is not limited to a resettable mechanical fuse, a one-time shear device, pin, a cotter pin, a breakaway tab, shear tab, or some other mechanism that allows the translating element <NUM> output to spin.

In one or more embodiments of the present invention, a mechanical fuse <NUM> is introduced to prevent damage to the translating element <NUM> caused by the additional torque, and allow a control input to continue to the PCS <NUM>. The mechanical fuse <NUM> allows a piston <NUM> to spin along with the tail rotor under high torque conditions while preventing damage to input/feedback linkages thereby permitting control for a limited time, for example to effectuate a safe landing.

With reference to <FIG>, the mechanical fuse <NUM> is incorporated with the clevis assembly <NUM>, which includes a shackle portion <NUM> with two ends. Each end includes at least one respective aperture <NUM>, <NUM> for receiving at least one shearing device <NUM>. It should be appreciated that although the shearing device <NUM> depicted in <FIG> and <FIG> is a pin, the shearing device <NUM> includes other structures operable to shear due to high torque transmitted by the TR system <NUM>. For illustration purposes, in <FIG> and <FIG> the shearing device <NUM> is a pin, however, it should be appreciated that the shearing device <NUM> includes other structures operable to shear in response to a high-torque transmitted by the TR system <NUM>. The apertures <NUM>, <NUM> are connected by a structure <NUM> having a hollow portion and extending from a first aperture <NUM> at an end of the clevis assembly <NUM> to a second aperture <NUM> at the other end of the clevis assembly <NUM>. In some embodiments, the structure <NUM> is cylindrical. The structure <NUM> includes an anti-rotation tab <NUM> and an anti-rotation plate <NUM> for countering torque generated by the main rotor. The structure <NUM> houses a bearing <NUM> with an inner raceway in operable communication with a TR system piston <NUM>. For example, the bearing <NUM> includes a journal bearing, an oil-impregnated bushing, or other bearing-type structure. The piston <NUM> includes splines, such that when inserted into the bearing <NUM> a spline joint is created. The TR system piston <NUM> is in operable communication with the PCS <NUM>. In some embodiments, the bearing <NUM> is housed in a central part of the structure <NUM> and substantially equidistant from the ends of the clevis assembly <NUM>. The outer raceway of the bearing <NUM> includes an opening for receiving the at least one shearing device <NUM>.

The at least one shearing device <NUM> is received by the aperture <NUM> of the clevis assembly <NUM> and the hollow portion of the structure <NUM>. A proximal end of the pin is arranged at the aperture <NUM> of the clevis assembly <NUM>. A portion of the at least one shearing device <NUM> extends beyond an outer surface of the clevis assembly <NUM> and is fastened to the clevis assembly <NUM> by a fastener <NUM>. A threaded insert <NUM> is arranged between the structure <NUM> and the at least one shearing device <NUM>. The threaded insert <NUM> further secures the at least one shearing device <NUM> to the structure <NUM>. A distal end of the shearing device <NUM> includes a mechanical fuse <NUM>. The mechanical fuse <NUM> has a frangible point <NUM>, such that when the outer raceway of the bearing <NUM> applies a force greater than a threshold amount (high torque), the at least one shearing device <NUM> breaks through a shear force at the frangible point <NUM>. In some embodiments, the at least one shearing device <NUM> is a cylindrical rod and the frangible point <NUM> is a portion of the rod having a diameter less than the diameter of the other portion of the rod. In other embodiments, the frangible point <NUM> is comprised of a material having a tensile strength less than a tensile strength of the other portion of the at least one shearing device <NUM>. In yet other embodiments, the frangible point <NUM> includes pre-made divots on an outer surface of the at least one shearing device <NUM>. In yet even other embodiments, the frangible point <NUM> is hollow, whereas the other portion of the at least one shearing device <NUM> is solid.

<FIG> is provided to illustrate the shearing of the mechanical fuse <NUM>. The inner raceway of the bearing <NUM> is in operable communication with a piston <NUM> via splines. The piston <NUM> is in operable communication with the PCS <NUM>. Under typical conditions, the piston <NUM> does not rotate. However, if the PCS bearing structure malfunctions, the resulting torque causes the piston <NUM> to rotate, which in turn causes the bearing <NUM> to rotate. The force applied by the outer raceway of the bearing <NUM> causes the at least one shearing device <NUM> to shear at the frangible point <NUM>. As seen in <FIG>, the shearing device <NUM> has sheared at the frangible point <NUM>, and the mechanical fuse <NUM> has dislodged from the shearing device <NUM> and is disposed within a compartment <NUM> in the piston <NUM>. The piston <NUM> is spinning due to the additional torque, however the shearing device <NUM> remains secure. Furthermore, the shearing caused by the additional torque is directed to the frangible point <NUM>, which limits the unpredictability of break points in a system due to additional torque.

In some embodiments, the clevis assembly <NUM> includes a second pin <NUM> that extends beyond an outer surface of the clevis assembly <NUM> and is fastened to the clevis assembly <NUM> by a fastener <NUM>. A threaded insert <NUM> is arranged between the structure <NUM> and the second pin <NUM>. The threaded insert <NUM> further secures the second pin <NUM> to the structure <NUM>. The second pin <NUM> increases the structural integrity of the clevis assembly <NUM>.

The mechanical fuse <NUM> localizes breaks caused by higher torques to a predetermined area of the at least one shearing device <NUM>. In this sense, the variability of break points is reduced, and breaks caused by higher torques can be more readily managed.

Claim 1:
A clevis assembly (<NUM>) for a tail rotor system of a rotary wing aircraft comprising:
a shackle (<NUM>) having two ends, each end respectively comprising an aperture (<NUM>, <NUM>);
a structure (<NUM>) connecting the apertures (<NUM>, <NUM>) and housing a bearing (<NUM>);
a shearing device (<NUM>) comprising a frangible point (<NUM>) and in operable communication with the bearing (<NUM>), wherein the shearing device (<NUM>) is housed in a hollow portion of the structure (<NUM>), and
a piston (<NUM>) in operable communication with the shearing device (<NUM>) and restrained from spinning, wherein an inner raceway of the bearing (<NUM>) is in operable communication with the piston (<NUM>);
wherein the shearing device (<NUM>) is operable to shear under a pressure greater than a threshold amount and allow the piston (<NUM>) to spin.