Rotary kinetic fluid motors or pumps – Including destructible – fusible – or deformable non-reusable...
Reexamination Certificate
2000-02-15
2001-11-06
Look, Edward K. (Department: 3745)
Rotary kinetic fluid motors or pumps
Including destructible, fusible, or deformable non-reusable...
C415S123000, C060S039091, C188S074000
Reexamination Certificate
active
06312215
ABSTRACT:
TECHNICAL FIELD
This invention relates to turbine engine rotors and particularly to a brake that inhibits windmilling of a fan rotor.
BACKGROUND OF THE INVENTION
Turbofan engines for powering commercial and military aircraft include a rotatable fan for compressing a stream of ambient air. The fan includes a hub with a series of blades projecting radially outwardly therefrom. A rotatable drive shaft connects the hub to a turbine so that the turbine can power the fan. The fan and drive shaft are supported from a bearing support by ball and/or roller bearings.
On rare occasions, a fan blade may separate from the hub during engine operation, usually as a consequence of a foreign object having been ingested into the fan. The separation of a blade severely unbalances the rotating fan which, in turn, transmits tremendous forces into the load bearing components of the engine and aircraft. Accordingly, engine and aircraft designers size these components to be able to withstand severe imbalance forces. Such design practices contribute appreciable, undesirable weight to the engine and aircraft. The added weight is especially unwelcome since it accommodates an event that may never occur throughout the entire life of an engine.
To address the problem of adding weight to account for excessive but rare rotor imbalance, engine designers have devised variable stiffness bearing supports. One such bearing support features dual, parallel load paths that normally act together to provide a relatively stiff, rigid fan support. One of the load paths crosses a breakaway connection engineered to fail if subjected to abnormally high forces, such as the imbalance forces arising from a fan blade separation. Once the connection fails, the surviving load path acts alone to provide a relatively flexible or soft support for the fan. The soft, flexible support attenuates the amplitude of the imbalance forces transmitted to the bearing support and to other engine and aircraft structural components. As a result, those components may be made lighter without compromising their structural integrity.
Despite the merits of the variable stiffness bearing support, it is not without one potential shortcoming. After the blade separation event, the aircraft flight deck crew normally shuts off the fuel supply to the now heavily damaged engine and proceeds to the nearest landing site under the power of one or more undamaged engines. During this phase of flight, the forward motion of the aircraft drives ambient air into the damaged engine, causing the unbalanced fan to rotate or “windmill” at a low rotational frequency. If the fan rotor windmilling frequency is equal or approximately equal to its resonant frequency, the windmilling rotor can generate undesirably high amplitude vibrations. These vibrations, which can persist for the remaining duration of the flight, propagate into the aircraft and can be unsettling to the aircraft occupants.
What is needed is a means for suppressing resonant vibrations of a windmilling rotor following a fan blade separation.
SUMMARY OF THE INVENTION
It is, therefore, an object of the invention to suppress resonant vibrations by providing a windmill brake that precludes rotor windmilling or at least ensures that windmilling is limited to subresonant rotational frequencies. Ideally, the brake engages passively in response to abnormal rotor imbalance.
According to the invention, a brake for a turbine engine rotor includes a brake drum, a set of brake shoes and a means, responsive to rotor imbalance, for engaging the brake to inhibit relative rotary motion between the drum and the shoes.
In one detailed embodiment of the inventive brake system, the brake drum is an interior surface of an engine rotor. The drum circumscribes a brake unit. The brake unit includes a brake ring with brake shoes each connected to the ring by respective springs. A leg extends from each shoe and toward the ring. The ring is sandwiched axially between bolting flanges on forward and aft carrier portions of a two piece, variable stiffness, dual load path rotor bearing support. The bearing support, in turn, is mechanically grounded to an engine case. Nonsacrificial fasteners secure the brake ring to one of the carrier flanges while frangible fasteners secure the carrier flanges to each other. The frangible fasteners also extend through En eye in each leg to deflect the springs and hold the brake shoes in an armed state out of contact with the drum. Upon being exposed to abnormal imbalance loads, the frangible fasteners fail in tension to disable one of the load paths. The failure also releases the springs, enabling them to force the shoes into a deployed state in which the shoes contact the drum and inhibit rotary motion of the rotor relative to the case.
The invention is advantageous in that it provides a windmill braking system that responds passively (i.e. without complex control systems or pilot action) to abnormal rotor imbalance to inhibit rotor windmilling and thus guard against resonant rotor vibrations.
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Baran Kenneth C.
Look Edward K.
Nguyen Ninh
United Technologies Corporation
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