Machine element or mechanism – Elements – Gear casings
Reexamination Certificate
1998-12-17
2001-04-10
Bucci, David A. (Department: 3682)
Machine element or mechanism
Elements
Gear casings
C267S064130, C244S054000
Reexamination Certificate
active
06212974
ABSTRACT:
TECHNICAL FIELD
The present invention relates to gas turbine engines, and more particularly to providing a variable stiffness mounting arrangement for a gearbox of the engine to minimize damage to the gearbox in the event of a high rotor imbalance condition such as a fan blade loss.
BACKGROUND ART
Modern gas turbine aircraft engines have gearboxes that are mounted on their external cases. The main gearbox is generally positioned under the engine's core or fan case and uses power from the engine to drive the major accessories needed for engine functions, such as generators for electricity, pumps for circulating fluids and heat exchangers for cooling oil or heating fuel. The accessories are frequently mounted on the gearbox.
The main gearbox of modern turbofan engines such as the Model PW4000, manufactured by the Pratt & Whitney Division of United Technologies Corporation, is mounted within the engine core cowl and is driven by an angle gearbox through a towershaft from the high-pressure compressor. Various components are mounted on the gearbox. The main gearbox drives the fuel pump, scavenge pump, the electrical generator for the electronic engine control, external deoiler, hydraulic pump, and the integrated drive generator (for producing electricity).
Various mounting arrangements are used to mount the gearbox to the external case. The gearbox is often connected to the engine case at two or more locations along the length of the gearbox. For example, a forward V-shaped link may be used to fasten the front of the gearbox to the engine. A plurality of side hanger links may be used on either side of the gearbox housing to support and stabilize the gearbox. Further, a mounting plate may also be used to provide alignment of the gearbox with respect to the engine case and prevent undesirable movement of the gearbox relative to the engine. Thus, the gearbox may be connected to the engine by a hard mount, that is, a connection which is essentially inflexible and immobile such that case deflections are transmitted to the gearbox. This hard mount arrangement imposes undesirable loads on the mounting hardware and on the gearbox itself.
One problem caused by such arrangements for mounting the gearbox to the engine case is subjecting the gearbox to high loads caused by rotor imbalances. Severe rotor imbalance can occur in an engine, particularly after a fan blade breaks off from the rotor assembly. One cause of fan blade loss is impact with foreign objects, such as birds, hailstones or other objects which, on occasion, are ingested into the engine. The detached fan blade is thrown outwardly and passes through the fan case, but is typically caught by the fabric wraps in the fan containment case assembly. Such blade loss produces an imbalance in the rotor and causes the rotor shaft to deflect radially outwardly. The more the rotor deflects, the greater is the radial load on the rotor bearing supports.
The rotor imbalance loads are transmitted from the bearing supports to the engine cases and ultimately to the gearbox fastened to the engine case. The gearbox is forced to move with the engine as the gearbox is fastened to the engine cases. Blade loss loads have been traditionally underestimated with respect to gearbox structural design. If the vibratory loads transmitted to the gearbox are high, the gearbox may not be able to sustain the imbalance loads and this may result in the failure of the gearbox housing itself, and often the liberation of the accessories themselves from the gearbox.
When the gearbox housing breaks, oil from within the gearbox may splash over the hot case of the engine and immediately ignite, causing a fire. The release of oil from within the gearbox also potentially damages the internal components of the gearbox as they would have to operate without lubrication. If an accessory breaks off from the box or pulls apart, spilled hydraulic fluid or fuel may cause a fire. If the generator pulls off, a very large component is liberated that can potentially break through the engine cowling itself and ultimately hit an aircraft control surface.
Consequently, gearbox housings have been made thicker, the mount points stronger, and other strength and durability requirements have been increased. However, these prior art solutions have resulted in substantially heavier hardware which in turn has resulted in the gearbox mount loads to increase. In turn, the efficiency of the engines is adversely impacted by such heavier hardware.
It is also known to use a mounting arrangement having some flexibility or mobility to absorb energy and thus prevent the gearbox from engine case deflections and distortions. However, the flexible mounting arrangements of the prior art adversely compromise the life of the hardware as the elastomeric materials used to introduce the desired flexibility harden when cycled in a high temperature environment.
Thus, the challenge for modern gas turbine engines, during fan blade loss events, is the limiting of damage to the gearbox mounts and the gearbox itself.
DISCLOSURE OF THE INVENTION
A primary object of the present invention is to provide a gearbox mounting arrangement that protects the gearbox from the undesirable effects of engine case deflections and distortions experienced during high rotor imbalance events such as a fan blade loss condition.
According to the present invention, a variable stiffness positioning link for a gearbox for a gas turbine engine includes a first portion connected to the engine case, a second portion connected to the gearbox and an isolation mechanism having a lower stiffness than the first and second portions and which releasably connects the first and second portion, wherein during a high rotor imbalance event such as a fan blade loss condition, the isolation mechanism becomes operational in response to the application of high axial loads and causes the first and second portions to partially disengage. The partial disengagement of the portion of the variable stiffness link results in partially isolating the gearbox from the deflections experienced by the engine case, thus partially isolating the gearbox from the bulk of the energy of the engine case. Even though the variable stiffness link still transmits some load from the engine case to the gearbox in the partially disengaged state, the lower stiffness of the link reduces the transmission of loads to the gearbox, thus minimizing damage to the gearbox due to high deflection loads.
In one embodiment of the present invention, the energy associated with the loads transmitted through the variable stiffness positioning links is dissipated via damping of a fluid which is disposed in the isolation mechanism.
In another embodiment of the present invention, the energy associated with the loads transmitted through the variable stiffness positioning links is accommodated by the compression of an elastomer disposed in the isolation mechanism.
A primary advantage of the present invention is the minimization of damage to the gearbox housing and accessories during large rotor imbalance conditions such as in the event of a fan blade loss. The variable stiffness positioning links of the present invention provide acceptable means of controlling the displacement of the gearbox. A further advantage of the gearbox links of the present invention is their ability to provide an appropriate positioning structure for the gearbox during normal operating conditions. Another advantage is the ease and cost of manufacturing and incorporating into the gearbox of the prior art the variable stiffness positioning links of the present invention. The simplicity of the structure of the gearbox links and the use of economic materials, allows for cost effective manufacturing processes. Further, gearboxes of the prior art can be retrofitted to include the present invention in a cost-effective manner. The gearbox housing can be made lighter as the strength of the gearbox housing does not have to be increased due to the present invention. As a result, the use of the variable stiffness positioning links of the present
Bucci David A.
Hansen Colby
United Technologies Corporation
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