DESCRIPTION OF THE DRAWINGS
Reference is now made to the accompanying figures depicting aspects of the present invention, in which:
FIG. 1 is a schematic, cross-sectional view of a gas turbine engine;
FIG. 2 is a schematic, side view showing part of an accessory gearbox, in relation with a central shaft of a gas turbine engine such as shown in FIG. 1;
FIG. 3 is a schematic, side view of a gear shaft of the gearbox shown in δFIG. 2;
FIG. 4 is a schematic, front view of a gear shaft of the prior art; and
FIG. 5 is a schematic, front view of the gear shaft of FIG. 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates a gas turbine engine 10 of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan 12 through which ambient air is propelled, a compressor section 14 for pressurizing the air, a combustor 16 in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section 18 for extracting energy from the combustion gases. The engine 10 includes a central shaft 20 which is driven by the turbine section 18 and which drives the compressor section 14.
As schematically shown in FIG. 1, the gas turbine engine 10 also includes an accessory gearbox 30 which is driven by the central shaft 20 of the engine 10. Referring to FIG. 2, the central shaft 20 includes a gear 32 integral and concentric therewith. An inclined tower shaft 34 has a bevel gear 36 integral therewith at a first end 38 and a pinion 40 integral therewith at a second end 42. The bevel gear 36 of the tower shaft 34 meshes with the gear 32 of the engine's central shaft 20 such that the tower shaft 34 is driven by the central shaft 20.
As can be seen in FIG. 3, the accessory gearbox 30 includes a starter gear shaft 44 having a spiral bevel gear 46 integral and concentric therewith, the bevel or pitch angle of the spiral bevel gear being shown as ε. The spiral bevel gear 46 is meshed with the pinion 40 of the tower shaft 34 such that the tower shaft 34 in turn drives the gear shaft 44 (see FIG. 2). The gear shaft 44 also includes at least one additional gear 48 which meshes with at least one accessory gear 50 to drive at least one accessory (not shown), such as for example a fuel pump, an oil pump, an hydraulic pump, a generator, an alternator, etc.
The spiral bevel gear 46 has a negative helix or spiral angle γ. Mating gears have to be compatible. Therefore, the negative helix or spiral angle of the bevel gear 46 is matched with a corresponding negative spiral angle of the mating pinion 40. As shown in FIG. 5, the negative helix angle γ allows the cutting and grinding tools 52, which are schematically shown in dotted lines, to remain outside of an area 54 defined by a cross-section of the starter gear shaft 44 while machining the spiral bevel gear 46. As such, the gear shaft 44 and spiral bevel gear 46 can be formed from a single piece of material, with portions of the gear shaft 44 protruding from each side of the spiral bevel gear 46, without interference between the machining tools 52 and the gear shaft 44.
As such, each tooth 56 of the spiral bevel gear 46 extends along an arc of a circle (represented by the circle 52 of the machining tool) which does not come into contact with the gear shaft 44, i.e. which is defined outside the cross-section 54 of the gear shaft 44.
In a particular embodiment, the starter gear shaft 44 and spiral bevel gear 46 have relative dimensions similar to those shown in FIG. 4, i.e. relative dimensions that would cause interference of the machining tools 52 with the gear shaft 44 if the spiral bevel gear 46 had a positive helix angle γ as in the prior art.
The spiral angle of a negative gear=360 degrees—the spiral angle of an equivalent positive angle gear. Spiral angles ranging from 1 degree to up to 35 degrees are typically used for positive spiral angles, the equivalent negative spiral angles are 360 degrees−1 degree=359 degrees and 360 degrees−35 degrees=325 degrees. Accordingly, a typical range of negative spiral angle would be comprised between about 325 degrees to about 359 degrees.
In a particular embodiment, the gear shaft 44 and spiral bevel gear 46 are manufactured from a single piece which is formed to define the gear shaft 44 and a body of the spiral bevel gear 46. The gear body is then machined, using for example cutting and grinding tools 52 as is schematically illustrated in FIG. 5, to define the teeth of the spiral bevel gear 46 with the desired pitch angle ε and negative helix angle γ(see FIG. 3).
The integral gear shaft 44 and spiral bevel gear 46 made from a single piece of material allow for a lower cost of manufacture for the gearbox 30, as well as a better manufacturing accuracy through the elimination of the joint between the gear shaft 44 and the spiral bevel gear 46. The negative helix angle γ of the spiral bevel gear 46, through the elimination of the interference between the gear shaft 44 and the machining tools 52, reduces the risk of damaging the machining tools 52 and/or the gear shaft 44 through accidental contact therebetween when machining the spiral bevel gear 46.
The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without department from the scope of the invention disclosed. For example, the starter gear shaft 44 and spiral bevel gear 46 can be used in other types of gearboxes for gas turbine engines. For instance, it could be used in a reduction gear box (RGB) of a turboprop engine. Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.
Patent Application
20080016880
