Bolt for gas turbine engine rotor

The bolt has, in sequence along a bolt axis: a threaded portion, a thread run-out portion, a shank, and a head, the threaded portion having a thread with a given root radius and a given depth, the thread run-out portion connected to the shank via a thread run-out fillet having a thread run-out fillet radius, the thread run-out fillet radius being between two and six times the thread root radius.

TECHNICAL FIELD

The application relates generally to the field of gas turbine engines and, more particularly, to bolts suited for used in rotating structures.

BACKGROUND OF THE ART

Bolts are sometimes the only suitable option for clamping small size rotors having low bore radii. The head of the bolt is on one side of the bore, the shank of the bolt extends inside the bore, and the threaded portion extends out the other side of the bore, to which a nut is secured in a “thru bolt” configuration. Thru bolts can provide satisfactory access for torquing/untorquing. Bolt weight, nut weight, tightening torques, distance between the bolt's axis and the rotation axis, and rotation speed can be high, which can result in significantly high stresses in the bolt which can limit the bolt life. Lightweight alloys for the bolt have been used to achieve acceptable low-cycle fatigue (LCF) life for the discs, but there are limits in the advantages gainable solely by alloy selection since some properties or features of lightweight materials are typically traded off for their lighter weight.

Accordingly, there remains room for improvement in addressing LCF life of bolts, especially in the context of high stress rotary environments.

SUMMARY

In one aspect, there is provided a bolt comprising in sequence, along a bolt axis: a threaded portion, a thread run-out portion, a shank, and a head, the threaded portion having a thread with a given root radius and a given depth, the thread run-out portion connected to the shank via a thread run-out fillet having a thread run-out fillet radius, the thread run-out fillet radius being between two and six times the root radius. The run-out profile can be a single radius or a combination of multiple radii or a curve.

In a second aspect, there is provided use of the bolt to assemble rotary components with the bolt axis parallel and spaced-apart from a rotation axis of the rotary components.

In a third aspect, there is provided a bolt comprising in sequence, along a bolt axis: a threaded portion, a thread run-out portion, a shank, and a head, the threaded portion having a thread with a predetermined root radius and a predetermined depth, the thread run-out portion connected to the shank via a thread run-out fillet having an average radius of curvature comprised between two and six times the root radius.

DETAILED DESCRIPTION

FIG. 1illustrates an example of a turbine engine. In this example, the turbine engine10is a turboshaft engine generally comprising in serial flow communication, a multistage compressor12for pressurizing the air, a combustor14in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section16for extracting energy from the combustion gases. The turbine engine terminates in an exhaust section. The compressor12and turbine section16include rotary components, or rotors, which revolve at high speeds around the main axis11of the engine.

FIG. 2Ashows a bolt20having a thru bolt configuration used in clamping two rotor components22a,22bof gas turbine engine10. More specifically, the bolt20is used to clamp a mixed flow rotor and the impeller flange together, in this specific embodiment. The bolt20can generally be seen to have a bolt axis24, which is parallel to and spaced apart from the main axis11of the engine. In sequence along the bolt axis11, the bolt can be seen to have a threaded portion26, a thread run-out28, a shank30, and a head32. A nut34is threadingly engaged onto the threaded portion26and torqued to secure the assembly together.

During operation of the engine10, centrifugal acceleration imparts a radially-outward force proportional to the weight of the nut34and the threaded portion of the bolt26, and proportional to the distance between the bolt axis24and the main axis11, onto the threaded portion26, which, in turn, generates a bending stress on the bolt20; these bending stresses peaking in the run-out area28.FIG. 2Bshows an example of the deformation which can occur in the bolt20due to this bending load, during operation of the gas turbine engine10.

In a traditional bolt, the transition between the thread run-out and the shank is sharp, which attracts relatively high concentrations of stress. Over time, fatigue occurring at that specific location limits the low-cycle fatigue life of the bolt.

An improved thread run-out design is shown more clearly inFIG. 3. In the embodiment shown inFIG. 3, the threaded portion26has a thread36of a given depth38and of a given pitch40. The thread36fades out into the thread run-out area28. A run-out radius48connects the shank30via a sequence of a radially concave and annular thread run-out fillet44and a radially convex and annular outer shank edge46. Numerical simulations demonstrated that by smoothening the transition between the last thread and the shank as compared with traditional bolts, the maximum amount of bending stress (peak stress) suffered by the bolt during use of the gas turbine engine was significantly reduced, which has a direct impact on the low-cycle fatigue life of the bolt10. More specifically, in the illustrated embodiment, the thread run-out fillet44has a radius48which was designed to be between two and six times the root radius50of the thread36. Preferably, the thread run-out fillet radius44can be between three and six times the root radius50of the thread36. The specific design of the embodiment is illustrated in greater detail in the views provided inFIGS. 4 and 5.

The resulting bolt20can be understood to be most suited to high speed turbo machinery applications where the bolted assembly is rotated at high speed and is clamping at an appreciable length of rotor stack. In the specific embodiment illustrated, a thread with a 0.036″ pitch and 0.006″ root radius was used, and the bolt was made of titanium, which allowed to achieve satisfactory LCF life both for the bolt and the bolt hole, in a gas turbine revolving at very high rotational speed. Other materials and thread dimensions can be used for the bolt in alternate embodiments.

Peak stress can also be addressed by providing a satisfactorily large shank edge radius52, although in the simulation environment, the shank edge radius52was not as directly relevant as the thread run-out fillet radius48in achieving satisfactory low-cycle fatigue life.

The simulation environment also demonstrated that low-cycle fatigue life can be affected by optimizing the length of the threaded portion. More specifically, the length of the threaded portion can be minimized on either side of the nut while satisfying the required adverse stack-up conditions outlined in design manuals.