Patent Application
20160010481
The present invention relates generally to rotor structures in gas turbine engines and, more particularly, to a rotor structure including a spindle bolt structure for reducing fatigue in turbine spindle bolts of gas turbine engines.
Turbomachines, such as gas turbine engines, generally include a compressor section, a combustor section and a turbine section. A rotor is typically provided extending axially through the sections of the gas turbine engine and includes structure supporting rotating blades in the compressor and turbine sections. In particular, a portion of the rotor extending through the turbine section comprises a plurality of turbine disks joined together wherein each turbine disk is adapted to support a plurality of turbine blades. Similarly, a portion of the rotor extending through the compressor section comprises a plurality of compressor disks joined together wherein each compressor disk is adapted to support a plurality of compressor blades. The portions of the rotor in the turbine and compressor sections are connected by a torque tube.
In a known construction of the rotor, the turbine disks are joined together by a plurality of spindle bolts extending longitudinally through the turbine disks in the axial direction. The spindle bolts are subjected to stresses which may comprise preload stresses and stresses resulting from thrust, centrifugal force, and/or thermal effects.
In accordance with an aspect of the invention, a spindle bolt structure is provided in a gas turbine engine. The gas turbine engine comprises a rotor including a plurality of turbine disks for supporting rows of blades, a torque tube located on a compressor side of the turbine disks, and a seal disk located between the torque tube and a first stage turbine disk. The spindle bolt structure comprises a spindle bolt extending through the turbine disks and disposed offset from a rotational axis of the turbine disks. The seal disk includes an upstream axial face and an opposing downstream axial face, and a bolt hole extending between the upstream and downstream axial faces. The spindle bolt extends through the bolt hole and includes a bolt shoulder engaged on the seal disk within the bolt hole. A pilot region is formed on the spindle bolt and is located within the bolt hole for effecting a reduction in fretting fatigue of the spindle bolt. The pilot region includes a circumferential pilot ridge located in the bolt hole adjacent to the downstream axial face of the seal disk and a circumferential trough portion located between the bolt shoulder and the pilot ridge. The trough portion defines a trough diameter that is less than a diameter of the bolt shoulder and that is less than a diameter of the pilot ridge.
The diameter of the pilot ridge may be less than the diameter of the bolt shoulder. The diameter of the pilot ridge may be 0.1 mm less than the diameter of the bolt shoulder.
An axial length of the pilot ridge may be less than an axial length of the bolt shoulder and less than an axial length of the trough portion.
The bolt shoulder and the pilot ridge may be formed with an applied compressive residual stress. The compressive residual stress may include a subsurface compressive layer that is formed by a Low Plasticity Burnishing process. The compressive residual stress may form a compressive layer into the bolt shoulder and the pilot ridge to a depth of 0.2 mm or greater. A compressive residual stress of at least 200 ksi may be applied to the bolt shoulder and the pilot ridge. The bolt shoulder and the pilot ridge can each define a smooth, mirror finish circumferential surface.
In accordance with another aspect of the invention, a spindle bolt structure is provided in a gas turbine engine. The gas turbine engine comprises a rotor including a plurality of turbine disks for supporting rows of blades, a torque tube located on a compressor side of the turbine disks, and a seal disk located between the torque tube and a first stage turbine disk. The spindle bolt structure comprises a spindle bolt extending through the turbine disks and disposed offset from a rotational axis of the turbine disks. The seal disk includes an upstream axial face and an opposing downstream axial face, and a bolt hole extending between the upstream and downstream axial faces. A bolt shoulder on the spindle bolt is located within the bolt hole, and the bolt shoulder is formed with an applied compressive residual stress and is positioned for engagement with the seal disk. The compressive residual stress effects a reduction in fretting fatigue of the spindle bolt at locations of contact between the bolt shoulder and the seal disk.
A pilot region may be formed on the spindle bolt and located within the bolt hole for effecting a reduction in fretting fatigue of the spindle bolt, the pilot region including a circumferential pilot ridge located in the bolt hole adjacent to the downstream axial face of the seal disk and a circumferential trough portion located between the bolt shoulder and the pilot ridge, the trough portion defining a trough diameter that is less than a diameter of the bolt shoulder and less than a diameter of the pilot ridge. The diameter of the pilot ridge may be less than the diameter of the bolt shoulder, and the pilot ridge may be formed with an applied compressive residual stress.
In accordance with a further aspect of the invention, a spindle bolt structure is provided in a gas turbine engine. The gas turbine engine comprises a rotor including a plurality of turbine disks for supporting rows of blades, a torque tube located on a compressor side of the turbine disks, and a seal disk located between the torque tube and a first stage turbine disk. The spindle bolt structure comprises a spindle bolt extending through the turbine disks and disposed offset from a rotational axis of the turbine disks. The seal disk includes an upstream axial face and an opposing downstream axial face, and a bolt hole extending between the upstream and downstream axial faces. The spindle bolt extends through the bolt hole and includes a bolt shoulder engaged on the seal disk within the bolt hole. A pilot region is formed on the spindle bolt and is located within the bolt hole. The pilot region includes a circumferential pilot ridge located in the bolt hole adjacent to the downstream axial face of the seal disk and includes a circumferential trough portion located between the bolt shoulder and the pilot ridge. The trough portion defines a trough diameter that is less than a diameter of the bolt shoulder and that is less than a diameter of the pilot ridge. The bolt shoulder and pilot ridge are formed with an applied compressive residual stress and are positioned for engagement with the seal disk. The compressive residual stress and the pilot region effect a reduction in fretting fatigue of the spindle bolt at locations of contact between the spindle bolt and the seal disk.
While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, it is believed that the present invention will be better understood from the following description in conjunction with the accompanying Drawing Figures, in which like reference numerals identify like elements, and wherein:
In the following detailed description of the preferred embodiment, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, a specific preferred embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention.
Referring to
The turbine section 16 includes a plurality of turbine stages, illustrated as first through fourth stages 28a, 28b, 28c, 28d. Each of the turbine stages 28a, 28b, 28c, 28d comprises a respective one of first through fourth turbine disks 30a, 30b, 30c, 30d that define a portion of the rotor 20, and each of the turbine disks 30a, 30b, 30c, 30d supports a plurality of blades 32 for converting the energy of the hot working gases into rotational movement of the rotor 20. The rotor 20 further comprises a torque tube 34 extending between the compressor section 12 and the turbine section 16 for transferring output power from the turbine section 16 to the compressor section 12, where a portion of the output power is used to drive the compressor disks 18 and blades 22, and the remaining portion of the output power is used to drive an output device, such as electrical generator (not shown) in a power generation plant.
In addition, the rotor 20 includes a seal disk 36 at a location between the turbine section 16 and the combustor section 14, supported between the first stage turbine disk 30a and the torque tube 34. The seal disk 36 includes a rotating seal 38 cooperating with a stationary seal structure 40 adjacent to the combustor shell 24 and forming a seal between the turbine section 16 and the combustor section 14.
A spindle bolt structure comprises a plurality of spindle bolts 42 (only one shown) that extend through the turbine disks 30a-d, and pass through the seal disk 36 and an end portion 80 (
In accordance with an aspect of the invention, it has been determined that a failure of a spindle bolt 42 may occur in existing turbine engines 10 having a rotor construction, such as is described above with reference to
Various factors are typically anticipated in specifying the design for the spindle bolts 42 in order to ensure that the spindle bolts 42 are capable of withstanding stresses exerted in the rotors 20. Such factors include possible effects from stresses induced by loads due to thrust, a centrifugal force, or thermal effects. In addition, a preload stress is typically present on the spindle bolts 42, which comprises a stress induced by a predetermined tension applied on the bolts 42 during assembly of the rotor 20 for maintaining the turbine disks 30a-d, seal disk 36 and torque tube 34 joined together.
Known design practice permits specification of the spindle bolts 42 to withstand the conventionally anticipated stresses experienced by the spindle bolts 42 as a result of forces generated by thrust, rotation of the rotor 20 and thermal effects, which are generally referenced herein as baseline or mean stresses. That is, the mean stresses generated by thrust, rotation of the rotor 20 and thermal effects, and substantially due to centrifugal forces with rotation of the rotor 20, are generally predictable, permitting the spindle bolts 42 to be designed, including a factor of safety, to withstand these predicted stresses.
The above noted area of potential failure in the spindle bolt 42, i.e., along the incremental length section FA, indicates an area where a failure may occur in spite of the application of conventional design practice to configure the spindle bolts 42 to withstand the predicted stresses applied through the rotor 20. In accordance with the present invention, a theory of failure and solution are presented to address the unexpected spindle bolt failures at the exemplary location along the incremental length section FA. Specifically, it may be observed that the rotor 20 normally experiences a sag between the bearings 46, 50, resulting in a certain amount of axial elongation of the spindle bolts 42 as the bolts 42 each rotate with the rotor 20 from top-dead-center (TDC) to bottom-dead-center (BDC). For example, a cyclically occurring bolt stretch or elongation can occur in the axial direction A1 (
It is believed that an aspect contributing to failure of the spindle bolt 42 comprises high cycle fatigue (HCF) that can result in a fretting effect (fretting fatigue) associated with the cyclically occurring lengthwise or axial movement of the spindle bolt 42 relative to the spindle bolt hole 58 in the seal disk 36 and the torque tube 34 as the length of the seal bolt 42 changes in the axial length section AL. The fretting effect comprises an alternating stress, S, produced by high traction forces formed at an interface between the contacting surfaces of the spindle bolt 42 and the spindle bolt hole 58. Cracks at or near the surface of the spindle bolt 42 can propagate under HCF loading and can lead to the spindle bolt 42 eventually fracturing under tension due to the axial preload. In accordance with an aspect of the invention, it has been observed that a location of fretting fatigue, and potential failure, can occur along the incremental length section FA (
Referring to
The spindle bolt 42a includes a pilot region 70 located axially within the bolt hole 58 for effecting a reduction in fretting fatigue of the spindle bolt 42a. The pilot region 70 includes a circumferential pilot ridge 70a located in the bolt hole 58 adjacent to the downstream axial face 56 of the seal disk 36, and a circumferential trough portion 70b located between the bolt shoulder 62S and the pilot ridge 70a.
Referring to
In accordance with a further aspect of the invention, the diameter D3 of the pilot ridge 70a is less than the diameter D2 of the bolt shoulder 62S. Preferably the diameter D3 of the pilot ridge 70a is 0.1 mm less than the diameter D2 of the bolt shoulder 62S, where the bolt shoulder has a diameter of about 66 mm. Further, the pilot ridge is a relatively narrow feature having an axial length LR that is less than the axial length LT of the trough portion 70b, as well as less than an axial length LS (
The provision of the pilot region 70, and specifically the reduced diameter of the pilot ridge 70a, is believed to allow the spindle bolt 42a to accommodate a greater bending load resulting from thermal growth of the seal disk 36 and the adjacent first turbine disk 30a. In particular, the smaller diameter of the pilot ridge 70a, and the associated additional clearance to the seal disk surface within the bolt hole 58, allows the spindle bolt 42a to take more bending load and reduces the peak contact stress resulting from edge effect. That is, the provision of the pilot ridge 70a results in reduced localized contact locations, redistributing the contact load over a greater portion of the surface area of the bolt shoulder 62S. Additionally, the increased contact area can result in lower membrane stress, i.e., an axial stress component, in the incremental length section FA, and provides an improved safety margin by lowering the mean stress in the spindle bolt 42a.
In an exemplary embodiment, the compressive residual stress may be applied up to a value of at least 200 ksi, and to a depth of up to 1 mm to provide an optimal compressive layer 82 that can substantially lower crack initiation and propagation in the spindle bolt 42a. Further, below the compressive layer 82, the maximum residual tensile stress is preferably no more than 20 ksi, i.e., the residual tensile stress can equal 20 ksi or less.
The diagram of
In addition to the above described improvements, the LPB process can provide a smooth, mirror finish to the outer surface 72, 73 of the spindle bolt 42a in the areas of contact within the bolt hole 58 of the seal disk 36. The smooth, mirror finish can provide a further resistance to HCF crack initiation. The LPB process can be performed by a burnishing or peening element that is brought into contact with the surface 72, 73 in a predetermined manner, such as a process performed by controlling the depth to which a surface treatment element, such as a burnishing or peening element, is impinged against the surface of the spindle bolt 42a. For example, the process for the forming the subsurface compressive layer 82 can be a process such as is described in U.S. Pat. No. 7,600,404, which patent is incorporated herein by reference in its entirety.
It may be understood that the pilot region 70 and the compressive layer 82, such as may be provided by the LPB process, are preferably provided to the spindle bolt 42a in combination. The combination of the spindle bolt 42a having the pilot region 70 and the compressive layer 82 can operate together to lower crack initiation under fretting in the area identified as subject to failure. Further, the improved surface finish lowers the probability of crack initiation from fretting, and the subsurface compressive stress field of the compressive layer 82 can stop crack propagation. The combined effects of the described aspects of the invention provide improved design safety margins for the spindle bolt 42a.
While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.
