# Patent

## Title
Turbine engine disk spacers

## Abstract

A gas turbine engine rotor stack includes one or more longitudinally outwardly concave spacers. The spacers may provide a longitudinal compression force that increases with rotational speed.

## Background
U.S. GOVERNMENT RIGHTS
The invention was made with U.S. Government support under contract F33615-97-C-2779 awarded by the U.S. Air Force. The U.S. Government has certain rights in the invention.


BACKGROUND OF THE INVENTION
(1) Field of the Invention
The invention relates to gas turbine engines. More particularly, the invention relates to gas turbine engines having center-tie rotor stacks.
(2) Description of the Related Art
A gas turbine engine typically includes one or more rotor stacks associated with one or more sections of the engine. A rotor stack may include several longitudinally spaced apart blade-carrying disks of successive stages of the section. A stator structure may include circumferential stages of vanes longitudinally interspersed with the rotor disks. The rotor disks are secured to each other against relative rotation and the rotor stack is secured against rotation relative to other components on its common spool (e.g., the low and high speed/pressure spools of the engine).
Numerous systems have been used to tie rotor disks together. In an exemplary center-tie system, the disks are held longitudinally spaced from each other by sleeve-like spacers. The spacers may be unitarily formed with one or both adjacent disks. However, some spacers are often separate from at least one of the adjacent pair of disks and may engage that disk via an interference fit and/or a keying arrangement. The interference fit or keying arrangement may require the maintenance of a longitudinal compressive force across the disk stack so as to maintain the engagement. The compressive force may be obtained by securing opposite ends of the stack to a central shaft passing within the stack. The stack may be mounted to the shaft with a longitudinal precompression force so that a tensile force of equal magnitude is transmitted through the portion of the shaft within the stack.
Alternate configurations involve the use of an array of circumferentially-spaced tie rods extending through web portions of the rotor disks to tie the disks together. In such systems, the associated spool may lack a shaft portion passing within the rotor. Rather, separate shaft segments may extend longitudinally outward from one or both ends of the rotor stack.
Desired improvements in efficiency and output have greatly driven developments in turbine engine configurations. Efficiency may include both performance efficiency and manufacturing efficiency.
Accordingly, there remains room for improvement in the art.
SUMMARY OF THE INVENTION
One aspect of the invention involves a turbine engine having a number of disks and a number of spacers. Each disk extends radially from an inner aperture to an outer periphery. Each spacer is positioned between an adjacent pair of the disks. A central shaft carries the disks and spacers to rotate about an axis with the disks and spacers as a unit. The spacers include one or more first spacers having a longitudinal cross-section. The longitudinal cross-section has a first portion being essentially outwardly concave in a static condition.
In various implementations, the first portion may have a longitudinal span of at least 2.0 cm. At least one of the first spacers may be essentially unitarily formed with at least a first disk of the adjacent pair of disks. At least one of the first spacers may have an end portion essentially interference fit within a portion of a first disk of the adjacent pair of disks. The engine may lack off-center tie members holding the disks and spacers under compression. The longitudinal cross-section first portion may be essentially outwardly concave in a running condition of a speed of at least 5000 rpm. The shaft may be a high speed shaft and the disks may be high speed compressor section disks.
Another aspect of the invention involves a gas turbine engine disk spacer having a first end portion, a second end portion, and an essentially annular intermediate portion. The first end portion is either integrally formed with a first disk or has a surface for engaging the first disk. The second end portion is either integrally formed with a second disk or has a surface for engaging the second disk. The intermediate portion has a concave outward longitudinal sectional median. The sectional median may be measured without reference to any seal teeth. The spacer lacks a radially inwardly extending structural bore.
In various implementations, the intermediate portion may have a longitudinal span of at least 2.0 cm. The first and second end portions and the intermediate portion may be unitarily-formed of a metallic material. The spacer may include at least one radially outwardly extending seal tooth. The spacer may be combined with the first and second disks. The spacer first end portion may be unitarily formed with the first disk. The spacer second end portion may be interference fit within a collar portion of the second disk.
Another aspect of the invention involves a turbine engine having a central shaft and a rotor carried by the central shaft. The rotor includes a number of disks. Each disk extends radially from an inner aperture to an outer periphery. Means couple the disks and provide an increase in a longitudinal compression force across the rotor from a first force at a static condition to a second force at a running condition.
In various implementations, the running condition may be characterized by a speed in excess of 5000 rpm. The compression force may essentially increase with speed continuously between the first force and the second force. The first force may be 50–200 kN. The means may comprise an annular spacer portion having a longitudinal cross-section that: in the static condition is outwardly concave with a characteristic concavity having a first value; and in the running condition is outwardly concave with the characteristic concavity having a second value less than the first value. The means may include at least three such annular spacer portions. There may be no off-center tie members holding the disks and spacers under compression.
Another aspect of the invention involves a method for engineering an engine. For at least a first condition characterized by a first speed, a first longitudinal compression force across a rotor stack is determined. For at least a second condition characterized by a second speed, a second longitudinal compression force across the rotor stack is determined. At least one of a number of spacers in the rotor stack is modified so that the second longitudinal compression force exceeds the first longitudinal compression force by a target amount.
In various implementations, the method may be performed as a simulation. The first speed may be zero. The method may be performed as a reengineering of an engine configuration from an initial configuration to a reengineered configuration. The first longitudinal compression force of the reengineered configuration may be less than the first longitudinal compression force of the initial configuration. The second longitudinal compression force of the reengineered configuration may be at least as great as the second longitudinal compression force of the initial configuration.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partial longitudinal sectional view of a gas turbine engine.
FIG. 2 is a longitudinal sectional view of a high pressure compressor rotor stack of the engine of FIG. 1.
FIG. 3 is a detail view of a portion of the rotor stack of FIG. 2.
FIG. 4 longitudinal sectional view of a leading portion of the rotor stack in a first stage of installation to the shaft of the engine of FIG. 1.
FIG. 5 is a longitudinal sectional view of the leading portion of the rotor stack in a second stage of installation.
FIG. 6 is a transverse sectional view of a retainer ring locking the rotor stack to the shaft.
FIG. 7 is a longitudinal sectional view of the leading a third stage of installation.



Like reference numbers and designations in the various drawings indicate like elements.
DETAILED DESCRIPTION
FIG. 1 shows a gas turbine engine 20 having a high speed/pressure compressor (HPC) section 22 receiving air moving along a core flowpath 500 from a low speed/pressure compressor (LPC) section (not shown) and delivering the air to a combustor section 24. High and low speed/pressure turbine sections (HPT, LPT—not shown) are downstream of the combustor along the core flowpath. The engine may further include a transmission-driven fan (not shown) and an augmentor (not shown) among other systems or features.
The engine 20 includes low and high speed shafts 26 and 28 mounted for rotation about an engine central longitudinal axis or centerline 502 relative to an engine stationary structure via several bearing systems 30. Each shaft 26 and 28 may be an assembly, either fully or partially integrated (e.g., via welding). The low speed shaft carries LPC and LPT rotors and their blades to form a low speed spool. The high speed shaft 28 carries the HPC and HPT rotors and their blades to form a high speed spool. FIG. 1 shows an HPC rotor stack 32 mounted to the high speed shaft 28. The exemplary rotor stack 32 includes, from fore to aft and upstream to downstream, seven blade disks 34A–34G carrying an associated stage of blades 36A–36G. Between each pair of adjacent blade stages, an associated stage of vanes 38A–38F is located along the core flowpath 500. The vanes extend radially inward from outboard platforms 39A–39F formed as portions of a core flowpath outer wall 40 to inboard platforms 42A–42F forming portions of a core flowpath inboard wall 46.
In the exemplary embodiment, each of the disks has a generally annular web 50A–50G extending radially outward from an inboard annular protuberance known as a “bore” 52A–52G to an outboard peripheral portion 54A–54G. The bores 52A–52G encircle central apertures 55A–55G (FIG. 2) of the disks through which a portion 56 of the high speed shaft 28 freely passes with clearance. The blades may be unitarily formed with the peripheral portions 54A–54G (e.g., as a single piece with continuous microstructure), non-unitarily integrally formed (e.g., via welding), or may be removably mounted to the peripheral portions via mounting features such as fir tree blade roots captured within complementary fir tree channels in the peripheral portions.
A series of spacers 62A–62F connect adjacent pairs of the disks 34A–34G and separate associated inboard/interior annular interdisk cavities 64A–64F from outboard/exterior interdisk annular cavities 66A–66F. In the exemplary embodiment, at fore and aft ends 70 and 72, the rotor stack is mounted to the high speed shaft 28 but intermediate (e.g., at the disk bores) is clear of the shaft 28. In the exemplary embodiment, at the fore end 70, an annular collar portion 74 at the end of a frustoconical sleeve portion 76 has an interior surface portion 78 engaging a shaft exterior surface portion 80 and a fore end rim surface 82 engaging a precompressive retainer 84 discussed in further detail below. In the exemplary embodiment, the collar and frustoconical sleeve portions 74 and 76 are unitarily formed with a remainder of the first disk 34A (e.g., at least with inboard portion of the web 50A from which the sleeve portion 76 extends forward). At the aft end 72, a rear hub 90 (which may be unitarily formed with or integrated with an adjacent portion of the high speed shaft 28) extends radially outward and forward to an annular distal end 92 having an outboard surface 94 and a forward rim surface 96. The outboard surface is captured against an inboard surface 98 of a collar portion 100 being unitarily formed with and extending aft from the web 50G of the aft disk 34G. The rim surface 96 engages an aft surface of the web 50G.
In the exemplary engine, the first spacer 62A is formed as a generally frustoconical sleeve extending between the fore surface of the second disk web 50B and the aft surface of the first disk web 50A. The exemplary first spacer 62A is formed of a fore portion 104 and an aft portion 106 joined at a weld 108. The fore portion is unitarily formed with a remainder of the fore disk 34A and the aft portion 106 is unitarily formed with a remainder of the second disk 34B. The exemplary second spacer 62B is also formed of fore and aft portions 110 and 112 joined at a weld 114 and unitarily formed with remaining portions of the adjacent disks 34B and 34C, respectively. However, as discussed in further detail below, the exemplary spacer 62B is of a generally concave-outward arcuate longitudinal cross-section rather than a straight cross-section. In the exemplary engine, the third and fourth spacers 62C and 62D are unitarily formed with the remaining portions of the fourth disk 34D.
FIG. 3 shows the exemplary third spacer 62C as extending forward from a proximal aft end portion 120 at the fourth disk fore surface to a distal fore end portion 122. The fore end portion 122 has an annular outboard surface 124 in force fit relationship with an inboard surface 126 of a collar portion 128 extending aft from the aft surface of the third disk web portion 50C. A forward rim surface 130 of the fore end portion 122 abuts a contacting portion 132 of the third disk web aft surface. In the exemplary embodiment, the surface pairs 124 and 126 and 130 and 132 are in frictional engagement (discussed in further detail below). Optionally, one or both surface pairs may be provided with interfitting keying means such as teeth (e.g., gear-like teeth or castellations). A central portion 140 of the third spacer 62C extends between the end portions 120 and 122. Along this central portion 140, the longitudinal cross-section is concave outward. For example, a median 520 between inboard and outboard surfaces 142 and 144 is concave outward. The spacer may have a series of annular teeth 146 extending outward from its outboard surface 144 for sealing with an abradable seal 148 carried by the associated vane inboard platform. In an exemplary definition of the median, the sealing teeth are ignored. The central portion 140 may have a longitudinal span L1 which may be a major portion of an associated disk-to-disk span or spacing L2. L1 and L2 may be different for each spacer. Exemplary L2 is 4–10 cm. Exemplary L1 is 2–8 cm. Exemplary thickness T along the central portion 140 is 2–5 mm.
In the exemplary engine, the fourth spacer 62D has a proximal fore portion 150, a distal aft portion 152 and a central portion 154. The distal portion 152 may be engaged with a forwardly-projecting collar portion 156 of the fifth disk in a similar manner to the engagement of the third spacer distal portion 122 with the collar portion 128. In the exemplary embodiment, the fifth and sixth spacers 62E and 62F are similarly unitarily formed with the remaining portion of the sixth disk as the third and fourth spacers are with the fourth disk. The fifth and sixth spacers engage the fifth and seventh disks in similar fashion to the engagement of the third and fourth spacers with the third and fifth disks. Other arrangements of the spacers are possible. For example, a spacer need not be unitarily formed with one of the adjacent disks but could have two end portions with similar engagement to associated collar portions of the two adjacent disks as is described above.
The arcuate nature of the spacers 62B–62F may have one or more of several functions and may achieve one or more of several results relative to alternate configurations as is discussed below.
In an exemplary method of manufacture, the disks may be forged from an alloy (e.g., a titanium alloy or nickel- or cobalt-based superalloy). In an exemplary sequence of assembly, the hub 90 (FIG. 2) is preformed with the shaft portion 56 (e.g., unitarily formed with or welded thereto). The shaft may be oriented to protrude upward from the hub. The hub may be cooled to thermally contract the hub and the seventh disk 34G heated to expand the disk. This allows the aft/last disk 34G to be placed over the shaft and seated against the hub, with the hub surface 94 initially passing freely within the disk surface 98 so that the hub surface 96 contacts the disk. Ultimately the two may be allowed to thermally equalize whereupon expansion of the hub and/or contraction of the disk brings the two into a thermal interference fit between the surfaces 94 and 98. However, in the exemplary embodiment, while the seventh disk 34G is still hot, the sixth disk, having been precooled, may promptly be similarly put in place with its sixth spacer distal portion being accommodated radially inside the collar portion of the seventh disk. Again, upon subsequent thermal equalization, there will be an interference fit. Similarly, while the sixth disk is still cool, the preheated fifth disk may be put in place and the precooled fourth disk put in place. The exemplary first through third disks are pre-formed as a welded assembly. While the fourth disk is still cool, this preheated assembly may be put in place.
After the assembly of the exemplary rotor stack, it is necessary to longitudinally precompress the rotor stack. The precompression method may be influenced by nature of the particular retainer 84 used. FIG. 4 shows the exemplary rotor stack in an uncompressed condition. In the exemplary uncompressed condition, the exemplary rim surface 82 is well forward of an aft surface/extremity 200 of an inwardly-extending annular rebate 202 in the shaft 28. The exemplary rebate 202 includes a forward surface 204 and a base surface 206. In the exemplary engine, the base surface 206 is moderately rearwardly divergent at a conical half angle θ1 (e.g., 5°–20°). The exemplary fore and aft surfaces 204 and 200 are close to radial (e.g., within 5° of radial). A compressive force 522 is applied to the first disk via a fixture portion 400 and an equal and opposite tensile force 524 is applied to the shaft 28 thereahead via a fixture portion 402. This precompresses the rotor stack into an intermediate condition shown in FIG. 5. In this intermediate condition, the rim surface 82 is shifted aft of the rebate aft surface 200. With the rotor stack in the intermediate condition, the retainer may be put in place. The exemplary retainer uses a segmented locking ring having a pair of segments 210A and 210B (FIGS. 5 and 6). In the exemplary retainer, there are two segments, each very slightly under 180° of arc to leave a pair of gaps 211A and 211B between adjacent segment ends. If present, the gaps may prevent interference and permit full seating of the segments. The gaps may, advantageously, be very small to minimize balance problems and are shown in exaggerated scale.
The exemplary segments are generally complementary to the channel having a fore surface 212 (FIG. 5), an aft surface 214, an inboard surface 216, and an outboard surface 218 in generally trapezoidal sectional configuration. The surface intersections may be rounded and the rebate surface intersections may be correspondingly filleted for stress relief. In the exemplary engine, the rebate is a full annulus as discussed above. Alternatively, the rebate may be a segmented annulus (e.g., two segments of slightly less than 180° each with a corresponding reduction in the circumferential span of the interfitting portions of the ring segments 210A and 210B). There also may be more than two retainer segments.
With the segments in place, a segment retaining means may be provided. In the exemplary retainer, this includes a full annulus retaining ring 220 (FIG. 7) having an outboard surface 222 and a stepped inboard surface having: an aft portion 224 of corresponding diameter and extent to the segment outboard surface 218; and a smaller fore portion 226. The fore portion 226 is separated from the aft portion 224 by a radial shoulder 228 and the fore portion 226 has a diameter corresponding to that of an adjacent portion 230 of the shalt. In the exemplary embodiment, the retaining ring may be slid (translated) into position and held in that position by the subsequent insulation of a bearing retainer 232 for the bearing system 30 thereahead. Alternatively or additionally, there may be a threaded or other locking engagement between the surface portions 230 and 226. With the precompressive retainer 84 thus installed, the applied force may be released, permitting the rotor stack to slightly decompress. The release brings the rim surface 82 into engagement with the segment all surfaces 214. With the rim surface 82 bearing against the retainer segments 210A and 210B, the retainer segment fore surfaces 212 bear against the rebate fore surface 204 to transmit force between the rotor stack and the shaft 28. The result is to leave the rotor stack with a residual precompressive force and the portion 56 of the shaft 28 within the rotor stack with an equal and opposite pretension force. An exemplary precompression force is 50–200 kN. Advantageous force will depend upon the size of the rotor stack, with longer stacks requiring greater force. To achieve this, the assembly precompression force may be slightly greater (eg., by 5–20%).
In operation, as the rotor stack rotates, inertial forces stress the rotor stack. The rotation-induced tensile forces increase with radius. Exemplary engine speeds are 5,000–20,000 rpm for smaller engines and 10,000–30,000 rpm for larger engines. At high engine speeds, the inertial forces on outboard portions of a simple annular component could produce tensile forces in excess of the material strength of the component. It is for this reason that disk bores are ubiquitous in the art. By placing a large amount of material relatively inboard (and therefore subject to subcritical stress levels) some of the supercritical stress otherwise imposed on outboard portions of the disk may be transferred to the bore. The supercritical tensile forces are particularly significant for the spacers. With non-arcuate spacers, the rotation tends to bow the spacer outward into a convex-out shape. This may produce very high tensile stresses near the outboard surface of the spacer. Care must be used to insure that this does not cause failure. This may constrain the use of non-arcuate spacers. For example, the spacer's length may be substantially restricted and thus the associated disk-to-disk span. The spacers may be restricted in radial position to relatively inboard locations. The spacer may require their own bores for reinforcement.
In the exemplary engine, the orientation and relative inboard location of the first spacer 62A permits its non-arcuate nature. The remaining spacers are concave outward. Outward centrifugal loading tends to partially straighten the spacers, reducing their characteristic concavity (e.g., a particular local or average inverse of radius of curvature). However, this straightening is resisted by the compression in the disk stack causing an increase in the compression experienced by the spacer rather than a supercritical tensile condition. Thus, as the rotational speed increases, the compression force across the stack will tend to increase. This increase in compression force has a number of additional implications. One set of implications relates to the spacer configuration. By countering the inertial tensile forces experienced by the spacers, the spacers may be shifted outboard relative to a corresponding engine (e.g., a baseline engine being reengineered) with straight spacers. This outward shift may increase rotor stiffness. The outward shift also permits the outboard interdisk cavities to decrease in size. This size decrease may help increase stability by reducing gas recirculation in these cavities. This may reduce heat transfer to the disks. Additionally, the arcuate spacers may permit an increase in the disk-to-disk spacing L2. This spacing increase may permit use of blade and vane airfoils with longer chords. For example, in a given overall rotor length, fewer disks may be used to obtain generally similar performance (e.g., dropping one or two disks from a baseline 7–10 disk rotor stack). This reduction in the number of disks may reduce manufacturing costs.
Other advantages may relate to the change in the compression profile (i.e., the relationship between speed and longitudinal compression force across the rotor stack). For example, the reengineered system may have compression that essentially continuously increases with engine speed from a static condition to an at-speed condition such as a maximum speed condition. This compression profile may be distinguished from a baseline configuration wherein the peak compression force is at a static condition and there is a continuous decrease with speed. One or more advantages or combinations may be achieved in such a reengineering. First, if the reengineered at-speed longitudinal compression force is higher than the baseline at-speed compression force, there is better engagement between the spacers and disks thereby reducing galling or other damage/wear at their junctions and prolonging life. Second, the static precompression force may be substantially reduced relative to the baseline configuration (e.g., to 20–50% of the baseline force). This reduction may also reduce stress-related fatigue and prolong life. This reduction may also ease manufacturing.
The configuration of the retainer 84 may have one or more advantages independent of or in combination with advantageous properties of the rotor stack. The exemplary retainer 84 may be contrasted with a simple nut retainer against which the rotor stack would bear and through the threads of which the precompression forces would be passed to the shaft. Nevertheless, it may be seen that such a nut retainer might be used in combination with inventive features of the rotor stack. One disadvantage which may be reduced or eliminated is the galling or fatigue-induced damage to the shaft and retainer threads. Eliminating or reducing this damage source may help prolong engine life. Other potential advantages involve ease of assembly and/or reducing the chances of damage during assembly. For example, the chances of damage to the threads from cross threading may be eliminated.
One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, when applied as a reengineering of an existing engine configuration, details of the existing configuration may influence details of any particular implementation. Accordingly, other embodiments are within the scope of the following claims.

## Claims

1. A turbine engine comprising:
a plurality of disks, each disk extending radially from an inner aperture to an outer periphery;
a plurality of spacers, each spacer between an adjacent pair of said disks; and
a central shalt freely passing through said inner apertures and carrying the plurality of disks and the plurality of spacers to rotate about an axis with the plurality of disks and the plurality of spacers as a unit,

wherein:
said spacers include one or more first spacers having a longitudinal cross-section, said longitudinal cross-section baying a first portion being essentially outwardly concave in a static condition.

2. The engine of claim 1 wherein:
said first portion has a longitudinal span of at least 2.0 cm.

3. The engine of claim 1 wherein:
at least one of said first spacers is essentially unitarily formed with at least a first disk of said adjacent pair of said disks.

4. The engine of claim 1 wherein:
at least one of said first spacers has an end portion essentially interference fit within a portion of a first disk of said adjacent pair of said disks.

5. The engine of claim 1 wherein:
there are no off-center tie members holding the plurality of disks and the plurality of spacers under compression.

6. The engine of claim 1 wherein: said longitudinal cross-section first portion is essentially outwardly concave in a running condition of a speed of at least 5000 rpm.

7. The engine of claim 1 wherein:
the shaft is a high speed shaft; and
the plurality of disks are high speed compressor section disks.

8. The engine of claim 1 wherein:
there is a precompression force across the plurality of spacers and a pretension force across an associated portion of the central shaft.

9. The engine of claim 1 wherein:
there is a precompression force across the plurality of spacers and an equal magnitude pretension force across an associated portion of the central shaft.

10. A gas turbine engine disk spacer comprising:
a first end portion either integrally formed wit a first disk or having a surface for engaging the first disk;
a second end portion either integrally formed with a second disk or having a surface for engaging the second disk; and
an essentially annular intermediate portion having a concave outward longitudinal sectional median, said longitudinal sectional median measured without reference to any seal teeth, the spacer lacking a radially inwardly extending structural bore.

11. The spacer of claim 10 wherein:
said intermediate portion has a longitudinal span of at least 2.0 cm.

12. The spacer of claim 10 wherein:
the first and second end portions and the intermediate portion are unitarily-formed of a metallic material; and
the spacer further includes at least one radially outwardly extending seal tooth.

13. The sparer of claim 10 in combination with said first and second disks and wherein:
the spacer first end portion is unitarily formed with the first disk; and
the spacer second end portion is interference fit within a collar portion at said second disk.

14. A turbine engine comprising:
a central shaft; and
a rotor carried byte central shaft to rotate wit the shaft as a unit and comprising:
a plurality of disks, each disk extending radially from an inner aperture to an outer periphery, the central shaft freely extending through the inner aperture of each disk; and
means coupling the plurality of disks, the means providing an increase in a longitudinal compression force across the rotor from a first force at a static condition to a second force at a running condition.

15. The engine of claim 14 wherein:
said running condition is characterized by a speed in excess of 5000 rpm; and
said compression force essentially increases with speed continuously between said first force and said second force.

16. The engine of claim 14 wherein:
said first force is 50–200 kN.

17. The engine of claim 14 wherein:
said means comprises an annular spacer portion having a longitudinal cross-section that:
in said static condition is outwardly concave wit a characteristic concavity having a first value; and
in said running condition is outwardly concave with said characteristic concavity having a second value less than the first value.

18. The engine of claim 17 wherein:
the means includes at least three such annular spacer portions.

19. The engine of claim 14 wherein:
there are no off-center tie members holding the plurality of disks and the plurality of spacers under compression.

20. The engine of claim 14 wherein:
in said static, condition, there is a pretension force on the central shaft equal in magnitude to the first force.