US 3737250 A
A fiber composite compressor blade having spanwise fiber elements cast within an appropriate composite material wherein at the root end of the blade the fiber elements are looped over transverse pins extending across a hollow root fitting. The root fitting is generally formed to dovetail into peripheral slots extending across a compressor disc. The exterior of each blade is enclosed by layers of crossply fiber cloth and the cavity within the root fitting is filled with an appropriate potting compound to respectively provide strength and rigidity to the blade.
Description (OCR text may contain errors)
United States Patent Pllpel et al. 1 June 5, 1973 54] FIBER BLADE ATTACHMENT 2,919,889 1/1960 Rube] ..416/230 x 2,929,755 3/1960 Porter ..416/230 x  Inventors. Walter Pilpel, West Hartford;
Spencer Tore, New Britain; 3,487,879 1/1970 McCarthy et a1. ..416/220 ux g p Manchester, all FOREIGN PATENTS 0R APPLICATIONS 0 01111.
787,500 12/1957 Great Britain ..416/230 The United States of America as represented by the Secretary of the Navy, Washington, D.C.
Filed: June 16, 1971 Appl. No.: 153,720
US. Cl. ..4l6/219, 416/230, 416/241, 416/248 Int. Cl ..F0ld 5/30 Field of Search ..416/220, 230, 249, 416/248, 215, 219, 241 A References Cited UNITED STATES PATENTS 11/1958 Warnken ..416/230 X 1/1959 Hampshire et a1. ..4l6/230 Primary Examiner-Everette A. Powell, Jr. Attorney- R. S. Sciascia and Henry Hansen A fiber composite compressor blade having spanwise fiber elements cast within an appropriate composite material wherein at the root end of the blade the fiber elements are looped over transverse pins extending across a hollow root fitting. The root fitting is generally formed to dovetail into peripheral slots extending across a compressor disc. The exterior of each blade is enclosed by layers of crossply fiber cloth and the cavity within the root fitting is filled with an appropriate potting compound to respectively provide strength and rigidity to the blade.
ABSTRACT 4 Claims, 4 Drawing Figures 17/] saw/1,1,
FIBER BLADE ATTACHMENT STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
BACKGROUND OF THE INVENTION The present invention relates to turbine apparatus, and more particularly, to replaceable turbine blades constructed of fiber composite material.
In the field of turbine powerplants and particularly in turbine powerplants adapted to aircraft use, one of the most critical design items is the turbine blade. Considerations of powerplant cycle efficiency and weight usually require high blade tip speeds with resulting high centrifugal forces tending to separate the blade away from the retaining fixture or blade carrying disc. Accordingly there is a constant search in this art to obtain blade structures which are relatively strong in tension as compared to their weight or cross section. Generally one technique of constructing high strength blades of an axial turbine or compressor is to construct the blades using fiber composite materials. In order to utilize to best advantage the characteristically high tensile properties of the fibers the fibers are laid spanwise along the blade. In this manner high rotational blade speeds can be attained with a relatively low blade mass and cross section. At the root end the fibers of each blade are retained in a dovetailed fitting shaped to conform to a corresponding broached slot formed in the periphery of a blade carrying disc, such that the individual blades together with the root fittings are replaceable. Thus the root ends of the fibers are attached to a fitting which in turn attaches to the disc and the root attachment of the fibers becomes the limiting consideration of fiber composite blades. Typically in the prior art the centrifugal load transferred through the fibers is collected at the root fitting by various squeezing and wedging combinations such that the load is carried from the fibers to the fitting in shear. It is one of the well-known properties of fiber composites that they usually exhibit relatively low shear stress allowables as compared to their particular tensile stress allowables, thus the prior art techniques of blade root attachment result in critical limitations on blade manufacture and maximum blade rotational speed.
SUMMARY OF THE INVENTION Accordingly it is the general purpose and object of the present invention to provide a root attachment technique for attaching fiber composite blades wherein the tensile properties of the fibers are used to best advantage. Other objects of the invention are to provide a blade root attachment means which will conveniently accommodate various blade profiles, as well as blade root attachment means which are easily assembled.
These and other objects are accomplished according to the present invention by providing mounting pins extending across a cavity formed in a blade root fitting around which fibers are looped extending outwardly therefrom along the span of the blade. The blade root fitting is generally shaped on the exterior surfaces thereof to conform with the convolutions of a corresponding broached slot fabricated in the periphery of a blade disc and when in position the lateral motion of the pins is restrained by the slot. The spanwise fibers are looped in bundles around the respective pins, positioned laterally along the pin in conformance with the profile of the blade, thus within the limits of the root fitting cavity various blade profiles can be accommodated. The respective fibers arranged according to the profile of the blade are then potted in place by appropriate potting compound and the blade is formed by surrounding the fibers with the composite material. A crossply woven fiber cloth covers the exterior of the blade to further strengthen the blade and to prevent fiber separation or fraying.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1. shows in plan view a compressor disc assembly having blades constructed according to the invention mounted on the periphery thereof;
FIG. 2 is a fragmentary view shown in partial cross section along line 22 of FIG. 1 of one blade constructed according to the present invention;
FIG. 3 is a cross section view of the blade drawn along lines 33 of FIG. 2; and
FIG. 4 is a cross section view taken along line 4-4 of FIG. 2 of a blade root fitting constructed according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention provides a technique of root attachment of fiber composite blades specifically directed at the transmission of tensile stress from the composite structure to the attachment fixture. It is particularly useful in applications where the tensile properties of fiber composite structures are found advantageous, such as, attachment of composite helicopter rotor blades or,as shown specifically in this embodiment, attachment of compressor blades in turbine machinery.
Referring to FIG. 1 a compressor disc assembly 10 is shown comprising a plurality of replaceable blades 11 each blade terminating in a dovetailed root fitting 13. Each root fitting 13 is shown inserted into respective broached slots 14 formed across a peripheral flange of a disc 12. Each blade 11 extends radially away from the disc 12 and during rotation of assembly 10 is pulled outwardly by the centrifugal force. Accordingly the major component force is in tension transmitted through fitting 13 to the convolutions of slot 14 to the peripheral flange of disc 12. I
As shown in more detail in FIG. 2 blade 11 is formed of a composite material comprising fiber elements 20, such as glass fibers impregnated in a suitable thermosetting resin, extending generally along a spanwise direction of the blade. The root end of the blade extends into a cavity 21 formed in fitting 13 wherein fitting 13 is generally similar in shape to a flowerbox or a bathtub. A plurality of pins 25 extend through corresponding openings 26 across and between the lateral sides of the root fitting 13 extending across the cavity 21 proximate the bottom surface thereof, in a plane generally orthogonal to the longitudinal axis of blade 11. Fiber elements 20, separated into bundles, are looped around the respective pins 25 extending radially outward therefrom generally along the platform of the blade 11. A corresponding number of ribs 28 are shown adjacent and parallel to the pins 25 extending between the lateral sides of fitting l3 inside cavity 21 proximate the upper opening thereof. Ribs 28 areprovided for the purpose of separating the fiber bundles thereby facili rating the assembly thereof and the installation of pins 25. Once disposed in their respective positions fibers 20 are surrounded by an appropriate composite material 45, such as potting compound or resin, generally formed or cast to conform to the dimensions of the blade. The exterior of blade 11 thus formed is then wrapped with one or more layers of crossply fiber cloth 30 which strengthens the blade laterally to reduce possibility of fraying. Cavity 21 is also filled with a composite material 45 such as resin or potting compound providing a relatively rigid attachment of the fiber elements 20 to fitting l3 and securing the root edge of cloth 30.
Referring to FIGS. 3 and 4 the fiber elements 20 are looped in bundles over the open sections of pins 25 generally filling the thickness of the blade 11, the individual bundles of fiber elements 20 being displaceable along pins 25, within cavity 21, such that they generally can be arranged to conform to any root profile section of blade 11. This particular feature allows for convenient assembly of various blade profiles which is particularly significant during the development stages of assembly for any specific application. The individual bundles are affixed in their respective location by the potting compound allowing for the positioning of the fiber elements in the best location for efficient tensile load transfer within the cross section of blade 11.
Some of the many advantages of the present invention should now be readily apparent. The invention provides a convenient attachment configuration for a composite blade which is typically critical in tensile load transfer at the root end thereof. The invention also provides convenient freedom of fiber deployment such that many blade profiles can be easily accommodated within the limits of a standardized root fitting. This the invention accomplishes without any trade-off in reduced structural simplicity and convenience in blade replacement.
Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.
What is claimed is:
1. A fibercomposite blade for a rotor disc having slots of dovetail shape, comprising:
a root fitting having cross sections of outer dovetail shape uniform between the ends of said fitting and corresponding to the shape of the disc slots for contiguous and slidable insertion therein, the cross sections being transverse to the direction of insertion, a radially outwardly communicating cavity, and pin attachment means extending across said cavity transverse to the direction of root fitting insertion fiber elements looped around said pin attachment means the free ends thereof extending outwardly from said cavity; and a composite material interstitially surrounding said fiber elements, filling said cavity, and formed into a blade configuraton radially extending from said cavity. 2. A fiber composite blade according to claim 1, further comprising:
said root fitting including a plurality of pairs of coaxial openings on opposite sides of the cavity, the axes of said openings being normal to the direction of insertion; said pin attachment means including pins inserted at either end into said openings; said fiber elements being grouped in bundles, each being looped around respective ones of said pins within said cavity. 3. A fiber composite blade according to claim 2, further comprising:
said root fitting including transverse ribs extending across the opening of said cavity adjacent and parallel to said pins. 4. A fiber composite blade according to claim 3, further comprising:
one or more layers of fiber cloth enclosing said composite material, contiguously attached to the exterior surfaces thereof.