US2917274A

US2917274A - Compressor and turbine blades

Info

Publication number: US2917274A
Application number: US304975A
Authority: US
Inventors: Davidson Ivor Macaulay
Original assignee: Power Jets Research and Development Ltd
Current assignee: Power Jets Research and Development Ltd
Priority date: 1951-08-31
Filing date: 1952-08-18
Publication date: 1959-12-15
Anticipated expiration: 1976-12-15
Also published as: CH313851A; FR1068776A; DE971286C; NL81240C

Description

2 Sheets-Sheet 1 Filed Aug. 18, 1952 krllllviv Attorney 5 1959 l. M. DAVIDSON 2,917,274

COMPRESSOR AND TURBINE BLADES Filed Aug. 18. 1952 2 Sheets-Sheet 2 INVENTOR /1 0 e 1/46/4007 r fiAV/flSO/V WMWfM ATTORNEYS United States Patent O- COMPRESSOR AND TURBINE BLADES Ivor Macaulay Davidson, Farnborough, England, assignor to Power Jets (Research and Development) Lmuted, London, England, a company of Great Britain Application August 18, 1952, Serial No. 304,975 Claims priority, application Great Britain August 31, 1951 9 Claims. (Cl. 253-77) This invention relates to blades for use in elastic-fluid turbines and more particularly in compressors, fans and it must withstand continuous vibration and possibly even periods of stalled flutter.

In normal practice the blade, fixed at its root and free at its tip, constitutes a loaded cantilever. This cantilever must be fairly substantial so as to.withstand both the static gas loading and also the dynamic loadingwhich tends to cause vibration, a condition which is in opposition to the two requirements of lightness and thinness.

With the primary'objectof damping out vibrations, the present invention consists essentially in a blade formed of at least two layers engaging each other by contact surfaces extending along the .blade'length, from tip to root, and fastened immovably together at one end but left free along the rest of the blade length for one layer to move longitudinally over another.

The invention provides in a bladed rotary fluid flow machine, a blade having an attachment end and a tip at its respective longitudinal extremities which blade comprises at least first and second elements coextensive in length into which the blade is divided from the attachment end to the tip thereof, at least said first element being initially bowed to afford a concave contacting surface said blade including means securing said blade elements together at the attachment end of the blade only with said contacting surface of said first blade element constrained into contact with said second blade element whereby said first blade element is deflected and parts of said blade elements radially outward of their attachment ends are free to move over one another under the mutual constraint of their contact. 7

The aforesaid mutual constraint under which the contacting surfaces of the partsof the blade elements are free to move over one another may be supplemented by means of an intervening layer serving to increase the friction between the contacting surfaces. Such means may comprise an intervening third element of rubbery material, the initially concave surface of one metal blade element and the other metal blade element being constrained into contact with opposite sides of the rubbery material.

Usually division substantially chordwise into two laminae will suffice. Such a laminated blade is shown in the accompanying drawings, of which:

Figure l is a view of a lengthwise section through the complete blade,

2,917,274 Patented Dec. 15, 1959 Figure 2 is a cross-sectional view taken on line 11-11 in Figure 1,

Figure 3 is a view of a lengthwise section through the complete blade before being completely assembled,

Figure 4 is a sectional view of a machine, for example a compressor, incorporating the blade of Figures 1-3,

Figure 5 is an isometric view of the mounted blade of Figures 1 and 2, I I

Figure 6 is a projected view showing a modification of the blade of Figures 1 to 3 in which the contact surface is roughened,

Figure 7 is a view corresponding to Figure 1 of a further modified blade of this invention having an intermediate layer of friction material,

Figures 8 and 9 are views corresponding to Figures 3' and 2 of a further modified blade of this'invention having the concave surface of one blade element coated with a rubber material.

In these drawings the'laminated blade consists of two metal blade elements, 1 and 2 into which the blade is divided along its length, from tip to root, and across its width (as shown in Fig. 2) substantially chordwise from leading to trailing edge. The division continues down through theroot of the blade, which is separated into more or less equal attachment ends 1a and 2a locked together against relative radial movement e.g. by key 2b of one element engaging groove 1b in the other bladeelement. 7

The metal blade elements are secured together at the root but not elsewhere and so the laminae are free to slide slightly over each other on flexure of the assembled blade, as in a laminated spring. For convenience in assembling, the attachment ends or root parts 1a and 2a are rivetted togethere.g. by rivet 3 through the base of the blade and n'vet 4 through the bottom of the'r'oot-- but these rivets will only need to be heavy enough to hold the parts together during assembly; when the blades are mounted in the machine the parts will be held together by the anchoring of the root in its seating. l

The blade elements '1 and 2 are initially bowed so that when brought together as in Figure 3 they are in- 'clined towards each other; when rivetted up and so clamped together in contact, the blade elements are each elastically deflected and pre-stressed to such a degree that they do not separate under the action of the gas load.

To determine the shape to which the blade elements should be pre-formed, as shown in Fig. 3, to give the necessary pre-stressing, a sample blade can be made up of laminae which are'shaped initially to the desired final shape shown in Fig. 1, so that they will fit together without deflection and pre-stressing, to give the desired blade shape. Each blade element, apart from the other one, is then separately loaded to subject it to the estimated contact force but in a sense which will produce stresses opposite in sign from the desired pre-stressing; this will deflect the blade element to the shape to which it should be pre-formed. Accordingly, this deflected blade element can be used as a master pattern of the correct shape to which the blade elements should be made. r

- As the invention is primarily intended for metal blades the

elements

1 and 2 will beof metal. The friction between the metal in contact can be artificially increased by roughening as shown in Figure 6 or coating the rubbing surfaces of the elements as shown in Figure 7. When the blade is used in a compressor and not subject to the high temperature of a'gas or steam turbine, a thin layer of frictional material, usually non-metallic, and possible textile or fibrous material, shown at 5 in Fig. 7, can be inserted between or form part of adjacent metal elements. In addition to the textile or fibrous material a thin layer 6 of rubbery materialsuch as natural orsynthetic rubber, can be bonded to at least'one of the metal elements. In

lieu of the textile or fibrous material and the thin layer of rubbery material only the thin layer of rubbery material may be employed. Forming the contact surface thereof, if continued down through the root, as shown in Figures 8 and 9, and with material omitted, this bonded layer may serve as a cement to hold the metal laminae temporarily together. The thickness and strength of the bonded layer may be either such that on application of a bending load to the blade, the cement fails in shear, after which it serves as the frictional coating between the relatively slidable layers, or such that the layer serves permanently to hold the metal elements together, the necessary relative longitudinal movement of the metal laminae being permitted by shear strain of the rubbery material, which thus serves to supply the necessary damping in place of frictional damping due to rubbing of the laminae.

Although the attachment ends forming the root can be of any of the suitable forms, such as a bulbous root, it is indicated in the drawings as a tapered and serrated root of so-called fir-tree form.

It will thus be clear that the abovedescribed blade, when held firmly in place by its root and loaded in the manner of a turbine or compressor blade, constitutes a built-in cantilevered leaf spring, of which one blade element can move over another with frictional damping due either to frictional rubbing or internal friction of a bonded layer. Vibration of the blade will accordingly be frictionally damped.

Stresses due to dynamic gas loads will not then be appreciable, since correct shaping of the elements and correct choice of the degree of pre-stressing can ensure, even under stalled flutter'conditions, that the damping between the elements is suificient to prevent serious vibration.

It will be seen that regarded from another aspect the invention consists in a fabricated metal blade permitting bending of the blade under load by very free but highly damped relative movement between layers of metal extending chordwise along the blade length from root to tip.

The division through the blade lies transversely to the blade thickness; the division is shown in the drawings as extending chordwise and passing through the leading edge and the trailing edge of the blade, but this is merely by way of example of one possible manner of division. The blade profile may be such that the surface which passes chordwise through each section of the blade and equally divides each section, from tip to root is substantially a surface generated by a radius of the rotor of the machine in which the blade is to be used; centrifugal force acting alone will then produce substantially no bending or twisting moment on an undeflected blade, and will tend to restore a deflected blade. In such case a dividing surface between

laminae

1 and 2 coincides with the said camber sheet. As however there may be some slight and uneven discontinuity of the external profile of the blade at the junction between

blade elements

1 and 2 the surface of separation may be chosen to cut the external surface of the blade at positions displaced from the blade edges, so as to maintain an unbroken external surface of the blade along its length at around at least the leading edge, especially of a compressor blade. Alternatively the edges of the blade elements where they meet at or near the leading edge of the blade may have their sharpness removed by slight rounding. Final machining of the blade and of its root to its finished shapee.g. by grindingmay be done after assembly of the separate parts.

The invention, by reducing the stresses in blades, may make possible the use of aluminium or like light alloy castings for compressor blades and these can then be precision die castings, requiring little or no external machining.

I claim:

1. A bladed rotary fluid flow machine including a blade having an attachment end and a tip at its respective longitudinal extremities which blade comprises at least first and second elements co-extensive in length into which the blade is divided from the attachment end of the tip thereof, at least said first element being initially bowed to afford a concave contacting surface, said blade including means securing said blade elements together at the attchment end of the blade only with said contacting surface of said first blade element constrained into contact with said second blade element whereby said first blade element is deflected and parts of said blade elements radially outward of their attachment ends are free to move over one another under the mutual constraint of their contact.

2. The machine of claim 1 wherein said second blade element is initially bowed to afford a second concave contact surface, said securing means constraining said second concave surface of said second blade element into contact with said first mentioned concave surface of said first blade element.

3. A bladed rotary fluid flow machine including a blade having an attachment end and a tip at its respective longitudinal extremities which comprises a plurality of blade elements. extending from the attachment end to the tip of the blade into which the latter is divided, said elements including at least two metal elements and an intervening third element of rubbery material, at least one of said metal elements being bowed to afford a concave contact surface on its side adjacent the other metal element, said machine including, means securing said blade elements together at the attachment end of the blade only with said initially concave surface and the said other metal element constrained into contact with opposite sides of said rubbery layer whereby said initially bowed element is deflected and parts of said metal elements radially spaced from said attachment ends are free to move relatively under the constraint of their contact with said rubbery layer.

4. A bladed rotary fluid flow machine, including a blade having an attachment end and a tip at its respective longitudinal extremities, which blade comprises at least first and second elements co-extensive in length into which the blade is divided from the attachment end to the tip thereof, at least said first element being initially bowed to afford a concave contacting surface, said machine including means securing said blade elements together at the attachment end of the blade only with said contacting surface of said first blade element constrained into contact with said second plate element whereby said first blade element is deflected and parts of said blade elements radially outward of said attachment end are free to move over one another under the mutual constraint of their contact.

5. A bladed rotary fluid flow machine, including a blade having an attachment end and a tip at its respective longitudinal extremities, said blade being divided in two substantially equal first and second blade elements from the attachment end to the tip, said blade elements being co-extensive in length and whereof at least said first element is initially bowed to aiford a longitudinally concave surface on its side adjacent said second element, said machine including means securing said blade elements together at the attachment end only with said first blade element being deflected by said securing means into frictional engagement with said second blade element, whereby parts of said first and second blade elements spaced from said attachment end are able to move over one another relatively to one another under their mutual friction constraint.

6. A bladed rotary fluid flow machine as claimed in claim 5 in which a third element constitued by a separate layer of friction material is interposed between said first and second blade elements.

7. A bladed rotary fluid flow machine including a blade having an attachment end and a tip at its respective longitudinal extremities which comprises a plurality of blade elements extending from the attachment end to the tip of the blade into which the latter is divided, said elements including at least two metal elements at least one of said metal elements being bowed to afford a concave contact surface adjacent the other element, said machine including means securing said blade elements together at the attachment end of the blade only thereby to draw said initially concave surface into frictional engagement with the adjacent surface of the other element whilst allowing parts of said elements spaced from said attachment end to move relative to each other under frictional constraint and including means between said elements for supplementing the frictional constraint tending to restrict such movement of said elements.

8. A bladed rotary fluid flow machine as claimed in claim 7 in which said means between said elements for supplementing frictional constraint includes an artificial roughening of one of the two adjacent surfaces of said blade elements.

9. A bladed rotary fluid flow machine as claimed in claim 7 in which a third element is interposed between said first and second elements, said third element comprising a layer of friction material and said securing means serving to constrain said first and second elements into frictional contact with said third element whilst allowing parts of said first and second elements spaced from said securing means free to move relatively under the constraint of their contact with said third element.

References Cited in the file of this patent UNITED STATES PATENTS 2,340,133 Martin Jan. 25, 1944 2,394,124 Warren Feb. 5, 1946 2,412,615 Howard Dec. 17, 1946 FOREIGN PATENTS 246,801 Great Britain Mar. 18, 1926