US3406760A - Flexible blade fan - Google Patents

Description

Oct. 22, 1968 T. J. WEIR FLEXIBLE -BLADE FAN Filed Sept. 18, 1967 .5 fi W 9 s m A I 0 z/mii/wl United States Patent 3,406,760 I FLEXIBLE BLADE FAN Thomas J. Weir, Indianapolis, Ind., assignor t0 Wallace- Murray Corporation, New York, N.Y., a corporation of Delaware Continuation-impart of application Ser. No. 613,464,

Feb. 2, 1967. This application Sept. 18, 1967, Ser. No. 668,340

Claims. (Cl. 170-160.5)

ABSTRACT OF THE DISCLOSURE A flexible blade fan structure in which the flexible blades are each folded along their length at their leading edge to provide a smooth, stiff leading edge on each blade and the trailing edge marginal portion of each blade is formed to provide a longitudinal rib, with the extending portion of the fold providing'more uniform stress distribution in the blade as it flexes under load with the longitudinal rib attenuating vibration of the fan blade without altering its ability to flex under load.

Cross reference to related application This application is a continuation-in-part of my US. patent application Ser. No. 613,464 filed Feb. 2, 1967, now abandoned and titled Flexible Blade Fan.

Background of the invention (1) Field of the inventi0n.The present invention relates to air moving fans, such as those used for cooling an internal combustion engine, particularly fans having flexible blades which flex to automatically reduce the fan blade pitch as the speedof the fan increases, thus limiting the air flow and the power required.

(2) Description of the prior art.Since the volume of air moved by a fan increases as the first power of the fan speed, at high engine speeds the fan may move more air than necessary to cool the engine, thus wasting power. Further, under these conditions, the noise level of the fan, which increases substantially with speed, will be unnecessarily high. A solution to this problem, not unknown in the prior art, is to provide the fan with flexible blades which distort or flex with increases in fan speed to a contour of reduced pitch, thereby unloading the fan, that is, lowering the volume of air moved per fan revolution. Flexible bladed fans of this type are disclosed in US. Patents 2,032,224 and 3,289,924. These prior art fans perform satisfactorily, however, two difficulties are involved with their use. One is related to the clamping of the blade to the fan spider arms. The required stiff leading edge for each flexible blade and the desirable stress-distributed attachment for the blade to the arm has, heretofore, required the use of an additional clamp component, and the resulting structure has not resulted in an entirely smooth and streamlined leading edge for the blade. The other difliculty arising in the prior art structures is the presence of destructive stress in the flexible blades caused by vibration inherent in the operation of the fan. The fan blades exhibit a second, mode vibration, that is, a vibration mode normal to the flexing mode which, particularly, causes a high stress in the blades, excessive vibration noises and often results in premature failure of the blades, typically by cracking along the blade-spider arm junction. Stiffening the blade throughout its area, by using less flexible stock to form the blades, for example, does transpose the destructive second mode vibration to a frequency beyond the operating speed range of the fan, however, since the blades are then relatively stifl, the automatic unloading characteristic of the fan is adversely affected.

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Summary 00 the invention The present invention provides a fan structure of the flexible blade type in which each flexible blade is folded on itself along its length adjacent the leading edge of the blade with the smoothly rounded, hollow bead thus formed giving-the blade a streamlined leading edge. The fold also strengthens and stiffens the leading marginal area of the blade so that the blade is essentially prevented from bending across its width while the remainder of the blade is free to flex along an axis approximately parallel to the bead; The multiple thickness of material of the folded section also distributes the stress, caused by flexing, away from the high stress area at the attachment to the spider. The advantages referred to are provided even though the structure is characterized by a reduced number of attachment components when compared to prior art flexible blade fan structures, and the fan structure of the present invention, therefor, can be manufactured more economically. Additionally, the flexible blades of the fan embodying the present invention are provided with a stiffening rib extending substantially the entire length of the fan blade and disposed outwardly of, that is, on the trailing side of, the area of flexure of the blade. The rib stiffens the blade lengthwise, that is, radially with respect to the rotational axis of the fan, without decreasing the flexibility of the blade transversely and thus permitting the blade to flex freely to unload the fan. The destructive second mode vibration is transposed to a frequency beyond the normal operating speed range of the fan without destroying the ability of the blades to flex freely to limit the loading of the fan.

Brief description of the drawing The full nature of the invention will be understood from the accompanying drawings and the following description and claims.

FIG. 1 is a front view of a fan assembly embodying the present invention.

FIG. 2 is a fragmentary, side view illustrating in detail the construction of the junction between the fan blades and the spider of FIG. 1.

FIG. 3 is a side, sectional view of a modified form of the flexible fan blade.

FIG. 4 is a side, sectional view of a further modified form of the flexible fan blade.

FIG. 5 is a side sectional view of a further modified form of the flexible fan blade.

FIG. 6 is a fragmentary front view of a further modified form of the flexible fan blade in which the stiffening rib has an arcuate or crescent contour.

Description of the preferred embodiments Referring initially to FIGS. 1 and 2, the fan assembly includes a spider having a hub portion 11 and a series of radially extending arms 12. The hub portion may be apertured as indicated at 13 for attachment to a driving pulley, and the arms 12 may, as indicated in FIG. 1, be spaced at various angles with respect to each other.

Referring now primarily to FIG. 2, each of the arms carries a flexible fan blade 14. Each of the blades extends beyond the ends of its corresponding arm and is folded longitudinally upon itself twice to bring its free end 20 into adjacent relationship with the smooth, hollow bead 17 which extend along one edge of the blade and is formed by the initial fold of the overlapping blade portion 18.

The overlapping portion 18 defines a triple-thickness, reinforced marginal area for the blade. Rivets 19 are spaced along the length of the spider arm and serve to rigidly join the arm and the reinforced marginal area of the blade.

The primary area of flexure of the blade when the fan is in operation is located outwardly of the reinforced or overlapping portion 18 of the blade. The portion of the blade outboard of, or beyond, this primary area of flexure of the blade is formed to provide a longitudinal rib 15, which, in the form of the structure shown in FIGS. 1 and 2 is located specifically adjacent the blade margin opposite the bead 17.

As the fan is rotated clockwise, as viewed in FIG. 1, at relatively low rotational speeds the fan blades 14 will be acted on by a force tending to deflect or bend the blades in the direction of the arrow shown in FIG. 2 principally in the primary area of flexure previously mentioned. If this force is of suflicient magnitude, the overlap portion 18 will, however, also deflect somewhat. As the speed of rotation of the fan increases, this deflecting force becomes larger and the blades 14 tend to flatten or unload. Visualized in cross-section, each of the blades can be considered to be a cantilever beam with the clamp being provided at the rivets 19, the beam being under a uniformly distributed load.

In prior art flexible blade fan structures without reinforcement in the attachment area, the stress distribution across the width of the fan blade is like that in a cantilever beam, fixed or clamped at one end and under a uniformly distributed load, that is, the stress is zero at the free tip of the beam and then varies exponentially to a maximum at the fixed end of the beam. Maximum stress thus concentrates at the rivet or weldment area in these prior art structures. The presence of the overlap portion 18 in the structure of the present invention serves to, in effect, increase the cross-section of the beam formed by the blade along a portion of its length nearest its clamped end. This more uniformly distributes the total stress along the width of the blade and avoids high concentration of stress at the riveted or welded attachment of the blade to the spider arm. The hollow bead 17 along the leading edge of each blade provides a smooth, streamlined surface which extends substantially completely over the rectilinear margin 12a of the spider arms 12. The blade leading edge is stiffened by this bead and aerodynamic advantages result from the smoothly curved surface it provides. The blade fold thus provides reinforcement at the area of attachment to the arms 12, essentially preventing bending across the blade width beyond arms 12, and provides a smooth leading edge without necessitating use of additional components such as clamps or backing plates.

As previously mentioned, the stiffening rib located outwardly of the primary area of flexure of the flexible blade serves to transfer the second mode vibration, which occurs when the fan is in operation, to a frequency which is beyond the operating speed range of the fan. Referring to FIG. 1, the second mode vibration referred may be considered to occur on each blade about the central point indicated at A, with the points indicated at B and C moving in the same direction, vibrating about the central point A as previously pointed out. If the stiffening rib is absent, this vibration can cause cracking and failure of the blades. The addition of this stiffening rib solves this vibration problem without destroying the ability of the blades to flex under load.

In the modified construction of FIG. 3, the overlap portion 18a differs from the overlap portion 18 of FIG. 2 in that the overlap portion 18a is formed by folding the blade upon itself only once and by having its marginal area 18b flanged outwardly at an acute angle away from the blade 14. This arrangement limits deflection of the blade and provides enhanced reinforcement at maximum deflection conditions, the position assumed by the blade and backing area under these conditions being shown in broken lines in FIG. 3.

The modified construction of FIG. 4 differs from that of FIG. 3 only in that the extending marginal portion 18b of FIG. 3 is eliminated and the structure is the same as that of FIG. 2 except that the marginal portion 180 of the blade is formed by folding the blade upon itself only once thereby providing an overlapping portion of double- 4 thickness, as in the structure of FIG. 3, and'as contrasted to the structure of FIG. 2.

The modified construction of FIG. 5 differs from that of FIG. 2 in that the trailing portion 18d of the overlapping, reinforcing area1 8e of ,the blade is flanged outwardly at an acute angle away from the blade portion 14. This outwardly flanged form of the reinforcing portion provides a deflection limiting function under maximum deflection conditions. The general position of the blade under these conditions is shown in broken lines in-FIG. 5.

The modified construction shown in FIG. 6ditfers from that of FIGS. 1 and 2 in that the stiffening rib 15a extends in a curved or arcuate contour across the blade 14, rather than rectilinearly as in the structure of FIGS. 1 and 2. The terminal ends, indicated at 25 and-26 in FIG. 6, of the stiffening rib are outward of, or beyond, the primary area of flexure of the blade, the entire rib thus being, as in the case of all of the forms of the structure disclosed, outwardly of the primary area of flexure of the blade as it flexes under load.

While the invention has been disclosed and described in some detail in the drawings and foregoing description, they are to be considered as illustrative and not restrictive in character, as other modifications may readily suggest themselves to persons skilled in this art and within the broad scope of the invention, reference being had to the appended claims. I

I claim:

1. A fan structure including a spider having arms extending radially outward from a central hub portion, a flexible fan blade overlying each of said arms, the improvement comprising the folding outwardly of each blade longitudinally upon itself away from said arm to present a smooth bead along substantially the full length of one edge of the blade, the overlapping portion of the blade defining a reinforced marginal area extending along the surface of said blade other than the surface in contact with said arm and adjacent said bead with the primary area of flexure of the blade under load being adjacent the reinforced marginal area, and fastening means spaced along the length of each arm to rigidly join the arm and the reinforced marginal area of the blade, whereby stress in the blades due to flexing of the blades is distributed across the width of the blade and said bead provides a stiff, streamlined leading blade edge which with said marginal area provides longitudinal reinforcement for the blade radially outwardly of the joined portion of the blade and arm.

2. A fan structure as claimed in claim 1 in which a stiffening rib is formed in each fan blade extending substantially the length of the blade and disposed outwardly of the said primary area of flexure of the blade under load.

3. A fan structure as claimed in claim 2 in which the stiffening rib formed in each fan blade is disposed adjament the trailing edge of the blade and extends rectilinearly substantially the length of the blade.

4. A fan structure as claimed in claim 2 in which the stiffening rib has a curved contour across the face of the blade and extends substantially the length of the blade.

5. A fan structure as claimed in claim 1 in which the margin of said-overlapping portion opposite said bead is flanged outwardly to provide a deflection limiting function.

6. A fan structure including a spider having arms eX- tending radially outward from a central hub portion, a flexible fan blade overlying each of said arms, the improvement comprising providing multiple overlapping folds longitudinally along each blade to present a smooth hollow bead along one edge of the blade, the folded overlapping portion of the blade defining a multi-ply marginal area adjacent said hollow bead with the primary area of flexure of the blade under load being adjacent the multi-ply marginal area, and fastening means spaced along the length of each arm to rigidly join the arm and the multi-ply marginal area of the blade, whereby stress 5 in the blades due to flexing of the blades is distributed across the width of the blade and said bead provides a stiff, streamlined leading blade edge.

7. A fan structure as claimed in claim 6 in which a stiffening rib is formed in each fan blade extending substantially the length of the blade and disposed outwardly of the said primary area of fiexure of the blade under load.

8. A fan structure as claimed in claim 7 in which the stiffening rib formed in each fan blade is disposed adjacent the trailing edge of the blade and extends rectilinearly substantially the length of the blade.

9. A fan structure as claimed in claim 7 in which the stiffening rib has a curved contour across the face of the blade and extends substantially the length of the blade.

10. A fan structure as claimed in claim 6 in which the margin of said overlapping portion opposite said bead is flanged outward to provide a deflection limiting function.

References Cited UNITED STATES PATENTS 1,241,166 9/1917 Swenson. 1,250,681 12/1917 Sheldon. 1,347,003 7/1920 Baumann et al.

3,044,557 7/1962 Posh 170-160.5 3,289,924 12/ 1966 Weir 170-1605 X FOREIGN PATENTS 225,195 7/1958 Australia.

26,604 1913 Great Britain. 565,262 11/ 1944 Great Britain.

EVERETTE A. POWELL, JR., Primary Examiner.

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