US2985245A

US2985245A - Propeller fan

Info

Publication number: US2985245A
Application number: US752583A
Authority: US
Inventors: Ralph P Maloof
Original assignee: NATIONAL FROST PROTECTION CO Inc
Current assignee: NATIONAL FROST PROTECTION CO Inc
Priority date: 1958-08-01
Filing date: 1958-08-01
Publication date: 1961-05-23
Anticipated expiration: 1978-05-23

Description

May 23, 1961 R. P. MALooF PROPELLER FAN Filed Aug. 1, 195s AIV-.

ALPH E /VA/.oo

A TroRNEY United States Patent PROPELLER FAN Ralph P. Maloof, Burbank, Calif., assiguor to National Frost Protection Co., Inc., Burbank, Calif, a corporation of California Filed Aug. 1, 1958, Ser. No. 752,583

3 Claims. (Cl. 170-173) The present invention relates to a propeller fan construction of large size, of high aerodynamic ei`nciency and low cost. The propeller fan of the invention is adapted for use on wind machines, air exhausters, and air inducers.

I have found that I may use an aluminum alloy extrusion of constant cross section, so shaped that p-ropeller fan blading could be made directly simply by cutting off lengths of extruded stock as the design required.

It is known that, while aluminum and other alloys, capable of being extruded, can be quite strong for the weight, there are limits to the stresses these alloys can endure, and such limits may be too low to allow the use of extruded blading of a practical cross section.

In any propeller fan, an impulse or thrust is produced by the action of the moving blades on the air impelled. This impulse is directly related to the change in energy level of the air passing through the plane of the fan. The impulse must be restrained completely in order to prevent axial travel of the entire fan mechanism. Therefore, the aerodynamic forces on the blading, must be balanced out by reacting forces iu the blades and fan structure.

The geometric arrangement giving the best aerodynamic eiciency is one that becomes well suited to a slender, thin, smooth, moderately twisted metal extrusion, except for the structural properties required of the region where the propeller fan blade joins the hub. In this connecting zone, large moments may appear which produce bending stress that actually exceed the tension stresses caused by centrifugal force,V It becomes necessary then to provide a mechanism which would make use of centrifugal force to overcome orrelieve `the bending moment. The connection between the hub and each blade forms a continuous, elastic,jdeflectable joint.

The technique of `using Ahinged .connections to sever the bending moment transfer path on aerodynamic lifting rotors has long been known. .The result of such a connection is a rotor whose blades are allowed to cone in thev direction of lift. The cone angle is automatically stabilized bythe balance of centrifugal restoring moment and aerodynamic lifting moment. The-pin of'ithe connecting hinge has only the centrifugal force tofcontend with. However desirable the hinge connection may be from a stress standpoint, the problems which attend are such that low cost is not possible, and inspection must be dependable and frequent. I have discovered that a bolted connection of very high strength and relatively low bending stiffness may be used to join hub and blade and whereby no moving parts such as hinges or auxiliary structure is required to produce a propeller fan rigid enough to support itself when not revolving.

An object of the invention is to provide an air propeller fan of superior mechanical design, economical to fabricate, of determined yaerodynamic efficiency, and having long trouble-free life.

With the -above mentioned and other aspects in View, the invention consists in the novel and useful provision,

formation, construction association and relative arrangement of parts, members and features all as disclosed in the drawing, described generally, and linally set forth in the claims.

In the drawing:

Figure l is a fragmentary partially sectional View showing the connection between the hub and the blades,

Figure 2 is a view, partially in section and in fragment, revolved substantially from the showing of Figure 1,

Figure 3 is a fragmentary sectional view on the line 3-3 of Figure 2,

Figure 4 is a sectional view on the line 4-4 of Figure 2, and

Figure 5 is a fragmentary view, in side elevation showing a motor and the improved blade construction with its hub secured -to the shaft of the motor.

Referring now to the drawing, and specifically to Figure l, I have shown by Way of illustration, an engine shaft 1 having the usual splined portion 2, and screw threaded end 3. The hub, designated generally as 4, includes two

annular plates

5 and 6, provided with the usual annular tapered keys 7 and 8 for securing the hub to the splined portion 2, particularly when the nut 9 is tightened on the screw-threaded end 3. In this hub construction, there is interposed between the

plates

5 and 6, tangs 10 yand 11 which, in the present construction, are diametrically opposed. These tangs, in each instance, have end portions of a given thickness secured between the

plates

5 and 6, by means of bolts designated generally as 12, passed -through aligned bores in both the tangs andv the plates. In the present instance, four bolts are utilized, with ends of said bolts carrying nuts 13. Each tang has a reduced thickness portion provided with a pair of opposedV flat surfaces 14 and 15 (Fig. 3) which are parallel and at an angle to a plane normal to the axis of the hub and engine shaft. As stated, in the present illustration, I use a pair of blades, 16 and 17, of identical constructionLand the blades, in the present instance,.

ing end is provided with a substantially plane or atl top surface 19 lwhichy isparallel to the flat bottom'surfaee V,20, with the splice'plates receiving said blade surface' therebetween. Both splice plates, top and bottom, l as well as the blade, are provided` with a series ofspaced' apart ,boresf designated Vgenerallyaas 22,*forreceivin'gf' bolts ,21` carrying end`nuts'23 and washers 24 whereby-- pairs"Y of splice plates will-secure ablade therebetwee The numberof iboltsiibetwe'en'the" splice plates "and a blade, as well as the number of bolts between the splice plates and a tang may .vary from that shown in the drawing.

The reason for providing the parallel flat surfaces i9 and 20 for each blade, is that this construction allows the blade to be joined to a hub tang with bolts or pins in double shear. Thus the bolt bending moments caused by a centrifugal force carried in the splice plates are equal and opposite, on each side, leaving a net moment of zero to be transmitted to the tang and blade. This construction reduces the number or machine operations, and provides a reference surface to use for machining and measuring.

In considering the type of hub employed, it isV quite obvious that the extruded aluminum blades would not Patented May 23, 1961 3 have sufficient strength to absorb the high bending moment from blade lift ifit were not for the relieving moment produced by centrifugal force. The centrifugal momentdoes not develop, however, until the blade cone is-festablishedqby'forward deflection lof the blade. lf the'blacle` were hingedwith respect to an axis perpendiouf lar to` Vthe rotation axis, that is, parallel to a 'blade chord, free coning would take place and all momentswould balance to zero at the edge, but because of mechanical difliculties arising from hinge bearings,retc., Yand because of manufacturing costs of. such machinery, an attachment having Ylow bending stiness which includes the tangs with the splice plates for hub to blade connection was devised. The thickness of the blade` attachment tang is established so that the high bending stresseswould be limited by. restrained coning of the blades; The aerodynamic bending moment is nowV only partially absorbed by elastic deiorrnation of the hub tangs ybecause the bending deformation of the blade provides affor-Y ward shift in the blade mass, allowing centrifugal force to act on the displaced mass in such a Way as to counteract the aerodynamic force to balance out the moment produced by them. While, from an ideal standpoint, the flexible connection of the hub appendage should have a neutral elastic plane lying exactly perpendicular to the rotation axis, I do not practice this, because the marginal advantage in dynamic stability is overbalanced by the cost advantage of allowing the tang to deflect in the blade inclination plane which is displaced-from perpendicular about 17. The detailsV described areV the ones that make it possible to produce these propeller fans at a low cost and without sacrifice of performance.

When l speak of a flexiblel connection between the hub and a blade I am referring tolow bending stiffness in the hub to blade connection such as may be accomplished by providing a steel bartang having a width tive to ten times its thickness.

A practical limit to the art of extruding large shapes is taken advantage of in the design of the blades for this propeller fan. It is known that the use of very slender, thin blade on propeller fans produces high aerodynamic eiiiciency. But such bladestend to flutter destructively if the torsional stiffness is not kept at a maximum, particularly if the blade chord-wise center of gravity moves too far toward the trailingedge. With extruded-blades of Vsolid shape, avery thin airfoil can beused without the need for auxiliary chord-wise balancing orartificial stabilizing` means, because .of .the high torsion stillness. The blade thickness ratio can be made as thin as a practical hubconnection will allow.

The propeller` fan described herein may orv maynot have twisted blades.V In thevcaseV of la. Wind machine propeller fan, a twisted blade is .used;however, certain air inducers need not have twisted blades.

Iclaim:

v1. In a two-blade propeller-fan construction, a hub comprising two annular plates for mounting on a driving shaft, a pair of tanks 180V apart, each tang having a portion bolted between the plates and having beyond the hub a reduced thickness portion with parallel sides making an angle of about 17 with a plane at right angles to the axis of the hub and shaft, a pair of elongated slender propeller blades located at the outer ends of the tangs, a pair of velongated splice plates at both sides of the hub for receiving therebetween the reduced thickness portion of the tang, and a portion of the blade to provide a exible connection between ,the hub and the blade, and means for securing each pair of splice plates together in double shear.

2. Av propeller fan of large lsize for wind machines including a pair Aof diametrically disposed blades formed from a metal alloy extruded stock of uniform cross section, a tangrfor leach blade including a thicker end portion with parallel sides and a reduced thickness portion with opposed flat surfaces parallelto each other and at an acute angle to a plane normal to the axis about which the fan revolves, said reduced thickness portion being from live to ten ,timesy asvwide as it is thick, a hub including a pair 'of annular. hub platesrsecured together with the `thicker end portions of thettwo tan'gs between them, four splice plates of uniform thickness and length` securing together the blades and the reduced thickness portions of thetangs with theinner ends of the splice plates spaced Vfr'omthe hub, vand means for mounting the hub on an engine shaft.Y

3. A propeller fanof large size for two-bladed Wind machinesV having long'thin exible blades,Y including a Y'pair 'of diametrically disposed blades formed from an aluminum alloy extruded stock of uniform cross section.

having a Vcurved upper face and a lower centrally plane face, a tang for each blade including a thicker inner portion with parallel sides and a reduced thickness portion with opposed flat surface parallel toeach other and at an acute angle of about 17 to a plane normal to the axis about which the fan revolves, said reduced-thickness portion being fromfve to ten times as wide as it is thick, withy its outer end proximate Ythe inner end of a blade, a hub including a pair of Vannular hub plate disks bolted together with the thicker inner portions of the two tangs between them, two pairs of elongated spliceplates Y the inner ends of the splice plates spacedfrom the hub,

the upper faceof each blade having a at portion adjacent the proximate splice plate, and meansfor mounting the hub on an engine shaft.' Y

References Cited in the file of this patent UNITED STATES PATENTS Y OTHER REFERENCES Y Mechanical Engineers Handbook, by L. S. Marks,

Vsecond edition, (page 718 relied upon), published 1925,