US2207948A - Fan - Google Patents

Description

J ly 16, 1940 F. R. SHERIDAN 2 07,948

FAN

Filed Nov. 11, 1937 A TTORNEY Patented July 16, 1940 PATENT OFFICE f FAN Frederick a. Sheridan,.l)etroit, Mica, assignor to The Detroit Edison Company, a corporation of New York Application November 11, 1937, Serial No. 173,980

1 Claim.

This invention relates to fluid impellers and more particularly to impellers for creating a quiet flow of a gaseous or otherwise resilient medium, and especially to fans for moving air.

In air conditioning and other similar equipment it has become of importance to create a relatively large flow of air, of the order of two to five thousand cubic feet of air per minute. In such an apparatus, especially those to be used in dwelling houses, the noise created by the fan becomes an acute objection to the use of the equipment. This is especially true of exhaust fans. It has been found that a fan having a blade which subtends a large angle, for example one hundred degrees or more, around the axis of the fan hub is quieter and will move more air at slower speeds of revolution than will a blade subtending a narrower angle. In such a fan only three or even two blades are necessary.

.The problem of mounting a wide-angle fan blade on a rotating shaft is one which presents difficulties when light weight is a desideratum. A practice widely used heretofore is to provide a large diameter hub and mount the blade, usually of single-layer sheet metal or solid cast construction, in a direction spiralling around the outer surface of the hub. In such constructions the blade is held near its inner edge and its outer portion is substantially unsupported. Such a construction tends to become unstable against the fluctuating stresses to which it is subjected during operation. These include centrifugal forces as well as Various torsional forces due to the resistance of the air, and vibrations tend to occur.

An explanation of the centrifugal forces alone, with which this invention is particularly concerned, follows: If the blade of a fan has a rectangular cross-section it will tend to vibrate with larger amplitude in a plane at right angles to the wider face, and at lesser amplitude in a plane parallel to this face. When the blade is pivotally mounted so that it can oscillate in a plane at right angles to its bread face, it may be likened to a pendulum moving in this plane. If such a blade is so attached to the hub that the kinetic energy of the blade beyond the point of support is balanced by that of the blade between the point of support and the hub, then vibration of this nature tends to be minimized. Gravity forces acting on the blade, however, may cause the blade to vibrate solely in a plane perpendicular to the wider face of the blade.

The reason for this is that, with a fan rotating about a horizontal axis, the center of gravity of the blade will be above the center of oscillation when the blade is below the hub, and below the center of oscillation when the blade is above the hub, and when the center of oscillation and the center of gravity are not on the same radius, as 5 is usually the case, thisrelative shifting of the location of the gravity force will, irrespective of such other forces as may be present, tend to produce vibration in a plane perpendicular to the face of the blade, having a period of vibration 10 proportional to the speed of rotation of the fan. One must regulate the speed of rotation so as to keep the period of this vibration below the limits of audibility.

The stresses set up in the material of the blade 15 of usual design are for the most part bending stresses and the amplitude of the resultant vibration is governed by the value of the force causing the original deflection and the ability of the blade to resist the force.

v 20 One of the objects of my invention is to pro- .vide a blade in which such forces acting at an angle to the face of the blade will, by developing compression and tension in the blade-fans, be transferred directly to the point of support of the blade, so that a minimum of bending will occur.

Another object of my invention is to providea mounting for a wide-angle fan blade which will support it against vibration due to centrifugal and torsional forces.

A further object of my invention is to provide an improved blade construction for a fan having wide-angle blades.

A still further object of my invention is to provide a flexible mounting for a wide-angle fan blade.

Other objects of this invention will more fully appear from the following detailed description taken in connection with the accompanying 4 drawing which illustrates the preferred embodiment thereof and in which:

Fig. 1 is an axial elevation of a fan having wide-angle blades constructed and mounted in accordance with my invention;

Fig. 2 is a side elevation of the fan shown in Fig. 1;

Fig. 3 is a section taken on the line 3-3 of Fig. 1;

Fig. 4 is a section taken on the line 4-4 of Fig. 1;

Fig. 5 is a section, similar to Fig. 3, of a modifled blade-mounting enlarged disproportionatelyto the blade, for the sake of clarity; and

Fig. 6 is a section taken on the line 6-6 of Fig. 5, the blade itself being somewhat distorted. Referring to the drawing l0 illustrates tubular bearing members together constituting a fan hub. These are afiixed to a spider I i, as by bolts I2, the

spider being in the form of a tube arranged with its axis perpendicular to that of the hub members and having apertures l3 aligned with the hub members for the reception of the fan shaft.

The tube H is slotted at either end as at M, the slots having one side 5 which is substantially parallel to the axis of the shaft H, and the other side I! is curved, as shown in Fig. 2. The two sides of each slot are adapted to conform to the outer curvature of a hollow Wide-angle fan blade which is inserted therein and held in place in a manner to be later described, the tube ll thus constitutes a tubular extension of the hub I0.

The blade proper comprises a leading face 18 and a trailing face l9 both formed of sheet metal. As shown the trailing face presents, to the air traveling thereover, a convex surface having a much smaller radius of curvature than the leading face iii.

The two faces are punched out to provide aligned apertures 20 and H and may then have their inner faces covered with an adhesive vibra tion dampening material such as canvas glued in place. A block 23 having a contour to fit the inner curvature of the two. faces is placed between them with its central aperture 24 in alignment with the punched holes in the faces.

This assembly is then clamped together in one of the slots [4 of the fan spider II. This is accomplished by a bolt 26 which is passed through washers 25 and an opening 21 in the tube II and through the openings in the faces and the block 23. On the trailing face of the fan is a washer 28 having a convex surface corresponding in contour to that of the trailing face. A nut 29 and washer 3| then serve to hold the blade against the straight side l6 of the slot l4.

As shown in Fig. 3 the tube l I may have a hole 32 to provide clearance for the threaded end of the bolt. Also the slot I4 is seen to be sufficiently wide so that there is no contact, nor any excessive clearance, between its rear edge I! and the convex trailing face l9.

With the faces clamped in position together their edges may be integrally secured together, and, after both blades have been secured in position on the tube l I suitable closure plates 33 may be attached as by bolts 34, these plates serving to bridge the gap between the two blades.

axis of the bolt 26 which, as is seen, constitutes the support for the assembled blade structure.'

With the center of oscillation substantially coinciding with the point of support there can be no tendency for the blade to pivot about any axis passing through the point of support. With the blade held against such rotation, which could be induced by centrifugal force or by the air forces, any undue motion, pf the pendulum type previously referred to, is inhibited and hence any tendency to cause noise is retarded.

The blade construction shown has been found to be inherently stable against bending about any axis perpendicular to that of the supporting bolt 26. This is due to the fact that any such bending as would be produced by the air forces tends to create a tension in the leading face l8 and a corresponding compression in the trailing face I 9, which, due to its truss-like curvature, tends to resist bending. Hence, a further factor contributing to noise, is reduced.

In the modification shown in Figs. 5 and 6, the tube H passes radially outwardly within the fan blades, the blades having, for this purpose, apertures 36 on their inner edges. In these figures the blades are somewhat distorted in order clearly to show the details of the mounting. In fact the blades, in these figures, may be considered as diagrammatical. The blocks are provided with radially extending apertures 31 through which the tube is passed. It is held within the apertures by bolts 38 lying in a plane substantially tangential to the path of movement of the center of oscillation of the blade assembly. Thus the kinetic energy of rotation of the part of the blade' above its point of support is equal to the kinetic energy of rotation of the part of the blade below its point of support.

With this construction the bolt 26 must be replaced by studs 39 which hold the leading edge against the block 23 and the stud 4| which holds the trailing face to the block.

The apertures 31 may have a diameter slightly greater than the outside diameter of the tube II bolts 38. These blocks 42 preferably have a high hysteresis loss which produces a retardation or lagging efiect which tends to absorb any vibration, should such occur.

One of the important features of my construction is that the center of gravity of the fan blade and the point of support are in substantially the same plane perpendicular to the axis'of the hub. This tends to minimize rocking of the blade about the axis of the bolts 38.

While I have herein described in some detail a specific fan embodiment of my invention, which I deem to be new and advantageous and may specifically claim, I do not desire it to be understood that my invention is limited to the exact details of the fan construction, as it will be apparent that changes suitable for water propellers may be made therein without departing from the spirit or scope of my invention.

I claim:

In a fan, a rotatable hub, a hollow blade, a tubular arm extending radially from said hub and having a portion of the wall thereof cut away, said blade lying in said cutaway portion and extending through said arm, and means for securing said blade to said arm.

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