US2876891A

US2876891A - Vibratory apparatus

Info

Publication number: US2876891A
Application number: US333108A
Authority: US
Inventors: Long George; Tsuchiya Takuzo
Original assignee: General Mills Inc
Current assignee: General Mills Inc
Priority date: 1953-01-26
Filing date: 1953-01-26
Publication date: 1959-03-10
Anticipated expiration: 1976-03-10

Description

March 10, 1959 F|G.,l A 1//// //////.l //////AV// (5. LONG ErAL .VIBRATORY APPARATUS Filed Jan. 26, 1953.

FIG. 2

'FIG. 4

IN l/E/VTORS GEORGE Lone TAKUZO TSUOHIYA BY arro mv 'r United tte s Pate t VIBRATORY APPARATUS George Long and Takuzo Tsuchiya, Minneapolis, Minn, assignors to General Mills, Inc, a corporation of Delaware Application January 26, 1953, Serial No. 333,108

14 Claims. (Cl. 198-220) The present invention relates to vibratory apparatus and more particularly to an improved type of spring for such apparatus.

Vibratory conveyors are well known in which a vibrating or working portion of the apparatus is supported on springs, such as spring beams, for back and forth movement along a path substantially perpendicular to the springs. In such devices the spring beams are generally clamped rigidly at some portion, such as one end or both ends, with the remaining free or effective length of the spring being subject to deflection under the particular applied forces and loads.

We have found that resilient means of this type are particularly subject to breakage or failure at the edges of the rigidly clamped portions. At these points, the

spring is ordinarily rigidly confined between the sharp edges of clamping members or plates which hold the spring ends substantially rigid and permit deflections of those portions of the spring outwardly of the clamped area. Since known springs of this type have, to the best of our knowledge, always been constructed with uniform cross section throughout their length, our initial attempts to avoid breakage at the edges of the clamped areas involved the use of a stronger cross section throughout the length of the spring. In the use of vibratory apparatus, however, it is often necessary to provide springs with a spring constant in a specified range, and with certain construction limitations on the available length of such springs. If springs which are designed to provide the spring constant and deflection for a particular application are made of stronger cross section, then the spring constant is increased. It is then necessary to increase the length of the springs if the same spring constant is desired with the stronger cross section. This increased spring length not only adds cost to the apparatus, but may also make it impractical to position such a device within the available space provided. Furthermore, the increased length of the springs proportionally increases the bending moment at the edges of the clamped areas of the spring ends. This offsets part of the decrease in stress due to greater thickness, so that extremely long springs would be necessary to obtain the desired stress reduction at the clamps.

With the above problems and findings in mind, it is one object of the present invention to provide an improved type of spring beam for vibratory apparatus.

A further object is the provision of resilient springs of novel construction in which the end portion of the spring which is adapted to be rigidly clamped is provided with a substantially stronger cross section than the remaining portions of the spring.

Another object is a spring support for vibratory apparatus in which at least one end portion of the spring is made of predetermined thickness or cross section providing a substantially higher safety factor at the edges of the clamped area than at the intermediate portions of the spring.

Another object is a spring construction in which an end portion of the spring is made of greater thickness than the intermediate portion of the spring and in which a tapering portion of gradually decreasing cross section and strength is provided between the end portion and central portion.

A further object is such a spring in which the portions of difierent cross section are formed by forging.

Other objects and advantages of the invention will be apparent from the following specification in which certain preferred embodiments of the invention are described.

In the drawings which form a part of this application and in which like reference characters indicate like parts,

Figure 1 is a side view of a vibratory apparatus incorporating features of the present invention.

Fig. 2 is a side elevation of one of the individual supporting springs of the apparatus of Fig. 1.

Fig. 3 is a plan view of the spring of Fig. 2, and

Fig. 4 is a view similar to Fig. 2 of another embodiment of the invention.

The present invention is illustrated in Fig. 1 in connection with vibratory apparatus adapted to convey material from one point to another. This conveyor includes a working member or conveyor tube 10 having an inlet at 12 and outlet at 14. This working member is supported on resilient spring units 16 which have their lower ends secured to an intermediate base or frame portion 18. This intermediate base 18 is in turn supported for resilient back and forth movement by means of upright beams 20 secured to an overhead support or frame 21.

The necessary driving forces for vibration of the working member 10 are applied in this case to the intermediate frame 18 by cyclical force-applying

units

22 and 24 which are connected together for simultaneous operation by suitable belting 26. These force-applying

units

22 and 24 are preferably of the counter-rotating weight type in which each unit includes a pair of weights rotating about transversely spaced axes such that all transverse forces (lateral or vertical) are cancelled out and the resulting force is one of reciprocation back and forth substantially longitudinally of intermediate base 13 parallel to working member 10. The counter-rotating weights of these force-applying units are driven by a motor or power source 28 through belting 30. In this case the motor 28 is illustrated as carried by a separate supporting frame 32 secured to the overhead foundation 21. In many cases, however, it may be desirable to mount the motor 28 directly on the intermediate frame 18.

The resilient supporting units 16 which connect the working member 10 to the intermediate base 18 are illustrated as including pairs of springs 34, the springs of each pair being spaced longitudinally of the conveyor unit and having their upper ends rigidly clamped to con necting members 36 which are in turn secured to the conveyor tube 10. The lower ends of the springs 34 are similarly rigidly secured to connecting members 38 fastened to the intermediate frame 18. For certain applications, the springs 34 may be rigidly connected at only one end, although in such cases it is necessary to provide different spring lengths if the same spring constants are desired. Certain features of the conveyor described above and illustrated in Figure 1 are claimed in the co-pending application of Long and Tsuchiya entitled Vibratory Conveyors, Serial No. 329,556, filed January 5, 1953, and an application of Tsuchiya and Long entitled Vibratory Apparatus, Serial No. 330,768, filed January 12, 1953, now abandoned.

According to the present invention, the individual springs or resilient beams 34 are constructed with a cross section which is not uniform throughout the length of the beam and which will provide a substantially greater safety factor at the clamped end or ends than at other esteem points. Figs. 2 and 3 illustrate the details of one spring beam of this type. This particular spring beam is adapted for rigid clamping engagement at each end, between suitable clamping members connected to the working or conveying member 19 of the intermediate base 18 as described.

In order to minimize the danger of spring failures immediately adjacent the clamped areas, the springs 34 are provided with at least one end portion of stronger cross section, which reduces the stresses due to bending at the area adapted to be received between the clamps. In this case, since the spring is designed to be clamped at both ends, portions of stronger cross section are provided at 49 and 42 at the respective ends of the spring. In this example, the stronger cross section is provided by increasing the thickness of the springs in the direction of relative vibration perpendicular to the springs. Thus

portions

40 and 42 are substantially thicker than the remaining portions of the spring, the total thickness at these portions being determined as described in detail below.

A substantial central portion 44 of the spring is then provided with a cross section of substantially less thickness or strength, in order that the desired resilience or spring constant can be obtained without substantially increasing the length of the spring. Between the

end portions

40 and 42 and this central portion 44- of smaller cross section, the spring is provided with tapering

portions

46 and 48 in which the cross sectional strength gradually decreases from the thickened end portions to the central portion. For convenience in discussion, it is noted that the tapering portion 46 of this example joins the thickened end portion 40 at a transition point or line 50, while the other end of the tapering portion 46 meets the central portion 44 at a transition line 52. Similarly, the tapering portion 48 meets the thickened end portion 42 at a line of transition 54, while the other end of the tapering portion 48 intersects the thin central portion of the spring at a transition line 56.

While these

transition lines

50, 52, 54, and 56 are shown in the drawing and are referred to herein as a matter of convenience in defining the boundaries of the various portions, it will be understood that in practice there will ordinarily be no sharp change in cross section at these points. The change in cross section from one area to the other should be accomplished with certain minimum radii of curvature in order to avoid stress concentrations at these transitional areas.

In the spring of Figs. 2 and 3, the thickness of the central portion 44 is uniform throughout the area, while the thickness of the

tapering portions

46 and 48 decreases not only gradually but also uniformly per unit length from the end portions of greater thickness to the edges of the central portion. For convenience in con struction, this particular spring is also illustrated with one flat side 57, so that the portions of different thickness and the tapering sections may be formed by suitable milling operations on only one surface of the spring.

In the design and construction of springs of the type illustrated in Figs. 2 and 3 for a particular vibratory application, one must first ascertain the particular spring constant, desired amplitude of vibration, and effective spring length for a particular case. These factors can be determined in known manner and may involve the following of the principles set forth in the earlier of the co-pending applications referred to above. Once the spring constant, length and desired amplitude are known, it is possible to compute the bending moment and in turn determine the stress at any given point along the length of the spring.

According to one feature of the present invention, and because we have found that there is a greater tendency for springs to fail at the edges of rigidly clamped portions corresponding to

lines

50 and 54, than at intermediate portions of the spring, we provide a substantially greater factor of safety at these points than at the remaining intermediate portions of the spring. Since the endurance limit of a particular spring material is generally known, a cross section can be provided for the end portions and 42 which will, for the particular spring constant, amplitude and spring length of the particular case, provide suihcient additional strength at point to obtain the desired safety factor of endurance limit over expected stress at this point.

While we do not wish to be bound to any particular theory or explanation of the unusual breakage at the boundary of the clamped area in prior forms of construction, it is our belief, based on experience, that the engagement of the edges of clamping members at these points may contribute to such spring failure. Whether this failure is due to abrasion or grooving of the spring surface at this point, or to other stress-raising factors, the fact remains that we have found it important to provide a greater safety factor at these areas than in the remaining intermediate portions of the spring. Thus the thickness or cross section of the

end portions

40 and 42 is modified so that a higher safety factor is provided in these areas. Depending on the nature of the spring and clamping materials and other factors, this safety factor is preferably at least in the range from two to four, said safety factor involving the ratio of endurance limit to combined calculated stresses at these points, with the bending stress computed on the basis of the predetermined desired spring constant, amplitude of vibration and spring length, and the stresses due to shear and/or column loading being computed in known manner.

The thickness or cross section of the central portion 44 must then be so chosen that it will provide the desired spring constant in spite of the increased thickness or cross section of the end portions of the spring. In other words, the portion 44 must be substantially less stifli than would be the case if springs 34 were of uniform increased cross section throughout their length. Obviously the spring constant will depend not only on the thickness or cross section of this central portion, but also on the manner in which the cross section changes between the thickened end portions and the thinner central portion. In the present case, where the tapering

sections

46 and 48 have a thickness which decreases uniformly per unit length from the thickened end portions of the springs to the thinner central portion, we have found that the length of the tapering portion as measured from 50 to 52 or from 54 to 56 should be in the range from one-third to one-sixth (and preferably of the order of one-fourth to one-fifth) of the free effective length of the spring be tween points 59 and 54.

Thus the exact location of the

transition points

52 and 56 may vary in a construction of this type, and it will be apparent that the shorter the tapering portions 46 and 43, i. e., the

nearer points

52 and 56 come to

points

50 and 54, respectively, the greater will be the bending stress at

points

52 and 56 for a given spring constant, amplitude and spring length. In determining the exact location of these points 52 and '56, the endurance limit of the material and the calculated stresses at these transition points will again be considered. In this case, however, since the spring is not rigidly clamped at either side of the transition point and immediately adjacent thereto, we have found that a much lower safety factor can be used. We have found that a safety factor in the range from 1.25 to 2.0 is sutficient at these points.

The reduction in cross section or thickness of the central portion 44 of the spring makes it possible to obtain a given desired spring constant with a relatively shorter spring. Throughout the central portion of the spring the safety factor required will be no greater than that re quired at the

transition points

52 and 56. The strength of this central portion must of course be adequate to spring under the particular working conditions to be encountered.

Stated in another way, the spring of Figs. 2 and 3 may be considered as one in which the central or intermediate portions are provided with a cross section of reduced size and stiffness such that the desired spring constant can be obtained with a spring length substantially less than the spring length which would result in the same resilience if the spring were of uniform cross section similar to the

strengthened end portions

40 and 42 throughout its length. The extent to which this central portion is reduced in strength can thus depend in any given case on the rigidity of the space requirements which control the spring length, as well as on a consideration of the extent to which the stronger cross section at the spring end is offset by increased bending moments due to increased spring length. In general, however, it is desirable to reduce the intermediate cross section sufticiently to provide the desired spring constant with a spring length of not more than 1.5 times the length of the originally determined uniform spring with unstrengthened ends. In some cases the non-uniform or tapered spring of the present invention might even be made shorter than such original uniform spring length.

By way of example, assume that it is determined for a given installation that a spring constant of 540 pounds per inch is needed for a desired deflection of 0.23 inch. A uniform steel spring substantially 2 inches wide, onehalf inch thick and 24 inches long will meet these limitations. In such a spring, however, the maximum end stress at the edges of the clamped ends is substantially 18,000 pounds per square inch, which does not provide the desired higher safety factor at these ends.

If the entire spring is thickened to 1 inch to increase the safety factor at the end, then it will be necessary to lengthen the spring to 48 inches to obtain the desired K of 540 pounds/inch. In such a spring, for the same deflections of 0.23 inch, the end stress will be reduced to 9,000 pounds per square inch, but the extra length would involve extra cost and might not fit the desired installation.

According to the present invention, however, it is possible to obtain similar or even greater stress reduction with spring lengths closer to the original 24 inch length, by decreasing the cross section of central portions of the spring. For example, a spring beam similar to that shown in Figs. 2 and 3 but cut away symmetrically on opposite surfaces such as 44 and 57, with a uniform width of 2 inches, a thickness of one inch at the ends and one-half inch at the central portion, a free effective length of 32.8 inches, and with uniformly tapered portions extending one-fourth of the free effective length, will also have a K of substantially 540 pounds/inch. For the same deflections of 0.23 inch, the end stress will be reduced to 6,100 pounds per square inch, while the stress at the quarter point, where the tapered portion meets the central portion will be substantially 12,200 pounds per square inch. Thus a substantially higher safety factor is provided at the edge of the clamped area than at the intermediate portions, such as the quarter point.

If the length of the tapered portion of such a spring is reduced to one-fifth of the effective length a shorter total free spring length of 31 inches will give the same K, with end stress of only 5780 pounds per square inch, and a stress at the one-fifth point (where the tapered portion meets the central portion) of 13,760 pounds per square inch.

Other variations in spring dimensions can be used for the same purpose of reduced end stress and shorter spring length. Thus, if the length of the tapered portion is one-fifth the free length, spring dimensions of two inches uniform width, thicknesses of three-fourths and threeeighths inches at the ends and center respectively, and a free effective length of 24 inches will again give essen- 6 tially the same K, with end stress of 7,500 pounds per square inch, and a stress at the one-fifth point (where the tapered section meets the central portion) of 17,800 pounds per square inch.

If the spring just described is modified to provide a tapered portion whose length is one-fourth the free effective length of the spring, with the same uniform width of two inches and thicknesses of three-fourths and threeeighth inches at the ends and central portion respectively, then the same K can be obtained with a free efiective length of 24.8 inches. In this case the end stress will be 8,120 pounds per square inch and the stress at the quarter point will be substantially 16,240 pounds per square inch.

Again, with the one-quarter taper, a spring of two inches width, thicknesses of five-eighths inch at the ends and five-sixteenths inch at the central portion, and a free effective length of 20.5 inches will still provide the same K, with end stress of 10,000 pounds per square inch and a stress of 20,000 pounds per square inch at the one-quarter point.

Or with the one-fifth taper, a spring of two inches width, thicknesses of five-eighths inch at the ends and five-sixteenths inch at the center, and a. free effective length of only 20 inches will have the same K, with maximum bending stress at the ends or edges of the clamped area of 8,690 pounds per square inch and a stress of 20,650 pounds per square inch at the one-fifth point.

In all these cases, it will be observed that a substantially higher safety factor can be provided at the spring ends, i. e., at the clamped edges, than at intermediate portions of the spring. Since the endurance limit for spring steels may range from 45,000 to 55,000 pounds per square inch, the desired high safety factor can be obtained at the clamped ends, with a lower but still adequate safety factor at the intermediate portions, even when stresses due to shear and column loading are added to the above bending stresses.

It will be understood that the use of a central portion of uniform cross section and the use of tapered portions in which the cross section changes uniformly per unit length, merely represent one solution which may be convenient from the standpoint of ease of milling and ease g of computation. It is also possible to design a spring in which the cross section varies non-uniformly, for example along a parabolic or hyperbolic curve, to provide the desired extra strength and safety factor at the ends, and yet obtain the required K with short springs. Fig. 4 illustrates such a spring 70, in which the end portions are thickened at 72 and 74 and provided with the

usual openings

76 and 78 for clamping bolts. The midpoint 80 has a much weaker cross section, with

opposite surfaces

82 and 84 recessed or cut away to provide tapering portions and 92 in which the cross section changes smoothly but non-uniformly per unit length from the ends to the center. The spring of Fig. 4 may be of uniform width as in Figs. 2 and 3, or may also have a variable width somewhat similar to the variable thickness of Fig. 4.

In all these cases the minimum cross section at the center will be dictated by the expected column loading and shear. The cross section at the ends will be great enough to provide the desired high safety factor at the clamps, while the change in cross section between the center and the strengthened ends will be determined so as to obtain the desired K and deflection, without the disadvantages of extremely long springs.

The spring construction of the present invention can also be used where the end stresses are already within safe limits for a given deflection and where it is desired to increase the allowable deflection without substantial increase in stress.

The provision of a spring beam with intermediate porti-ons of reduced or less stilt cross section may thus be '7 considered either as a means for reducing excessive end stresses to obtain a desired high' safety factor with short springs, or as a means for obtaining greater allowable deflections for the same end stresses where such stresses are already within the prescribed safety limits for the desired spring length.

It should be noted that laminated springs with more leaves at the ends than at the center and with progressively difierent lengths for the extra laminations are not effective for the purposes of this invention because of the high stress concentrations where the cross section changes abruptly and discontinuously at the ends of the laminations.

As another specific example of one form of spring according to the present invention, a conveyor like that shown in Fig. l was designed with a total of 20 individual supporting springs each shaped as in Figs. 2 and 3. The springs were made of aluminum alloy and had a uniform width of two inches, with an over-all efiective free length of 22 inches, the thickness of the end portions was five-eighths inch, the thickness of the central portion five-sixteenths inch, and the length of each uniformly tapered portion was 5% inches.

In connection with the determination of stresses in springs of non-uniform cross section as described herein, reference is made to the standard text by Timoshenko on Strength of Materials (1940 edition). in this connection we prefer the use of the modified bending moment diagram and the conjugate beam theory as a means of determining deflections for such springs.

We have found that springs of variable cross section as described and claimed herein are advantageously made from metallic stock of uniform cross section by forging. The forging step effectively causes the metal to flow longitudinally away from the portions of smaller cross section with minimum damage to the grain or fiber orientation of the metal.

Where it is desired to form the springs by milling away part of the cross section of a metal beam of uniform cross section, it is possible to mill opposite sides of the beam symmetrically or to cut away only one side and leave the opposite surface fiat as at 57 in Fig. 2. Even in this latter case, however, it is desirable to mill away a thin layer of metal along the flat surface 57, in order to remove all abrasions or other surface defects which might provide points of stress concentration.

The provision of a spring construction with end portions of sufficient thickness to provide an extra or fourfold safety factor at the edges of the clamped end portions, together with a reduction in thickness of the central or intermediate portions of the spring toward the minimum required for the expected column loading and shear, and the provision of variable or tapering sections between these two portions in such a Way as to provide a substantially smaller safety factor at the intermediate unclamped regions of the spring, all combine to make possible the construction of conveyors in which the desired spring constant can be obtained with minimum spring length and with an outstanding reduction in the likelihood of breakage or spring failure at the edges of the clamped spring ends. While the above discussion and examples have been limited to spring beams which defleet laterally, the principles and teachings set forth here-v in are also useful in certain other applications whereother types of springs, such as torsion bars, are utilized.

The construction described above thus accomplishes the objects set forth at the beginning of this application and makes possible greater economies in the space re quired by vibratory apparatus, the size of the parts, partieularly with respect to spring length, substantially longer spring life and less maintenance of the machine in the way of replacement of springs which have approached the fatigue point. Since minor variations and changes in the exact details of construction will be apparent to-.persons skilled in this field, it is intended that this invention shall cover all such changes and modifications as fall within the spirit and scope of the attached claims.

Now, therefore, we claim:

1. A vibratory device having a vibratory member and at least one integral individual spring operatively supporting said member for vibration along a given path, at least one clamping member rigidly secured to one portion of said spring and engaging a surface of the spring, said one portion of the spring having a construction providing a substantially greater safety factor at the edge of said clamping member than the construction at the unclamped portion of the spring spaced from said clamped portion, said spring having a construction of gradually decreasing strength between said clamped and unclamped portions.

2. A vibratory device having a vibratory member and at least one integral spring beam operatively supporting said member for vibration along a path generally perpendicular to the spring beam, at least one clamping member rigidly secured to one end of said spring beam and engaging a surface of the beam which rs generally perpendicular to said path, said one end of the beam having a stronger cross section at the edge of said clamping member than the cross section at the intermediate portions of the spring beam a gradually tapered portion between said stronger cross section and said cross section of the intermediate portion of the spring beam.

3. A vibratory device having a vibratory member and a plurality of spaced individual spring beams operatively supporting said member for vibration along a path generally perpendicular to the spring beams, each of said individual beams consisting of a single integral piece of material and having at least one clamping member rigidly secured to one end of said spring beam and engaging a surface of the beam which is generally perpendicular to said path, said one end of the beam having a greater thickness along said path at the edge of said clamping member than at the intermediate unclamped portions of the spring beam a tapered portion between said one end of the beam having a greater thickness and said intermediate unclamped portion of the spring beam.

4. A vibratory device according to claim 3 in which said clamping member is secured to the end of the beam adjacent said vibratory member and is also rigidly secured to the vibratory member.

5. A vibratory device according to claim 3 in which the intermediate portion of the spring beam includes a substantial intermediate length having a uniform thickness less than the beam thickness at the edge of said clamping member, and the thickness of said beam decreases gradually from the edge of the clamping member to said portion of uniform thickness.

6. A vibratory device according to claim 5 in which the beam includes a tapered portion in which the thickness of said beam decreases gradually and uniformly per unit length from the clamping member to said portion of uniform thickness.

7. A vibratory device according to claim 6 in which the length of said tapered portion is in the range from one-third to onesixth of the free effective length of the beam.

8. A vibratory device according to claim 7 in which the length of said tapered portion is one-fourth to one fifth of the free elfective length of the beam.

9. A vibratory device having a working member, a base member, and a plurality of spaced individual spring beams operatively connecting said members for vibration of the working member along a path generally perpendicular to the spring beams, the improved combination comprising at least one pair of clamps rigidly secured to one of said members and engaging opposite surfaces of the corresponding end of one of said beams with the clamps generally perpendicular to said path, and said beam having a greater cross section between said clamps than at the intermediate portions of the spring beam between said members, a substantial intermediate length of the spring having a uniform cross-section less than the cross-section between the clamps, and the spring includes a tapering portion in which the cross-section of said beam decreases gradually from the edge of the clamps to the nearest end of the intermediate length.

10. An integral supporting spring beam for vibratory apparatus of the type in which at least one end of the spring is rigidly clamped to an associated portion of the apparatus, said spring having an end portion of predetermined cross-section providing an area to be clamped, a central portion of lesser cross-section, and a tapering portion of decreasing cross-section from said end portion to said central portion.

11. A spring beam according to claim 10 in which the portions of diflerent cross section are of dilferent thickness in the direction of expected vibration, the thickness of said end portion at the edge of the area to be clamped providing a substantial predetermined safety factor of endurance limit over combined calculated stresses, the thickness of the central portion being small enough to provide the required spring constant and great enough to support the expected shear and column loading of the spring, and the relative length and thickness of said tapering portion providing a substantially smaller safety factor at the transition point between said tapering portion and said central portion.

12. A spring beam according to claim 11 having an end portion of said predetermined thickness at each end of the beam, and a tapering portion between each end portion and the central portion, said tapering portions being of equal length.

13. A spring beam according to claim 12 in which the thickness of each end portion is substantially twice the thickness of said central portion, and in which the length of each tapering portion is substantially one-fourth to one-fifth the free effective length of the spring.

14. An integral metallic supporting spring beam for vibratory apparatus of the type in which at least one end of the spring is clamped to an associated portion of the apparatus, said spring having an end portion of predetermined cross section providing an area to be clamped, and an intermediate portion of smaller cross section, said spring having said portions of different cross section formed from a single metal strip of uniform cross section corresponding to the desired predetermined cross section of said end portion by a forging step in which the metal is forced to fiow longitudinally from said intermediate portion thereby providing said smaller cross section with minimum damage to its grain flow or fibrous structure.

References Cited in the file of this patent UNITED STATES PATENTS 1,022,332 Roth Apr. 2, 1912 1,200,497 Hosford Oct. 10, 1916 1,534,892 Beaumont Apr. 21, 1925 1,728,657 Binte Sept. 17, 1929 2,062,760 Overstrom Dec. 1, 1936 2,540,832 Piron Feb. 6, 1951 FOREIGN PATENTS 584,435 Germany Sept. 20, 1933