US2757007A - Shank and bushing assembly for pneumatic tools - Google Patents

Description

S. N. LEVY July 31, 1956 SHANK AND BUSHING ASSEMBLY FOR PNEUMATIC TOOLS Filed May 29, 1953 United States Patent SHANK AND BUSHING ASSEMBLY FOR PNEUMATIC TOOLS Sylvan N. Levy, Wilmington, Del., assignor to Delaware Tool Steel Corporation, Wilmington, Del., a corporation of Delaware Application May 29 1953, Serial No. 358,421

1 Claim. (Cl. 279-19.6)

This invention relates to pneumatic percussion tools such as chisels, riveters, and the like.

In tools of this class, the chisel or other working bit is detachably supported in the barrel of the pneumatic hammer through the medium of a bushing shrink-fitted to the barrel and forming a socket for the shank of the chisel or other bit. It has been proposed to form the said shank with a cylindrical end and to provide a corresponding cylindrical bore in the bushing, the said cylindrical end fitting closely but not tightly within the socket. This type of shank and bushing assembly, while affording maximum strength in the bushing, is subject to breakage in the shank at the shoulder where the cylindrical terminal end portion joins the body of the shank, such shoulder being unavoidable in that the cylindrical portion must necessarily be substantially smaller in diameter than the said body. This design also permits the bit to rotate freely in the bushing so that the operator is obliged to guide the bit by hand in addressing the work, an operation which requires the skill of an expert.

In order to overcome this undesirable characteristic of the cylindrical shank and socket assembly it was proposed to form the outer end portion of the cylindrical extremity of the shank of hexagonal or other non-cylindrical form and to provide the bore of the bushing with an inner corresponding hexagonal or other portion to receive the said terminal end of the shank and to thereby preclude rotational movements of the bit in the socket. This form of assembly, now in common use, exhibits weakness in excessive shank breakage usually at its weakest section where the hexagonal extremity joins the cylindrical por tion of the shank. It is necessary in this case also that the cylindrical portion of the shank be accurately ground to size so as to closely fit the bore of the bushing to act as an air seal, and if not accurately fitted to the bushing there is a resultant loss of compressed air and a weakening of the hammer blows transmitted to the bit by the pneumatic mechanism. The cost of producing this shank and bushing assembly, therefore, is relatively high.

In this respect it should be noted that when pneumatic chisels are dressed to the form of gouges for billet chipping and as cape or round nose chisels, the leverage in the chipping operation falls most heavily on the shank of the chisel. In making heavy cuts it is not unusual for the operator to apply his body weight to the hammer in order to sink the chisel into the metal. When a section of maximum weakness is inherent in the structure of the shank as in the cases described above, breakage will frequently occur at the weak section before the chisels have been redressed to the point Where their usefulness is ended.

In an attempt to overcome this weakness of the standard hexagonal shank it was proposed to form the shank of the bit of hexagonal form with a frusto-conical extremity, and to form the bore of the bushing with an outer hexagonal portion to receive the hexagonal section of the shank and with an inner conically tapered portion closely fitted to the frusto-conical shank extremity. This 'ice type of bit when taking the form of a chisel is conventionally made throughout of hexagonal stock and is provided with the required chisel cutting edge at one end with the frusto-conical extremity at the other end. Shoulders are thus avoided. The Weakness of this assembly lies in a tendency toward excessive bushing breakage.

The outside diameter of a pneumatic hammer bushing is strictly limited and to enlarge the diameter of the bushing would necessitate enlarging also the diameter of the barrel of the hammer in which the bushing is mounted. This is impractical. Thus, as a practical matter, the outside diameter of the standard hammer bushing cannotbe machined and ground larger than 1.0625 1-.005" and the tolerance for a new tool is 1-.0005 in order that the bushing may be properly press fitted in the hammer. It will be apparent, therefore, that when the bushing is provided with a hexagonal socket for reception of the hexagonal shank the distance between the corners of the socket and the outer peripheral surface of the bushing will be so small as to constitute a serious weakness in the assembly. Each of the sharp corners of the hexagonal socket constitutes a point of potential fracture, both in tlie hardening operation, since the bushings must be hard to resist Wear, and also in service. The high number of strokes per minute, approximately 3,000, rapidly develops fatigue strains in the chisel and in the bushing. Since the cost of the bushing is relatively high and the cost of replacement excessive as measured in terms of lost time and in the difiiculty of releasing the bushing from its shrink fit, this weakness of the assembly is a serious one.

The primary object of the present invention, then, is to provide a shank and bushing assembly which shall be relatively free from the weaknesses of the prior types described above and which shall be characterized by relatively low production cost and relatively high inherent strength and durability.

The invention will be more readily understood by reference to the attached drawings wherein:

Figure 1 is a side elevational view partly in section of a shank and bushing assembly made in accordance with the new invention;

Figure 2 is a sectional view on the line 2-2, Figure 1;

Figure 3 is a sectional view on the line 33, Figure 1;

Figure 4 is a view in perspective of the portion of the shank shown in Figure 1 which occupies the socket of the bushing, and;

Figure 5 is a sectional perspective view of the two-part bushing.

With reference to the drawings the shank 1 of the bit, a chisel in the present instance, is generally square in cross section, as clearly shown in Figure 3, the corners of the shank being rounded as indicated at 2. In the present instance these corners are arcuate in form and collectively define the periphery of a circle which centers in the longitudinal axis of the shank. The rounded corners 2 are of such width that the distance between opposite arcs is measured on a line intersecting the axis 3, which distance corresponds in the present instance to the diameter of the aforesaid circle, constitutes the major cross sectional dimension of the shank but is substantially less than the correspondingly measured distance between the opposite corners of a hexagon whose sides are equal in length to the width of the respective sides 4 of the shank between the rounded corners. The shank is provided with a frustoconical extremity 5 and the arcuate corners 2 merge smoothly into this conical extremity at the juncture thereof with the generally rectangular body portion of the shank. As shown in Figure 4 the fiat sides 4 of the shank extend in their respective planes into the frusto-conical extremity the present instance is made in two parts, 8 and 9 respeclively. The inner surface of the bushing part 8 conforms in cross sectional form to the rectangular portion of the shank and is closely but not tightly fitted to the latter. The bore 11 of the bushing part 9 is conically tapered so as to accurately fit the frusto-conical extremity 5 of the shank. It will. be noted that the shank, except for the tapering extremity 5, is of the same cross sectional shape and dimensions throughout and istherefore devoid of shoulders and other critical fracture areas. It will be noted alsoby reference to Figure 2 that the bushing wall, even in the areas, ofthe arcuate corners, is sufiiciently thick to insure adequate bushing strength and is free from sharply angular recesses constituting pointsof major Weakness and potential fracture. It has been found in practice thatthis assembly possesses materially greater inherentstrengththan any of the prior assemblies described above and that its use will reduce incidence of breakage both in the shank and in the bushing to a practical minimum.

In a pneumatic percussion tool of the character described}. a shankand bushing assembly whereof the shank comprises a portion of generally polygonal cross section and, at the butt endthereof, a longitudinal portion of circular cross section and reduced diameter tapered in the area immediately adjoining the portion of polygonal cross section and atthe juncture of said portions having a major diarneter corresponding to the major cross sec tionall dimension of the polygonal portion, the flat side 30 surfaces of the polygonal portion being extended in their individual respective planes into and merging with the said adjoining tapered surface so as to avoid the formation of a shoulder at said juncture, and a bushing forming a socket for the said butt end of the shank, said socket comprising an inner polygonal portion conforming in shape and cross sectional dimensions to the said polygonal portion of the shank, and an adjoining frustoconical portion conforming in shape and cross sectional dimensions-to the tapered portion ofthetshank, said frustoe conical portion embracing an area of the tapered surface of the shank which is intersected by the flat sides of the polygonal portion and extending outwardly toward the butt end of the shank beyond the terminal ends of the said flat sides.

References Cited in the file of this patent UNITED STATES PATENTS 836,856 Borque Nov. 27, 1906" 942,869 Waugh n Dec. 7,, 1909- l',49 8,59' 8 Williams June 24,1924 1,540,868 Scully June 9, 1925 1,882,520 Pollard Oct. 11, 19-32 2,028,993 Pollard June 28, 1936 2,458,289 Morrison J an. 4, 1949 FOREIGN PATENTS 308,198 Great Britain Nov. 28, 1-929

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