US 3656764 A
An earth boring drill bit employing roller cutters is provided with an improved seal for inhibiting ingress of abrasive materials into the bearing surfaces and egress of lubricant. The improved seal is between an outwardly facing re-entrant corner on the journal and an inwardly facing re-entrant corner on the cutter. The seal is made by a pair of O-rings engaging the opposed re-entrant corners and separated by a floating rigid ring having opposed bearing surfaces for seating the O-rings into the corners. This seal accommodates radial, axial, and angular displacements of almost twice the magnitude that can be accommodated by a single O-ring of the size of one of the O-rings without significantly increasing the length of journal needed for the seal.
Claims available in
Description (OCR text may contain errors)
ilite tates Robinson ate  3,656,766 [451 Apr. E6, 1972  DRILL BET SEAL ASSEMBLY  Appl. No.: 68,140
Primary Examiner-Robert 1. Smith Attorney-Christie, Parker & Hale [5 7] ABSTRACT An earth boring drill bit employing roller cutters is provided with an improved seal for inhibiting ingress of abrasive materials into the bearing surfaces and egress of lubricant. The improved seal is between an outwardly facing re-entrant corner on the journal and an inwardly facing re-entrant corner on the cutter. The seal is made by a pair of O-rings engaging the opposed re-entrant comers and separated by a floating rigid ring having opposed bearing surfaces for seating the O-rings into the comers. This seal accommodates radial, axial, and angular displacements of almost twice the magnitude that can be accommodated by a single O-ring of the size'of one of the 0- rings without significantly increasing the length of journal needed for the seal.
14 Claims, 8 Drawing Figures SHEET 10F 2 PATENTEU APR] 8 m2 1 N VEN TOR. r ROBWS 0 1/ III) PATENTEUAPR 18 1912 v SHEET 2 OF 2 DRILL BIT SEAL ASSEMBLY BACKGROUND An earth boring drill such as employed for drilling oil or gas wells or the like is a rugged piece of equipment that operates in a very severe environment. In such a drill, a drill bit is threaded onto a pipe and lowered into a well bore in which it is rotated for cutting through rock and earth formations. Rotatable cutters mounted on the drill bit fragment the rock, and the fragments are removed from the bore in a drilling fluid. Normally such a drill bit is operated until the rolling cutters or the bearings supporting the cutters are excessively worn. For most economical operation, it is desired to operate the drill bit for a long time between bit changes. The operating lifetime of a drill bit is enhanced by lubricating the bearings mounting the cutter on the drill bit and seals are normally provided for containing lubricant in the region of the bearings, and also for preventing the ingress of abrasive rock fragments or corrosive materials into the bearing surfaces. Either the excessive loss of lubricant or the ingress of deleterious materials can unduly shorten the operating lifetime of the earth boring drill. It is, therefore, of considerable importance that the seals in an earth boring drill have a long operating lifetime.
The operation of a seal in an earth boring drill is complicated by the fluctuations in position and pressure that are encountered as the drill is operated. The rolling cutters and the journals on which they are mounted are subject to relative displacements in an axial direction, in a radial direction, and also in angular alignment. These deviations from concentricity and axial position are exaggerated as the bit is operated and normal wear occurs on the various bearing surfaces. It is of importance that the seals have sufficient accommodation or compliance to remain effective even in the presence of large axial, radial and angular displacements.
BRIEF SUMMARY OF THE INVENTION In practice of this invention there is provided a seal for relatively rotatable parts comprising a first compressible ring bearing on one part, a second compressible ring bearing on a second rotatable part, and a rigid ring between the two compressible rings for seating the compressible rings against the relatively rotatable parts.
DRAWINGS Features and advantages of the present invention will be better understood by reference to the following detailed description of a presently preferred embodiment when considered in connection with the accompanying drawings wherein:
FIG. 1 illustrates a portion of an earth boring drill incorporating principles of this invention;
FIG. 2 illustrates in transverse cross section a ring employed in a seal of the drill of FIG. 1;
FIG. 3 illustrates schematically the sealing arrangement employed in the drill of FIG. 1;
FIG. 4 illustrates schematically another embodiment of seal between relatively rotatable parts;
FIG. 5 illustrates schematically another embodiment of seal between relatively rotatable parts; and
FIGS. 6, 7 and 8 illustrate schematically other embodiments of seals between relatively rotatable parts.
DESCRIPTION FIG. 1 illustrates an earth boring drill provided with a seal incorporating principles of this invention. As illustrated in this preferred embodiment, the drill comprises a conventional drill bit 11, only a fragment of which is shown, since such a bit is conventional and the means for attachment, means for lubricating and other portions not illustrated are not of significance in practice of this invention. A conventional earth boring drill has one or more (usually three) diagonally depending cutter mounting pins 12, one of which is shown in cross section in FIG. 1. Mounted on each of the pins 12 is a hollow conical rotatable cutter 13 on the outer surface of which are hard cutting teeth 14 for fragmenting rock as the drill is used.
As the earth boring drill is used there is very high pressure on the conical cutters, which are mounted on relatively short pins. To accommodate this load, as much length as possible of the pin and cutter is employed for bearing surfaces. Thus in the illustrated embodiment at the outer or free end of the pin 12 is a disk 16 of bearing material such as tool steel for accommodating thrust loading between the end of the pin and the inside end of the cutter 13. A sleeve bearing 17 is provided around a cylindrical portion 18 of the pin for bearing against the inside of the cutter. Conventional ball bearings 19 are provided between an inner race 21 on the pin and an outer race 22 on the cutter. The ball bearings serve to hold the cutter 13 on the pin 12 and must therefore be inserted in the races after assembly of the cutter onto the pin. In order to insert the balls 19, a hole 23 extends through the body of the bit 11, and after the balls are inserted a pin 24 closes the hole and is secured in place by a weld 26. A slot 27 along the length of the pin 24 provides fluid communication between a lubrication channel 28 in the body of the bit and the several bearings mounting the cutter on the pin.
A segment bearing having a plurality of segments 29 of bearing material between an outer cylindrical surface 31 on the pin and inner cylindrical surface 32 in the cutter provide additional bearing support for the cutter on the pin.
The pin 12 on which the cutter is journalled has a seal seat formed of a surface 33 facing radially outwardly and an intersecting surface 34 facing longitudinally along the length of the pin towards its free end. These two surfaces 33 and 34 extending around the journal cooperate to define a seal seat in the form of a re-entrant corner with a substantially cylindrical surface 33 and a substantially plane surface 34. It might be noted that in the illustrated embodiment the radially outwardly facing surface 33 is not exactly a cylinder but is actually in the form of the surface of a cone having an included half angle of about 10. Similarly, the substantially flat surface 34 is actually the surface of a cone having an included half angle of so that the two surfaces 33 and 34 intersect at a right angle. A slight fillet is also provided in the re-entrant corner between the surfaces 33 and 34. It should be understood that the seal seat formed by the intersecting surfaces can just as well be in the form of a cylindrical and flat surface, respectively, or if desired a continuous curve in the form of a sector of a torus can be employed in other embodiments.
Opposed to the seal seat on the pin or journal 12 is a corresponding internal seal seat on the cutter 13 formed of a substantially flat surface 36 facing towards the broader open end of the conical cutter. An inwardly facing substantially cylindrical surface 37 intersects the flat surface 36 and cooperates to form the seal seat. The seal seat surfaces 36 and 37 are actually portions of conical surfaces opposed to the seal seat surfaces 33 and 34 on the journal. The seal seat on the cutter can also be made with cylindrical and flat intersecting surfaces or with a continuously curved surface defining the re-entrant corner, as may be desired.
Mounted between the seal seats on the journal and cutter, respectively, is a sealing assembly comprising an inner elastomeric compressible O-ring 38 in engagement with the seal seat 33, 34 on the journal. A second elastomeric compressible O-ring 39 having a larger diameter than the inner O- ring 38 is in engagement with the seal seat 36, 37 on the cutter. In between the two O-ring 38 and 39 is a rigid ring 41, also illustrated in transverse cross section in FIG. 2 and schematically in FIG. 3. The rigid ring has a first or outer curved bearing surface 42 facing diagonally outwardly from one face 43 of the ring. Opposed to the first bearing surface 42 is a second or inner curved bearing surface 44 facing diagonally inwardly on the opposite face 46 of the rigid ring. Each of the bearing surfaces 42 and 44 on the rigid ring is in the form of a continuous or smooth curve having a radius about the same as the O-ring cross section for engagement with the two O-rings 39 and 38, respectively.
In order for an O-ring or similar elastomeric seal to remain in sealing engagement, the O-ring becomes loaded in compression upon assembly so as to be in intimate contact with both of the surfaces between which it is compressed. No matter what its cross section, the extent of compression of an O-ring is limited, and in a rotary seal excessive compression leads to rapid wear and deterioration of the O-ring. When a single Iii-inch cross section O-ring is used, it may tolerate a compression of, for example, about 0.015 inch for prolonged periods without excessive wear; however, a substantial increase in compression may cause accelerated wear, significantly shortening the operating lifetime of the O-ring. A somewhat larger compression can be tolerated by employing an O-ring having a larger cross section. If, however, one goes to a larger cross section O-ring, more space must be allowed to accommodate it. In an earth boring drill, for example, the additional space required for a larger cross section O-ring can be provided only by sacrificing the length of journal and cutter available for bearing surfaces. Because of the very high pressures and severe operating conditions of an earth boring drill,
it is desirable to have as much bearing surface as possible between the cutter and its mounting journal, and it becomes impractical to provide space in a direction along the axis of the bearings for a large cross section O-ring.
It is found in practice of this invention that the compression that can be tolerated by the seal assembly is nearly doubled without significantly increasing the space required for the seal. This effect is achieved by employing two O-rings of smaller cross section separated by a rigid floating ring so that substantially the complete compression accommodation of each of the rings is obtained and the total compression accommodation of the seal assembly is substantially twice the accommodation of either of the O-rings alone. This increased accommodation is obtained without substantial increase in the length of journal or cutter required for the seal assembly.
In an earth boring drill high pressures and extreme wear are involved and a cutter may be displaced radially, axially, or both from its intended position relative to the journal on which it is mounted. A radial displacement, that is, where the cutter is no longer coaxial with the journal, may either increase or decrease the compression of the O-rings from the initially applied preload. If a single O-ring is employed in a seal, the amount or change of compression that it can accommodate is determined by its size and material and there is some acceptable limit, such as, for example, 0.015 inch. In the illustrated embodiment of FIG. 1, if the cutter 13 displaces radially relative to the pin or journal 12, the compression of the outer O-ring between the cylindrical surface 33 and the rigid ring 41 will increase or decrease and, at the same time, the compression of the inner O-ring 38 between the cylindrical surface 37 and the rigid ring will also increase or decrease by substantially the same amount due to the fact that the rigid ring 41 floats free of either the journal or cutter and can shift slightly in position so that the compression of each of the rings is approximately equal. The floating rigid ring between the O-rings assures approximately equal compressions, and the total available compression is the sum of that of the two 0- rings, such as, for example, nearly 0.030 inch.
In the same manner, an axial displacement may occur as the drill bit is operated. If this occurs, the outer O-ring 39 and inner O-ring 38 are compressed between the substantially flat surfaces 36 and 34, respectively, and the rigid ring 41. The floating rigid ring between the two O-rings assures that the compression on the two O-rings is substantially equal and therefore the total accommodation by the seal for axial displacement is substantially equal to twice the compression of each of the O-rings individually.
It is apparent that since the seal assembly provides enhanced accommodation for axial displacement and radial displacement that it also accommodates angular displacement where the axes of the cutter and journal are no longer aligned since this merely involves increased compression on one side of the seal and decreased compression on the opposite side. Thus in the illustrated seal assembly, the accommodation of radial displacement, axial displacement and angular displacement is substantially doubled as compared with a single O-ring and the total length of journal and cutter required for the seal assembly is increased only a small amount, corresponding to only a minor fraction of the cross-sectional diameter of the 0- rings.
FIG. 4 illustrates schematically in a manner similar to the enlarged view of FIG. 3 an arrangement of another embodiment of double O-ring seal incorporating principles of this invention. As illustrated in this embodiment, an inner cylindrical member of journal 51 is surrounded by an annular outer member or race 52, and the two members 51 and 52 are rotatable relative to each other about an axis 53. Either of the members 51 or 52 may be fixed and the other rotatable about the axis 53 as may be desired in a particular application. A sea] assembly between the relatively rotatable members has an inner O-ring 54 in engagement with the journal 51. An outer O-ring 55 is in engagement with the outer race 52. In between the inner O-ring 54 and outer O-ring 55 is a floating rigid ring 56 extending around the axis 53 having a substantially S- shaped cross section for providing an inwardly facing O-ring groove in the crook of the S for the inner O-ring 54 and an outwardly facing O-ring groove in the other crook of the S for the outer O-ring 55. In an actual application as distinguished 1 from the schematic illustration of FIG. 4, some means, such as a shoulder, or shoulders (not shown), may also be provided for assuring that the seal assembly remains in its intended axial position. It will be apparent that just as in the illustrated preferred embodiment, the compression accommodated in each of the O-rings 54 and 55 is available for assuring proper sealing and the total radial displacement of the relatively rotatable parts that can be accommodated by the seal assembly is substantially twice the compression available in each of the O-rings alone. It will also be noted that the total distance between the relatively rotatable parts 51 and 52 is not substantially increased beyond the cross-sectional thickness of either of the O-rings since the cross sectional thickness of the S-shaped rigid ring 56 is not great.
FIG. 5 illustrates schematically a seal assembly for a pair of relatively rotatable members in face to face proximity. As illustrated in the embodiment, a member 61 has a flat face opposite a flat face on a second member 62. The members 61 and 62 are rotatable relative to each other about an axis 63. Between the opposed faces of the rotatable members 61 and 62 is a seal assembly comprising a first larger diameter O rings 64 and a second smaller diameter O-ring 65. The larger diameter O-ring is in sealing engagement with the surface of the second member 63, and the smaller diameter O-ring 65 is in sealing engagement with the surface of the first member 61. In between the two O-rings 64 and 65 is a floating rigid ring 66 extending around the axis 63. The rigid ring 66 also has an S- shaped cross section providing an O-ring groove for the outer O-ring 64 adjacent one face of the ring and an O-ring groove for the inner O-ring 65 adjacent the opposite face of rigid ring. In a manner similar to that hereinabove described, the seal assembly illustrated in FIG. 5 accommodates an axial displacement substantially twice the accommodation available in either of the O-rings 64 or 65 alone without increasing the axial space required for the seal assembly by an amount significantly more than the space required for a single O-ring.
Referring again to the rigid ring 41 of FIG. 2 and the schematic illustration of FIG. 3 it will be seen that the rigid ring 41 also has a generally S-shape with the ends of the S somewhat truncated. The two O-rings 38 and 39 are accommodated in the respective opposed crooks of the S-shaped cross section of the rigid ring.
FIG. 6 illustrates schematically a seal assembly usable between a first part 71 and a second part 72 that are rotatable relative to each other. In this embodiment the two members 71 and 72 may be in face to face engagement and rotate relative to each other about an axis 73, or in the alternative may comprise an outer race and inner cylinder, respectively, rotatable about an axis 74. it will be apparent that in either case the principles of operation are substantially the same.
A groove 75 in the first relatively rotatable member 71 accommodates a first O-ring 76. Directly opposed to the first groove 75 is a second groove 77 in the other rotatable member 72 which accommodates a second O-ring 78. In between the two O-rings 76 and 78 is a floating rigid ring 79 extending around either the axis 73 or the axis 74, depending on the axis of relative rotation of the two parts 71 and 72. The rigid ring 79 assures that each of the O-rings 76 and 78 can accommodate substantially its full available compression, and the total compression that can be accommodated by the seal assembly is substantially twice the accommodation of either of the O-rings alone. it will be noted in this embodiment that the opposed surfaces of the two relative rotatable members 71 and 72 each include a small step at the O-ring groove so that the floating ring 79 is retained in position between the compressible O-rings.
FIG. 7 illustrates in a fragmentary view another embodiment of seal having a pair of elastomeric O-rings between relatively rotatable parts. The illustrated seal is analogous to that hereinabove described and illustrated in FIG. 1 and is particularly well suited for use in an earth boring drill or the like. Since the elements of this embodiment are particularly closely related to the embodiment of FIG. 1, the corresponding elements are designated by reference numerals differing from those employed in P16. 1 only by the addition of 1110. Thus, the earth boring drill comprises a cutter mounting pin 112 corresponding to the cutter mounting pin 12 of FIG. 1.
Mounted on the cutter mounting pin 112 is a cutter cone 113 and bearing segments 129 are provided therebetween for bearing support of the cutter on the pin. The pin 112 includes an outwardly facing substantially cylindrical surface 133 against which an O-ring 138 seats. The cutter 113 includes an inwardly facing substantially cylindrical surface 137 opposed to the cylindrical surface 133 and against which an O-ring 139 seats. The cutter cone also includes a substantially flat face 136 opposed to a flat face 134 on the pin 112 with the O-rings 133 and 139 therebetween.
In between the two O-rings 138 and 139 is a rigid ring 141 having an L-shaped cross section. The base 145 of the L is in the form of a thin cylindrical ring between the two O-rings 138 and 139, and provides opposed surfaces against which the two O-rings seal. The leg 150 of the L is in the form of a flange extending inwardly from the base 145 of the L-shaped ring, in between the inner O-ring 138 and the bearing segments 129 so as to prevent contact therebetween and excessive wear of the O-ring. A recessed portion 155 is provided in the plane surface 134 on the mounting pin 112, having a depth substantially equal to the thickness of the flange 1511 so that the space available for accommodation of the inner O-ring 138 is substantially equal to the space available for the outer O-ring 139.
During use, any radial shift in position between the cutter 133 and pin 112 is accommodated by the compression of the two O-rings 138 and 139, which, as in the other embodiments, is cumulative for providing greater compression than could be obtained with either O-ring alone. Any axial shift in position between the cutter and pin is accommodated by mere sliding of the two parts relative to each other without any change in the compression on the O-rings. This axial movement can be accommodated since the distance between the flat faces 134 and 136 and the recessed portion 155 and flange 150 are slightly greater than the required space for compression of the two O-rings. Thus, in the illustrated embodiment having an L- shaped rigid ring between the O-rings, a shaft seal is provided between substantially concentric cylindrical surfaces rather than a corner seal between both cylindrical surfaces and fiat faces as is provided in the embodiment of FIG. 1.
FIG. 8 illustrates schematically another embodiment of seal employing a pair of resilient O-rings between relatively rotatable parts. As illustrated in this embodiment, a shaft 91 is provided within a cylindrical surrounding part 92 and these two parts 91 and 92 are rotatable relative to each other about an axis 93. A deep groove 94 in the shaft 91 accommodates a pair of O-rings 95 and 96 that are essentially in series insofar as their sealing relation between the two parts 91 an 92 is concerned. A floating ring 92 is provided between the two O-rings 95 and 96 and within the O-ring groove 94.
By having the two O-rings 95 and 96 serially arranged, the available compression of the two O-rings is substantially double the available compression of either of the O-rings alone, thereby providing substantial accommodation of radial displacement between the two relatively rotatable parts. The use of two O-rings also permits use of two O-rings with difierent properties such as compressibility, hardness, wear resistance, and coefficient of friction. It will be apparent to one skilled in the art that some conventional disassembly means must be provided on the shaft 91 so that the ring 97 can be assembled into the groove 94. It will also be apparent that, if desired, the deep groove 94 can be provided in the surrounding cylindrical member 92 instead of the cylindrical shaft.
By providing a seal having a pair of elastomeric sealing rings with a rigid floating ring therebetween, the full compression of the elastomeric rings is obtained with no danger of the rings slipping past each other and losing engagement with the sealing surfaces. If two O-rings were employed in a single sealing space without an interposed rigid ring, the two sealing rings could displace so as to both be on a diagonal in the sealing space, thereby substantially relieving the desire compression of the sealing rings which is needed for obtaining a tight seal.
Various arrangements of seals incorporating principles of this invention, particularly in an earth boring drill subject to severe operating conditions and having limited space, have been set forth hereinabove. Many modifications and variations of the present invention will be readily apparent to one skilled in the art. Thus, for example, the seal assembly having a pair of opposed O-rings with a rigid ring therebetween can be formed with O-rings having different cross-sectional diameters rather than the equal cross-sectional diameters illustrated in the preferred embodiment. Similarly, if desired, the two 0- rings employed in the seal can be of different materials or shapes (such as rectangular) so as to provide different hardnesses, degrees of compressibility, frictional properties or the like. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.
What is claimed is:
1. A rotary seal comprising:
a first member having a first substantially flat face and an outwardly facing cylindrical face;
a second member cooperating with the first member and having a second flat face opposite the first flat face and an inwardly directed cylindrical face opposite the outwardly directly cylindrical face;
a first compressible seal ring having a portion in engagement with the outer cylindrical face;
a second compressible seal ring having a portion in engagement with the inner cylindrical face; and
a rigid floating ring between and in engagement with the first and second seal rings.
2. A rotary seal as defined in claim 1 wherein the first flat face and the second flat face are spaced apart by a distance greater than the cross section of the seal rings so that each of the seal rings is in engagement with no more than one flat face.
3. A rotary seal as defined in claim 1 wherein the rigid ring comprises:
a cylindrical section between the first and second seal rings and a flange portion transverse to the cylindrical portion and adjacent one of the seal rings.
4. A rotary seal as defined in claim 3 wherein the rigid ring has an L-shaped cross section with the flange portion extending inwardly from the cylindrical portion.
5. A rotary seal as defined in claim 4 wherein the first flat face further comprises a recessed portion recessed from the fiat face a distance approximately the same as the thickness of the flange portion of the L- shaped rigid ring so that both seal rings have substantially the same clearance from the first and second faces.
6. A rotary seal as defined in claim 3 wherein the rigid ring has a generally S-shaped cross section with a second flange portion extending transverse to the cylindrical portion and in an opposite direction therefrom from the first mentioned flange portion.
7. A rotary seal as defined in claim 1 wherein the rigid ring has a generally S-shaped cross section, the first and second rings being accommodated in the respective opposed crooks of the S-shaped rigid ring.
8. A seal as defined in claim 7 wherein a first crook of the generally S-shaped cross section of the rigid ring faces diagonally outwardly from the rigid ring and the other crock of the S-shaped cross section of the rigid ring faces diagonally inwardly from the rigid ring in substantially the opposite direction from the first crook.
9. A seal for a pair of relatively rotatable parts comprising:
a first elastomeric compressible ring bearing on one rotatable part;
a second elastomeric compressible ring bearing on the other rotatable part; and
a rigid floating ring having a generally S-shaped cross section between the first ring and the second ring, the first and second rings being accommodated in the respective opposed crooks of the S-shaped rigid ring.
10. A seal as defined in claim 9 wherein a first crook of the generally S-shaped cross section of the rigid ring faces diagonally outwardly from the rigid ring and the other crook of the S-shaped cross section of the rigid ring faces diagonally 8 inwardly from the rigid ring in substantially the opposite direction from the first crook.
11. A seal as defined in claim 8 wherein a first crook of the S-shaped cross section of the rigid ring faces one face of the rigid ring and the second crock of the S-shaped cross section of the rigid ring faces the other face of the rigid ring; and wherein the first crook is radially further from the ring center than the second crook is.
12. A seal as defined in claim 9 wherein a first crook of the S-shaped cross section of the rigid ring faces radially outwardly from the rigid ring and a second crook of the S-shaped cross section rigid ring faces radially inwardly from the rigid ring, and wherein the first compressible ring is in a plane parallel to the plane of the second compressible ring and is displaced from the second compressible ring in a direction normal to the plane of the rings.
13. A seal for a pair of relatively rotatable parts comprising:
a first elastomeric compressible ring bearing on one rotatable part;
a second elastomeric compressible ring bearing on the other rotatable part; and
a rigid floating ring between the first ring and the second ring, the rigid ring comprising a diagonally inwardly facing recessed portion on one face of the ring and adjacent the inner perimeter of the ring for accommodating the first compressible ring.
14. A seal as defined in claim 13 wherein the rigid ring further comprises a second diagonally outwardly facing recessed portion on the opposite face of the ring and adjacent the outer perimeter for accommodating the second compressible ring.
UNITED STATES PATENT OFFICE QERTIFICATE OF CORRECTION Patent No. 3,656,76 Dated April 18, 1972 lnventm-(s) William P. Robinson It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
Assignee not shown Assignee:Smith International 1 Incorporated Col. line 14 i 7 "of" should be -or---;
Col. 4, line ,6 "the" should be --t'his--;
Col. 4, line 49 "rings" should be --r ing-?-.
Col. 6, line C "an" should be --and--; Col. 6, line 7 "92 should be 97" Col 6, line 29 I "desire" should be desired- Col. 6, line 56 directly should be --directed--. Col. 8, line 13 insert --of the-- between "section and "rigid Signed and sealed this 3rd day of October 1972.
EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK' Attesting Officer Commissioner of Patents