US3595323A - Exhaust means for percussion tool motors - Google Patents

Abstract

In one type of percussion drilling tool, the motor has an anvil which is slidable in the forward end of the motor casing between a normal working position and an abnormal advanced position when the tool is hanging off bottom and the working fluid is caused to bypass the piston and ''''blow'''' continuously through the anvil. In order to prevent ''''tapping'''' of the hammer piston at such time, due to leakage of the working fluid into the forward working chamber, this chamber is vented by means of recesses and passages in the anvil and its mounting chuck and/or the casing which are interconnected when the tool is ''''blowing,'''' and which are disconnected in the normal operating position of the anvil. Special configurations of parts provide in part the interconnecting recesses and passages.

Description

United States Patent 1,157,605 10/1915 Waugh Inventors Marvin E. Schindler;

Vernon G. Stewart, both of Houston, Tex.

Appl. No. 835,594

Filed June 23, 1969 Patented July 27, 1971 Assignee Misson Manufacturing Company Houston, Tex.

Continuation-impart of application Ser. No. 752,990, Aug. 15, 1968, now abandoned.

EXHAUST MEANS FOR PERCUSSION TOOL MOTORS 14 Claims, 12 Drawing Figs.

U.S. C1 173/66, 173/73, 173/80 Int. Cl E2lb l/06 Field of Search 173/15- References Cited UNITED STATES PATENTS 2,859,733 11/1958 Bassingeretalummh 3,136,375 6/1964 Learetal ABSTRACT: In one type of percussion drilling tool, the motor has an anvil which is slidable in the forward end of the motor casing between a normal working position and an abnormal advanced position when the tool is hanging off bottom and the working fluid is caused to bypass the piston and blow continuously through the anvil. in order to prevent tapping" of the hammer piston at such time, due to leakage of the working fluid into the forward working chamber, this chamber is vented by means of recesses and passages in the anvil and its mounting chuck and/or the casing which are interconnected when the tool is blowing, and which are disconnected in the normal operating position of the anvil. Special configurations of parts provide in part the interconnecting recesses and passages.

PATENTEDJULZYIBH 3,595,323

SHEET 1 OF a ATTOHIVEYJ PATENTEflJuLzmn 3' 595 323 SHEET 3 OF 4 Mar v/n 5.

Vernon 6.

ATTORNEY EXHAUST MEANS FOR PERCUSSIONTOOL MOTORS I CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of our copending application Ser. No. 752,990 filed Aug. 15, I968 and now abandoned. Certain features of the present application are disclosed and claimed in a copending application Ser. No. 748,413 filed July 29, 1968, and now US. Pat. No. 3,503,459 in the name of Marvin E Schindler. This latter copending application discloses and claims a fluid-operated percussion motor in which the working fluid supply passages extend through portions of the casing which functions as a whole as load-carrying structure, as distinguished from conventional prior art motors in which separate, nonload-carrying sleeves are provided for forming the fluid-carrying passages. The reciprocating piston has a central exhaust passage which is aligned with a similar passage extending through the anvil slidably received in the forward end of the casing. Finger valves project from the motor back head and the anvil for alternately entering and sealing the central exhaust passage in the piston during reciprocation thereof and the piston also having annular passages which alternately register with ports in the casing to direct the working fluid to the opposite ends of the casing for causing the piston to reciprocate and beat upon the anvil.

BACKGROUND OF THE INVENTION I. Field of the Invention This invention relates to fluid-operated, percussiontype drill motors and consists particularly in novel means for preventing operation of the hammer piston and for causing the bypassing of working fluid through the anvil when the tool is hung off bottom.

II. Description of the Prior Art Various prior art means have been devised for achieving a generally similar purpose. For instance, in Collier et al. U.S. Pat. No. 3,3l I,I77 there is shown a type of percussion drill motor in which both the working fluid and the exhaust are conducted through a central porting stem which extends through the piston and into the axial exhaust duct in the anvil. The anvil, and particularly the exhaust duet therein, is specially configured to provide for exhausting of the forward end of the motor chamber when the tool is suspended off bottom. However, the means disclosed in the mentioned patent cannot be utilized in the present type of percussion motor in which the central porting stem is omitted. In other prior art devices, a centrally bored hammer piston, in its abnormally advanced, hanging position, uncovers a "blowing" port whereby working fluid is bypassed through the anvil and bit.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a fluid motor having an axially bored piston in a casing, an anvil slidably received in a chuck member mounted in the forward end of the casing, and valving for alternately directing pressured working fluid into opposite ends of the working chamber in the casing. The anvil advances abnormally when the tool is hung off bottom. Recesses are provided in the abutting faces of the anvil, the chuck member, and/or the easing for cooperating to exhaust the forward working chamber of the motor to the exterior thereof, in the abnormally advanced position of the anvil.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings,

FIG. 1 is a longitudinal section illustrating the lower (forward) portion ofa percussion drill motor embodying one form of the present invention, the anvil being shown in its abnormally advanced position, as when the tool is suspended off bottom, and half in central longitudinal section and half in elevation.

. position, as when the tool is resting on bottom.

FIG. 3 is a section taken on broken line 3-3 of FIG. 1.

FIG. 4 is a half longitudinal section showing a modified tool, in part. FIG. 5 is a view of the structure in FIG. 4, but with the anvil in the abnormally advanced position.

FIG. 6 is a transverse section taken on line 6-6 of FIG. 5.

FIG. 7 is a transverse section taken on line 7-7 of FIG. 5.

FIGS. 8 and 9 are partial vertical transverse sections with the anvil, respectively, in the normal or working position and the abnormally advanced or hlowing" position.

FIG. 10 is a transverse section taken on line I0-I0 of FIG. 9.

FIG. II is a view similar to FIG. I0, but showing another modification.

FIG. I2 is a fragmentary longitudinal section as in FIG. I, but showing still another form ofthe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The casing 5 of the motor (FIGS. I3), while incorporating working fluid supply passages, as at 6, and forward supply ports 7, is of structurally rugged design, as described in the above-mentioned copending application Ser. No. 748,413 filed July 29, I968. The forward end of the casing is internally threaded as at 8 for attachment of the externally threaded intermediate portion 9 of a generally annular chuck or driver sub" member designated 10. A thickened collar 11 at the forward extremity of the chuck member, in the normal operating position of the tool as in FIG. 2, rests on a horizontal shoulder 12 in the anvil-bit device, generally designated 13. Between the upper end of intermediate portion 9 of chuck member 10 and a thickened upper portion 16 thereof having oil grooves 17 in its inner surface, there is provided an internal shoulder I5. An anvil retainer ring 19, which is formed of two split halves, is secured in position by the chuck member against an internal shoulder 18 in the casing. An additional resilient split ring 22 is mounted on an internal groove in the casing just above ring 19.

The composite anvil-bit device 13 includes a forward (lowermost) cutter bit portion 25 of the solid-type having hardened cutter inserts 26, an intermediate shank portion 27 with splines and splineways 28 and 29, and an uppermost (rearward) impact-receiving head portion having an upper striking surface 31 which receives impact blows from the hammer piston 32, and shouldered, as at 33. Between impactreceiving surface 31 and splined shank portion 27, the outer surface of the anvil is provided with flutes 37 terminating at a cylindrical surface 34 connected by one or several transverse passages 35 with an exhaust duct 36 extending axially through the anvil member 13 and terminating in divergent passages 39 for directing fluid flow against the faces of the formation 55 being drilled for removing cuttings, as is well known.

The anvil is guided in its sliding movements entirely by engagement of the splined shank portion 27 thereof with the abutting portion or mating surface of chuck member 10,

which is complementarily splined, so that the anvil and chuck members, as well as casing 5, are constrained for rotary movements together. Splines 28 in the anvil terminate rearwardly at 38, that is, at the lower edge of surface 34. Matching internal splineways 40 in the chuck member terminate somewhat short of shoulder I5 to form an annular slot 41. Flutes 37 in the anvil are formed in alignment with chuck splineways 40 so that continuous longitudinal passageways are provided therethrough in the suspended position of the tool, as shown in FIG. I Split stop ring I9 is rather loosely received between casing 5 and the confronting portion of the anvil below head 30 so that this ring does not function as a limiting guide for the shank, this function being performed entirely by chuck member 9.

Resiliently lodged in the countersunk rearward extremity 46 of anvil exhaust duct 36 is a short tubular element 47 which serves as a finger valve, as will be explained. This finger valve is flexibly anchored by a packing collar 49 of resilient material which permits some disalignment ofthe finger valve and anvil.

In order to assemble the tool, tubular chuck member is first inserted over the rearward extremity of anvil member 13 and the splineways engaged. Split retainer stop ring halves 19 are then inserted in place between overhanging percussion shoulder 33 and the rearward extremity of chuck member 10. This subassembly unit or device is then threaded into the forward extremity of easing 5 until the chuck, in effect, forms both the forward part of the casing and a part of the anvil. The limitation on this threading will be established by the annular shoulder at the top of chuck collar ll. Casing 5 will be secured by suitable means to the lower extremity of a string of drill pipe through which working fluid, in this instance, compressed air or other gas, will be supplied.

In running the tool into a well bore 50, anvil member I3 will hang in its abnormally advanced position, as shown in FIG. 1, with rearward percussion head shoulder 33 resting against stop ring 19. In this suspended position of the anvil, hammer piston 32 likewise will be in an abnormally forward position and, as explained in the above-mentioned copending application Ser. No. 748,413 filed July 29, I968, working fluid will be constantly bypassed through central axial passage 52 in the piston and through registering finger valve element 47 and exhaust duct 36 in the anvil. At this time, ports 7 will be covered by the hammer piston (FIG. I) and the forward (lower) working chamber 53 within the casing will be vented through flutes 37 in the anvil, annular groove 41, splineways 40 in the chuck member, transverse passages 35, and exhaust duct 36. Such venting of the forward chamber effectively prevents any leaking working fluid which may enter the forward working chamber 53 from causing continued operation (tapping") of the hammer piston during this period. The suspended position of the anvil is also utilized in so-called blowing" action for cleaning out debris in the bottom of the hole.

When bit rests on the bottom 55 of the bore hole and the drill string is dropped to cause collar 11 at the bottom of the chuck member to bear against shoulder 12 on the bit, the tool will be in operating or working condition in which the working fluid first will be directed through casing passages 6 and ports 7 to lift hammer piston 32. When the hammer piston is lifted clear of finger valve member 47, forward working chamber 53 will be exhausted through valve member 47 and exhaust duct 36, while the working fluid applied to the rearward end ofthe hammer piston will drive the piston forward for striking the anvil. Reciprocatory action of the hammer piston will continue as long as the tool is resting on bottom and working fluid is supplied thereto at the proper pressure. The percussive blows thus applied to the anvil will be segregated from the easing because of the sliding action of the ahvil permitted by its splined assembly with the casing through chuck member 10.

The guiding of the anvil solely within chuck member 10 has substantial advantages both in manufacturing and in durability of the tool. Split ring 22 in the casing normally is cleared by the hammer piston, even in the suspended position of FIG. I. However, when the anvil-chuck subassembly capsule is removed, this ring 22, as long as it is in position, prevents dropping out of the hammer piston from the casing. Other types of drill bits may be utilized, and the bit may be integral with the anvil shank, as shown, or may be secured thereto by threaded or other types of couplings.

The form in FIGS. 4, 5,6, and 7 differs from the previous form in that cross passages in the anvil are omitted and splines and splineways 28a and 29a in the anvil and complementary splines a in chuck member 10 are extended downwardly or forwardly somewhat beyond the corresponding parts in the first form so that, when the tool is suspended off bottom, as in FIG. 5, the lower ends of anvil splineways 29a communicate with an internal annular recess 6] at the bottom of chuck enlargement ll. Sufficient clearance is provided between the matching splines for venting lower working chambcr 53 through longitudinal recesses or flutes 37 in the upper end of anvil l3, annular internal groove 41 in the chuck member, space 63 adjacent cylindrical surface 34 on the anvil, the clearance in matching splines and splineways 28a, 29a, and 40a, and annular countersink recess 61, all of which interconnect in the FIG. 5 position to form continuous exhaust passaging. The exhaust from chuck groove 6], of course, flows into the well annulus above bit shoulder 12.

The form in FIGS. 8, 9, and I0 is similar to that in FIGS. 4- 7 except for the omission of one of the splines 2811 on the anvil 13 to provide a longitudinal passageway 65 in the inner surface of chuck member 10. In the onbottom or normal operating position of the tool, as in FIG. 8, the lower end 66 of Iongitudinal passage 65 is closed by lower enlargement II on the chuck member 10 (FIG. 8). However, when the tool is suspended off bottom as in FIG. 9, opening 66 is exposed beneath the lower end of chuck part 11 for exhausting lower working chamber 53, as in the previous form.

The form in FIG. 11 is substantially the same as that in FIGS. 8l0 except that in FIG. II, one or more, in this instance, two, of the splines 28a on the anvil I3 are partially cut away, in this instance, half cut away longitudinally, to provide the longitudinal passageways 650 corresponding to passageway 65 in the previous form. This, of course, permits normal radial engagement between the anvil and chuck splines and splineways and does not materially affect the rotational interconnection of the parts due to the usual rotation of the tool in the direction to cause lateral abutment of the anvil and chuck splines and splineways. The lower ends of longitudinal passageways 65a are exposed beneath chuck enlargement II, in the hanging position of the tool, as in FIG. 9.

The form in FIG. I2 is quite similar to that in FIG. 1 except for the provision of radial passages 67 and 68, respectively, in chuck member 10 and easing 5. An internal annular groove 69 in the casing abreast the passages or ports 68 insures the interconnection of passages 67 and 68 in the hanging position of the tool for venting the lower working chamber through flutes 37, annular groove 41, and these latter passages 67 and 68. In this form, the complementary splines and splineways in the anvil and chuck members need not be utilized in exhausting the forward working chamber ofthe motor.

In each of the forms, specially constructed anvil and chuck members provide for exhausting of the forward working chamber in the hanging position of the tool. Details of the fluid motor itself as well as the anvil, chuck, and easing portions more immediately involved in the present invention may be altered as will occur to those skilled in the art.

We claim:

I. The combination in a percussion drilling tool having a casing forming a working chamber, a connection at the rearward end of said chamber for working fluid, a chuck member secured in the forward end of said casing, an anvil member slidable in said chuck member between an abnormally advanced position when the tool is suspended off bottom and normal working positions, said anvil member having a striking end face and an axial exhaust duct, a hammer piston rcciprocable in said working chamber, and valving means for controlling the application of working fluid from said connec tion to the opposite ends of said chamber and for exhausting the same to cause said piston to beat upon said anvil member end face, of means for exhausting the forward end of said working chamber when said anvil member is in said abnormally advanced position comprising first, second, and third recesses in the mating surfaces of said members, said first and second recesses being in the wall of said anvil and extending, respectively, through said striking face and exteriorly of said members and said third recess being in said chuck member, said first and second recesses being positioned in said abnormally advanced position of said anvil member to form with said third recess a continuous passage from the forward portion of said working chamber to the exterior of the tool for exhausting said working chamber, there being structure on one of said members positioned to close said passagcin said normal working position of said anvil member.

2. The combination described in claim 1 in which said second recess includes a part extending axially along said anvil member and a part extending transversely from said first part to said axial duct.

3. The combination described in claim 1 in which said engaging faces of said anvil member and said chuck means include complementary spline structure, there being clearance in said spline structure forming one of said recesses.

4. The combination described in claim 3 in which a portion of said spline structure is cut away to form one of said recesses.

5. The combination described in claim 3 in which the spline structure in said anvil member extends forwardly beyond said chuck means in the abnormally advanced position of said anvil member for discharging exhaust from said working chamber exteriorly of the tool.

6. An anvil device for use in a percussion drilling tool of the type having a casing forming a working chamber and a hammer piston reciprocable in said chamber for beating upon said device, said anvil device including a chuck member for attachment in the forward portion of said casing and an anvil received in said chuck member for limited sliding movements between normal working and abnormal advanced positions, there being a first recess in the outer wall of said anvil and extending through the striking face thereof for communication with said chamber, an exhaust duct extending axially through said anvil, and a passage extending transversely through said anvil from said duct through a portion of said anvil outer wall spaced axially from said recess, and there being a second recess in the inner wall of said chuck member positioned to connect said passage and said duct with said first recess and said chamber, in said abnormal advanced position of said anvil, for exhausting the working chamber forward portion.

7. A chuck device for attachment to the forward portion of a fluid pressure drill tool of the type having a casing forming a working chamber, an anvil, and a hammer piston reciprocable in said chamber for heating upon said anvil, said device comprising a generally annular body with a threaded external surface for screwing into a matching thread in the casing, a rearwardly facing casing-abutting upper end shoulder rearwardly of said threaded surface, and spline and splineway structure in the inner surface of said body, there being inner surface recessing in .said body spaced longitudinally from said end thereof forming part of passaging for exhausting the motor working chamber.

therethrough, there being first, second, and third recesses in the mating surfaces of said members, said first and second recesses being spaced apart and extending, respectively, through said striking face for communication with said working chamber and to the exterior of said device, said first and second recesses being in said anvil member and being movable when said anvil member is moved to said abnormally advanced position to interconnect all of said recesses for exhausting said working chamber.

9. An anvil device as described in claim 8 in which said third recess is formed in said chuck member and said first and second recesses are longitudinally spaced apart formed in said anvil member and movable therewith to intersect said third recess when said anvil member is moved to said abnormally advanced position.

10. An anvil for mounting at the forward end ofa drill tool of the type having a casing forming a working chamber, anvil and chuck members at the forward end of said casing, and a hammer piston reciprocable in said chamber for beating upon said anvil, said anvil having a striking end face, and axial exhaust duct, an annular shoulder for engaging and supporting the end of said casing in the normal working positioning of said anvil, a first outer surface recess extending through said striking end face, and a second recess spaced from said first recess for cooperation therewith in exhausting said working chamber.

11. An anvil for mounting at the forward end ofa drill tool of the type having a casing forming a working chamber, anvil and chuck members at the forward end of said casing, and a hammer piston reciprocable in said chamber for beating upon said anvil, said anvil having a striking end face, an axial exhaust duct, an annular shoulder for supporting said casing in the normal working position of said anvil, a first outer surface recess extending through said striking end face, and a second recess spaced from said first recess and extending transversely into said axial exhaust duct for cooperation with said first recess in exhausting said working chamber.

12. An anvil for mounting at the forward end of a percussion drill tool of the type having a casing forming an operating cylinder and with splines at its forward end for slidably encompassing said anvil, and a reciprocating hammer piston in said cylinder for beating upon said anvil, said anvil having a shank with a rear end portion including a rear end striking face and a generally cylindrical sidewall and a forward portion forming external longitudinal splineways for cooperating with said casing splines, there being at least one longitudinal flute in said cylindrical sidewall opening through said striking face for cooperating with said splineways, when said anvil is abnormally advanced in said casing, to vent said cylinder.

13. An anvil as described in claim 12 further including a stop shoulder at the rear end thereof, said flute extending through said shoulder.

14. An anvil as described in claim 12 further including a bit portion formed integrally at the forward extremity thereof and in which the external radius of said stop shoulder does not exceed the radius of said cylindrical surface.

Claims (14)

1. The combination in a percussion drilling tool having a casing forming a working chamber, a connection at the rearward end of said chamber for working fluid, a chuck member secured in the forward end of said casing, an anvil member slidable in said chuck member between an abnormally advanced position when the tool is suspended off bottom and normal working positions, said anvil member having a striking end face and an axial exhaust duct, a hammer piston reciprocable in said working chamber, and valving means for controlling the application of working fluid from said connection to the opposite ends of said chamber and for exhausting the same to cause said piston to beat upon said anvil member end face, of means for exhausting the forward end of said working chamber when said anvil member is in said abnormally advanced position comprising first, second, and third recesses in the mating surfaces of said members, said first and second recesses being in the wall of said anvil and extending, respectively, through said striking face and exteriorly of said members and said third recess being in said chuck member, said first and second recesses being positioned in said abnormally advanced position of said anvil member to form with said third recess a continuous passage from the forward portion of said working chamber to the exterior of the tool for exhausting said working chamber, there being structure on one of said members positioned to close said passage in said normal working position of said anvil member.

2. The combination described in claim 1 in which said second recess includes a part extending axially along said anvil member and a part extending transversely from said first part to said axial duct.

3. The combination described in claim 1 in which said engaging faces of said anvil member and said chuck means include complementary spline structure, there being clearance in said spline structure forming one of said recesses.

4. The combination described in claim 3 in which a portion of said spline structure is cut away to form one of said recesses.

5. The combination described in claim 3 in which the spline structure in said anvil member extends forwardly beyond said chuck means in the abnormally advanced position of said anvil member for discharging exhaust from said working chamber exteriorly of the tool.

6. An anvil device for use in a percussion drilling tool of the type having a casing forming a working chamber and a hammer piston reciprocable in said chamber for beating upon said device, said anvil device including a chuck member for attachment in the forward portion of said casing and an anvil received in said chuck member for limited sliding movements between normal working and abnormal advanced positions, there being a first recess in the outer wall of said anvil and extending through the striking face thereof for communication with said chamber, an exhaust duct extending axially through said anvil, and a passage extending transversely through said anvil from said duct through a portion of said anvil outer wall spaced axially from said recess, and there being a second recess in the inner wall of said chuck member positioned to connect said passage and said duct with said first recess and said chamber, in said abnormal advanced position of said anvil, for exhausting the working chamber forward portion.

7. A chuck device for attachment to the forward portion of a fluid pressure drill tool of the type having a casing forming a working chamber, an anvil, and a hammer piston reciprocable in said chamber for heating upon said anvil, said device comprising a generally annular body with a threaded external surface for screwing into a matching thread in the casing, a rearwardly facing casing-abutting upper end shoulder rearwardly of said threaded surface, and spline and splineway structure in the inner surface of said body, there being inner surface recessing in said body spaced longitudinally from said end thereof forming part of passaging for exhausting the motor working chamber.

8. An anvil device for use in a percussion drilling tool of the type having a casing forming a working chamber, an anvil device, and a hammer piston reciprocable in said chamber for beating upon said device, said anvil device comprising a chuck member externally configured for attachment to the forward portion of said casing and an anvil member received in said chuck member and slidable therein between retracted normal working positions and an abnormally advanced position, said anvil member having a rear end striking face for presentation in said chamber and an exhaust duct extending longitudinally therethrough, there being first, second, and third recesses in the mating surfaces of said members, said first and second recesses being spaced apart and extending, respectively, through said striking face for communication with said working chamber and to the exterior of said device, said first and second recesses being in said anvil member and being movable when said anvil member is moved to said abnormally advanced position to interconnect all of said recesses for exhausting said working chamber.

9. An anvil device as described in claim 8 in which said third recess is formed in said chuck member and said first and second recesses are longitudinally spaced apart formed in said anvil member and movable therewith to intersect said third recess when said anvil member is moved to said abnormally advanced position.

10. An anvil for mounting at the forward end of a drill tool of the type having a casing forming a working chamber, anvil and chuck members at the forward end of said casing, and a hammer piston reciprocable in said chamber for beating upon said anvil, said anvil having a striking end face, and axial exhaust duct, an annular shoulder for engaging and supporting the end of said casing in the normal working positioning of said anvil, a first outer surface recess extending through said striking end face, and a second recess spaced from said first recess for cooperation therewith in exhausting said working chamber.

11. An anvil for mounting at the forward end of a drill tool of the type having a casing forming a working chamber, anvil and chuck members at the forward end of said casing, and a hammer piston reciprocable in said chamber for beating upon said anvil, said anvil having a striking end face, an axial exhaust duct, an annular shoulder for supporting said casing in the normal working position of said anvil, a first outer surface recess extending through said striking end face, and a second recess spaced from said first recess and extending transversely into said axial exhaust duct for cooperation with said first recess in exhausting said working chamber.

12. An anvil for mounting at the forward end of a percussion drill tool of the type having a casing forming an operating cylinder and with splines at its forward end for slidably encompassing said anvil, and a reciprocating hammer piston in said cylinder for beating upon said anvil, said anvil having a shank with a rear end portion including a rear end striking face and a generally cylindrical sidewall and a forward portion forming external longitudinal splineways for cooperating with said casing splines, there being at least one longitudinal flute in said cylindrical sidewall opening through said striking face for cooperating with said splineways, when said anvil is abnormally advanced in said casing, to vent said cylinder.

13. An anVil as described in claim 12 further including a stop shoulder at the rear end thereof, said flute extending through said shoulder.

14. An anvil as described in claim 12 further including a bit portion formed integrally at the forward extremity thereof and in which the external radius of said stop shoulder does not exceed the radius of said cylindrical surface.

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