US3664435A - Hydraulic hammer with automatic stopping action - Google Patents

Hydraulic hammer with automatic stopping action Download PDF

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US3664435A
US3664435A US3664435DA US3664435A US 3664435 A US3664435 A US 3664435A US 3664435D A US3664435D A US 3664435DA US 3664435 A US3664435 A US 3664435A
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hammer
inlet
valve
outlet
fluid
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Ernest F Klessig
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Worthington Corp
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Worthington Corp
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    • BPERFORMING OPERATIONS; TRANSPORTING
    • B25HAND TOOLS; PORTABLE POWER-DRIVEN TOOLS; MANIPULATORS
    • B25DPERCUSSIVE TOOLS
    • B25D9/00Portable percussive tools with fluid-pressure drive, i.e. driven directly by fluids, e.g. having several percussive tool bits operated simultaneously
    • B25D9/06Means for driving the impulse member
    • B25D9/12Means for driving the impulse member comprising a built-in liquid motor, i.e. the tool being driven by hydraulic pressure
    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10STECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S173/00Tool driving or impacting
    • Y10S173/04Liquid operated

Abstract

An improved hydraulic hammer assembly including an automatically operable system for connecting the inlet to the outlet and bypassing the control system of the hammer when the hammer moves past a predetermined point as when the work performing element of the assembly is not in contact with the work. In the exemplary embodiment, a bypass conduit extends from the inlet to the outlet and is normally closed by a valve. When the hammer does not move past the position where it would normally encounter the tool, the application of fluid under pressure to first and third surfaces of the valve maintains the valve closed notwithstanding the application of pressure to a second surface thereon in opposed hydraulic relation to the first and third surfaces. However, when the hammer moves past the position mentioned above, the third surface on the valve is vented to the outlet and the application of fluid under pressure to the second surface causes the valve to open to establish a bypass flow path for the hydraulic fluid which avoids entirely the control system used to control reciprocation of the hammer.

Description

United States Patent [151 3,664,435
Klessig May 23, 1972 [54] HYDRAULIC 1 il' =-l R WITH: Primary Examiner-Ernest R. Purser AUTOMATIC SOPPING ACTION Attorneyl-lofgren, Wegner, Allen, Stellman & McCord [72] Inventor: Ernest F. Klessig, Racine, Wis [57] ABSTRACT Assign: Worthington corpofation "8f An improved hydraulic hammer assembly including an auto- Qompmr and Engme lmemm'onal matically operable system for connecting the inlet to the outsmn) Holyoke, Mass let and bypassing the control system of the hammer when the [22] Filed: N0 9, 1970 hammer moves past a predetermined point as when the work performing element of the assembly is not in contact with the pp 87,965 work. In the exemplary embodiment, a bypass conduit extends from the inlet to the outlet and is normally closed by a valve. 52] us. Cl ..173/17, 91 /220, 173/134 when the hammer Past the Q P" 51 lm. Cl ..B25d 9/18 wuld many amount" the ml, the aPPhcmm fluld [58] Field of Search 173/16 17; 91/220, 235 u der pressure to first third surfaces of the valve mamtains the valve closed notwithstanding the application of pressure to a second surface thereon in opposed hydraulic relation 56 References Cited to the first and third surfaces. However, when the hammer 1 moves past the position mentioned above, the third surface on UNITED STATES PATENTS the valve is vented to the outlet and the application of fluid under pressure to the second surface causes the valve to open 2,837,317 6/1958 l-lulshlzer ..l73/17 to establish a bypass fl path f the hydraulic fl id which 3,207,043 1965 y 173/17 X avoids entirely the control system used to control reciproca- 3,399,602 9/1968 Klessig et a1 ..9l/235 tion fth hammer 8 Claims, 2 Drawing Figures 68' l I III 54 56 62 2 76 J 4 72 in I81 70 44 l6 7 52 l t I 4 l2 92 45l42 A3293 I 50 I 4; 96 9 84 66 HO I02 r l 86 I08 ll lO6 7 05 p4 7 a SHEET 2 OF 2 FIGZ BACKGROUND OF THE INVENTION Hydraulic hammers are now available to do work as efficiently and more economically than air hammers and without the high noise level. Manually controlled hydraulic hammers include a control valve for controlling operation. Frequently, a hydraulic hammer. will be mounted on a boom or other support and moved relative to the work such as concrete or the like. Operation of the boom requires attention of the operator and rather than also require attention of the operator to operate a control valve for the hammer, it is preferable to have the hammer operate automatically dependent on its engagement with the work. This ensures that the hammer will only operate when engaging the work and avoids unnecessary vibrations.
It has been suggested that the problem be overcome by the use of valve channels on the hammer which will permit fluid under pressure from the inlet in passing through the control valve to flow about the hammer to an outlet when the hammer descends beyond a predetermined point in the bore as, for example, when the tool is suspended from its mounting means at the end of the bore due to the fact the tool is not in contact with the work. 1
One difficulty with the foregoing approach is that the machining of the valve channels on the hammer is a relatively costly operation. A second difiiculty is that such an approach does not employ a positive bypass of the hammer control system and the system may be rendered ineffective if the valve channels, for any reason, become clogged.
SUMMARY OF THE INVENTION It is the principal object of the invention to provide a new and improved hydraulic hammer including means for automatically terminating operation of the hammer when the hammer descends beyond a predetermined point within a casing bore, as when the tool associated therewith is no longer in engagement with the work.
The exemplary embodiment accomplished the foregoing object by means of a bypass conduit within the hammer casing extending from the inlet directly to the outlet. A bypass valve normally closes the bypass conduit and includes three pressure sensitive surfaces. The first two are associated with the inlet and are hydraulically opposed while the third is hydraulically parallel to the first and will be in fluid communication with the inlet whenever the hammer has not moved beyond the position mentioned previously. The three surfaces are arranged so that when inlet pressure is applied against the first and third surfaces, the bypass valve will be closed while when inlet pressure is applied only against the first and second surfaces, the bypass valve will be opened.
Fluid communication between the third surface and the inlet is normallv established through the bore in which the hammer reciprocates by means of a hammer shank having a lesser diameter than the inside diameter of the bore. Fluid communication between the third surface and the inlet may be precluded when an end portion of the hammer, which also serves as a piston, and which has a diameter equal to that of the bore, descends below a predetermined point in the bore as when not restricted in movement by the tool and blocks the fluid path mentioned previously. At the same time, a reduced diameter portion on the hammer used for other valve purposes in conjunction with a control valve which controls the reciprocation of the hammer vents the third surface to the outlet thereby permitting the bypass valve to open.
Other objects and advantages of the invention will become apparent from the following specification taken in conjunction with the accompanying drawings.
DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation, with parts shown in section, of a hydraulic hammer made according to the invention with the elements shown in one position wherein the bypass valve is closed; and
FIG. 2 is a side elevation, with parts shown in section with the elements in the position wherein the bypass valve is opened.
I "DESCRIPT ION OF THE PREFERRED EMBODIMENT An exemplary embodiment of a hydraulic hammer made according to the invention is illustrated in FIGS. I and 2 and is seen to include a casing, generally designated 10, having a central bore 12 therein. Disposed within the bore 12 for reciprocation therein is a hammer element, generally designated 14, having an upper valve end including an annular reduced diameter portion 16 and a pressure responsive piston 18. The hammer element 14 also includes a lower, enlarged hammer 20 which is disposed within an enlarged diameter portion 22 of the bore 12.
Secured to the lower end of the casing 10 is a tool mount 24 having a central bore 26 receiving a movable tool 28 which is disposed to be struck by the hammer 20 and which has a lower, work performing end (not shown). Alternatively, that portion of the tool 28 illustrated could be an anvil having its lower end operatively associated with a selected tool to be used in conjunction with the hammer. For example, the lower end of such an anvil may ride on the upper end of the tool. The tool 28 includes a key way 30 and a key 32 which extends through the tool mount 24 and the key way 30 to restrict the movement of the tool 28 within the tool mount 24 to the length of the key way 30.
The casing 10 includes an inlet 34 which may be connected to any suitable source of hydraulic fluid under pressure to serve as a source of energy for reciprocating the hammer 20 within the casing 10 to impart energy to the tool 28. In fluid communication with the inlet is a conventional accumulator 36. The casing 10 further includes an outlet 38 and a second accumulator 40 in fluid communication with the outlet 38.
A port 42 transverse to the bore 12 in the casing 10 and in fluid communication with the inlet 34 is arranged to permit the application of fluid under pressure to a lowermost surface 44 of the piston 18 of the hammer element 14. As will be seen, this application of fluid under pressure serves to move the hammer element 14 upwardly in the bore 12 for its return stroke. A series of small grooves 45 extend upwardly partially to the port 46 from the port 42 for purposes to be seen.
A port 46 normally in fluid communcation with the port 42 through the bore 12 and reduced diameter portion 16 provides fluid under pressure to a hydraulically operated control valve spool, generally designated 50, which is operative to control the reciprocation of the hammer element 14 within the bore 12. The spool 50 includes a reduced diameter portion 52. The spool 50 is shiftable within a bore 54 to a first position (illustrated in FIG. 1) wherein fluid under pressure from the inlet 34 passing through the port 42 and the port 46 is fed along the reduced diameter portion 52 to the upper end of the bore 12 via a conduit 56 in communication with both the bore 54 and the bore 12 and applied to an upper surface 58 on the piston 18 of the hammer element 14. The surface 58 is larger than the surface 44 so that when fluid under pressure is applied to the former, notwithstanding the fact that fluid under pressure is also applied to the latter, the hammer element 14 will be driven downwardly to strike the tool 28.
A second position of the spool 50 within the bore 54, shown in FIG. 2, is one wherein the spool 50 is shifted upwardly from the position shown in FIG. 1 and wherein an enlarged lower end 60 blocks off the path of fluid under pressure from the inlet 34 to the upper end of the bore 12 while an enlarged upper end 62, in conjunction with the reduced diameter portion 52 permits fluid in the upper end of the bore above the surface 58 to exit to the outlet 38 via a conduit 64. Interposed between the conduit 64 and the outlet 38 and hydraulically prior to the point of connection of the accumulator 40 to the outlet 38 is a restriction or orifice 65 for purposes to be seen.
When the spool 50 is in the just-mentioned position, the fact that the surface 58 is subjected to outlet pressure while the surface 44 is subjected to inlet pressure will cause the hammer to move upwardlv for its return stroke.
Positioning of the spool 50 is controlled by a differential pin device including pins 66 in the lowermost end of the spool 50 and pins 68 at the opposite end thereof. The pins 66 have a lesser effective area than the pins 68 and are continuously subjected to inlet pressure. The pins 68 are intermittently subjected to inlet pressure dependent upon the position of the hammer 14 within the bore 12 and when such is the case, the spool 50 will be driven by the pins 68 to the position shown. When such is not the case, the pins 68 will drive the spool 50 upwardly to the second-mentioned position above. For a more detailed statement of the operation and specific construction of the differential pin device, reference may be had to U.S. Pat. No. 3,399,602 to Klessig et al., the details of which are herein incorporated by reference.
As mentioned previously, one of the difficulties encountered in the operation of hvdraulic hammers is variations in the frequency of operation principally due to varying back pressure. Specifically, the rate of return of the hammer element during the return stroke will be generally dependent upon the ratio of the area of the smaller surface 44 and the pressure supplied thereto to the area of the larger surface 58 and the pressure applied thereto. Since, during return, the latter surface is subjected only to outlet pressure, an increased back pressure will cause a slower rate of return than would be the case with little or no back pressure present. Accordingly, the exemplary embodiment of the invention includes means by which the back pressure against the surface 58 during the return stroke of the hammer element 14 is maintained constant so that cycling frequency will be constant.
Specifically, a restriction of constant size is located in the flow path from the upper end of the bore 12 to the outlet 38 and which is established when the valve 50 is in the second position mentioned above. More particularly, the restriction is in the form of a venturi, generallv designated 70 and carried on the upper end of the reduced diameter portion 52 of the spool 50. The venturi 70 is generally defined by a land 72 having a diameter less than the diameter of the bore 54 and greater than that of the reduced diameter portion 52; an upstream tapered short side 74; and a downstream tapered long side 76. That is, the longitudinal extent of the upstream side 74 of the land 72 is less than the longitudinal extent of the downstream side 76.
When the spool 50 is moved upwardly to the second-mentioned position, the same in conjunction with a portion 78 of the bore 54 interposed between the conduit 64 define an annular venturi. The annular venturi effectively isolates the pressure in the conduit 56 from pressure variations normally encountered in the return conduit 64.
It should be specificallv noted that while the exemplary embodiment employs a venturi on the spool 50, the same could be located at any one of a variety of downstream points in fluid communication with the outlet 38. However, because the casing is generally formed as a casting and the various conduits contained therein are frequently defined by bores, the forming of a restriction in one of the conduits would require a rather complex machining or casting operation. In contrast, the formation of the venturi 70 on the spool 50 may be accomplished relatively easily during the formation of the reduced diameter portion 52 thereon.
The exemplary embodiment further includes a bypass conduit 80 which extends between the inlet 34 and the outlet 38 and which is normally closed by a bypass valve, generally designated 82. The bypass valve 82 is mounted for reciprocation within a bore 84 in the casing 10. One end of the bore 84 is closed by a plug 86 which serves to position a spring 88 in a recess 90 formed in the body of the valve member 82 to bias the valve member 82 against a seat 92 surrounding the bypass conduit 80 as shown in FIG. 1.
The bypass valve 82, intermediate its ends, includes a reduced diameter portion 93 which normally permits fluid under pressure to flow from the inlet 34 to the port 42 for use in driving the hammer in the manner mentioned previously.
The bypass valve 82 further includes a pair of pressure responsive surfaces 94 and 96 with the surface 94 being larger than the surface 96. The recess terminates in a third pressure sensitive surface 98 on the valve 82. The arrangement is such that the combined areas of the surfaces 96 and 98 is greater than the area of the surface 94 but the surface 94 is greater in size than the surface 96 alone. More particularly, the surfaces 94, 96, 98 and the spring 88 act such that when inlet pressure is applied against the surface 96 and the surface 94 (as will always be the case) and is applied to the surface 98 (as will usually be the case) the valve 82 will be in the position shown in FIG. 1. However, when inlet pressure is not applied to the surface 98, the valve will shift downwardly to the position shown in FIG. 2 to open the bypass 80 whereupon fluid under pressure from the inlet 34 will flow directly to the outlet 38 bypassing entirely the control valve 50. As a result, reciprocation of the hammer element will cease.
Returning to FIG. 2, fluid under pressure from the inlet 34 is normally applied to the surface 98 via the port 42 and a section 100 of the bore 12 interconnecting the port 42 and a port 102. The bore section 100 is of slightly greater diameter than the shank of the hammer element 14 but, for purposes to be seen, is of the same diameter as the diameter of the piston 18.
Referring now to FIG. 2, the relief of fluid pressure against the surface 98 so as to permit the valve 82 to open for bypass purposes is accomplished by means of a port 104 in the bottom of a stationary sleeve 106 embracing the shank of the hammer element 14. Also included are a series of small grooves extending upwardly a short distance from the port 104 to permit the establishment of fluid communication to the port 104 through the bore section 100 slightly before the reduced diameter portion moves to the u per boundary of the port 104. A channel 108 is in fluid communication with the port 104 and a bore 110 which empties into the hollow center 112 of the spool 50. From the hollow center 112, fluid may flow through a radial bore 114 in the uppermost end of the spool 50 to the conduit 64 to the outlet 38. The arrangement is such that when the hammer element 14 has been driven downwardly past a position wherein it would normally encounter the tool 28 and not yet has encountered the tool as shown in FIG. 2, the side of the piston 18 will enter the bore portion 100 thereby blocking fluid communication between the inlet 34 via the port 42 to the surface 98. Simultaneously, the reduced diameter portion 16 on the hammer element 14 will establish fluid communication between the port 102 and the port 104 through the grooves 105 thereby permitting the fluid under pressure bearing against the surface 98 to flow to the outlet 38 via the path mentioned previously.
This relieving of the pressure against the surface 98 will cause the inlet pressure to shift the valve 82 downwardly thereby permitting fluid from the inlet 34 to flow directly to the outlet 38 via the control valve 50. Furthermore, the presence of the orifice 65 in the return line will, when the accumulator 40 dumps, cause a large back pressure area 58 of the piston 18 through the port 64, the venturi 70 and the port 56 to tend to keep the hammer in its down position. Also, the grooves 45 permit flow of fluid from the pressure port 42 into the upper portion of the bore 12 to tend to fill the cavity caused by cavitation as the piston 18 cuts off flow through the port 46 during downward movement.
As a result of the foregoing, reciprocation of the hammer 14 will cease. It will therefore be apparent that whenever the tool is at a predetermined position within its bore 26 and lower than the desired position, as for example, when it is not bearing against work, the hammer will automatically cease to operate.
From the foregoing, it will be appreciated that a hydraulic hammer made according to the invention possesses a number of significant advantages over those heretofore known. For example, a positive bypass is provided as opposed to one wherein the hydraulic fluid will flow through a control system for the hammer. Secondly, the exemplary construction does not require any special machining operations on the hammer element. Specifically, the reduced diameter portion 16 on the hammer element, which is operative to relieve pressure against the third surface 98 of the control valve 92 when hammer reciprocation is to cease would normally be employed in the hammer in any event for controlling the application of hvdraulic fluid to the large piston surface 58. It is therefore believed apparent that the invention provides an improved hammer of economical construction and which is positive in its operation for ceasing, automatically, the reciprocation of the hammer when the same has moved beyond a predetermined point in the bore.
I claim:
1. A hydraulic hammer comprising:
a casing having a bore;
a hammer received within said bore for reciprocating movement therein;
a tool adjacent one end of the bore and normally positioned to be struck by said hammer, said tool being movably mounted and movable to a position wherein it cannot be struck by said hammer;
a hydraulic fluid inlet in said casing;
a hydraulic fluid outlet in said casing;
means, including a hydraulically operated control valve for intermittently directing hydraulic fluid from said inlet to the hammer to drive the same in one direction within said bore;
means for driving the hammer within said bore in a direction opposite said one direction; and
means associated with said inlet and said outlet and responsive to movement of the tool to said position thereof for directing hydraulic fluid from said inlet to said outlet in a flow path that bypasses said control valve.
2. A hydraulic hammer according to claim 1 wherein said directing means includes a bypass conduit within said casing extending from said inlet to said outlet and a valve normally closing said bypass conduit.
3. A hydraulic hammer according to claim 1 wherein said directing means comprises a bypass conduit within said casing extending from said inlet to said outlet, a valve member positioned to close said bypass conduit, said valve having a pair of opposed pressure sensing surfaces with one of said surfaces having a larger area than the other of said surfaces, said valve further including a third pressure sensitive surface hydraulically parallel to said other surface, said pair of surfaces being arranged with respect to each other and to said inlet and said bypass conduit so that said valve will be moved to open said bypass conduit whenever pressure is not applied to said third surface, and means for applying fluid under pressure to said third surface whenever said anvil is not in said position wherein it cannot be struck by said hammer.
4. A hydraulic hammer according to claim 3 wherein said means for applying pressure to said third surface includes conduit means on said hammer normallv establishing fluid communication between said inlet and said third surface and cooperating means within said casing for blocking said conduit means when said hammer moves within said bore past a portion wherein it would normally strike said tool.
5. A hydraulic hammer comprising:
a casing having a bore;
a tool movably mounted adjacent one end of a bore and normally positioned to be struck by a hammer;
a hammer received within said bore for reciprocating movement to a position wherein it would normally strike said tool;
a hydraulic fluid inlet in said casing;
a hydraulic fluid outlet in said casing;
means, including a hydraulically operated control valve for intermittently directing hydrualic fluid from said inlet to said hammer to drive the same towards said tool; and means associated with said inlet and said outlet and responsive to movement of said hammer past said position wherein it would normally strike said tool for directing hydraulic fluid from said inlet to said outlet in a flow path that bypasses said control valve.
6. A hydraulic hammer according to claim 5 wherein said directing means comprises a bypass conduit extending between said inlet and said outlet, a hydraulic operated bypass valve normally closing said bypass conduit and means for normally directing fluid under pressure from said inlet to said bypass valve to bias the same to a closed position, and means on said hammer for blocking the application of fluid under pressure from said inlet to said b pass valve when said hammer moves beyond the position at which it would normally strike said tool.
7. A hydraulic hammer according to claim 6 further including means on said hammer for directing fluid from said bypass valve to said outlet when said hammer moves beyond said position at which it would normally strike said too]; said bypass valve further including pressure responsive surface means in fluid communication with said inlet and responsive to the fluid under pressure to cause movement of said bypass valve to an open position when said hammer has moved beyond said position wherein it would normally strike said tool.
8. A hydraulic impact device having a casing having a bore, a hammer element reciprocable in said bore in a cycle between normal limit positions by controlled application of pressure fluid to opposed areas of the hammer, said casing having a pressure fluid inlet and a fluid outlet, a control valve for controlling the intermittent application of fluid to one of said areas of the hammer, a bypass passage in said casing connecting said inlet and outlet and bypassing said control valve, a bypass valve closing said bypass passage, and means, including a valve section on said hammer, responsive to movement of the hammer beyond a normal limit position to open said bypass valve and stop cycling of the hammer element.

Claims (8)

1. A hydraulic hammer comprising: a casing having a bore; a hammer received within said bore for reciprocating movement therein; a tool adjacent one end of the bore and normally positioned to be struck by said hammer, said tool being movably mounted and movable to a position wherein it cannot be struck by said hammer; a hydraulic fluid inlet in said casing; a hydraulic fluid outlet in said casing; means, including a hydraulically operated control valve for intermittently directing hydraulic fluid from said inlet to the hammer to drive the same in one direction within said bore; means for driving the hammer within said bore in a direction opposite said one direction; and means associated with said inlet and said outlet and responsive to movement of the tool to said position thereof for directing hydraulic fluid from said inlet to said outlet in a flow path that bypasses said control valve.
2. A hydraulic hammer according to claim 1 wherein said directing means includeS a bypass conduit within said casing extending from said inlet to said outlet and a valve normally closing said bypass conduit.
3. A hydraulic hammer according to claim 1 wherein said directing means comprises a bypass conduit within said casing extending from said inlet to said outlet, a valve member positioned to close said bypass conduit, said valve having a pair of opposed pressure sensing surfaces with one of said surfaces having a larger area than the other of said surfaces, said valve further including a third pressure sensitive surface hydraulically parallel to said other surface, said pair of surfaces being arranged with respect to each other and to said inlet and said bypass conduit so that said valve will be moved to open said bypass conduit whenever pressure is not applied to said third surface, and means for applying fluid under pressure to said third surface whenever said anvil is not in said position wherein it cannot be struck by said hammer.
4. A hydraulic hammer according to claim 3 wherein said means for applying pressure to said third surface includes conduit means on said hammer normally establishing fluid communication between said inlet and said third surface and cooperating means within said casing for blocking said conduit means when said hammer moves within said bore past a portion wherein it would normally strike said tool.
5. A hydraulic hammer comprising: a casing having a bore; a tool movably mounted adjacent one end of a bore and normally positioned to be struck by a hammer; a hammer received within said bore for reciprocating movement to a position wherein it would normally strike said tool; a hydraulic fluid inlet in said casing; a hydraulic fluid outlet in said casing; means, including a hydraulically operated control valve for intermittently directing hydrualic fluid from said inlet to said hammer to drive the same towards said tool; and means associated with said inlet and said outlet and responsive to movement of said hammer past said position wherein it would normally strike said tool for directing hydraulic fluid from said inlet to said outlet in a flow path that bypasses said control valve.
6. A hydraulic hammer according to claim 5 wherein said directing means comprises a bypass conduit extending between said inlet and said outlet, a hydraulic operated bypass valve normally closing said bypass conduit and means for normally directing fluid under pressure from said inlet to said bypass valve to bias the same to a closed position, and means on said hammer for blocking the application of fluid under pressure from said inlet to said bypass valve when said hammer moves beyond the position at which it would normally strike said tool.
7. A hydraulic hammer according to claim 6 further including means on said hammer for directing fluid from said bypass valve to said outlet when said hammer moves beyond said position at which it would normally strike said tool; said bypass valve further including pressure responsive surface means in fluid communication with said inlet and responsive to the fluid under pressure to cause movement of said bypass valve to an open position when said hammer has moved beyond said position wherein it would normally strike said tool.
8. A hydraulic impact device having a casing having a bore, a hammer element reciprocable in said bore in a cycle between normal limit positions by controlled application of pressure fluid to opposed areas of the hammer, said casing having a pressure fluid inlet and a fluid outlet, a control valve for controlling the intermittent application of fluid to one of said areas of the hammer, a bypass passage in said casing connecting said inlet and outlet and bypassing said control valve, a bypass valve closing said bypass passage, and means, including a valve section on said hammer, responsive to movement of the hammer beyond a normal limit position to open said bypass valve and stop cycling of the hammer element.
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