US 3605915 A
Description (OCR text may contain errors)
3 Shoots-Sheet 2 FIG. 7
T. J. GATELY EIAL PNEUMATIC RAPPER FOR ELECTROSTATIC PRECIPITATORS Filed April 1]., 1969 Sept. 20, 1971 FIG. 6
p 1971 'r. J. GATELY A PNEUMATIC RAPPER FDR ELECTROSTATIC PRECIPITATORS 3 Shuts-Shoot 3 Filed April 11, 1969 FIG. II
FIG. I0 PRIOR ART FIG. 9 PP/o/e APT 'United States Patent 3,605,915 PNEUMATIC RAPPER FOR ELECTROSTATIC PRECIPITATORS Thomas J. Gately, Ellicott City, Bartholomew F.
Quintilian, Baltimore, and Charles H. Rodgers, Sykesville, Md. (all Koppers Company, Inc., 440 College Park Drive, Monroeville, Pa. 15146) Filed Apr. 11, 1969, Ser. No. 815,418 Int. Cl. B03c 3/76; Bd 9/00 US. Cl. 173-116 13 Claims ABSTRACT OF THE DISCLOSURE A pneumatic single-impulse rapper for electrostatic precipitators comprising a piston made from dry-lubricating material within a unitary, flangeless, cylindrical housing having no bolted connections, with the piston being returned to its pre-pulse position by a spring having reduced diameter end coils, said piston including a spherical or alternate striking surface for localized impact against the end of the housing, said housing including a non-removable cap and an air inlet adapted to secure a strengthened air fitting to the housing. A braided wire provides electrical grounding of the rapper.
BACKGROUND OF THE INVENTION Field of the invention This invention relates generally to gas separation devices and particularly to vibrating, jarring or rapping means for the collector and discharge electrodes of an electrostatic precipitator.
Description of the prior art Conventionally, in electrostatic precipitators, a particle laden gas stream is directed through banks of large vertically suspended sheet metal collector electrode plates. Between these collector plates are vertically suspended discharge electrode Wires. High voltage applied to the electrode wires ionizes the gas. Dust particles in the gas become charged and are thus attracted to the collector electrode plates. The plates and wires must be kept free of accumulated dust to function properly.
Pneumatic rappers are often used to transmit vibrations to the electrodes thus causing the deposited dust particles to break loose and fall into a hopper at the bottom of the precipitator. An example of one such rapper is shown and described in Penningtons Pat. No. 3,030,753.
It has been found that the repeated high intensity vibrations of the rappers cause substantial acceleration and fatigue stresses in the rappers thus causing damage to the components thereof. One recurring problem in conventional rappers has been the high stresses imposed on the bolted joints between the mating portions of the parts forming the rapper cylinder. These stresses cause eventual failure of the bolts and thus replacement is necessary.
It has also been found that repeated high impact loads cause damage to the piston return spring due to coil clash of the end coils causing them to break off. For purposes of illustration only, this description refers to the reduced diameter end coils as the inactive coils.
Patented Sept. 20, 1971 end in a flush contact and the subsequent side thrusts cause asymmetrical loads on the rapper. A further result is that mushrooming of the piston striking surface occurs because of the angular contact which necessitates eventual piston replacement.
SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a single impulse pneumatic rapper having improved reliability thereby making repair and replacement less frequent. This object is achieved in the present invention by providing a single impulse pneumatic rapper including a fiangeless body unit which eliminates the need for interconnecting bolts; an improved spring having inactive end coils wound to a smaller pitch diameter than the active coils thus preventing coil clash and breaking off of the ends of the coils, the spring being provided so as to maintain a tight fit against the cylinder Wall's when it is compressed thus eliminating looseness and subsequent vibrations after impact which are known causes of spring failure; an air inlet fitting of increased crosssection having a chamfered seat provided to mate with a correspondingly chamfered opening in the rapper cylinder for absorbing cantilever loads applied to the fitting and greatly reducing the effect of such loads previously applied to the threads of conventional fittings causing the threads to fail; a piston made of conventional ductile iron and having sufiicient nodular graphite dispersion to eliminate the need for either fluid lubrication or a synthetic lining, said piston having a spherical or alternate striking surface thus reducing side thrusts and mushrooming and further providing greater energy output from the piston due to its localized contact with the cylinder end thereby directing the impact substantially along the axis of the rapper; the use of a tightly fitting cap closure for the rapper cylinder providing a good shock resistant joint not subject to the variations of preload found in bolted joints; a dependable electrical grounding connection achieved by twisting the end of a conventional fiat braided grounding wire to increase its strength and securing the wire by means of a screw.
The above and further objects and novel features of the invention will appear more fully from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are not intended as a definition of the invention but are for the purposes of illustration only.
BRIEF DESCRIPTION OF THE DRAWINGS In the drawings wherein like parts are marked alike:
FIG. 1 illustrates a cross-sectional side elevation of a preferred embodiment of the pneumatic rapper assembly of the invention;
FIG. 2 is an enlarged section of the air inlet fitting of FIG. 1;
FIG. 3 is a top view of FIG. 1;
' FIG. 4 is an end view of the piston return spring of FIG. 1;
FIG. 5 is an enlarged partial elevation of the piston return spring of FIG. 4;
FIG. 6 is a partial cross-sectional elevation taken substantially along line VI-VI of FIG. 4 showing the piston return spring in compression;
FIG. 7 is an enlarged side elevation of the rapper piston of FIG. 1;
FIG. 8 is an enlarged section showing the grounding wire connection of FIG. 1;
FIG. 9 is a diagrammatic illustration of a piston of the type heretofore known having a flat striking surface and being misaligned within a rapper cylinder;
FIG. is a diagrammatic illustration of a piston of the type heretofore known having a flat striking surface not square with the piston walls and being coaxial with a rapper cylinder;
FIG. 11 is a diagrammatic illustration of an embodiment of a piston made in accordance with the invention having a striking surface comprising a truncated cone and being misaligned within a rapper cylinder;
FIG. 12 is a diagrammatic illustration of another embodiment of a piston having a spherical striking surface and being misaligned with a rapper cylinder;
FIG. 13 is a diagrammatic illustration of a further embodiment of a piston having a conical striking surface and being misaligned with a rapper cylinder;
FIG. 14 is a diagrammatic illustration of still another embodiment of a piston having a striking surface comprising a ball and being misaligned within a rapper cylinder; and
FIG. 15 is a diagrammatic illustration of a piston made in accordance with this invention showing three alternate radii configurations and being misaligned within a rapper cylinder.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, a rapper assembly generally designated by numeral includes a cylinder housing 22 having a cylinder cavity 24 within which a rapper piston 26 reciprocates. Reciprocation of the piston 26 within the cylinder cavity 24 is achieved by high pressure air pulses supplied by a suitable source (not shown) and entering the cylinder cavity 24 by means of an air inlet fitting 28 secured to the cylinder housing 22. Air entering the upper portion of cylinder cavity 24 drives the piston 26 downward (from a primary to a secondary position) until a lower spherical striking surface 30 of piston 26 strikes the anvil 32 of cylinder cavity 24. Piston 26 is returned to the upper portion of the cylinder cavity 24 by means of a piston return spring 34 which extends until an upper striking surface 36 (preferably spherical) of piston 26 strikes surface 38 of a cap closure 40.
Rapper assembly 20 is mounted to a tapered end 42 of a rod 44 which extends into a correspondingly tapered cavity 46 of cylinder housing 22. conventionally, rod 44 is secured to a supporting structure (not shown) from which are suspended either the discharge electrode wires or the collector electrode plates of an electrostatic precipitator (also not shown). Rod 44 is the means by which vibrations produced by the rapper assembly are imparted to the supporting structure of the wires or the plates.
The rapper cylinder housing 22, shown in FIG. 1, is of unitary steel construction and includes a cylinder cavity 24. During assembly of the rapper, an end of cylinder housing 22 is left open to permit spring 34 and piston 26 to be placed within cylinder cavity 24. The open end of cylinder housing 22 is then closed by placing end cap 40 within an enlarged bore which forms seating shoulder 54 as best shown in FIG. 2. Referring again to FIG. 1, end cap 40 is pressed to fit within cylinder housing 22 tight enough to resist separation upon impact of piston 26 on its return to its primary position. Preferably a fail safe means of retaining cap closure 40 is provided by a conventional snap ring 56 in snap ring groove 58 adjacent to end cap 40 as shown in FIG. 2. This type of cylinder construction provides a rapper wherein there is no need for bolted connections. Such connections as exist in conventional rapper cylinder housings are known to fail due to fatigue caused by intense vibrations resulting from impact of the piston 26 against anvil 32.
A tapered cavity 46 is located along the central axis of the cylinder housing 22 as shown in FIG. 1. A correspondingly tapered end 42 of rod 44 Wedges into the tapered cavity 46 of the cylinder housing 22. The wedging action between the co-acting tapers increases each time the piston 26 hammers against the anvil 32 of the cylinder cavity 24 so as to retain the rapper assembly 20 on the rod 44 and thereby provides a reliable connecting means between rod 44 and rapper assembly 20.
A threaded opening 60 is provided for securing air inlet fitting 28 in the cylinder housing 22 as will be later described in greater detail. Air-relieving port 62 is provided in the lower portion of cylinder cavity 24 to allow air in the lower portion of cylinder cavity 24 to be vented into the atmosphere during the power or pressure stroke of piston 26.
A common shortcoming in pneumatic rappers is that the solid wire conventionally used to prevent an electrical potential between the rapper and the supporting structure, breaks due to the repeated vibrations. Braided grounding cable may also break if used in the conventional way. conventionally these grounding wires were secured to the rapper assembly by a lug, however, this method has proved to be inadequate. It has been found that, as shown in FIG. 8, this problem can be overcome by twisting an end of a braided cable 48 and placing the twisted end into a cavity 50 and securing it therein by a screw 52. This means for securing the cable was found to withstand the intense and prolonged vibrations with very high reliability.
FIG. 2 shows an enlarged view of the threaded opening 60 in the cylinder housing 22 in which fitting 28 is seated. Conventional fittings usually have small crosssectional areas where the body of the fitting butts against the housing 22 which tend to fail in bending and shear from intense vibrations of the rapper. Standard pipe threads on such fittings also become loosened as a result of the intense vibrations of the rapper. To overcome these shortcomings, fitting 28 is provided with a frusto-conical or chamfered portion 64 held in wedging engagement with a mating chamfer 66 provided in the threaded opening 60. This construction provides a greater cross-sectional area of metal of the fitting 28 along the line of shear at the juncture of the fitting 28 and the cylinder 22 thereby decreasing the possibility of breakage induced by rapper vibrations. An added advantage is obtained since tightening of the threads will wedge chamfer 64 into tight engagement with mating chamfer 66 thereby reducing the tendency of the threaded connection to become loosened by rapper vibration. A torque of foot pounds, applied to fitting 28 has been found to be sufficient to firmly seat the chamfered section 64 of the fitting 28 in the mating chamfer 66 of the opening 60.
In the conventional rapper the repeated compression of the piston return spring caused failure of end coils due to coil clash and abrasion, particularly when the end of the spring would normally gouge into the side of the adjacent coil upon compression of the spring. The novel piston return spring of this invention overcomes this problem. From FIGS. 4, 5 and 6 it can be seen that the pitch radius R of the active coils 39 of piston return spring 34 is greater than the pitch radius R of inactive end coils 37, by slightly more than one wire diameter. By referring of FIG. 6, which is a view along line VIVI of FIG. 4, it may be seen that upon compression active coils 39 overlap the inactive end coil 37. Thus, axial abrasion between the active coils 39 and the end coils 37 is avoided.
In addition, continued vibrations of the piston return spring of a conventional rapper after impact of the piston, caused premature damage to the spring. From FIG. 1 it can be seen that the spring 34 of this invention, when extended, is proportioned so as not to touch the wall of cylinder cavity 24. As is well known, a spring coil will increase in diameter as the spring is compressed. Spring 34 is proportioned so that when it is compressed to the point where piston 26 reaches its secondary position, the diameter of spring 34 will have increased so as to contact the wall of cylinder cavity 24 thus providing a damping effect upon spring 34 thereby increasing the life of the spring.
Two other problems long associated with conventional rappers are:
(A) Providing lubrication, and
(B) Increasing the magnitude of vibrations of the rap- 1 er. p Providing oil lubrication requires periodic servicing which makes rappers unreliable and increases maintenance costs. Reducing the need for lubrication by providing the piston with a polytetrafluoroethylene band is satisfactory but is expensive.
Furthermore, the effective output of the conventional rapper is often reduced because of piston misalignment upon impact due to the tendency of the piston to cock in the cylinder from the required clearances between the piston and the cylinder walls. The result is less impact energy transmitted when the edge of the piston hits the cylinder end or anvil 32 first and then levels out against it. In accordance with this invention, a piston which maintains its efficiency is provided. As best illustrated in FIG. 7, piston 26 is preferably formed as a symmetrical cylinder so that it can be used with either end against anvil 32. The piston 26 includes spaced apart bearing lands 21 separated by an undercut groove 23 with the lands 21 adapted for sliding contact with the interior wall 19 of housing 22. Spacing the lands 21 apart tends to guide piston 26 within the cylinder cavity 24. The ends of the piston extend axially beyond lands 21 and are undercut to provide a relief 27 between ends 25 and the interior wall 19 of housing 22. Relief 27 is proportioned so that the end coils 37 of Spring 34 can be overlapped by the active coils 39 within the relief as was previously explained. If desired, relief 27 may be undercut to provide an additional relief 29 of a smaller diameter than relief 27. Spherical striking surfaces 30 and 36 are rounded sufficiently so as not to allow a corner portion thereof to contact the anvil 32 should piston 26 be cocked in cylinder cavity 24 at the maximum possible angle. Furthermore, the spherical striking ends 30 and 36 of piston 26 localize stresses and thereby provide greater energy output due to their localized contact with the anvil 32.
The contrast between the piston arrangement of this invention and the prior art pistons which have had flat striking surfaces is diagrammatically illustrated in FIG. 9. When piston 260 strikes anvil 32 the resultant impact force should ideally pass through the center of gravity 92 of piston 260 while simultaneously acting along the axis 98 of the cylinder cavity 24. If the piston 260 is cocked because of clearances between piston and cylinder, an edge portion of the flat striking end of piston 260 will strike the anvil 32. A resultant force will act on the piston 260 through the point 90 at which the piston 260 and anvil 32 meet. This resultant force acts along line 94 and does not pass through the center of gravity 92 of piston 260. Line 94 is parallel to the axis 98 of cylinder cavity 24 and the perpendicular distance between lines 98 and 94 is moment arm 96 which when created acts as a righting moment on piston 260. Thus, part of the energy of the piston 260 striking the anvil 32 is dissipated in moving the mass of piston 260 about the center of gravity 92. The result is that the line of force is not co-axial with the rapper assembly and the full energy output of the piston is not realized. Also, the action of the righting moment thrusts the piston against the interior wall of the cylinder cavity causing damage to both the piston and the cylinder wall and also produces undesirable vibrations which fatigue the rapper assembly.
A similar condition would result if, as FIG. 10 illustrates, piston 260 moves co-axially within cylinder cavity 24 but due to piston 260 having a striking surface not square with the piston walls, moment arm 96 is produced.
FIG. 12 illustrates a preferred embodiment of the invention wherein the resultant forces act through the center of gravity of piston 26. First, the length of the piston 26, of course, is selected to provide the mass desired. In accordance with this invention, the radius 104 of the spherical end of piston 26 is struck from the center of gravity 92 and extends to the end of the piston. Thus, the line of force 94 of piston 26 coincides with its line of action 98, both of these acting through the center of gravity 92 and no moment arm is produced to cause side thrust on the piston. The nominal clearances which permit the piston 26 to cock in the cylinder cavity 24 are small so that piston 26 cannot cock enough to allow the point of contact between the are 10 0 and anvil 32 to occur too near the point where are intersects the piston side wall 106.
Referring now to FIG. 15 it can be seen that if radius 104 is made greater than radius 104 and is not struck from the center of gravity 92 of piston 26, it will cause undesired side thrusts produced by the line of force 94' and moment arm 96.
Conversely, if radius 104" is less than radius 104 and also not struck from the center of gravity 92 then a line of force 94" and moment arm 96" are also produced.
Thus, the preferred construction utilizes a spherical end whose radius is struck substantially from the center of gravity of the piston 26. Less desirable, but still workable, are spherical ends whose radii are longer or shorter than the preferred radius provided that they are not so large as to shift the line of force 94 beyond the intersection of are 100 with the edge 106 of the piston 26, or so small that the resultant spherical end would be subject to destruction upon impact caused by material limitations. The larger limit can be easily calculated by graphic illustration of a piston within a cylinder and taking into account the maximum clearances provided for.
Other constructions for reducing the magnitude of the undesirable moment arms are illustrated in FIGS. 11, 13 and 14. FIG. 11 shows piston 26 provided with a truncated conical tip; FIG. 13 shows piston 26 with a conical tip; and FIG. 14 shows piston 26 with a ball imbedded in the piston striking end. In each of these cases the moment arm 96 would be reduced, however, the result would not be as desirable as provided by the ideal case shown in FIG. 12.
If the condition is reversed, that is, placing the spherical striking surface on the anvil 32 and having a fiat striking surface on piston 26, the desirable effect is not completely obtainable. It may be graphically shown that merely reversing the geometry of the co-aeting striking surfaces. it does not hold that the exact opposite action will occur. For example, placing a spherical tip on anvil 32 having the ideal radius and striking such a surface with a flat piston end produces a moment arm greater than the ones shown in FIG. 15.
It was originally thought that repeated contact by the spherical striking surface 30 shown in FIG. 1 would cause noticeable deformation in the anvil 32. Surprisingly. the anvil 32 deforms very little under prolonged impacts from piston 26 even though the surface 30 of the piston 26 is spherical. It was also thought that the spherical striking surface 30 would deform due to repeated contact with anvil 32 but no significant deformation was found. Apparently, controlling the geometry of the piston to move the impact area inward from the edge of the piston limits the impact area of both the piston 30 and anvil 32 so that the impact area on each surface is completely surrounded by material of the component thereby providing greater support upon impact. As a result, anvil 32 does not deform and spherical end 30 does not mushroom nearly as much as flat ended pistons tend to do.
It has been found that making piston 26 of conventional ductile iron and making the cylinder housing 22 of steel or other non-ductile iron material overcomes the lubrication problem. It is postulated that the nodular graphite dispersion in the ductile iron of piston 26 is a factor in providing lubrication under these conditions. As the lands 21 wear from sliding contact with the cylinder wall, nodules of graphite are exposed and as these nodules are released by abrasion, they seem to condition the surface of the cylinder wall. Thus, the graphite becomes operative as a dry lubricant between the sliding surfaces so that no further lubrication is required.
In operation, the rapper assembly 20 is actuated by a pulse of high pressure air from a suitable conventional source (not shown) which enters the cylinder cavity 24 through an air hose (not shown) connected to the air inlet fitting 28 in the known manner. The pulse of air rapidly fills the upper portion (as shown in FIG. 1) of the cylinder cavity 24 which drives the piston 26 downward until it impacts against anvil 32. Air in the lower portion of the cavity 24 is allowed to escape through the port 62 in the cylinder housing 22 into the atmosphere. The resulting vibrations are transmitted to rod 44 connected to the bottom of the rapper assembly 20.
The high pressure air pulse is mostly expended at the end of the power stroke so that the compressed Spring 34 can overcome the reduced air pressure in the upper cylinder cavity 24 and gradually extend itself until the travel of the piston 26 is halted by contact of the upper spherical striking end 36 of piston 26 with the striking surface 38 of the cap closure 40. Any residual air above piston 26 slowly leaks past piston 26 into the lower portion of cavity 24 as the piston moves upwardly. This entire cycle takes approximately seconds to complete, and may be repeated thereafter at preselected intervals.
The foregoing has described a novel rapper assembly which is desirable where it is diflicult to provide continuing lubrication between co-acting surfaces of a piston and a cylinder housing. Having thus described the invention in its best embodiment and mode of operation, that which is desired to be claimed by Letters Patent is:
1. A pneumatically operated single-impulse rapper assembly for producing vibrations in a structure connected to said assembly, comprising:
a tubular housing having a first end connected to said structure;
a piston reciprocable within said housing and urged against a second end thereof by resilient means between said piston and said first end; and
pressure means for directing a pulse of air pressure against said piston to drive it against said first end,
whereby vibrations are produced in said structure,
said housing including:
an integral portion of said housing closing said first end and forming an anvil for impact by said piston;
an enlarged recess formed in said second end providing a shoulder therein for seating a cap closure proportioned for an interference fit within said recess; and
venting means adjacent said first end for exhausting air from within said housing when said piston is driven toward said anvil by said pulse of air,
said piston including a striking surface on its end adjacent said resilient means for impact against said anvil;
localizing means providing a localized impact area between said striking surface and said anvil for directing impact forces substantially along the central axis of said rapper;
said resilient means comprising a spring having active coils with in active coils at each end thereof, said active coils having a larger diameter than said inactive end coils whereby said end coils telescope within said active coils upon compression of said spring by said piston;
said pressure means including an air fitting with a tapered portion adapted for wedging engagement with a correspondingly tapered opening provided in said housing adjacent said second end, said fitting being maintained in Wedging engagement by co-acting threads in said opening and on said fitting, said fitting being connected to a source for supplying a pulse of air pressure,
whereby pulses from said source flow into said housing between said piston and said second end to drive said piston against said anvil.
2. The apparatus of claim 1 wherein:
said piston having spaced apart bearing lands separated by a groove undercut in said piston with an end adjacent each of said lands having a relief of lesser diameter than said lands for seating the ends of said spring between said piston and the interior wall of said housing and with the ends of the piston further including a striking surface for engagement with said anvil,
whereby either end of said piston is operable against said anvil.
3. The apparatus of claim 1 wherein:
said localizing means includes a flat surface formed on said anvil normal to the path of travel of said piston cooperating with said striking surface upon impact by said piston, and said striking surface comprises a spherical surface formed on the end of said piston whose radius limits said impact area to a point of tangency between said spherical surface and said anvil when said piston is disposed to its maximum misaligned position within said housing,
whereby forces resulting from impact between said piston and said anvil are directed substantially along the central axis of said rapper.
4. The apparatus of claim 1 wherein:
said localizing means includes a flat surface formed on said anvil normal to the path of travel of said piston cooperating with said striking surface upon impact by said piston and said striking surface comprises a spherical surface formed on the end of said piston wherein the radius defining said spherical end is struck from the center of gravity of said piston,
whereby forces resulting from impact between said piston and said anvil are directed substantially along the central axis of said rapper through said center of gravity regardless of axial misalignment of said piston within said housing.
5. The apparatus of claim 1 wherein:
said localizing means includes a fiat surface formed on said anvil normal to the path of travel of said piston cooperating with said striking surface upon impact by said piston, and said striking surface comprises a truncated cone formed on the end of said piston with a flat nose portion proportioned to limit said impact area to a point within the truncated portion of said truncated cone when said piston is disposed to its maximum misaligned position with said housing,
whereby forces resulting from impact between said piston and said anvil are directed substantially along the central axis of said rapper.
6. The apparatus of claim 1 wherein:
said localizing means includes a flat surface formed on said anvil normal to the path of travel of said piston cooperating with said striking surface upon impact by said piston, and said striking surface comprises a ball whose maximum radius is less than the radius of the end of said piston and whose minimum radius limits said impact area to a point of tangency between said ball and said flat surface, said impact area being the only point of contact between said piston and said anvil when said piston is disposed to its maximum misaligned position within said housing,
whereby forces resulting from impact between said piston and said anvil are directed substantially along the central axis of said rapper.
7. The apparatus of claim 1 wherein:
said localizing means includes a flat surface formed on said anvil normal to the path of travel of said piston cooperating with said striking surface upon impact by said piston, and said striking surface comprises a conical tip formed on the end of said piston to limit said impact area to a point of tangency between said anvil and the converging end of said conical tip, said impact area being the only point of contact between said piston and said anvil when said piston is disposed to its maximum misaligned position within said housing,
8. The apparatus of claim 1, and in addition:
a groove provided within said recess for receiving a retaining ring to abut said cap closure thereby further retaining said closure against said shoulder.
9. The apparatus of claim 1 wherein:
the end of said piston adjacent said spring has a reduced outer diameter to form a relief between said piston and said housing in which the inactive coils on one end of said spring are seated; and
said first end of said housing includes an annular relief between said anvil and the wall of said housing in which the inactive coils on the other end of said spring are seated.
10. The apparatus of claim 1 wherein:
said housing includes a frusto-conical cavity extending axially inward from said first end for wedging engagement with a correspondingly tapered portion of said structure.
11. The apparatus of claim 1 wherien:
the active coils of said spring are proportioned to prevent contact between the outer diameter of said active coils and the interior wall of said housing when said piston in in contact with said second end and to cause contact with the interior wall of said housing when said piston is in contact with said anvil,
whereby vibrations in said spring are reduced during the impact of said piston against said anvil.
12. The apparatus of claim 1,
having a grounding means providing a current path between said housing and said structure comprising 10 a braided grounding strap having a twisted end inserted and secured in an opening in said first end. 13. A pneumatically operated single-impulse rapper assembly for producing vibrations in a structure con- 5 nected to said assembly, comprising:
a tubular housing having a first end adapted for connection to said structure;
a piston reciprocable within said housing, and resilient means between said piston and said first end for urging said piston against a second end; and
pressure means for driving said piston against said first end,
whereby vibrations are produced in said structure,
said piston including a spherical striking surface formed on its end adjacent said resilient means for impact against said first end, said spherical surface defined by a radius struck substantially from the center of gravity of said piston,
whereby forces resulting from impact between said piston and said first end are directed substantially along the central axis of said rapper through said center of gravity regardless of axial misalignment of said piston within said housing.
References Cited UNITED STATES PATENTS 2,699,224 1/1955 Schmitz 112 2,722,918 11/1955 Kimball 173-12l 2,854,089 9/1958 White et al. 55-112 2,985,802 5/1961 Drenning 55-ll2 JAMES A. LEPPINK, Primary Examiner US. Cl. X.R.