US2726640A - Pneumatic rock drill with selective drill steel rotation - Google Patents

Description

FUEHRER DRILL STEEL ROTATION 3 Sheets-Sheet 1 Filed Sept. 9, 1953 5 mm ex u Hm e m N 0 mm 7 Q @N H mm ow mm 3 Q E w. w N i i 42/ m 0Q mm hm. \Q G mo 5 kwm 6 mo J 0 B P5. $9 \xmwmmxm w Q 8 4 wv /mm M mm @m .I l. Em i N Q. mm mm h s 6 cm m W :Kfi 274/ V m 7 Dec. 13, 1955 G. H. FUEHRER 2,726,640

PNEUMATIC ROCK D L WITH SELECTIVE DRILL STE ROTATION 3 Sheets-Sheet 2 Filed Sept. 9, 1955 IN V EN TOR.

650 965 b. FUE'H/QEIQ, W

Dec. 13, 1955 cs. H. FUEHRER 2,726,640

PNEUMATIC ROCK DRILL WITH SELECTIVE DRILL STEEL ROTATION 3 SheetS-Sheet 3 Filed Sept. 9, 1955 INVEN TOR.

/? QA/EV 660865 H FUf/VQEQ,

United States l atent PNEUMATIC ROCK DRILL WITH SELECTIVE DRILL STEEL ROTATION George H. Fuehrer, Downey, Calif., assignor to Thor Power Tool Company, Aurora, IlL, a corporation of Delaware Application September 9, 1953, Serial No. 379,207

7 Claims. (Cl. 121-7) Steel Rotation, now Patent No. 2,678,636, issued May 18,

1954, I have disclosed a relatively light air hammer for use in drilling, chipping and the like, and which incorporates means to prevent steel rotation when it is desired to form a starting hole or when the apparatus is to be used as a chipping hammer. In the air hammer referred to, manually operated means were provided to efiect shifting of the rotation pawls longitudinally of the drill and into engagement with a cam surface adapted to pivot them away from the associated ratchet ring and thus prevent steel rotation. The present invention also involves the broad principle of longitudinal pawl shifting for the purpose of preventing drill steel rotation while permitting the hammer action to continue. However, the present invention is specific to the embodiment of the principle in a relatively large drilling apparatus such as a stoper or sinker employed in mining and tunneling operations.

It is evident that the problem encountered in connection with relatively large drilling apparatus, such as is used for mining and tunneling, are somewhat different than those which must be overcome in connection with small air hammers such as the one described in the copending application referred to above. For example, the relatively large size of the drill makes it impractical and inconvenient to effect pawl shifting by manual action alone. Furthermore, the camming and spring return means for the pawls must be associated with the valve guide, rifle bar and other elements of the drill in such a way that the rotation release device will not interfere with normal drill operation.

In view of the above factors characteristic of relatively large pneumatically operated drilling apparatus, it is an object of the present invention to provide novel means for pivotally mounting the rotation pawls in the rifle bar flange of a rock drill, the means being such that longitudinal shifting of the pawls is permitted to effect their pivotal movement out of operative engagement with at ratchetring.

An additional object is to provied cam and spring return means for associating the longitudinally shiftable rotation pawls and the stationary valve guide of the rock drill.

A further object is to provide pneumatically operated piston means for shifting the pawls under the control of a manually operable .valve, the latter being mounted on the main throttle valve for the rock drill but operating independently thereof.

These and other objects and advantages of the invention will be more fully set forth in the following specification and claims, considered in connection with the attached drawings to which they relate.

2,726,040 Patented Dec. 13, 1955 In the drawings: a g

Figure 1 is a longitudinal central section of the backhead and adjacent portions of a rock drill of the side-feed stoper type; I

Figure 2 is a vertical section along line 2-2 of Figure 1, with portions broken away to illustrate the air passages connecting the control valve and the pawl-shifting piston;

Figure 3 is a fragmentary sectional view along line 33 of Figure 2 and showing the control valve in its vent position, at which the rotation pawls are engaged with the associated ratchet ring;

Figure 4 is a sectional view along line 44 of Figure 3, and illustrating the mounting of the rotation control valve on the main throttle valve of the drill;

Figure 5 is a sectional view of the rotation control valve, corresponding to Figure 3 but illustrating the valve as shifted to its rotation-preventing position;

Figure 6 is a fragmentary longitudinal section illustrating the rotation pawls as shifted forwardly from the Figure 1 position to the rotation-release position;

Figure 7 is a transverse section of the drilling apparatus, taken along line 77 of Figure l and showing the pawls in the operative positions assumed when the parts are in the positions shown in Figures 1 and 3; and

Figure 8 is a view corresponding to Figure 7 but show ing the pawls after pivoting to the disengaged positions assumed when the parts are in the positions shown in Figures 5 and 6.

Referring to the drawings, and particularly to Figure l, the invention is illustrated as embodied in a stoper-type rock drill of the offset telescopic feed leg type. The offset feed leg, indicated fragmentarily at 10, is associated through a bracket arm 11 with the backhead 12 which contains the valve elements of the drill. Backhead 12 is, in turn, associated through suitable side bolts 13 (Figures 2, 7 and 8) with the main drill cylinder 14 containing conventional piston hammer, chuck and drill steel elements. The piston hammer, indicated at 15, is adapted to percussively actuate the drill steel which is mounted in the chuck, both of these elements being unshown.

The rear end portion of main cylinder 14 is provided with an internal shoulder 16 to seat in stacked abutting relationship, starting from the left in Figure 1, a forward valve chest 17, a rear valve chest 18, a valve guide 19, and a ratchet ring 20, the latter being also seated on an internal shoulder 22 formed at the forward portion of backhead 12. All of the elements 17-20 are locked I ratchet ring 20 relative to cylinder 14 and backhead 12 is efiectively prevented not only by the side bolts but by a key pin 23 disposed in suitable registered grooves in these elements.

It is to be understood that the valve chests 17 and 18 form part of a conventional distributing valve embodying a longitudinally reciprocable valve element 24 which is slidable on a forwardly extending cylindrical stem portion 26 of the valve guide 19. The distributing valve operates to alternately direct air through suitable air passages and to the forward and reverse sides of the piston hammer 15 in order to efiect its reciprocation for the purpose of imparting both percussion and rotation to the drill steel. The flow of air to the distributing valve is from a suitable inlet air hose, not shown, and through the main throttle valve 27 which is mounted transversely of backhead 12 in a rounded boss portion 28 at its under surface. The throttle valve 27 comprises a hollow sleeve or cylinder 29 rotatably mounted in a transverse bore in boss portion 28 and associated through a necked-down end portion 31 with a handle 32. As illustrated in Figure 4, the handle 32 may be removably mounted by use of a nut ,33 which is threaded over the Outer and Of P t and is ada ted to bear nst th base or hub of handle 32, the latter being seated over a squared section of portion 31 inwardly adjacent its threaded end.

Referring again to Figure 1, the piston hammer is associated through its rifle nut 34 with a rotatable rifle bar 35 which extends through an axial bore in valve guide stem 26. Rifle bar 35 is provided at its rear end'with a circular flange 36 rotatably seated between the flange portion 37 of valve guide 19 and a stationary plug 38, the latter being press fit into an axial bore in backhead 12. To further stabilize the rotational relationship between rifle bar 35, valve guide 19'and'plug 38,-'the rifle bar is provided at its rear end with a cylindrical protuberance 39 which seats in a corresponding cylindrical recess in plug 38. It will be noted that the rifle bar 35 and plug 38 are axially bored to receive the conventional water tube 41 adapted to supply cleansing water to the bit end of the drill steel; The flow'of water through tube 41 is under the control of a pneumatically operated water valve 42 of the type described and'claimed in my co-pending application Serial No. 355,679 filed May 18, 1953, for an Automatic Water Valve for Rock Drills.

According to the present invention, a plurality of rotation pawls 46 are mounted in'rifle bar flange 36 for movement longitudinally and forwardly of the drill and into engagement with a cam surface provided on flange 37 of valve guide 19. As best shown in Figures 7 and 8, the rifle bar flange 36 is formed with four peripherally spaced notches 47 each having a radial edge 48 and a somewhat longer edge 49 extending along a chord of the circle formed by the flange periphery. The notches are adapted to receive rectangular lug plates or portions 51 of pawls 46 and to permit their pivotal movement about axes extending longitudinally of drill cylinder 14. To provide for the pivotal movement of the lug plates as indicated, and also to permit shifting of the p'awls'longitudinally of the drill, the inner edge portion of each lug plate 51 is integral at its center with a cylindrical journal portion 52 adapted to be 'slidably'received-in a longitudinally extending'cylindri'cal bore o'r recess 53formed in the corner of each notch 47. In orderto bias the lug plates 51 radially outwardly into "engagement with the internal teeth 55 of ratchet ring 20, and which are'outwardly adjacent the periphery of rifle bar flange 36, a plurality of resilient plunger mechanisms 56 are mounted one for each pawl in cylindrical recesses 57 extending perpendicularly of the respective chordal notch edges 49. The plunger mechanisms 56, which are respectively mounted in two transverse planes, each comprise a metallic plunger cylinder 58 slidably mounted in its associated recess 57 and enclosing a helicalcompres'sion spring 59, there being a bore 61 communicating With'the bottom of each recess 57 to permit exhausting of air in the recess when plunger 58 is shifted inwardly;

In the normal operation of the drill with both rotation and hammering action, the plunger mechanisms 56 are operative to pivot pawl lug plates-51-outwardly into engagement with the ratchet-teeth '55 to effect a ratchet action between the ratchet ring and rifle flange 36; To effect cessation of the ratchet action, and consequent stoppage of drill steel rotation, the pawls 46 are shifted forwardly through notches 47 and recesses 53 toprovide a camming action as will next be described.

It will be observed upon reference to Figure 1 that the rear surface of valve guide flange 37 is capped to form an annular recess 63 between the peripheral flange portion and the valve guide stem 26. Recess 63 is provided longitudinally opposite the -'rim' Qfriflebar flange 36 with a frustoconical cam wall 64 converging toward the forward or front part of the drill. A radially extending wall 66 is provided at the cup bottomto form"a "seatfor a flat wire helical compression spring 67 which extends between wall 66 and the forward edges 68 of pawl lug plates 51. The compression spring 67 operates to bias the pawls 46 rearwardly to the position shown in Figure 1, and at which the plunger mechanisms 56 are operative to pivot the pawls outwardly into engagement with the ratchet ring teeth 55. However, upon shifting of pawls 46 forwardly against the bias of spring 67, the outer longitudinal corners 70 of lug plates 51 ride on cam surface 64 to efiect inward pivoting of the pawls to the disengaged position illustrated in'Figu'res 6 and 8. The ratchet mechanism is thus maintained in free-wheeling or inoperative position until the pawls are permitted to return to their rearward-shifted positions under the'bias of spring 67.

In accordance with the present invention, the means for shifting the pawls 46 forwardly to provide with cam surface 64 a cam action effecting retraction of pawl portions 51 out of engagement with ratchet teeth 55, comprise a cup-shaped piston 72 which is slidably mounted over plug 38 for movement longitudinally of the drill. Piston 72 is shaped and disposed with its rim adjacent the rear edges 74 of pawl lug plates 51, and with its base 75 necked-down toslidably engage a relatively small diameter portion 76 of plug 38. It'will be observed (Figures l and 6) thatthe forward travel of piston 72 is limited by the radially extending plug wall 77 which separates small diameter portion 76 from the mai'nbody 78 of the plug, and the rearward travel by a radial backhead wall 79 which forms the bottom of a'recess in backhead 12 for piston 72. The radial backhead wall 79 forms the forward surface of an internal shoulder or neck 80 in backhead 12 and which receives the relatively small diameter portion 76 of plug 38 in press-fit relationship as previously indicated.

Proceeding next with the description of the air passage and valve means for shifting piston 72 forwardly'to the rotation-release position shown in' Figure 6, it will be observed upon reference to Figures 2 and 4 that a relatively small rotation control or'release valve 81 is mounted circumferentially of the necked-down end 31 of throttle valve sleeve 29 and inwardly of the throttle valve handle 32. The rotation release valve 81' comprises a sleeve'82 mounted on a cylindrical section of throttle valve portion 31 and adapted to rotate relative to it, there being a boss or flange 83 provided on' sleeve 82 between the main body of throttlevalve sleeve 29 and the neck-down end portion of boss 28. A radially extending handle 84 is mounted over sleeve 82 between boss 28 and throttle valve handle 32 and is provided with a screw 86 projecting through an aperture in sleeve 82 and into an angular groove 87 in throttle valve extension 31, the arrangement being such that turning of handle 84 will rotate sleeve .82 independently of the throttle valve. A spring-pressed detent mechanism 88, mounted in backhead boss 28, is adapted to cooperate with handle 84 to maintain the valve 81 in a's'el'ected controlling position except during intentional shifting of the valve by the operator. With the described construction the op erator may independently control the throttle valve 27 and the rotation release valve 81 by manipulating valve handles 32 and 84 which are disposed closely adjacent each other forease of access.

Referring again to Figure 4 of the drawings, it is to be assumed that the valve chamber 90 defined by throttle valve sleeve 29 is supplied: with compressed'air from a suitable air hose, notshown,'for conduction to the distributing valveas previously indicated. According to the present invention the compressed air in chamber 90 is also employed,"under the control, of valve 81 to shift rotation-release piston 72 forwardly and effect 'clisengagementof pawls 46 from r mnajr i aa Eorthis' purpose, a bore 91 is formed in throttleyalve sleeve 29 between chamber 90 and an annulangroove 9 2at the meeting'faces of sleeve 2 9ja'nd "flang'e 83. In addition, the flange 83 is provided with a passage 93 leading from groove 92 to the cylindricall'wall offthe backhead bore into which throttle valve sleeve 29 is inserted. When the sleeve 82 is rotated by handle 84 to the rotationrelease position illustrated in Figure 5, passage 93 registers with a passage 94 (Figures 2, 3 and leading upwardly through backhead 12 to a radially inwardly extending passage 96. Passage 96, in turn, communicates with an annular groove 97 formed in the rear surface of piston 72 as shown in Figures 1, 2 and 6. It may be seen that when the valve is in the Figure 5 position, air pressure is transmitted to groove 97 and will operate to shift piston 72 forwardly until piston base 75 engages wall 77 of plug 38.

As the piston is thus shifted forwardly, the air between the wall 77 and piston base 75 is vented to the atmosphere by means of communicating passages 98 and 99 in plug portion 76, the latter passage leading man annular groove 101 in portion 76 which registers with a passage 102 in backhead 12. Passage 102 communicates with a laterally extending passage 103 (Figures 1 and 2) which leads to the atmosphere and completes a vent path for the air chamber between piston base 75 and plug wall 77.

It is to be understood that air pressure admitted to annular piston groove 97 and thus to the space between piston base 75 and backhead wall 79 is sufficient to press the piston and pawls 46 forwardly against the bias of spring 67 and also against the air pressure admitted from throttle valve 27 to the chamber 104 surrounding the pawl and ratchet mechanism and the piston. The chamber 104 includes in its forward portion the recess 63 in valve guide flange 37 and in its rear portion. a recess formed in backhead 12 outwardly of the cylindrical body of piston 72. Chamber 104 forms part of the conduit means through which air passes from throttle valve 27 on its way to the distributing valve mechanism and thus to the piston hammer 15, there being a port 106 in throttle valve sleeve 29 and a passage 107 in backhead 12 for the purpose of connecting valve chamber 90 with chamber 104.

In order to eifect recommencement of drill steel rotation after the pawls 46 have been shifted forwardly to their rotation-release positions, the rotation control valve 81 is turned to the position illustrated in Figure 3. The flow of air from valve chamber 90 to backhead passage 94 is thus blocked, since passage 93 in flange 83 then leads only to a Wall of the bore for sleeve 29. A vent path from the chamber between piston base 75 and backhead wall 79 is, however, created through groove 97 and passages 96 and 94 to a recess or groove 109 in the outer surface of flange 83. Recess 109 communicates, when the rotation control valve 81 is in the Figure 3 position, with upwardly and transversely extending back head passages 111 and 112 which lead to the atmosphere as indicated in Figure 2. When the flow of air to piston groove 97 is blocked and the vent path completed as described, the piston 72 and pawls 46 are shifted rearwardly from their Figure 6 positions to their original positions shown in Figure l. The rearward shifting is efiected by the combined action of compression spring 67 and of the air pressure in chamber 104, with the spring acting on the pawls 46 and the air pressure acting on the rim of piston 72.

In the operation of the rock drilling apparatus, let it be assumed that the valve 81 is in the position illustrated in Figure 3, so that piston 72 is in its rearward shifted position (Figure l) at which the pawl lug plates 51 are in engagement with ratchet teeth 55 as shown in Figure 7. Let it also be assumed that the main throttle valve 27 is turned to a suitable position at which air is admitted through port 106 and passage 107 to chamber 104 and thence to the distributing valve to effect reciprocation of valve element 24 for alternate transmittal of air to the forward and rear sides of piston hammer 15. As previously stated, the piston hammer 15 is provided, as in conventional rock drilling apparatus of the type indicated, with the rifle nut 34 which rides on the spirally fluted rifle bar 35 and tends to eflect relative rotation between the rifle bar and piston hammer during reciprocation of the latter. During intervals when rifle bar 35 is held against rotation the piston will rotate, and during intervals when the rifle bar is permitted to move freely the rifle bar will rotate and the piston will not. Accordingly, the pawl and ratchet mechanism is provided to lock rifle bar 35 against rotation during each rearward piston stroke, so that the piston hammer rotates to rotate the associated chuck and drill steel, but to permit rifle bar rotation during each forward stroke to effect temporary cessation of piston and drill steel rotation. The piston hammer, chuck and drill steel are thus rotated during each rearward piston hammer stroke and in a single direction as is desired. It will be observed that when the parts are in the assumed positions the pawl lug plates 51 are in engagement with ratchet teeth 55, under the resilient bias of plunger mechanisms 56. The necessary ratchet action is thus effected between the stationary ratchet ring 20 and the rotatable rifle bar 35 to effect drill steel rotation in the conventional manner indicated above.

Should it be desired to effect cessation of drill steel rotation, for example when it is necessary to form a starting or collar hole or when the drill steel is caught in a seam, rotation control valve handle 84 is rotated until the valve is in the position illustrated in Figure 5. An air 'path is then completed from throttle valve chamber 90 (Figure 4) through passage 91 in valve sleeve 29, passage 93 in flange 83, passages 94 and 96 in backhead 12 and to the annular groove 97 in piston 72. Piston 72 is then shifted forwardly against the air pressure in chamber 104, which operates to shift the pawls 46 forwardly against the bias of spring 67 and to the rotation-release positions shown in Figure 6. As the pawls are thus shifted, the outer corners 70 of pawl plates 51 ride along the frustoconical cam wall 64 of valve guide flange 37 and are drawn inwardly against the bias of plunger mech-' anisms 56 to the positions shown in Figure 8, at which they are in engagement with the chordal notch walls 49. The pawl lug plates 51 are then out of engagement with ratchet teeth 55 to effect free-wheeling between the rifle bar flange 36 and ratchet ring 20. The rifle bar 35 is then rotated back and forth during reciprocation of the piston hammer to slide the pawl edges on the cam wall 64, there being no rotation of the piston hammer, chuck and drill steel since the rifle bar is not locked against rotation in either direction.

To again start the drill steel rotation, the valve 81 is shifted to its Figure 3 position, at which a vent path is completed from groove 97 and through backhead passages 96 and 94 to recess 109 in the valve flange 83 and thus to passages 111 and 112 leading to the atmosphere. The spring 67 and the air pressure in chamber 104 are then operative to shift pawls 46 and piston 72 rearwardly until piston base engages backhead wall 79 as shown in Figure 1. During rearward shifting of the pawls 46, the plunger mechanisms 56 act on the respective pawl lug plates 51 to pivot them outwardly into engagement with ratchet teeth 55. The pawls 46 then operate to prevent rotation of the rifle bar 35 in a clockwise direction as viewed in Figures 7 and 8, so that the piston hammer, chuck and drill steel will instead rotate as desired.

Referring to Figure 1, it is pointed out that the pawl return spring 67 is completely expanded and at free length when pawls 46 are in their rearward-shifted positions. The piston 72 is also in its rearward-shifted position due to the action of the air pressure in chamber 104,. and the arrangement is such that clearance gaps are then formed between pawl ends 74 and the piston rim. In this. manner, the air pressure in chamber 104 is utilized to completely disengage piston 72 from the pawls and pre-- vent any interference with the normal pivoting operation of the latter.

I i Pointed u tha th t r l v e 27 y b turned to any controlling position independently of the rotation control valve 81 and without affecting the latter, since the annular groove 92 (Figure 4) is engaged with valve passage 93 during all positions of the rotation-release valve. Completely independent control of the hammer action and rotation action is thereby achieved, yet the valve handles 32 and 84 are sufficiently close to permit an operator to manipulate them without shifting his grip on the drill.

While the particular apparatus herein shown and described in detail is fully capable of attaining the objects and providing the advantages hereinbefore stated, it is to be understood that it is merely illustrative of the pres ently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as defined in the appended claims.

I claim:

1. A pneumatic rock drilling apparatus, comprising an elongated casing adapted to mount a drill steel, a piston hammer mounted in said casing for reciprocation toward and away from said drill steel to percussively actuate the same, a valve guide mounted longitudinally in said casing and adapted to slidably mount the distributing valve of the apparatus, a flanged rifle bar journalled longitudinally in said valve guide and adapted to cooperate with a nut on said piston to effect relative rotation between said piston and rifle bar during reciprocation of said piston, an internally toothed ratchet ring mounted peripherally of the flange of said rifle bar and locked against rotation relative to said casing, a pawl mounted on said rifle bar flange for pivotal movement about, an axis extending longitudinally of said casing and for shifting along said axis, a flange provided on said valve guide adjacent said rifle bar flange, said valve guide flange being recessed to form a cam surface adapted when engaged by said pawl to pivot the same out of engagement with the teeth of said ratchet ring, and means to shift said pawl along said axis and into engagement with said cam surface.

2. The invention as claimed in claim 1, wherein a compression spring is seated between the bottom of said recess and said pawl to bias the latter toward its initial longitudinal position.

3. The invention as claimed in claim 2, wherein a compression spring is mounted on said rifle bar flange and adapted to bias said pawl into engagement with said ratchet ring teethexcept when said pawl is held in disengaged position.

4. In a pneumatic drilling apparatus of a type employinga rotatable rifle bar to effect rotation of the piston hammer during its reciprocation, means to define a chamber adapted to contain air under pressure, a rifle bar flange mounted in said chamber and rotatable with said rifle bar, a ratchet ring fixedly mounted adjacent said rifle bar flange, a plurality of pawls mounted on said rifle bar flange for pivotal movement into engagement with said :8 ratchet ring and for movement longitudinally of the drilling apparatus, a cam surface provided longitudinally adjacent said pawls and adapted when engaged thereby to eflect pivoting thereof out of engagement with said ratchet ring, spring means to bias said pawls toward a position out of engagement with said cam surface, and pneumatically operated piston means to shift said pawls longitudinally into engagement with said cam surface; said piston means being normally held in inoperative position by the air pressure in said chamber, and being constructed and adapted to overcome both said air pressure and the bias of said spring means when operated to longitudinally shift said pawls.

5. The invention as claimed in claim 4, wherein said chamber defining means includes a flange on the valve guide for said apparatus, said flange being also constructed to seat said spring means and to form said cam surface.

6. The invention as claimed in claim 4, wherein said spring means are at free length when said piston means are in said inoperative position, and wherein clearance is then provided between said piston means and pawls to prevent interference with the normal operation of the latter.

7. A pneumatic rock drilling apparatus, comprising an elongated casing adapted to mount a drill steel, a piston hammer mounted in said casing for reciprocation toward and away from said drill steel to percussively' actuate the same, a valve guide mounted longitudinally in said casing and adapted to slidably mount the distributing valve of the apparatus, a flanged rifle bar journalled longitudinally in said valve guide and adapted to cooperate with said piston to eflect relative rotation between said piston and rifle bar during reciprocation of said piston, an internally toothed ratchet ring mounted peripherally of the flange of said, rifle bar and locked against rotation relative to said casing, a pawl mounted on said rifle bar flange for pivotal movement about an axis extending longitudinally of said casing and for shifting along said axis, a flange provided onsaid valve guide adjacent said rifle bar flange, said valve guide flange being shaped to form a cam surface adapted when engaged by said pawl to pivot the same out of engagement with the teeth of said ratchet ring, a compression spring seated on said valve guide flange and adapted to engage one end of said pawl and bias said pawl toward an initial longitudinal position, a piston mounted in said casing and slidable to contact the other end of said pawl to shift said pawl into engagement with said cam surface, and a valve-controlled.pneumatic system selectively operable to actuate said piston.

References Cited in thefileof this patent UNITED STATES PATENTS 1,905,474 Lear Apr. 25, 1933 2,001,729 Huffman May 21, 1935 2,251,269 Curtis Aug. 5, 1941 2,678,636 Fuehrer May 18, 1954

Images