US3540246A - Ram return for mechanical press brakes - Google Patents

Description

Nov. 17, 1970 c. o. JONES, JR 3,540,246

RAM RETURN FOR MECHANICAL PRESS BRAKES Filed Dec. 16, 1968 2 Sheets-Sheet 1 INVENTOR.

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A T TOPNE YS Nov. 17., 1970 c. o. JONES, JR 3,540,245

RAM RETURN FOR MECHANICAL PRESS BRAKES Filed Dec. 16, 1968 2 Sheets-Sheet 2 l 1 -T66 FLYWHEEL FORWARD b L STOP gar 331 75 %f 7 c2 IMR 77 0L o o L -u \lL L gv I/h cR q /I 70 l I REVERSE 67 l/ 9 7 CR l IMF v Y FL 69 7s 7 1 m I I I 9 86 P I P I I CR5 RAM RETURN I To TOP CR6 I 90 85 I I CR (R4 21 i l cns I 1/ ATTORNEYS.

United States Patent O1fice 3,540,246 Patented Nov. 17, 1970 ABSTRACT OF THE DISCLOSURE A control for mechanical press brakes which enables the operator to command the automatic return of the press brake ram to a top stop position without completing an entire cycle. A manually actuatable relay circuit deenergizes the press brake main motor and actuates a braking means such as a motor-mounted or flywheel brake and after a delay determined by a timing relay simultaneously re-energizes the motor for rotation in the reverse direction and releases the brake. A limit switch in response to the return of the ram to the top stop position actuates a relay circuit which de-energizes the motor and activates the brake and then after a delay again determined by the timing relay simultaneously re-energizes the motor for rotation again in a forward direction and releases the brake.

BACKGROUND OF THE INVENTION This invention relates to a control for mechanical press brakes and, more particularly to a command-responsive control for automatically returning the press brake ram to the top stop position without completing a full working cycle and then conditioning the arm for the beginning of a new working cycle.

Sheet metal bending operations performed on a press brake sometimes require withdrawal of the ram from the workpiece without completing a full working cycle. For example, when operating a press brake it is not uncommon to find that the bend is started slightly off the desired line, which bend if completed as a result of the ram traveling through an entire working cycle would ruin the workpiece. It is also common to inch the ram toward the workpiece whereupon the pressing operation is completed before the press brake working stroke is finished. In such situations a saving of both time and material will result if the press brake operator can quickly and automatically withdraw the ram and return it to the top stop position without completing a full Working cycle.

In hydraulic press brakes wherein the ram is moved toward and away from a part by the operation of two hydraulic motors acting on laterally spaced apart points on the ram, an automatic return of the ram to the top stop position without completing a full working cycle is easily accomplished by means of a valve which functions to reverse the direction of flow of operating fluid to the hydraulic motors. In mechanical press brakes wherein the ram is reciprocated by conventional transmission means driven by an electric drive motor, such a return of the ram heretofore has not been possible automatically but only through a cumbersome manipulation of the press brake controls.

SUMMARY OF THE INVENTION The present invention provides a command-responsive control for a mechanical press brake whereby in response to a command given at any point in a working cycle the ram is first stopped, then returned in a reverse direction to the top stop position, whereupon the ram is conditioned for the beginning of a new working stroke. The control provided by this invention enables the press brake operator to return the ram to the top stop position by a single manual operation which provides a command to the control.

Other objects and advantages of the invention will be apparent from a consideration of the following specification and the accompanying drawings which disclose a representative embodiment of the principles of the invention. Various modifications which may be made within the scope of the present invention will be apparent to those skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS moived and showing the components illustrated in FIG. 2; an

FIG. 4 is a wiring diagram of one arrangement of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a mechan form of the control ical press brake viewed r the front and including, briefly, a stationary bed llfsiiib frame members 12 and -'13 and a reciprocable ram 14 movable toward and away from the bed 11 by electromechanical drive means connected to the ram and located between the frame members 12 and 13, the latter being connected by a crown plate 15. Located on the front of the crown plate is aconventional switch housing 16 in cluding forward, reverse and stop command buttons 17 18 and 19, respectively, for controlling the electro mechanical drive means. A conventional speed control knob 20 is also located on the crown plate 15.

Located on the ram 14 is another switch housing 21 having buttons 22 and 23 which when depressed by the press brake operator effect an upward or downward ad ustment, respectively, of the ram. The control arrangement associated with these buttons, however, forms no part of the present invention. Also located in housing 21 is a button 24 which when depressed by the operator provides a command for activating the control provided by this invention. If during a forward (downward) stroke of the ram 14 the press brake operator wishes to return the ram to the top stop position without completing the entire forward stroke, he simply depresses button 24 whereupon the ram is stopped, returned to the top stop position, and then conditioned for a successive working stroke.

FIG. 2 shows the electromechanical drive means for reciprocating the press brake ram 14 as viewed from the rear of the press brake and with a portion of the crown plate 15 broken away. The drive means includes a reversible electric drive motor 25 having a shaft 26 which motor is rigidly attached to a mounting bracket 27 having two upwardly extending portions 28, 29 which, in turn, are each rigidly attached to two support arms 30 and 31, respectively. The support arms are connected to suitable supporting means such as member 32 shown in FIG. 2, which is rigidly attached at each end thereof to the housing members 12, 13. The drive means also includes a flywheel 33 having a shaft 34 which is connected to the input of suitable reduction gearing in a gear box 35. The shaft 34 is driven by motor shaft 26 through suitable coupling means, such as belts, engaging between a motor sheave 36 and the flywheel. The flywheel 33 together with the motor sheave 36 are supported in a suitable manner, as by attachment to the supporting member 32 While the reduction gearing and gearbox 35 may be supported conveniently by attachment to the support arms 30, 31.

The output of the reduction gearing is drivingly connected to a shaft 37 which is rotatably supported in housing members :12 and 13 and is connected to a pair of eccentrics 39 which reciprocate the press brake ram 14 in a well-known manner. The electromechanical drive means herein described is conventional with mechanical press brakes and thus will be familiar to those skilled in that art.

In accordance with this invention there is provided a braking means adapted to operate on the drive means in a controlled manner. An example of one such braking means included within the scope of this invention is a pneumatic brake '45 adapted to operate on the press brake flywheel. As shown in FIG. 3 also, which is a side view of the press brake taken with the housing member 12 removed, the brake 45 includes a shoe 46 operable to act against the periphery of the flywheel when moved by a pneumatically operated cylinder 47 reciprocable within a housing 48. The brake housing is supported in a suitable manner, for example by attachment to a bracket 49 which is rigidly secured to the crown plate 15.

It is to be understood that braking means of other types and adapted to operate on other parts of the drive means in a controlled manner could be used without departing from the spirit and scope of this invention. For example, a motor-mounted brake of the commercially available variety could be substituted for the flywheel brake shown by way of illustration.

The invention further includes sensing means operable in response to the ram 14 reaching a predetermined upper rest position relative to the bed 11. This position is known in the art as the top stop position. The sensing means, in preferred form, comprises a limit switch 50 including a stationary housing 51 and a plunger 52 movable therein. The switch housing is shown in FIGS. 2 and 3 mounted on the bracket 49. The switch 50 is operated by a switch operator in the form of cam 53 secured to the shaft 37 at a location so that the cam surface 54 engages the plunger 52. The cam surface is provided with a recessed portion 55 rotatively orientated on shaft 37 so as to coincide with the ram top stop position. The arrangement as illustrated is such that the switch is operated only when the plunger 52 engages the recessed portion 55 of the cam surface.

It is to be understood that other arrangements may be substituted without departing from the spirit and scope of the invention. For example, switch 50 could be located near the ram 14 so as to be operated by a switch operator on the ram when the ram reaches the top stop position.

The invention finally includes command responsive means connected in controlling relation to the drive means and to the braking means and in controlled relation to the sensing means for stopping the ram upon receiving a command and then returning the ram to a predetermined position, the top stop position, and for subsequently causing the' ram to begin a new operating stroke. The invention contemplates, as a specific embodiment of the command responsive means, an electrical control circuit which functions in response to actuation of the command button 24, shown in FIG. 1, on a forward stroke of the ram to first de-energize the electric motor 25 and operate the braking means and then, after a predetermined time needed for braking, to release the braking means and re-energize the motor 25 but for rotation in a reverse direction so as to return the ram to the top stop position. When the ram reaches that position, as indicated by operation of the limit switch 50, the control circuit then functions to de-energize the motor 25 and operate the braking means and then, after the same predetermined time, release the braking means and re-energize the motor but for rotation in a forward direction so that the ram is conditioned to begin a new forward working stroke. It will be observed that these operations all occur sequentially in response to a single depression of the command button 24 so that the press brake operator no longer is faced with the cumbersome manipulation of the forward, reverse, and stop command buttons 17 19 requiring human observation of when braking is completed and attention to the proper sequence of depressing the buttons.

FIG. 4 shows a wiring diagram of a control circuit contemplated by this invention having a plurality of branches each including control windings and controlled contacts of various relays and including various switches which branches are connected in parallel across control circuit supply conductors 65, 66. A contact 67 of a conventional overload relay is connected in supply line 66. For purposes of description and explanation the relays in the various branches will be designated generally by numbers and the control windings and controlled contacts will be given letter designations.

A first branch connected in controlling relation to the press brake drive means, specifically motor 25, comprises lines 68 and 69 which are connected in parallel with another line 70 across the supply lines 65, 66. Line 68 includes a relay 71 having a control winding 'IMF which, when energized, will through a controlled contact establish a power circuit to motor 25, in a well-known manner, causing the motor to rotate in a forward direction. A relay 72 having control winding CR4 is connected in parallel with winding IMF for simultaneous energization or deenergization so as to provide various additional control steps in response to forward motor rotation or to deenergization of the motor. The forward command switch designated 17 in FIG. 1 is connected in line 68 and mechanically interconnected to another branch as will be described further. Similarly, line 69 includes a relay 73 having a control winding designated IMR which when energized likewise establishes a power circuit to the motor 25 but so as to cause motor rotation in a reverse direction. A relay 74 having a control winding CR5 is connected in parallel with winding IMR for simultaneous energization or deenergization so as to provide various additional control steps in response to reverse motor rotation or to deenergization of the motor. The reverse command switch designated 18 in FIG. 1 is connected in line 69, and the stop command button 19 is connected in line 70 and mechanically interconnected to another line as will he described. A switch 75 responsive to the operation of the braking means provided by this invention is connected in line 70 and, for the particular example of a flywheel brake, is a pressure switch which opens when the brake is operative and closes when the brake is released.

A branch connected in controlling relation to the braking means comprises line 76 connected across supply lines 65, 66 and having a solenoid 77 connected therein. The flywheel brake in this particular example is operated in response to deenergization of solenoid 77 and is released when the solenoid is energized. The solenoid operation is controlled, in turn, by the sets of relay contacts shown connected in parallel in line 76 in response to the operation of their corresponding relay control windings as will be described.

The circuit branches controlling energization of the motor for forward rotation as Well as operation and release of the brake are, in turn, controlled by a branch comprising line 78 having connected therein a relay 79 provided with a control winding CR. An energizing circuit for the winding CR is established between supply lines 65, 66 through a series of relay contacts shown in line 78 in response to the automatic operation of the control circuit. Alternatively, the control winding CR is energized upon the closing of the forward command switch 17 in line 68 by virtue of its mechanically interconnected contact 80 included in a line 81 connected between supply line 65 and winding CR.

The control circuit includes five remaining relays 8286 having control windings designated CR1, CR2, CR3, CR6 and TR1, respectively, each connected in separate branches across the supply lines which branches comprise lines 8791, respectively. The branch comprising line 87 having control relay 82 connected therein functions, briefly, to cause the branch comprising line 76 to release the brake and simultaneously to cause the branch comprising line 69 to energize the motor for reverse rotation and hence return the ram 14 to the top stop position. The limit switch shown in FIGS. 2 and 3 is also connected in line 87 and opens when the ram 14 reaches the top stop position so that the motor is deenergized and the brake operated as will be described.

The purpose of the branch comprising line 88 and having relay 83 connected therein will be understood more clearly when the operation of the entire control circuit is described in more detail further on in the specification, A contact 92 connected in line 88 is mechanically connected to the stop command button 19 in line 70. The branch comprising line 89 having relay 84 connected therein functions, briefly, after the ram 14 has been returned to the top stop position in accordance with this invention, to enable the branch comprising line 78 to cause the branch comprising line 68 to initiate forward rotation of the motor and the beginning of a new forward working stroke of the ram.

The ram return command button designated 24 in FIG. 1 is connected in line 90, and the function of the branch comprising that line and relay 85 is to initiate the sequence of operations performed in accordance with this invention in response to manual actuation of the button 24. The branch comprising line 91 and including relay 86 functions, by virtue of the timed contact TR1 connected in line 78 and controlled by the TR1 winding in line 91, to provide the time delay needed for braking before a change in the direction of motor rotation is provided by the control circuit.

When it is desired to begin a working cycle with a forward stroke of the ram 14, the press brake operator merely depresses the forward command button 17 which closes the mechanical interconnected contact 80 in line 81 so as to energize control winding CR of relay 79 from the supply lines 65, 66. Encrgization of the winding closes the normally open relay contacts controlled thereby, specifically one CR contact is closed in line 78, one CR contact in line 76 closes so as to energize the flywheel brake solenoid 77, and the two CR contacts connected in and across line 68 are likewise closed. Energization of the brake solenoid causes the flywheel brake to be released at which point the brake pressure switch 75 in line 70 is closed. A circuit is now completed from supply line 65 through the normally closed stop command switch, through the two closed relay contacts CR, through the normally closed IMR relay contact, and through the IMF and CR4 control windings to supply line 66 through normally closed contact OL. The energization of winding IMF completes a power circuit to the drive motor 25 causing it to rotate in a forward direction. When the motor subsequently comes up to speed the operator may engage the standard press brake clutch causing the ram to begin a forward stroke toward the workpiece.

The energization of winding CR4 establishes a holding circuit for the relay winding CR, the contacts of which must be kept closed to keep the IMF winding energized. This, in turn, permits a release of the forward command button 17 without a resulting shut-down of the motor. In particular, the energization of winding CR4 closes the CR4 contact in line 90 which completes a circuit from supply line 65 through the normally closed ram return switch 24, through CR4 contact and through control winding CR6 of relay 85 to supply line 66. A CR6 contact in line is thus closed and establishes a holding circuit for the relay. Another CR6 contact is closed in line 78, and a circuit through that line is now completed, specifically from supply line 65 through CR4 contact connected thereacross which is now closed, through normally closed contact CR5, through CR6 contact now closed, through CR contact, and through the control winding CR to supply line 66. The forward command button 17 at this point in time may be released without deenergizing the control winding CR. It should be understood that this sequence of operations occurs relatively rapidly thus allowing the operator to release the button quickly.

The energization of control winding CR4 of relay 72 also closes a CR4 contact in line 91 so as to energize the control winding TR1 of timing relay 86 which, after a predetermined time, opens the normally closed TR1 timed contact in line 78. In addition, a normally closed CR4 contact in line 87 is opened so that control winding CR1 of relay 82 remains de-energized. The energization of control winding IMF of relay 71 establishes a circuit in line 88 through normally closed contact 92 of the stop button 19 and normally closed CR5 contact to energize control winding CR2 of relay 83. A CR2 contact in line 88 is thus closed to establish a holding circuit, and a CR2 contact connected across line 87 is also closed. Control winding CR3 of relay 84 in line 89 remains deenergized.

The control circuit thus functions as described to cause the motor 25 to rotate in a forward direction so as to begin a forward operating stroke of the press brake ram 14. To facilitate an understanding of the further operation of the control circuit it may be helpful to note that after this sequence of operations initiating a forward movement of the cam, the IMF and TR1 control windings and the CR, CR2, CR4, CR6 control windings as well as the brake solenoid 77 remain energized while the IMR, CR5, CR1 and CR3 windings remain deenergized.

After a forward stroke of the ram is begun, if for any reason it is desired to return the ram to its top stop position, the operator simply depresses the ram return to top command button 24 which opens the circuit in line 90 and deenergizes control winding CR6 of relay 85. This, in turn, opens the CR6 contact in line 78 and since the CR3 contact in parallel therewith is open because control winding CR3 of relay 84 is de-energized, the circuit in line 78 is opened and control winding CR of relay 79 is de-energized.

At this point, the brake is rendered operative and the motor is de-energized. This results because the opening of the CR6 and CR contacts in line 76, CR1 contact being already open, de-energizes the flywheel brake solenoid 77 causing the brake to operate on the flywheel. The deenergization of the CR control winding of relay 79 opens the CR contacts in line 68 which de-energizes the control winding IMF of relay 71 so as to cut off power to the motor 25.

The control winding CR4 of relay 72 across line 68 is also de-energized which opens the CR4 contact in line 91 so as to de-energize the control winding TR1 of the timing relay 86 and which closes the normally closed CR4 contact in line 87. The TR1 contact in line 78, however, will remain opened for a predetermined time necessary for adequate braking, and thereafter will close so as to establish a circuit from supply line 65-, through the TR1 contact in line 78, down to line 87 and through the normally closed CR4 contact, through the normally closed limit switch 50, through contact CR2 which is closed since CR2 control winding or relay 83 has remained energized, and through the CR1 control winding of relay 82.

The energization of this relay winding causes the brake to be withdrawn from the flywheel and the motor to be operated in the reverse direction so as to return the ram to the top stop position. In particular, relay contact CR1 in line 76 is closed so as to energize the brake solenoid 77 and thus release the brake. The CR1 contact connected to line 69 also closes and a circuit is completed from supply line 65 through the normally closed stop command switch 19, the brake pressure switch 75 which closes upon deactivation of the brake, the CR1 contact of line 69, the normally closed IMF contact, and through the IMR control winding of relay 73 and the CR control winding of relay 74 to supply line 66. Energization of the IMR control winding thus causes the motor to rotate in the reverse direction so as to raise the ram.

Energization of the IMR winding of relay 73 also closes an IMR contact across line 89, and since the CR1 contact connected thereto was previously closed, an energizing circuit is established through these contacts and the normally closed contact CR6 to control winding CR3 of relay 84. This, in turn, closes the CR3 contact connected across line 78 for a reason which will become apparent. Energization of the CR5 control Winding of relay 74 closes the CR5 contact connected to line 91 so as to energize the TR1 control winding of timing relay 86. This, in turn, opens after a predetermined time the normally closed TR1 contact in line 78 so as to prevent the establishment of an energizing circuit to the CR control winding at this stage of the operation. The normally closed CR5 contact in line 88 is opened so as to de-energize the CR2 control winding of relay 83. This opens the CR2 contact connected to line 87.

These switching operations have occurred simultaneously with the upward return travel of the ram. When the ram reaches its top stop position, limit switch 50 in line 87 is opened and the motor is de-activated and the brake rendered operative. In particular, the opening of ram return limit switch 50 opens the circuit in line 87 so as to de-energize CR1 winding of relay 82. This opens the CR1 contact connected to line 69 which de-energizes the IMR and CR5 control windings. The motor is shut off in response to the de-energization of IMR control winding of relay 73. The brake solenoid is de-energized since the CR1 contact in line 76 is opened, the CR and CR6 contacts being already open, and thus the brake is rendered operative.

The de-energization of CR5 control winding on line 69 opens a CR5 contact connected to line 91 so as to deenergize the TR1 control winding of timing relay 86. The TR1 contact in line 78 remains open, however, for the same predetermined time so as to insure adequate braking, whereupon it closes to establish an energizing circuit to CR control winding of relay 79. In particular, a circuit is completed from supply line 65 through the TR1 contact, the normally closed CR5 contact, the CR3 contact and CR control winding to supply line 66. It should be noted that although the IMR and CR1 contacts connected to line 89 are now opened, CR3 control winding remains energized by the CR3 contact and normally closed CR6 contact in line 89.

The energization of CR control winding of relay 79, this time automatically by the operation of the control circuit rather than manually by means of the forward command button 17, results in a new forward stroke of the ram as previously described. The CR contact in line 76 closes so as to energize brake solenoid 77 and release the brake whereupon the brake pressure switch 75 in line 70 will close and establish, in conjunction with the CR contacts in line 68, a circuit to energize the IMF and CR4 control windings. As a result the motor is rotated in a forward direction. The CR4 contact on line 90 closes and the CR6 winding is energized which closes the CR6 contact in line 78 which with the CR4 contact connected thereto establishes a holding circuit for CR control winding of relay 79 as previously described.

At any time during this new forward stroke of the ram the operator may again return the ram to the top stop position by depressing button 24. A sequence of operations identical to those described will again occur. Alternatively, the operator may wish merely to stop the ram in which case he depresses the stop command button 19. This immediately opens the circuit in lines 70 and 68 so as to de-energize the IMF and CR4 control windings. The motor is turned off and the opening of CR4 contact connected to line 91 de-energizes the CR control winding of relay 79. The opening of CR4 contact in line 91 de-energizes TR1 control winding of relay 86. The pressing of the stop command button 19', through the mechanically connected contact 92 also de-energizes CR2 winding of relay 83. The CR6 control winding of relay in line which remains energized holds the CR6 contact on line 76 closed so that the brake solenoid 77 remains energized. Thus in response to a stop command made through button 19 the motor 25 is not braked but rather is allowed to coast to a stop.

While the control circuit has been described in conjunction with a flywheel brake, it may be modified easily to operate with other types of braking means without departing from the spirit and scope of this invention. For example, if a motor mounted brake were used instead, the control winding of a relay would be substituted for the brake solenoid in line 76. A controlled relay contact would be substituted for the pressure switch in line 70. Controlled contacts would be included also in a separate brake power circuit. The operation would be quite similar in that the brake would be released and the controlled contacts closed when the control winding, as the brake solenoid, is energized.

As mentioned previously, the timing relay 86 functions to prevent re-energization of the motor 25 in an opposite direction prior to the time needed for adequate braking. Typically this is about 2-3 seconds, but is subject to variation with difierent press brake designs.

I claim:

1. In a mechanical press brake having a ram reciprocable toward and away from a bed by reversible electromechanical drive means connected to said ram:

(a) braking means operable against said electromechanical drive means;

(b) sensing means operable in response to said ram reaching a predetermined position relative to said bed; and

(c) command responsive means connected in controlling relation to said drive means and to said braking means and in controlled relation to said sensing means for stopping said ram upon receiving a command and after a predetermined time returning said ram to said pre-determined position and stopping said ram at said position and after a pre-determined time conditioning said ram to begin a new operating stroke.

2. Apparatus as set forth in claim 1 wherein said drive means includes a reversible electric motor drivingly connected to mechanical rotary-to-reciprocating motion conversion means and having a circuit for supplying electrical power thereto and wherein said braking means is operated or released by electrically responsive means.

3. Apparatus as set forth in claim 2 wherein said command responsive means is an electrical circuit comprising:

(a) a first branch operatively connected to said motor supply circuit for controlling the energization and direction of rotation of said motor:

(b) a second branch operatively connected to said brake for controlling the operation and release thereof;

(0) a third branch for controlling said first and second branches;

(d) a fourth command responsive branch connected in controlling relation to said third branch;

(e) a fifth branch connected in controlled relation to said sensing means and in controlling relation to said first and second branches; and

(f) a time delay branch connected in controlling relation to said third branch.

4. Apparatus as set forth in claim 3 wherein said fourth branch includes manually-actuated means for providing a command.

5. Apparatus as set forth in claim 3 wherein said sensing means comprises a switch connected in said fifth branch and operable when said ram reaches said predetermined position.

6. Apparatus as set forth in claim 3 wherein said time delay branch includes a timing relay.

7. In a mechanical press brake having a ram reciprocable between a predetermined top stop position and a stationary bed by drive means comprising a reversible electric motor drivingly connected through rotary-to-reciprocatory motion conversion means to said ram:

(a) electrically controllable braking means operable against said drive means;

(b) sensing means operable in response to said ram reaching said top stop position; and

(c) a control circuit connected in controlling relation to said motor and to said braking means and in controlled relation to said sensing means for stopping said motor and operating said brake in response to a command during a forward stroke of said ram, and after a pre-determined time releasing said brake and reversing said motor, and for stopping said motor and operating said brake in response to the operation of said sensing means, and after a predetermined time releasing said brake and re-energizing said motor. 8. Apparatus as set forth in claim 7 wherein said braking means operable on said motor.

9. Apparatus as set forth in claim 7 wherein said braking means is operable on said rotary to reciprocatory motion conversion means.

References Cited UNITED STATES PATENTS 2,496,574 2/1950 Boger 318-443 3,025,938 3/1962 Silberger 192-2 2,491,363 12/1949 Dehn 83526 2,840,135 6/1958 Fowler 7226 3,165,140 1/1965 Hazelton et al. 72-2 2,797,724 7/1957 Walldow 72-441 2,237,170 4/1941 Williamson 721 FOREIGN PATENTS 750,046 6/ 1956 Great Britain.

CHARLES W. LANHAM, Primary Examiner M. J. KEENAN, Assistant Examiner US. Cl. X.R. 72-441; -53; 1922

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