US2798583A

US2798583A - Electric eye barrier guard

Info

Publication number: US2798583A
Application number: US381092A
Authority: US
Inventors: Kenneth S Jeszka; Allen A Kudeck; Paalu Alex
Original assignee: AO Smith Corp
Current assignee: AO Smith Corp
Priority date: 1953-09-18
Filing date: 1953-09-18
Publication date: 1957-07-09
Anticipated expiration: 1974-07-09

Description

July 9, 1957 K. s. JE'SZKA EIAL ELECTRIC EYE BARRIER GUARD Filed Sept. 18, 1955 PRESS r loo 'INVENTORSI Alex Paalu Kenneth $.JeszKa Allen A.Kudeck ATTORNEYS.

United States Patent ELECTRIC EYE BARRIER GUARD Kenneth S. Jeszka, Allen A. Kudeck, and Alex Paalu, Milwaukee, Wis, assignors to A.Q. Smith Corporati n, Milwaukee, Wis, a corporation of'New York Application September 18, 1953, Serial No. 381,092 15 Claims. (Cl. 192-430) This invention relates to a safety guard for the protection of press operators.

An object of the invention is to provide a press guard without component parts which may interfere with the press operators.

Another object of the invention is to provide a press guard for covering openings into large work areas.

Another object of the invention is to provide a press guard, the operation of which may belimited to any portion of an operating cycle of the press.

A further object of the invention is to provide a press guard that cannot be disconnected by the press operator.

Another object of the invention is to provide a photoelectric press guard operable to stop the press when the light barrier is decreased or increasedfrom normal condition. I

A further object of the invention is to provide a press guard that does not obstruct the visibility of the die area of the press.

In general, the guard consists of a plurality of phototubes on one side of the press work area energized by a similar number of exciter lamps on the opposite side, thereby forming a light barrier across the entrance to the Work area. The phototubes control an amplification bridge circuit similar to a Wheatstone bridge. The amplification bridge circuit governs the current flow in a double contact meter relay. The meter relay in turncontrols the energization of a relay which is operable to stop the press. A cam limit switch is preferably inserted in circuit with the relay confine the effective operation of the guard to the downstroke of the press;

In the drawing:

Figure 1 is a diagrammatic view of the invention as connected to the'clutch control of the press; and

Pig. 2 is a diagrammatic view of the press and the press control.

Referring particularly to Figure 1, a plurality of phototubes 1 are connected in parallel with anodes 2 thereof connected by a lead 3 and a movable tap 4 to resistorf5. The cathodes 6 of tubes 1 are connected by a lead 7 to the grid 8 of vacuum tube 9 or other suitable amplification device. The cathodes 6 are also connected to resistor in series with a load resistor 10.

Although the guard is illustrated and described with three phototubes, the guard may be constructed utilizing any number of phototubes. The number of phototubes used will be governed by the size of the press opening to be covered by the light barrier.

A plurality of exciter lamps 11 are preferably connected in series circuit with the terminals 12 of a suitable source of voltage. The source of voltage is preferably a direct current source. The series circuit connection is preferable so that if one light fails they will all fail and insure stopping of the press. There is normally one exciter lamp for each phototube, as shown.

As shown in Fig. 2, suitably mounted on'opposite sides of the work area 13 of the press 14, are the photo- 2,798,583 Patented July 9, 1957 tubes 1 and the exciter lamps 11 to form a light barrier across the entrance to the press bed. The beam is preferably positioned directly in front of the work area of the press at such a level that the light will be interrupted by the operators hands whenever he reaches into the work-area.

Referring again to Figure 1, voltage is applied to the .phototubes 1 from the resistor 5 which is connected across the terminals 15 of a direct current voltage source by

supply lines

16 and 17.

The phototubes 1 control an amplification bridge circuit comprising two parallel electrical paths connected across

supply lines

16 and 17 Each path is divided into two portions or legs as follows: the upper left leg is the vacuum tube 9 with its anode 18 connected to supply line 16 and its cathode 19 connected to a resistor 20.

The bridge circuit is completed by a signal winding 25 of a relay meter 26 connected across the two parallel paths at the junction of vacuum tube 9 and resistor 20 and at the junction of vacuum tube 21 and resistor 24.

1 Current flow in tube 21 is governed by the voltage applied to its grid 27 from a resistor 28 which is con nected across

supply lines

16 and 17. A fine adjustment resistor 29 is preferably connected in parallel circuit with resistor 28 by the two movable taps 30 and 31. The grid 27 is connected to the resistor 29 by-the movable tap 32thereby allowing a fine adjustment of the voltage applied to the grid 27. in this manner, the current flow through tube 21 may be manually varied and the relative potential between the cathode 19 and cathode 23 varied to allow different current flow through winding 25. Tube 21 may be replaced by a suitable variable resistor why a nonvariable resistor if the relative potential noted above need not be subject to manual adjustment.

The filaments, not shown, of

vacuum tubes

9 and 21 are heated in any desired manner and no further description of the vacuum tubes and their construction is believed necessary for a full understanding of the invention. Further, although illustrated as normal triode tubes, any other variety may be employed within the scope of the invention.

A current indicating and contact needle 33 is pivotally secured adjacent the signal winding 25 on a shaft 34. The needle 33 rotates in response to the magnetic field set up by a current flow in Winding 25. A suitable scale 35 may be employed to indicate the current flow in the signal Winding 25. The needle 33 is biased to a normal zero current flow condition as by a coil spring 36. The spring 36 is preferably concentric with shaft 34 and is attached to the needle 33 and to the bracket 37 and biases the needle 33 to an initial position against a stop pin 38.

Positioned on opposite sides of needle 33 are the contacts 39 and 40 on pivotal members 41 and 42, respectively, which are pivotally supported on electrically

nonconducting pins

43 and 44, respectively. The contacts 39 and 40 may be set at any desired position by the rotation of members 41 and 42 so as to be in the path of needle 33 for engagement therewith.

A contact locking winding of meter 26, illustrated as asplit winding 45 and 46, is connected between coil spring 36 and a common electrical point 47 with capacitors 48 and 49 "for energization when the respective contacts 39 and 40 close, as subsequently described. The split Windinghas the one element 45 on the low current readingside of needle 33 and the other element 46 on the high current reading side of the needle 33 to hold the respective contacts 39 and 40 in a closed position after initial contact is made. The capacitors 48 and 49 are connected across the locking winding to relay contacts 39 and 40, respectively, to assist in the initial energization of the locking winding.

Although a double contact relay meter 26 is illustrated and described, other suitable relays may be employed. For example, if it is desired to actuate the apparatus only in response to variations of potential between the cathodes in only one direction a single contact relay may be employed. In any event, it is desirable to have a locking winding so that thepress is stopped, as hereinafter described, until the guard has been manually reset.

A series branch comprising contacts 50 of push button 51, relay winding 52 of relay 53, current limiting resistor 54 and cam limit switch 55 is connected between common point 47 of the circuit and one side of the secondary winding 56 of transformer 57. The opposite terminal of the secondary winding 56 of transformer 57, in series with a current limiting resistor 58, is connected to cathode 59 of rectifier 60 and also to the anode 61 of rectifier 62. The

rectifiers

60 and 62 are also connected to movable contacts 39 and 40, respectively. A capacitor 63 is connected across rectifier 60, the secondary winding 56 and resistor 58 to aid in rectification of the current. Similarly, capacitor 64 is connected across rectifier 62, winding 56 and resistor 58.

The primary winding 65 of transformer 57 is connected to any suitable alternating current source.

The transformer 57 and

rectifiers

60 and 62 may be replaced by a suitable source of direct current in which event the

capacitors

63 and 64 would be unnecessary.

Therefore, when either contact 39 and 40 engages needle 33, the relay winding 52 is energized through the following circuit: transformer 57, the above mentioned series branch, the locking winding, either closed contact 39 or 40, the

rectifier

60 or 62 connected to contact 39 and 40, respectively, and then back to the transformer 57.

When the relay winding 52 is energized, an armature 66 retracts and opens the contacts 67 which are in the electrical circuit controlling the stopping and starting of the press 14, as subsequently described.

The push button 51 allows the operator to reset the guard by opening the relay winding 52 energizing circuit as described above. If the light barrier has not returned to normal, the relay winding 52 will be energized as soon as the. operator releases the push button 51 and allows its contacts 50 to close.

The push button 51 also controls a second set of normallyclosed contacts 68. The contacts 68 are in the electrical circuit controlling the stopping and starting of the press 114 in order to prevent the operator from tying the push button down and thereby holding the energization circuit for relay winding 52 open.

press through the

sprockets

74 and 75 and connecting chain 76. The sprocket 75 is keyed to ram crankshaft 77 and rotates therewith. The chain 76 transmits the motion to cam 72 through sprocket 74 keyed to cam shaft 78.

The cam 72 will thus make one revolution for each revolution of the crankshaft 77.

The circumference of the fan-like projection 73, as illustrated, is preferably slightly less than 180 and therefore engages the arm 69 for slightly less than one-half revolution of the crankshaft 77. The cam 72 is so positioned that the projection 73 engages arm 69 to close contacts 70 shortly after a press ram 79 starts to descend and remains engaged therewith until just before the ram completes its downstroke.

The cam limit switch 55 may taken any other desired construction which opens the energization circuit of relay winding 52 for the desired period of the ram movement.

Movement of the press ram is shown for purposes of illustration as governed by an air-operated clutch and brake as illustrated in Figure l.

A rotating member 80 having an annular extension 81 disposed centrally thereof is keyed to the clutch shaft 82 as by key 83. One surface of annular extension 81 of member 80 is provided with an annular recess to form an air pressure chamber 84 which is adapted to receive a movable annular piston 85. The piston 85 is provided with a flange 86 which engages and compresses a series of spaced clutch discs 87 when the piston moves outwardly of chamber 84. The discs 87 are securely attached to the rotating member 80.

A cylindrical shaped fly-wheel 88 is supported by the clutch shaft 82 and houses the rotating member 80. A plurality of clutch discs 89 are secured to the interior surface of the fly wheel 88 and are alternately disposed between clutch discs 87 carried by the rotating mem ber 80.

The fly wheel 88 is rotated by a suitable motor 90 which is operatively connected to the fly wheel 88 by belts 91 or other suitable means, as shown in Fig. 2. When therpiston 86 is forced outwardly by fluid pressure within chamber 84, the

clutch discs

87 and 89 and compressed and the rotary motion of the fly wheel 88 is transmitted to the clutch shaft 82.

The annular extension 81 of member 80 is also recessed as at 92 on the surface opposite the air pressure recess 84 to receive a coil spring 93. An opening extends through the extension, concentric with the recess 92, to accommodate a shaft 94 extending therethrough. The shaft 94 has its one end threadedly attached to the flange 86 of air piston 85 and its opposite end attached to a braking plate 95. The coil spring 93 is concentric with shaft 94 and acts between the base of the recess 92 in member 80 and the plate 95.

Adjacent the braking plate 95, the member 80 has secured thereto a suitable number of spaced brake discs 96. A stationary cup member 97 is supported by the clutch shaft 82 with the shank thereof disposed between a clutch shaft bearing 98 and the clutch shaft 82. The

cup member 97 extends outwardly around the brake discs 96 and carries a plurality of stationary brake discs 99 which are alternately disposed between discs 96.

The brake discs are compressed by outward movement of the braking plate to stop rotation of the shaft 82.

air pressure in chamber 84, the spring 93 is compressed and the braking plate 95 moved inwardly and relieves the pressure on the brake discs so that shaft 82 may rotate. When air pressure chamber 84 is exhausted, the spring 93 expands and moves the braking plate 95 outwardly to compress the brake discs and consequently stop the shaft 82. As previously noted, outward movement of piston 85 compresses the clutch disc, while inward movement of piston 85 relieves the clutch discs. Therefore, when the brake discs are compressed, the clutch discs are relieved and similarly when the brake discs are relieved, the clutch discs are compressed.

The clutch shaft 82 is connected to the ram crankshaft 77 through a suitable gear train more clearly shown in Fig. 2 and thereby controls movement of the ram. A driving gear 100 is attached to clutch shaft 82 and drives the gear 101 on crankshaft 77 Suitable reduction gears, shown as

gears

102 and 103, connect gears and 101 to cause suitable rotation of the crankshaft 77.

To control the movement of the piston 85, an axial air "channel 104 extends through the clutch shaft 82, op-

positioned to communicate with the air channel 104.

The position of the valve 106 is controlled by a solenoid 109 which may be supported by the valve as by bracket 110. The solenoid plunger 111 is connected to air valve 106 such that the inlet pipe 107 communicates with air channel 104 when the solenoid is energized. The solenoid 109 is energized by asuitable source of voltage, such as from transformer 112 which is connected to any suitable A. C. voltage source.

The safety guard is connected in the solenoid energization circuit by the relay contacts 67 of relay 53 and also by the second set of contacts 68 of push button 51; both sets of contacts being in a normally closed position.

The operation of the safety guard is as follows:

A voltage is taken oif of resistor 5 and applied to the phototubes 1 by the movable taps such that the maximum rated voltage of the phototubes is not exceeded. Excitation of the exciter lamps 11 forms a light barrier across the press opening.

Excitation of the phototubes 1 places a voltage on the grid 8 of tube 9 and controls the current flow in that branch of the circuit. Therefore, as the excitation of the phototubes 1 is increased or decreased from the normal condition the current flow through tube 9 varies. Variations in current through tube 9 varies the potential of .cathode 19 with respect to cathode 23. A voltage is placed on the grid of tube 21 by adjustment of resistors 29 and 30 such that a predetermined current flows in the winding of the bridge circuit previously described when the light barrier across the press is in normal condition.

The current flow in winding 25 sets. up a magnetic field which attracts needle 33 and holds it a reading on the scale proportionate to the current flow in the winding, shown in the drawing at the center of the scale. The contacts 39 and 40 are placed on opposite sides of the needle and spaced a suitable distance therefrom so that a predetermined change in current flow through winding 25 occurs before either contact 39 or 40 is closed. The

safety guard is then in a standby condition.

Assume the light barrier is interrupted by the operators hand; the voltage applied to the grid 8 increases and the bridge is further unbalanced with a decrease of cur rent in the signal winding 25. The magnetic field in the signal winding 25 decreases and the needle 33 engages the contact 39.

When the contact is made, the locking winding is energized and locks the contacts independently of the signal winding 25 so that contact 39 remains closed until the circuit is opened by the manually operated push button 51. The capacitor 48 discharges through the locking winding to produce an initial surge of current to lock in the winding.

Simultaneously, the closing of contact 39 energizes the relay winding 52 and the relay contacts 67 open. Upon the opening of relay contacts 67 the solenoid 69 is deenergized and the solenoid plunger 111 is released. The air valve 106 then connects the air channel 104 with exhaust outlet 108 and the spring 93 acts to disengage the

clutch dics

87 and 89 and to compress the

brake discs

96 and 99, thereby stopping the press.

To reset the press guard the push button 51 is actuated by the operator to open the energization circuit of relay winding 52 and to thereby allow its contacts 67 to close. The solenoid 109 is again energized and it's plunger 111 places the air inlet 107 in communication with air channel 104 and chamber '84. Piston '85 moves to compressjthe

clutch discs

87 and 89 and disengage the

brake disc

96 and 99 and the press is once again in operating condition. Opening thecircuit also de-energizes the locking winding and the contact 39 is opened with needle 33 returning to its normal position if the light barrier has returned to normal such that the normal operating current flows in signal winding 25. If the light barrier has not returned to normal condition, as soon as the operator releases the push button 51the relay winding 52 will be energized and the press stopped.

The push button 51 is positioned away from the operator so that heha's to remove himself from his operating position to actuate the same to reset the guard. Further, the push buttons second set of contacts 68 are connected in the solenoid 109 energizing circuit and are opened when the push button is depressed so that the solenoid operator cannot tie the push button down to disconnect the safety guard and still operate the press.

The above operation of the press is further modified by the cam limit switch 55. If the cam 72 is disengaged from contact arm 69 the relay energizing circuit is opened and cannot be closed by closing of the meter contacts 39 and 40. Therefore, with the cam connected to operate, as previously described, to hold contact arm 69 closed only duriiig the time the press ram is moving toward the work, the guard cannot stop the press except during that time. The utilization of the cam allows the operator to reach beyond the barrier into the die area for the removal and positioning of pieces without stopping the press when there is no danger of injury from the press ram.

Similarly, if the light barrier is increased in intensity from an external source, the amplification bridge is unbalanced in the opposite direction. An increased flow of current in the signal Winding 25 causes the needle 33 to engage the contact 40 and the press is stopped, as previously described, for a breaking of the light barrier. Therefore, the operator cannot focus an additional light source on the electric eye to make the guard inoperative. Further, if a blinding flash should occur, the press would stop and the operator would not be in danger of injury by the press ram.

Various modes of carrying out the invention are contemplated as Within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

We claim:

1. In a safety guard for the protection of the operator of a press having an electrical circuit controlling the stopping and starting of the press, light responsive means disposed to one side of the entrance to a work area in said press, a source of light on the opposite side of the entrance to said work area to energize said light responsive means, an amplification bridge circuit electrically connected to said light responsive means, said light responsive means controlling the current flow in said bridge circuit, electromagnetic means connected in circuit with said bridge circuit and actuated by predetermined variations of current how in said bridge circuit, and means connecting said electromagnetic means in the electrical circuit controlling the stopping and starting of the press, said means being operable to prevent the injury of the press operator by the press in response to predetermined increases and decreases in the energization of said light responsive means.

2. In a safety guard for the protection of the operator of a press having an electrical circuit to regulate the ram movement of the press, a light sensitive device on one side of the entrance to the work area of the press, a source of light on the opposite side of the entrance to the work area forming a light barrier across said entrance, an amplification bridge circuit electrically connected to said light sensitive device with the current flow in a portion of said bridge circuit being controlled by said light sensitive device, a relay winding connected in circuit with said amplification bridge circuit, relay contacts controlled by said relay winding and closed in response to predetermined increases and decreases of current flow in said amplification bridge circuit, and a second relay winding in circuit with said contacts and with a source of voltage, and

a source of light disposed on the opposite side of said entrance to energize said light responsive means, an amplification bridge circuit connected to said light responsive means with the current distribution in the bridge controlled by said light responsive means, said amplification bridge circuit including a double contact relay connected as the bridging element, said double-contact relay closing a first set of contacts in response to a predetermined increase in the illumination of the light responsive means and closing a second set of contacts in response to a predetermined decrease in the illumination of the light responsive means and means connecting said double contact relay in the electrical ram-control circuit of the press to open said electrical circuit and render the press inoperative and protect the operator against injury by the press ram in response to the predetermined changes in illumination.

4. In a press safety guard for a press having an electrical circuit operable to regulate the press ram movement, photoelectric means to form a light barrier across the entrance to the work area of the press, an amplification bridge circuit having a vacuum tube as one element of the bridge, said photoelectric means connected to said vacuum tube to vary the current flow through said vacuum .tube and thereby vary the potential across the bridging branch of said bridge, a double contact relay in the bridging branch of said amplification bridge circuit, the contacts of said relay being in an open condition for a predetermined value of current flow through said relay and closing one set of contacts when the current increases by a predetermined amount and closing a second set of contacts when the current decreases by a predetermined amount, a second relay having its Winding in circuit with .said contacts and being energized when said contacts are closed, and a second set of contacts controlled by said second relay and connected in the electrical circuit of the press operable to regulate the press ram movement and protect the operator of the press from injury by the ram in response to variation of said photoelectric means.

5. In a safety guard for a press having electrical control circuit to control the movement of the press ram, photoelectric means to form a light barrier across the entrance to the work area of the press, an amplification bridge circuit having a vacuum tube with a control grid as one of the elements of the bridge circuit and having an electrical winding as the bridging element, said photoelectric means being connected to the control grid of said vacuum tube and controlling the current flow through said tube, an armature positioned adjacent said winding and moving in response to the current flow in said winding, 21 contact positioned on each side of said armature and one of said contacts being closed by the armature in response to increased current flow in said winding and the opposite contact being closed by the armature in response to decreased current flow in said winding, a relay connected in circuit with said pair of contacts and with a source of voltage with said relay being energized when either of said pair of contacts is closed, and normally closed contacts operated by said relay and being in the electrical control circuit for movement of the press ram to render said ram inoperative to injure the operator of the press when said contacts open 6. In a safety guard to protect an operator of a press having an electrical press ram movement control circuit, photoelectric means to form a light barrier across the entrance to the press, an amplification bridge circuit electrically connected to said photoelectric means and having the current flow therein controlled by said photoelectric means, a relay as the bridging branch of said circuit with the contacts of said relay closed in response to predetermined variations in current flow insaid bridge circuit, a locking winding connected in circuit with said contacts to hold said contacts closed independently of said relay after the initial closing thereof, a second relay in circuit with the contacts of said first named relay, said second relay having its contacts connected in the electrical press rammovement control circuit to render the ram inoperative to injure the press operator in response to variation in said photoelectric means, and timing means to hold the energization circuit of said second named relay open for a predetermined period of the ram movement.

7. In a press safety guard for a press having an electrically controlled press ram movement apparatus, photoelectric means forming a light barrier across the entrance to a work area of the press, an electronic tube having a control grid connected to the electrical output of said photoelectric means, a resistance connected in series with said electronic tube, a resistance circuit in parallel with said series connected electronic tube and resistance, an electrical winding connected across said series connected electronic tube and resistance and said resistance circuit to form an amplification bridge circuit, an armature disposed adjacent said electrical winding and movable in response to current flow in said winding, sets of contacts disposed adjacent opposite sides of said armature and closed by said armature in response to respective increased and decreased current flow in said electrical winding, and electromagnetic means connected in circuit With said contacts and a source of voltage, said electromagnetic means connected in the electrically controlled press ram movement apparatus to stop or reverse the movement of said ram in response to variations in said photoelectric means.

8. In press safety guard for a press provided with an electrical circuit controlling the movement of the press ram, a plurality of electric eyes disposed on one side of an entrance to the work area of the press, a source of light disposed on the opposite side of said entrance to form a light barrier across the entrance, an electronic tube having a grid connected to the output of said electric eyes, a resistance connected in series circuit with said electronic tube across a direct current source, a second electronic tube and resistance connected in series across said direct current source and connected in parallel with said first named electronic tube and resistance, a coil connected across said parallel connected electrical elements to form an amplification bridge, contacts positioned adjacent said coil and closed in response to predetermined increased and decreased current flow in said coil, and electromagnetic means connected in series circuit with said contacts and a source of voltage, said electromagnetic means having contacts connected in said electrical circuit controlling the movement of the press ram to control the latter in response to predetermined variations in the output of said electric eyes.

9. In a press guard for a press having an electrically controlled ram movement device, light responsive means disposed on one side of the entrance to a work area of the press, a light source disposed on the opposite side of said entrance and focused on said light responsive means, an amplification bridge circuit having a vacuum tube in a leg of the circuit, said light responsive means having its output connected to said vacuum tube and controlling the current flow therethrough to regulate the current flow in said amplification bridge circuit, a relay winding connected in the bridging branch of said amplification bridge circuit, a set of normally open contacts operably associated with said relay winding and closed in response to a predetermined current flow in said relay winding, a set of normally closed contacts operably associated with said relay winding and opened in response to a predetermined current flow in said relay winding, and arela'y connected in circuit with a source of voltage and with said first and second named contacts to effect energization of said relay wheneither set of contacts are closed, said relayhaving a set of contacts in the electrically, controlled ram movement device to stop or reverse the press ram movement and protect an operator of the pr'essfrom injury by the ram in response to variations of the output of said light responsive means.

10. In a safety guard for a press, light responsive means disposed to one side of an entrance to the work area of the press, a light source disposed to the opposite side of said entrance, an amplification bridge circuit connected connected to the output of said light responsive means and having the current flow therein controlled by the output of said light responsive means, a relay connected in said amplification bridge circuit as the bridging element therein, the current fiow through the relay being oppositely varied by an increased and a decreased illumination of the light responsive means respectively, said relay having a first set and a second set of normally open contacts closed in response to a responsive predetermined increase and decrease in current flow through said relay, electromagnetic means connected in circuit with a source of voltage and the contacts of said relay, a fluid pressure actuated clutch and spring actuated brake mechanism for controlling the movement of the press ram, a solenoid operated valve to control operation of said clutch and brake mechanism, and a source of voltage connected in series circuit with the solenoid and said electromagnetic means to energize said solenoid when said electromagnetic means is de-energized, said solenoid actuating said clutch and brake mechanism to effect movement of the press ram when energized and to prevent movement of the press ram when de-energized.

11. In a safety guard for a press having an entrance to a work area of the press to permit the manual position of Work in the press, photoelectric means to form a light barrier across the entrance to the work area of the press, an amplification bridge connected in circuit with the electrical output of said photoelectric means and having the current flow in one leg of said amplification bridge controlled by the electrical output of said photoelectric means to vary the potential across the bridging branch of said amplification bridge, a relay in the bridging branch of said amplification bridge, contacts actuated by the relay, electrically controlled locking means associated with the contacts to maintain the contacts in an actuated position after an initial actuation thereof, electromagnetic means connected in series circuit With a source of current and said contacts with the energization of said electromagnetic means controlled by said contacts, a set of contacts actuated by the electromagnetic means, a solenoid connected in circuit with a source of current in series with the second named set of contacts associated with said electromagnetic means, a valve connected to and operated by said solenoid, a pneumatic clutch and brake mechanism connecting a ram of the press to a source of power to control the movement of the ram, said valve regulating the pneumatic clutch and brake to discontinue the movement of the ram when said solenoid is de-enerigized and protect the operator of the press from injury by the ram, manually operated means to break said locking means and restore the first-named contacts to a normal actuated position, and a set of contacts included in said manually operated means and being connected in the circuit of the solenoid to hold the ram inoperative when the manually operated means is actuated.

12. In a safety guard for a press having a cyclic press ram movement with an electrically controlled pneumatic clutch and brake mechanism, photoelectric means to form a light barrier across an entrance to a work area of the press, an amplification bridge circuit having an electronic tube in one leg and having a relay winding as the bridging element, the output of said photoelectric means heingconnected tosai'd electronic tube to vary the current flow therethrough and tothereby vary the potential across said'relaywind'ing-relay contacts controlled by said relay winding andnor'mal'ly "in open "position, said relay contacts closing in response to both a decrease and an increase in the intensity ofth'e li'ghtbarrier, electromagnetic means connected in circuitwith a source of direct current and said relay contacts to form an energization circuit, said electromagnetic means electrically connected to said electrically controlled pneumatic clutch and brake mechanism to prevent ram movement when energized, a cam limit switch biased to open position in the energization circuit of said electromagnetic means to prevent energization of said electromagnetic means and consequent prevention of ram movement, 'and a cam mechanically geared to the press for rotation in synchronism with the press ram cycle and disposed adjacent said cam limit switch, said cam holding said cam limit switch closed for a predetermined period of a cycle of the press ram movement to permit energization of said electromagnetic means when the ram is in position to injure the operator of the press.

13. In a safety guard for a press having a solenoidoperated pneumatic clutch and brake mechanism, light responsive means disposed to one side of an entrance to a work area of the press, a light source disposed on the opposite side of the entrance, an amplification bridge circuit connected to said light responsive means with the current flow in said bridge circuit controlledby the output of said light responsve means, said amplification bridge circuit including a relay as the bridging element, said relay having normally open contacts closed in response to predetermined variations in said light responsive means, electromagnetic means connected with a source of current and the contacts of said relay and operable to open a set of contacts when energized, means to maintain said electromagnetic means energized independently of said first named contacts after the initial closing of the latter, said last named contacts being connected in an energization circuit of the solenoid operating: the pneumatic clutch and brake mechanism to open the energization circuit when said electromagnetic means is energized, a timing means operable to maintain said electromagnetic means de-energized for a predetermined period of a cycle of operation of the press, and means to manually disconnect said electromagnetic means from the source of voltage connected thereto to permit resetting of the safety guard, said last named means including contacts connected in the energization circuit for said solenoid to prevent operation of the press when said last named means are actuated.

14. A safety guard for a press having an entrance to a work area and an electrical control circuit, comprising light responsive means disposed to one side of the entrance to the Work area of the press, a light source disposed to the other side of the entrance to the work area and adapted to energize said light responsive means, electromagnetic means connected in circuit with said light responsive means, said light responsive means controlling the energization of the electromagnetic means, and switch means operatively associated with the electromagnetic means and actuated in response to energization of the electromagnetic means, said switch means being operatively connected in the electrical control circuit of the press and being operable to prevent injury of the press operator in response to a predetermined increase and decrease in the illumination of the light responsiveTHfeans.

15. A safety guard for a press having an electrical control system, comprising a set of normally closed contacts connected in the electrical control system and adapted to prevent operation of the press when actuated, means operatively associated with the contacts to actuate the contacts in response to a dangerous condition of the press, locking means to maintain the contacts in an actuated position independently of the last named means, a manual reset control operatively connected to the locking means to efiect a return of the contacts to a normally closed position and thereby allowing operation of the press, and a switch interlocked with the manual control and connected in the electrical control system to prevent operation of the press when the manual control is in an actuated position whereby the manual control cannot be maintained in an actuated position to prevent operation of the guard.

References Cited in the file of this patent UNITED STATES PATENTS Jongedyk June 12, 1934 Fair-field Feb. 18, 1936 McMaster June 1, 1937 Thompson Aug. 5, 1941 Schick Dec, 24, 1946