GB2188390A - Safety latch - Google Patents

Abstract

A safety device for fitting to a mechanical press comprises a hooked latch member 2 which is pivotally mounted to the press frame. The latch member 2 is capable of engaging a latch plate 2 rigidly attached to the crankshaft 25 of the press, in order to prevent the rotation of the crankshaft 25. The latch member 2 is raised and lowered by means of a fluid pressure cylinder 29. <IMAGE>

Description

SPECIFICATION Safety device The present invention is related to a safety device for fitting to a mechanical press, and in particular to a mechanical press comprising a flywheel capable of being coupled by a friction clutch to a crankshaft which drives the ram of the press. One complete rotation of the crankshaft causes the ram to move downwards and subsequently upwards again to effect a complete press stroke. Once a stroke has been completed, the press control system causes the clutch to be disengaged and a brake to be applied to the crankshaft to halt the ram and hold it at its top position.

Conventionally, it has been usual to supply mechanical presses with a physical guard around the working area. This guard is automatically locked when the press is operating, and remains so until the press cycle is complete, when it is automatically unlocked so that the work piece can be removed from the press. This means that the operator cannot inadvertently get his hands or fingers caught beneath the press slide.

However, the presence of this guard detracts from the efficiency of operation of the press, and it is for this reason that presses are now fitted with a photo-electric safety device instead of a physical guard.

This safety device comprises a continuous light beam monitor which detects any break in the light beam resulting from say an operative's hand. If such a break does occur, the press is deactivated and cannot be operated until the light beam has been restored. This photo-electric safety device is a proprietory device built to health and safety executive recommendations, which is fully safeguarded against all possibilities of failure by means of built-in safety features supplied with the device.

Such a press also includes a control interface which is self monitoring, and which provides an electrical link between the press controls and the photo-electric safety device controls. All the functions of both systems are checked during each press cycle, and the control interface signals an immediate press stop in the event of any malfunction within either of the two systems. The press stop signal acts directly on a solenoid valve which causes the clutch to disengage, so removing drive to the crankshaft, and for a brake to be applied to the crankshaft.

This means that the safety of the operator is dependant ultimately on the proper functioning of the clutch and brake systems of the press. Any deterioration of the clutch or brake is evidenced by a gradual overrun of the normal stop position of the press, which is monitored by the control system interface. When such overrun exceeds a certain safe limit further operation of the press is prevented.

However, should any of the mechanical parts within the clutch or brake system fail, the flywheel may cause continued rotation of the crankshaft and this may result in an unexpected press stroke occuring, which would not be prevented by the photo-eiectric safety controls. This presents a considerable safety hazard, and the object of the invention is to reduce or even eliminate this hazard.

According to the present invention there is provided a safety device for fitting to a mechanical press having a fly wheel, a crankshaft connectable by means of a clutch, to be driven from the fly wheel, and a ram connected to the crankshaft, the safety device comprising a latch member pivotally mounted on the press frame and movable between a first latched condition in which it engages with a correspondingly shaped latch plate rigidly secured to the crankshaft to prevent rotation of the crankshaft, and a second, unlatched condition in which rotation of the crankshaft is permitted.

Preferably, the latch member is raised from its latched condition into its unlatched condition, and subsequently returned to its latched condition, by means of a fluid-operated piston and cylinder.

Preferably, in the event of fluid-supply failure, the latch returns under gravity to its latched condition.

More preferably, the latch member is assisted in its return to the latched condition by means of one or more springs acting to the lower latch member. Upward movement of the latch member may be limited by an adjustable stop.

Desirably, the latch member includes an element engageable by the latch plate when the latch member is in its latched condition, said element being operative when so engaged to initiate an emergency stop of the press. Such element could be an electrical switch, a valveoperating member in a fluid pressure system, or a plunger or other mechanical member for operating a remote switch and/or valve.

Preferably, there are included two switches which are closed by the latch member when in its fully unlatched condition, to cause a press start signal controlling engagement of the clutch and removal of the brake.

Thus, with a press fitted with a safety device of the present invention, when the latch member is in its latched condition, even if the clutch and/or the brake should fail, rotational movement of the crankshaft beyond a given limit will be prevented by engagement of the latch plate with the latch member. Such engagement desirably also initiates an emergency stop, which preferably involves depressurising the clutch in the crankshaft drive, applying the crankshaft brake, stopping the main drive motor of the press and applying a brake to the flywheel.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is an end view of the crankshaft of a press, showing the safety device of the present invention; and, Figure 2 is a front elevation of the safety device shown in Fig. 1.

Referring to the drawings, a safety device of the present invention comprises a hooked latch member 2 which is mounted, by means of a pivot block 5 and fulcrum pin 7, to a press frame.

At the hooked end of the latch member 2 there is a plunger 16 which is connected to a cam plate 34 and which is biased to the position shown in Fig. 1 by a compression spring 18. When moved against the bias the cam plate can engage and open a plunger 19 of a dump valve 14, the plunger normally being closed by a compression spring 21. The plunger 19 has sections of two different diameters, against which an operating member of a microswitch 28 bears, the state of the switch thus being changed by movement of the plunger.

The opposite end of the latch member 2 is connected to a piston 29 working in a fluid pressure cylinder, and biased upwardly as seen in the figure by a compression spring 29a. This same end of the latch member 2 is attached, by means of a tension spring 29b, to the press frame. An adjustable stop 35 limits the upward movement of the latch member 2.

Rigidly attached to the crankshaft 25 of the press is a latch plate 1 which is capable of engaging the latch member 2 when in its lowered position in such a manner as to prevent rotation of the crankshaft 25.

In operation, when the ram is at rest at the top of the stroke, the cylinder 29 is exhausted and the latch member 2 is consequently in the lowered or latched condition, engaged with the latch plate 1 and held in that position by gravity and the effect of the compression spring 29a and tension spring 29b. Should any malfunction of the crankshaft clutch or brake occur, the crankshaft 25 will tend to be driven clockwise causing the ram to descend but such movement is limited by the presence of the latch member 2 and dangerous movement of the crankshaft and the ram is thus prevented. The limited crankshaft rotation will move the latch plate 1 to engage the plunger 16, and the cam plate 34 will thus move to depress the plunger 19 in the dump valve 14.

Valve 14 is in the fluid-pressure circuit for the clutch that couples the flywheel to the crankshaft, and opening of the valve causes fluid to be dumped from that circuit, so releasing the clutch pressure and applying the crankshaft brake. Operation of the switch 28 causes the press motor to be switched off and the fly wheel brake actuated.

Should the fluid supply to cylinder 29 fail, the compression spring 29a, tension spring 29b and the effect of gravity will maintain the latch in its latched condition.

When the press is to be operated to achieve a press stroke, a start signal (initiated by the operator) causes the cylinder 29 to be activated via a solenoid operated air valve.

This causes the latch member 2 to be raised clear of the latch plate 1.

A limit switch 32 is closed by upward movement of the hook shaped end of the latch member 2, and a similar limit switch 27 is actuated by the downward movement of the tail end of the latch member. The closing of these two switches causes a press start signal to be sent to the clutch and brake solenoid valve, thus engaging the clutch and releasing the crankshaft brake so that the flywheel can drive the crankshaft and ram through a subsequent press stroke.

Shortly after the ram reaches the bottom of its stroke, a non-repeat limit switch 32a is activated by a cam 32b on the crankshaft.

This disconnects the start circuitry so that a further stroke cannot be made unless specifically signalled after completion of the first stroke.

This non-repeat switch also provides a signal to a latch operating solenoid valve, which exhausts fluid from cylinder 29 so bringing the latch into its latched condition by the influence of gravity, the compression spring 29a and the tension spring 29b.

Shortly after the non-repeat switch is activated, a stop switch 32d is activated by a second cam 32c in the crankshaft. This gives a stop signal to the clutch and brake solenoid valve, which disengages the clutch and causes the crankshaft to be braked to stop the ram at the top of the stroke. A further stroke cannot be made without reactivation of a start signal by the operator.

Claims (11)

1. A safety device for fitting to a mechanical press having a fly wheel, a crankshaft connectable by means of a clutch, to be driven from the fly wheel, and a ram connected to the crankshaft, the safety device comprising a latch member pivotally mounted on the press frame and movable between a first latched condition in which it engages with a correspondingly shaped latch plate rigidly secured to the crankshaft to prevent rotation of the crankshaft, and a second, unlatched condition in which rotation of the crankshaft is permitted.

2. A device according to Claim 1 in which the latch member is moved from its latched condition into its unlatched condition, and subsequently returned to its latched condition, by means of a fluid-operated piston and cylinder.

3. A device according to Claim 1 or Claim 2 in which the latch may return under gravity to its latched condition.

4. A device according to any of the preceding Claims in which the latch member is assisted in its return to the latched condition by means of one or more springs acting to lower the latch member.

5. A device according to any of the preceding Claims in which upward movement of the latch member is limited by an adjustable stop.

6. A device according to any of the preceding Claims in which the latch member includes an element engageable by the latch plate when the latch member is in its latched condition, said element being operative when so engaged to initiate an emergency stop of the press to which the device is fitted.

7. A device according to Claim 6 in which the element engageable by the latch plate comprises or is operatively connected to a switch.

8. A device according to any of the preceding Claims in which there is included two switches which are closed by the latch member when in its fully unlatched condition, to cause a press start signal controlling engagement of the clutch and removal of the brake.

9. A device according to Claim 1 and substantially as herein described.

10. A safety device for fitting to a mechanical press, the safety device being substantially as herein described with reference to the accompanying drawings.

11. A mechanical press incorporating a safety device in accordance with any of the preceding Claims.