GB2042387A - Drive and control arrangement for an eccentric press - Google Patents

Abstract

A drive and control arrangement for an eccentric press comprises a d.c. electric motor 2, reduction gear 3, an eccentric 4 coupled for rotation with the output shaft of gear 3, a connecting rod 7 journalled eccentrically to the eccentric 4, a limit switch 10 operable by a cam element 22 adjustably mounted on the eccentric 4, and an electric control device 9. The control device 9 includes a starter unit 12 for supplying d.c. to the motor 2, a braking unit 15 connected to short-circuit the motor armature through a resistance 13 for braking the motor and the press, and a timing device 16 for switching off the press after a predetermined, adjustable time period in the event of failure of the switch 10 or the press becoming blocked in the bottom dead centre position. <IMAGE>

Description

SPECIFICATION Actuating and control device for eccentric presses The present invention relates to a driving and control arrangement for a mechanical eccentric press.

With eccentric presses it is common to transmit the pressing force to a pressing member by means of a flywheel driven by an alternating current electric motor, the flywheel being mounted on a flywheel shaft and connected to the eccentric shaft via a shift and brake coupling, and control being carried out by means of a clutch.

The known eccentric press drive systems however exhibit various deficiencies. In particular they show poor efficiency, which means that to produce a particular force at the eccentric, correspondingly oversized drive elements are required. The volume and weight of such presses thereby becomes disproportionately great and the stroke cycles long, so that a jerky operation cannot be avoided.

A further disadvantage consists in that the drive elements, such as reducing gear mechanism and clutch, are subjected to greater wear. Due to actuation of the clutch, an additional amount of inertia is created and can cause disproportionately high clutch wear, as well as result in overrunning of the press.

The present invention aims at overcoming the above disadvantages and accordingly provides a drive and control arrangement for a mechanical eccentric press, comprising a direct current electric motor, an eccentric coupled for rotation with a shaft driven by the motor and supported by rolling bearings, a connecting rod of the press being connected to the eccentric for pivotal movement about an axis offset to the axis of rotation of the eccentric, and control means for controlling the supply of electric current to the motor including a limit switch for interrupting the supply of current at a selected, adjustable angular position of the driven shaft and eccentric, the limit switch being electrically connected to a timing device.

With the arrangement of the invention, the torque of the direct current motor can be increased before the rod reaches bottom dead centre so that, for a short time, an increased pressing force is achieved without overloading the motor, and switching off or on again of the eccentric press ensures smooth procedure of operation.

A further advantage of the actuating and control arrangement consists in that neither a clutch, nor a flywheel or the like are required, whereby a wear-free operation of the eccentric press is ensured.

By selection of the time of operation of the limit switch during the press cycle, precise stopping at top dead centre can be ensured, taking into consideration motor power, cycle time and brake path.

The timing device can initiate braking of the motor and press at an adjustable, predetermined point during the press cycle in the event of failure of the limit switch. In a preferred arrangement said predetermined point is approximately 240 after top dead centre.

Reliable switching off of the eccentric press is thereby guaranteed, even on failure of the limit switch or on blocking of the press at bottom dead centre.

It is particularly advantageous if the control means includes a braking resistance and means for short-circuiting the armature of the direct current motor through the braking resistance for braking the motor and the press.

The short-circuiting means may be actuated either by the limit switch or the timing device.

By this means, over-run of the direct current motor, which is indefinite with normal switching off, can be variably shortened, so that the connecting rod reaches top dead centre in all cases.

In a preferred embodiment, a starting device is provided for controlling the supply of electric current to the motor and is connected to the short-circuiting means for interrupting the braking process initiated by operation of the limit switch, and the limit switch is arranged to be operated to interrupt or restart the starting device when the position of the driven shaft and eccentric corresponds to a point in the press cycle of approximately 220 after top dead centre.

The advantage of this embodiment is that the eccentric press can be operated at higher numbers of strokes.

To reduce the motor speed, a normally open switch can be connected in parallel with the starting device and in series with a resistor.

Due to the voltage reduced by the resistor, crawling speed of the eccentric press is possible, wherein the connecting rod can be stopped in any position and even in case of erroneous adjustment, due to the low pressure applied, no damage can occur to the tool, if for example, in case of a tool change, the relation between press, pressing ram and die is optimally balanced.

It is preferable for an off switch to be provided for stopping the motor and hence the press at any point during the press cycle.

The invention is described in more detail below by way of example with reference to the accompanying drawings, in which: Figure 1 shows a longitudinal section through an arrangement according to the invention; and Figure 2 is a diagram showing the electrical control circuit.

A direct current electric motor 2 with a control device 9 is connected to a reduction gear mechanism 3, preferably a spur gear mechanism, and mounted thereby on a frame 1. An output shaft of the gear mechanism is non-rotatably coupled to an eccentric 4, which is mounted on the frame 1 for rotation about a common axis with the shaft 8. A connecting rod 7 carrying a pressing ram (not shown) at its lower end is pivotally connected at its upper end to the eccentric for rotation relative thereto about an axis parallel to be offset from the common axis of shaft 8 and eccentric 4. A limit switch 10 of the control device 9 is mounted on the frame for switching off the direct current supply to motor 2, preferably after the connecting rod 7 and ram have passed through bottom dead centre. An actuating cam 22 carried by an actuating ring 21 mounted on eccentric 4 serves to operate the switch 10.The characteristics of a direct current motor, in particular its high starting torque, precise braking effect obtained by short-circuiting the armature, and the reduction of speed which can be achieved by reduced voltage, are taken advantage of in the invention as will become clear. During a pressing operation, the connecting rod 7 is moved downwards. On reaching bottom dead centre the torque of the direct current motor 2 is increased by increasing intensity of the supply current, and the current intensity may for a short time reach twice the nominal current intensity without overloading the direct current motor 2.Precise stopping of the connecting rod 7, for example at top dead centre position, can be guaranteed by short circuiting the armature of the direct current motor 2 after the connecting rod 7 has passed through bottom dead centre, so that the direct current motor 2 operates as a generator, whereby the moment of inertia is immediately and automatically braked. The braking distance can be selectively varied to an accuracy of + 1 mm in a known manner by appropriate choice of braking resistance 1 3 connected in series with the armature and the instant during the press cycle at which the limit switch 10 is operated.

By changing the d.c. voltage, various stroke speeds can be achieved. Thus, it is possible to move the connecting rod 7 slowly downwards in steps and stop it at any time and, for example, in case of a tool change to balance the relation between press, pressing ram and die optimally. Crawling speed control of the direct current motor 2 and hence the connecting rod 7, induced by means of low voltage, allows movement thereof in stages, wherein the connecting rod 7 can be stopped in any position. Direct current motors 2 with permanent magnet or field winding, d.c. disc-type rotor motors and the like may be used for a voltage of 70 to 380 V, preferably 1 80 to 220 V.

Referring to Fig. 2 the armature of direct current motor 2 is connected by a rectifier 1 7 to an alternating current source 1 8. Between the negative terminal of the rectifier 1 7 and the motor armature is connected a starting logic unit 1 2 having a starter 11 connected to its input, and a resistor 14 is connected in series between the armature and the positive terminal of the rectifier 1 7. The starting logic unit 1 2 is electrically connected to the limit switch 10 and a braking unit 1 5 which is connected in series with braking resistance 1 3 across the armature of the motor 2.Connected in parallel with the braking logic unit 1 5 is a timing unit 16 which is also connected to the starting logic unit 1 2 through a manual switch 24. The limit switch 10 is connected on the one hand to the braking logic unit 1 5 and on the other hand, as already meritioned, to the starting logic unit 1 2. A normally open switch 1 9 and a resistance 20 in series therewith are connected in parallel with the starting logic unit 1 2. By closing the switch 1 9 a reduction in motor speed and hence, as already described, crawl-ing speed control of the connecting rod 7 can be obtained.

Parallel to the limit switch 10 is connected an emergency switch device 23 for switchingoff the motor 2 in any position of the connecting rod 7.

Switching on of the motor 2 and hence the eccentric press is carried out by the starter 11 acting on the starting logic unit 12, preferably by a pedal switch, a manual key, an actuation sensor, for example proximity switch, a photoelectric cell and the like, and by pulse control of machines on the supply side. The starting signals can be fed to the starting logic unit 1 2 as individual pulses or as continuous signals.

The starting logic unit 1 2 is programmed so that after each starting signal, it must be turned off by the limit switch 10 and a continuously applied starting signal supplied from the starter 11 must be interrupted before the direct current motor 2 can be started again by the starting unit 1 2. Switching on of the direct current motor 2 preferably takes place via a low-voltage signal, for example 24 V, fed to a semiconductor power switch, for example a thyristor, transistor, triac and the like. A continuous signal from the starter 11 to the starting logic unit 1 2 could be interrupted for a short time by the limit switch 10 via an electrical connection (not shown), whereby continuous operation of the eccentric press would also be possible.

The limit switch 10 is fixed to the machine frame 1 and is actuated by a cam 22 carried on actuating ring 21 which is adjustable on the driven shaft 8, preferably after the con necting rod 7 has passed through bottom dead centre, for example at an angle of ap proximately 220 of the eccentric 4 from the top dead centre position. After each switch-on of the starter 11, the braking circuit is inter rupted by a signal emitted from the starting logic unit 1 2 to the braking logic unit 15, whereby after each starting signal the shaft 8 is always driven until the limit switch 10 is operated.The limit switch 10, preferably an electronic proximity switch, switches on the braking circuit consisting of the braking resistance 1 3 and the braking logic unit 15, as well as switching-off of the starting logic unit 1 2. The braking logic unit 1 5 includes a semiconductor power switch which is turned on by a low-voltage signal to short-circuit the armature 2 through the resistance 1 3 so that the motor 2 and hence the press are braked.

The kinetic energy is discharged via the braking resistance 1 3. Connected in parallel with the braking logic unit 1 5 is the timing device 1 6 which, after an adjustable, preset period, switches on the braking circuit, but the timing device only comes into effect when the limit switch 10 fails or the eccentric press is blocked at bottom dead centre. The timing device 1 6 may be set so that the braking process is initiated after passage of the driven shaft 8 through between 1 90 and 300 from the top dead centre position. Thee is no signal emitted automatically from the timing device 1 6 to the starting logic unit 1 2 so that the direct current motor 2 is not restarted after operation of the timing device. Such a fault in the operation is indicated optically and the eccentric press must be restarted by a closing switch 24 manually for providing a signal to the starting logic unit 1 2 from the timer.

The output of an eccentric press according to the invention, compared with known eccentric presses, can be doubled to quadrupled with very short stroke times of up to 0.1 seconds. The press is suitable for use in stamping, pressing, riveting, separating and similar operations.

Claims (10)

1. A drive and control arrangement for a mechanical eccentric press, comprising a direct current electric motor, an eccentric coupled for rotation with a shaft driven by the motor and supported by rolling bearings, a connecting rod of the press being connected to the eccentric for pivotal movement about an axis offset to the axis of rotation of the eccentric, and control means for controlling the supply of electric current to the motor including a limit switch for interrupting the supply of current at a selected, adjustable angular position of the driven shaft and eccentric, the limit switch being electrically connected to a timing device.

2. An arrangement according to claim 1, wherein the timing device is so arranged that on failure of the limit switch braking of the motor is initiated by the timing device at a point in the press cycle after bottom dead centre.

3. An arrangement according to claim 2, wherein said point in the press cycle is approximately 240 after top dead centre.

4. An arrangement according to any one of claims 1 to 3, wherein the control means includes a braking resistance and means for short-circuiting the armature of the direct current motor through the braking resistance for braking the motor and the press, and the limit switch and timing device are each arranged to actuate the short-circuiting means.

5. An arrangement according to claim 4, wherein a starting device is provided for controlling the supply of electric current to the motor and is connected to the short-circuiting means for interrupting the braking process initiated by operation of the limit switch.

6. An arrangement according to claim 5, wherein the limit switch is arranged to be operated to interrupt or restart operation of the starting device and actuate the shortcircuiting means when the position of the driven shaft and eccentric corresponds to a point in the press cycle of approximately 220 after top dead centre.

7. An arrangement according to any one of claims 1 to 6, wherein a normally open switch is connected in parallel with the starting device and has a resistor connected in series therewith, whereby said switch can be closed to reduce the motor speed.

8. An arrangement according to any one of claims 1 to 7, wherein the control means includes an off-switch operable to interrupt operation of the motor and press at any point during the press cycle.

9. A drive and control arrangement for a mechanical eccentric press, substantially as herein described with reference to the accompanying drawings.

10. A mechanical eccentric press having a drive and control arrangement as claimed in any one of claims 1 to 9.