GB2070180A - Machine safety device - Google Patents

Abstract

Device for preventing the application of full working force used to move a pair of co-acting elements into an operating relationship, e.g. the electrodes of a spot or resistance welding machine, except in a defined zone of their path of relative travel including said relationship, the effective width of said zone in said path being too small, e.g. 6mm wide, to allow an operative's finger or other part of the body to be trapped under full force. The zone position is adjustable and a pneumatic control circuit (Fig. 1 not shown) is described including the provision of controlled cushioning pressure to regulate the descent of a ram piston by gravity. The zone may be defined by the width of a control member 44a or of a gap, groove or aperture therein which co-acts with a proximity sensor 23 of the control circuit. <IMAGE>

Description

SPECIFICATION Machine safety device This invention relates to safety devices for preventing or reducing the likelihood of accidental damage or personal injury during operation of mechanisms in which a first element is urged towards a second element to effect an operation or in association with an operation. It has particular but not exclusive application to automatic or semi-automatic spot or other resistance welding machines as widely employed in the automotive industry but may have application to other types of machine such as presses, clamps and the like particularly where they are actuated pneumatically.

The invention may be applied to the safe control of actual operating parts of the machine, possibly in such form that automatic safety guards or shields are unnecessary and /or to the operation of an automatic guard or shield as such.

Safety regulations and requirements are constantly being reviewed and made more stringent. It is desirable that any safety device be effective and reliable under the most extreme operating conditions and should be proof against deliberate interference or misuse, yet should be easy to instal or adapt to existing machines and should so far as practicable not hinder efficient operation and maintenance. Spot welding machines are an example of the problems involved, the various contours and sizes of workpieces to be welded and the need to keep the fullest possible range of movement renders the fitting of any kind of really effective and reliable guard or shield to prevent trapping of e.g. an operator's hand or finger between the electrodes as they close together impracticable in many production applications.

Attempts have been made to control the closing pressure applied, usually pneumatically, to the electrodes by holding it at a safe level sufficient to effect the necessary relative movement and only applying the full operating pressure when they are in close proximity and finally brought into co-action to effect the weld The term "safe level" is used herein in this context to mean a pressure which will cause no substantial injury, damage, or trapping of a part of an operator's body present between the electrodes or other co-acting elements.

In this type of arrangement the recommendation or requirement is that full pressure is only applied when the electrodes or other elements are within Srnm from'each other.

Even with complex modifications to the operating system past proposals have not proved wholly reliable and safe.

The object of the invention is to provide a safety device which is relatively simple to install (even as an adaptation of an existing machine), which is easily, maintained and adjusted, which is substantially "foolproof" i.e.

proof against malfunction due to tampering or damage, and which does not substantially affect the efficiency of operation of the machine so that high production levels can be maintained with much improved standards of safety.

According to the invention there is provided a safety device for a machine in which an actuating means operably applies pressure to urge a first element into predetermined relationship with a second element including control means limiting the effective force acting to move the first element towards said relationship to a safe level (as hereinbefore defined) throughout the maximum range of travel of the first member except in a predetermined zone of said travel in which full operating pressure at a greater level is applied, and means for selectively adjusting the positioning of said zone along said range to enable it to include the position at which said relationship occurs; whereby a force greater than the safe level will not be applied at any position on either side of said zone.

Where the actuating means operates by fluid pressure the control means may act to regulate said pressure and/or to provide or control a back pressure or cushioning pressure limiting said force to the safe level.

Said zone may be defined by a physical dimension of a control formation, there being relative movement between said formation and a proximity responsive sensor of the control means in a path which is a function of the range of travel of the first member, response of said sensor causing or permitting application of the full operating pressure.

One embodiment of the invention is now more particularly described with reference to the accompanying drawing. wherein: Figure 1 is a diagram of a pneumatic control circuit of a spot welding machine; Figure 2 is a diagrammatic sectional view of one form of control formation and sensor arrangement for said circuit; Figure 3 is a like view of a second form of said arrangement; Figure 4 is a perspective view of a cable guared for the arrangement of Fig. 3; Figure 5 is a diagram of the activation sequence; Figure 6 is a diagrammatic sectional view of the application of the invention to a movable guard or shield; and Figure 7 is a sectional view of a third form of control formation and sensor arrangement.

The safety device now described takes the form of an accessory kit or unit which can easily be installed on existing pneumatically actuated spot welders as well as being built into or onto new welders.

Referring to Fig. 1 the welder itself includes a double-acting pneumatic ram (piston and cylinder) 10 which moves a first electrode (not shown) of the machine into and out of coacting relationship with a second electrode (not shown) for performing successive welding operations, pressurised air for actuating ram 10 being supplied from a main line 11 by way of a regulator and lubricator 12 and a four-way main control valve 1 3 of the machine. When a weld is to be formed air at full pressure is supplied from valve 13 by way of line 14 to the upper end (as viewed in the drawing) of ram 10 to urge the piston downwards and bring the electrodes together.

The safety device of the machine includes a three-way foot actuated valve 15 connected by a branch line 16 to main line 11 which will initiate single or successively repeated operations of the machine as described hereafter.

A second output from valve 13 is connected by a line 18a through a three-way air actuated with spring return valve 19 to a junction with a line 18 connected to a balance pressure device in the form of a diaphragm valve 17 (described in greater detail below).

The latter valve is connected to the lower end of ram 10. A connection 1 8b in- parallel with valves 13 and 19 and line 18a connects said juncton with line 18 to main line 11 by way of an adjustable pressure regulator 20 and, on the downstream side of the latter, a nonreturn check valve 21. A flow regulator could be substituted for regulator 20 and valve 21 would not then be needed.

Diaphragm valve 17 incorporates a flexible diaphragm located in a chamber of the valve between an inlet port with which line 18 is connected and an outlet port vented to atmosphere, with a side connecton to the lower end of ram 10. One face of the diaphragm coacts with a valve seat to close the outlet port when pressure on its opposite face exceeds pressure acting on said one face, while if the opposite pressure differential applies, the diaphragm will be urged into co-action with the inlet port so as to balance the pressure in the lower end of ram 10 against regulated pressure applied through line 18.

An output line from foot actuated valve 15 leads to a four-way connection 22 one branch of which is connected to a sensor 23 in the form of a pneumatic proximity switch by way of a further pressure regulator 24 and a 20 micron air filter 25. Said sensor could also be a gap sensor or some other form of noncontacting air limit switch. The output side of sensor 23 is connected to operate an air pressure actuated electrical switch 26 and a visual pneumatically actuated indicator 27.

A second branch from connection 22 leads to a three-way time delayed air valve 28 whose output is connected for the actuation of the three-way valve 19 in line section 18a.

Valve 28 incorporates a timer device holding it open for a predetermned period after pilot pressure initiating its operation has been cut off.

To ensure that sufficient pressure is available to operate timer valve 28 under the conditions of this pneumatic circuit, air pressure for its actuation is provided by way of a diaphragm amplifier valve 29 connected to the third branch of connector 22 and responsive to pressure in line 14 by way of a connection through a further pressure regulator 30. This arrangement ensures that the pressure needed to operate valve 28 (at least 2.8 kg/cm2 (40 Ib/in2) for this particular pattern of valve) is available even though pressure in line 14 may be well below that level under certain conditions. The connection between valve 29 and timer valve 28 is by way of a selectively operable threeway valve 31 used to switch the device between single operation and repeat operation modes and incorporating a non-return check valve 32.

Referring to Fig. 2 a control rod 40 is directly coupled to the piston rod of ram 10 for rectilinear movement with it and the associated electrode. A section of rod 10 is threaded and carries a tube 41 whose longitudinal location on the rod can be selectively adjusted by means of nuts 42 at each end.

Tube 41 slides through a box 43 mounted at a convenient position on the machine and enclosing the sensor 23 of the pneumatic circuit. Fixed to tube 41 within box 43 is a control formation in the form of a control member 44 which passes in close proximity to sensor 23 and which has an effective width of 6 mm, thus sensor 23 is only activated when member 44 is opposite to it i.e. only during passing of a zone 6 mm in the range of travel of rod 40 (i.e. travel of the associated electrode); the position of said zone along said path of travel being adjustable.

The operating cycle of the device will now be described.

In an initial "at rest" condition with no operation of foot valve 15 and the electrodes fully separated, air from main line 11 passes through valve 13, along section 18a through valve 19 and creates a pressure differential across valve 17 urging its diaphragm to the position at which it closes its outlet port and at the same time admits air pressure to the lower end of ram 10 to urge the piston upwards. The upper end of the ram cylinder is vented to atmosphere via line 14 and an outlet from valve 13. Foot actuated valve 15 is spring loaded to remain closed so that no pressure is reaching connection 22 and the other components of the circuit.

The positioning of control member 44 will have been adjusted so that its zone of coaction with sensor 23 includes the "strike" position at which the electrodes will engage the workpiece to perform a weld, adjustments being made as necessary to suit the particular operation and the setting and/or wear of the electrode tips, i.e. when the piston of ram 10 has been lowered from the rest position.

When foot actuated valve 15 is operated to initiate a welding operation, air at main pressure passes through connector 22 to the various parts of the circuit. Sensor 23 will receive air pressure but is not yet activated thus switch 26 is still open. This switch controls the operation of valve 13, when energised it changes valve 13 over to apply main pressure to the top of the piston of ram 10 by way of line 14.

When valve 15 is opened three-way valve 19 in line 18a closes, having been actuated by air fed through valve 28, and regulated pressure only is applied to diaphragm valve 17 through pressure regulator 20 and nonreturn valve 21. This regulated or balance pressure controls or cushions the descent of the ram piston under its own weight (line 14 being still open to atmosphere). The action of the diaphragm of valve 17 is to allow air from the lower end of ram 10 to vent to atmosphere without application of any back pressure within the lower part of ram 10 itself, the exhaust rate being determined by said regulated pressure acting on the diaphragm, the latter in turn determining the effective aperture of the outlet port of valve 17.

This arrangement allows the piston to drop quickly yet safely towards its operating position. While the balance pressure used in this construction is air pressure, other forms of balance pressure devices might be employed, e.g. a spring-loaded diaphragm or other form of spring or weight loaded valve, in substitution for diaphragm valve 17, whereby the pressure of the exhaust air from the lower end of the ram is cushioned by control of the effective opening of an outlet port.

If the moving electrode should meet with any obstruction, e.g. the operator's hand or finger, in this condition it would be halted without injury or damage as no air pressure is being applied by ram 10 and the downward movement is being cushioned.

Only when the moving electrode is slowed down considerably or halted within the 6mm zone embracing the "strike" position will sensor 23 be activated to close switch 26 and change over valve 13 for full main pressure on ram 10. the maximum 6mm gap making it virtually impossible that a finger or other part of the operator's body should be trapped.

When sensor 23 is activated indicator 27 changes colour and the electrical circuit of the machine is energised for the weld operation.

If only a single welding operation is required for each actuation of fodt valve 15 the single/repeat valve 31 will be kept closed and components 29-32 will not be used and valve 28 will remain open, indeed where a machine is required for a single operation mode only those items may be dispensed with, the outlet from foot valve 15 being connected directly to actuate valve 19 and to the sensor part of the circuit without the third branch leading to valve 29 etc. In this case when foot valve 15 is released and closes air is cut off from the sensor, valve 13 returns to its initial position and valve 19 is opened to pass air to the bottom of -ram 10 for returning the electrode to the rest position.

If repeat operation is required, i.e. formation of a succession of welds while foot valve 15 is held open, the manual valve 31 will be switched to open. In this case sufficient pressure to initiate operation of timed valve 28 will be passed from diaphragm valve 29 thus de-energising valve 19, the latter being held de-energised by the delay built into valve 28.

When valve 28 times out three-way valve 19 will be actuated once more and the operations are repeated successively until valve 6 is opened by removing foot pressure. When that occurs the machine returns to the "rest" condition described above.

The particular safety feature of this arrangement derives from the built in limit on the period and positioning at which full (or indeed any) line pressure is applied to ram 10 shown diagrammatically by Fig. 5. If the positioning of control member 44 is incorrectly set (deliberately or inadvertently) in either direction relative to the "strike" position, or the relationship of the latter to the control positioning changes e.g. due to electrode wear or damage, the machine will not operate for, if member 44 is moved beyond sensor 23, no pressure is applied, no weld will be formed, and ram 10 will remain in the condition at which its upper end is vented and its lower end has the controlled cushioning provided by the balance pressure device (valve 17).The safety device cannot be switched out of the circuit nor can initiation of a weld operation take place without the foot actuated valve being operated. The speed of operation of the welder is not substantially affected and adjustment is simple, the only settings needed being the positioning of control member 44 and the adjustment of pressure regulator 20 to give the required balancing pressure.

An alternative arrangement of sensor and control member is shown in Figs. 3 and 4 where it may be necessary to mount the box 43a enclosing these items more remotely from ram 10. In this case movement of the ram piston is transmitted by a Bowden cable 50 to a rod 51 carrying the control member 44a. Rod 51 is the piston rod of a small pneumatic ram 52 to which main line air pressure is constantly applied to maintain constant tension on the Bowden cable, a return spring also being incorporated. Sensor 23 is mounted on an adjustment screw 53 enabling its positioning to be altered within box 43a parallel to the path of movement of member 44a.This arrangement may not be so desirable due to the Bowden cable 50 being susceptible to accidental damage or interference. to reduce this possibility a tubular guard 54 may be mounted so as to cover the portion of the inner cable which would be otherwise exposed at ram 10 as indicated in Fig. 4. It is also possible that sensor 23 might be mounted e.g. on rod 51 for movement relative to a static control member, the positioning of either or both the sensor and member being selectively adjustable.

Fig 6 shows the invention applied to a guard shield 60 which is moved into and out of protective relationship with the weld or other operating area in each operating cycle.

The guard is mounted directly on a sliding rod 40a extending through box 43 in the same manner as shown in Fig. 2, the positioning of the control element being adjustable in the same way. A small ram under main line air pressure raises the guard, this being directly connected to the outlet from valve 28 of the circuit shown in Fig. 1 in place of the valve 19 and the latter, together with line 18.

valves 17 and 21. and pressure regulator 20 are not required.

If a gap sensor is used it may coact with a flat strip or bar having a slot or aperture whose width determines the zone in which full pressure can be applied. The sensor could also take the form of a limit switch arrangement, infra red gap sensor or other proximity detector device though for consistency and reliability of operation highly sensitive devices necessitating the use of electrical relays may not be so desirable.

One example of such an arrangement is illustrated in Fig. 7. Sensor 23 is a plunger actuated pneumatic valve mounted in a guide block 50 fixed to the machine head. The plunger 51 is resiliently biassed to project into a through bore of block 50 in which is received a tubular slider 52 mounted on a threaded control rod 53. Rod 53 is coupled to the piston rod of the ram as previously described, the longitudinal location of slider 52 being selectively adjustable by means of nuts 54 at top and bottom. A groove 55 6mm in axial width is formed in the wall of slider 52 defining the "striking zone" in the range of longitudinal movement of rod 53 as referred to previously.

When groove 55 aligns with plunger 51 the latter is displaced to close sensor valve 23 (otherwise open to exhaust) and so cause application of full pressure to ram 10 for the welding operation. To avoid time lag due to length of connections and response of servo elernerits such as switch 26 a valve directly controlling pressure application to the top of ram 10 could be mounted on block 50 for immediate actuation by plunger 51.

The invention may also function by direct control of pneumatic or other forces acting to move the electrodes or other elements towards each other e.g. in cases where said movement is not effected by gravity as described above. Also instead of merely controiling a cushioning pressure by regulating its release, control may be effected by admitting and exhausting positive back pressure to the lower end of ram 10 in some applications to limit the effective force acting on the first element to the safe limit.

Claims (22)

1 Asafety device for a machine in which an actuating means operably applies pressure to urge a first element into predetermined relationship with a second element including control means limiting the effective force acting to move the first element towards said relationship to a safe level (as hereinbeforedefined) throughout the maximum range of travel of the first member except in a predetermined zone of said travel in which full operating pressure at a greater level is applied, and means for selectively adjusting the positioning of said zone along said range to enable it to include the position at which said relationship occurs; whereby a force greater than the safe level will not be applied at any position on either side of said zone.

2. A device as in Claim 1 wherein the actuating means is operated by fluid pressure and the control means operates to regulate said pressure by preventing any application thereof for urging the first member towards said relationship except in said predetermined zone.

3. A device as in Claim 2 wherein the force urging the first member towards said relationship outside said zone is gravity.

4. A device as in Claim 1, 2 or 3 including a balance pressure device providing or controlling a back pressure or cushioning pressure acting in opposition to said travel of the first member to limit the effective force to the safe level.

5. A device as in Claim 4 wherein the actuating means is a double-acting fluid pressure operated ram and the balance pressure device is a valve having a resiliently loaded closure element acting to regulate outflow of fluid from one side of the ram to provide a controlled cushioning pressure.

6. A device as in Claim 5 wherein the closure member is a flexible diaphragm.

7. A device as in Claim 6 wherein the resilient loading of the diaphragm is provided by regulated fluid pressure applied to one face thereof in opposition to said fluid outflow.

8. A device as in any one preceding claim wherein said zone of travel is defined by a physical dimension of a control formation, there being relative movement between said formation and a proximity responsive sensor of the control means in a path which is a function of the range of travel of the first member, response of said sensor causing or permitting application of the full operating pressure.

9. A device as in Claim 8 wherein the position of the control formation is adjustable longitudinally of said path to effectively preselect the positioning of said zone along the range of travel of the first member.

10. A device as in Claim 8 or 9 wherein the control formation is directly coupled to the first member for movement therewith.

11. A device as in Claim 8, 9 or 10 wherein said physical dimension of the control formation defines said zone as being 6mm long in said range of travel of the first member.

12. A device as in any one of claims 8 to 11 wherein the control formation is a control member whose effective width defines said zone and which co-acts with said proximity sensor.

13. A device as in any one of claims 8 to 11 wherein the control formation is a gap, aperture or groove in a control member, the effective width of said gap, aperture or groove defining said zone and which co-acts with said proximity sensor.

14. A device as in any one of claims 8 to 13 wherein the proximity sensor co-acts with the control formation without physical contact therewith.

15. A device as in Claim 14 wherein said sensor is a pneumatic proximity switch.

16. A device as in any one of Claims 8 to 13 wherein the proximity sensor includes a resiliently loaded plunger in contact with the control formaton so as to be displaced in response to passage of said physical dimension.

17. A safety device substantially as herein before described with reference to and as shown in Figs.1, 2 and 5, or Figs. 1, 3, 4 and 5; or Fig. 6; or Fig. 7 of the accompanying drawings.

18. A machine including a safety device as in any one preceding claim.

19. An automatic or semi-automatic spot or other resistance welding machine including a safety device as in any one of Claims 1 to 16, the first and second elements being the co-acting electrodes of the machine.

20. A machine including a safety device as in any one of Claims 1 to 16, the first element being a movable safety guard or shield.

21. A spot welding machine substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

22. A movable safety guard or shield substantially as hereinbefore described with reference to and as shown in Fig. 6 of the accompanying drawings.