- Email: [email protected]
Resistance Welding
14 Resistance Welding
In resistance welding, the two pieces to be welded are pressed together by electrodes (usually copper or copper alloys) and a large electric current is passed through them. This current can be tens of thousands of amps, but the voltage could be as low as 3–4 V. The place where the two workpieces are clamped in contact has the highest resistance: hence the heating effect of the welding current is highest there and the metals melt to form a weld, see Fig. 14.1 The pressure between the electrodes may be exerted by an axial compression, or the electrodes may be rollers, where the workpiece is fed into the nip between them, see Fig. 14.2. Once it is correctly set up, the welding cycle is fully automatic and there should be little or no spatter. However, during set up, or if the current is set too high, spatter can be produced from the weld. There are many different designs of machine, some being fixed on a pedestal and others portable, so that they can, for instance, be mounted on a robot arm. Flash butt welding is included in this section, since it is described in BS 499154 as a resistance welding process. In this process two parts are brought together and a current passed across the joint until they are red hot. They are then separated and an arc is established between the ends until the metal begins to melt. The parts are then brought together under high pressures to form a joint, see Figs. 14.3 and 14.4.
Hazards The hazards likely to be encountered in the operation of resistance welding are: 141
142
– – – –
Health and Safety in Welding and Allied Processes
14.1
Schematic of resistance welding.
14.2
Seam welding.
electrical magnetic field hot metal mechanical hazards cutting hazards from sheet metal, crushing parts of the body between the electrodes, being drawn into the nip point in the seam welder, failure of the support system for portable machines. – flying particles – fume – noise.
Resistance Welding
143
14.3 Detail of flash butt weld region.
14.4 Flash welding.
A brief outline of the safety measures is given below. The equipment should be subject to periodic inspection and maintenance and the operators should be properly instructed and judged to be competent before using the equipment. Advice is given in an AWS document.155 Electrical hazards The machinery must be installed according to the requirements of the electrical regulations of the locality by a competent person.18,19,117,156 The machines often operate from a three-phase supply, which is extremely hazardous. There is a British Standard specific to the safety of this equipment.157 All the access panels and doors to electrical equipment which operates at hazardous voltages should be locked or interlocked, if
144
Health and Safety in Welding and Allied Processes
they are accessible at the production floor, so that they cannot be opened without a key or tool. They should not be open unless the machine is being serviced and this must only be done by a competent person. Any machine that has capacitor banks at high voltages should have an interlock so that if the door is opened the bank is discharged. External weld initiating control circuits should be at reduced voltage – not more than 120 V AC on stationary equipment or 36 V AC for portable equipment. There should be no danger from shock during the normal operation of a machine, because the voltages that are applied to the exposed and touchable current-carrying electrodes seldom exceed 20 V. The transformer secondary should be permanently grounded, or equivalent protection should be provided to protect against it becoming live as a consequence of any breakdown of insulation in the transformer.157 Doors are sometimes provided to give access to tap changing connections and control cabinets. Such doors should be fitted with safety interlocks, if these allow access to potentially hazardous voltages. There is one process, HF resistance welding, in which there are high voltages on exposed welding electrodes. The frequency of the electric current is so high, usually about 400 kHz, that there is little danger of electric shock if the terminals are touched. However, small but very deep and painful burns are likely to occur where the current enters and leaves the body and for this reason adequate precautions should be taken to prevent contact with these terminals while they are live.
Magnetic fields The high currents used in resistance welding give rise to correspondingly high magnetic fields, particularly in large machines of the DC or three-phase (low-frequency output) types. Static magnetic fields are subject to advisory exposure limits (see Chapter 1). When the current is pulsed, as for repetitive spot welding or for seam welding, the changing magnetic fields could affect the operation of some body implants, including some electronic heart pacemakers. Changing fields induce small voltages in the pacemaker unit, which could indicate normal heart function to the pacemaker, inhibiting pacemaker pulses. The patient’s heart might slow down or stop, causing him or her to faint. Fortunately, normal functioning
Resistance Welding
145
should be rapidly recovered on removal of the field. Normal walls or partitions will not screen off the low frequency magnetic fields most likely to cause adverse effects, but the fields fall off to negligible values at 10 m. Similar but smaller effects may arise from arc welding, with a corresponding distance of 3 m. A suggested plan of action is: –
Advise known pacemaker users to keep clear of the arc and resistance welding if possible. – Include advice in induction procedure for new employees. – Place notices to warn visitors of the risk. Since adverse effects are rare in practice, if a pacemaker user wishes to work near or with welding equipment they should ask their general practitioner or hospital consultant for advice; they may be able to arrange a trial run under medical supervision.
Hot metal Sometimes no visible heat is produced during the welding operation, consequently there is a tendency to think the welded parts are not hot. Burns will result from careless handling of hot assemblies.
Mechanical hazards Resistance welding is extensively used for guillotined, blanked or pressed components which may have very sharp edges or burrs. The incidence of cuts and lacerations can be minimised by deburring the components before they are taken to the welding position and by wearing protective gloves or gauntlets. Resistance welding machines invariably have at least one electrode which moves with considerable force; in this respect they closely resemble a power press. If a machine is operated while a finger or hand is between the electrodes or platens, severe crushing will result. The risk of crushing should be considered for each machine and a suitable means must be devised to safeguard the operator. Seam welding machines require similar protection against nipping hazards between the wheel electrodes. Unless the workpieces are of such configuration that the hands are remote from the point of operation, then the safeguards should be one of the following:
146
– – – – –
Health and Safety in Welding and Allied Processes
guards or fixtures to prevent the hand entering the danger zone two-handed controls latches presence sensing devices or any similar measure.
A recent development, applicable to a wide range of machines and with little disturbance to normal operation, uses a modified air pressure system. The initial approach is with a low pressure, which cannot produce sufficient force to cause significant injury to a trapped hand or finger. When the electrode gap has been reduced to 6–8 mm, in the absence of any obstruction, the full air pressure is applied to produce the normal weld squeeze force. An alternative is a probe near or surrounding the electrode which switches off the machine if it encounters an obstruction. For those machines that have a sequence that takes 3 s or more to complete, and that have mechanical movements that could be hazardous if the guards were removed, emergency stop buttons should be provided within reach of the operator and anyone who may get trapped, unless the buttons themselves would create additional hazards to persons. Some production jobs on assemblies of a complex shape, such as used in the construction of motor vehicles, may be carried out by robots; the hazards presented are discussed in Chapter 9.
Flying particles Particles of hot or molten metal will not fly out of spot, seam or projection welds if the material and welding conditions are ideal, but such particles are frequently expelled in production work and the greatest danger is to the eyes of the operator or a passer-by. Loose metal parts should not be left in the throat of the machine because they are liable to be projected from it with some velocity by electromagnetic forces. The flash welding process inherently produces a considerable quantity of flying red hot particles which may travel up to 6 m and which may enter unprotected eyes and exposed skin with some force. Fire-resistant screens should be used to protect persons in the vicinity and precautions should be taken against an outbreak of fire. Whenever there is a risk from flying particles people entering the area should wear goggles and protective clothing.
Resistance Welding
147
Fume If the articles to be resistance welded are free from dirt, oil and other extraneous material, little fume should arise during the operation. The amount of fume is unlikely to be great and will disperse quickly in a well-ventilated shop, but in small and inadequately-ventilated workplaces it may be necessary to provide local exhaust ventilation. Paint will usually prevent the welding current flowing and so will be cleaned off the work in the weld region. It will be necessary to ascertain the nature of any fume to which workers may be exposed, and establish that it does not exceed the limits laid down (Chapter 5). The use of cadmium is now restricted, but if this material is present on the parts to be welded extra care should be taken in controlling exposure to the fume since it is extremely toxic. Noise Certain resistance welding machines, for instance some that are used for flash welding, produce excessive noise which may be harmful to hearing. The noise that arises from the machine, such as compressed air exhausts, actuator cylinders or loose transformer laminations should, wherever it is reasonably practicable, be reduced at source. If this fails to reduce the noise exposure to employees enough to comply with the standards, then ear protection will be required, see Chapter 8.
In resistance welding, the two pieces to be welded are pressed together by electrodes (usually copper or copper alloys) and a large electric current is passed through them. This current can be tens of thousands of amps, but the voltage could be as low as 3–4 V. The place where the two workpieces are clamped in contact has the highest resistance: hence the heating effect of the welding current is highest there and the metals melt to form a weld, see Fig. 14.1 The pressure between the electrodes may be exerted by an axial compression, or the electrodes may be rollers, where the workpiece is fed into the nip between them, see Fig. 14.2. Once it is correctly set up, the welding cycle is fully automatic and there should be little or no spatter. However, during set up, or if the current is set too high, spatter can be produced from the weld. There are many different designs of machine, some being fixed on a pedestal and others portable, so that they can, for instance, be mounted on a robot arm. Flash butt welding is included in this section, since it is described in BS 499154 as a resistance welding process. In this process two parts are brought together and a current passed across the joint until they are red hot. They are then separated and an arc is established between the ends until the metal begins to melt. The parts are then brought together under high pressures to form a joint, see Figs. 14.3 and 14.4.
Hazards The hazards likely to be encountered in the operation of resistance welding are: 141
142
– – – –
Health and Safety in Welding and Allied Processes
14.1
Schematic of resistance welding.
14.2
Seam welding.
electrical magnetic field hot metal mechanical hazards cutting hazards from sheet metal, crushing parts of the body between the electrodes, being drawn into the nip point in the seam welder, failure of the support system for portable machines. – flying particles – fume – noise.
Resistance Welding
143
14.3 Detail of flash butt weld region.
14.4 Flash welding.
A brief outline of the safety measures is given below. The equipment should be subject to periodic inspection and maintenance and the operators should be properly instructed and judged to be competent before using the equipment. Advice is given in an AWS document.155 Electrical hazards The machinery must be installed according to the requirements of the electrical regulations of the locality by a competent person.18,19,117,156 The machines often operate from a three-phase supply, which is extremely hazardous. There is a British Standard specific to the safety of this equipment.157 All the access panels and doors to electrical equipment which operates at hazardous voltages should be locked or interlocked, if
144
Health and Safety in Welding and Allied Processes
they are accessible at the production floor, so that they cannot be opened without a key or tool. They should not be open unless the machine is being serviced and this must only be done by a competent person. Any machine that has capacitor banks at high voltages should have an interlock so that if the door is opened the bank is discharged. External weld initiating control circuits should be at reduced voltage – not more than 120 V AC on stationary equipment or 36 V AC for portable equipment. There should be no danger from shock during the normal operation of a machine, because the voltages that are applied to the exposed and touchable current-carrying electrodes seldom exceed 20 V. The transformer secondary should be permanently grounded, or equivalent protection should be provided to protect against it becoming live as a consequence of any breakdown of insulation in the transformer.157 Doors are sometimes provided to give access to tap changing connections and control cabinets. Such doors should be fitted with safety interlocks, if these allow access to potentially hazardous voltages. There is one process, HF resistance welding, in which there are high voltages on exposed welding electrodes. The frequency of the electric current is so high, usually about 400 kHz, that there is little danger of electric shock if the terminals are touched. However, small but very deep and painful burns are likely to occur where the current enters and leaves the body and for this reason adequate precautions should be taken to prevent contact with these terminals while they are live.
Magnetic fields The high currents used in resistance welding give rise to correspondingly high magnetic fields, particularly in large machines of the DC or three-phase (low-frequency output) types. Static magnetic fields are subject to advisory exposure limits (see Chapter 1). When the current is pulsed, as for repetitive spot welding or for seam welding, the changing magnetic fields could affect the operation of some body implants, including some electronic heart pacemakers. Changing fields induce small voltages in the pacemaker unit, which could indicate normal heart function to the pacemaker, inhibiting pacemaker pulses. The patient’s heart might slow down or stop, causing him or her to faint. Fortunately, normal functioning
Resistance Welding
145
should be rapidly recovered on removal of the field. Normal walls or partitions will not screen off the low frequency magnetic fields most likely to cause adverse effects, but the fields fall off to negligible values at 10 m. Similar but smaller effects may arise from arc welding, with a corresponding distance of 3 m. A suggested plan of action is: –
Advise known pacemaker users to keep clear of the arc and resistance welding if possible. – Include advice in induction procedure for new employees. – Place notices to warn visitors of the risk. Since adverse effects are rare in practice, if a pacemaker user wishes to work near or with welding equipment they should ask their general practitioner or hospital consultant for advice; they may be able to arrange a trial run under medical supervision.
Hot metal Sometimes no visible heat is produced during the welding operation, consequently there is a tendency to think the welded parts are not hot. Burns will result from careless handling of hot assemblies.
Mechanical hazards Resistance welding is extensively used for guillotined, blanked or pressed components which may have very sharp edges or burrs. The incidence of cuts and lacerations can be minimised by deburring the components before they are taken to the welding position and by wearing protective gloves or gauntlets. Resistance welding machines invariably have at least one electrode which moves with considerable force; in this respect they closely resemble a power press. If a machine is operated while a finger or hand is between the electrodes or platens, severe crushing will result. The risk of crushing should be considered for each machine and a suitable means must be devised to safeguard the operator. Seam welding machines require similar protection against nipping hazards between the wheel electrodes. Unless the workpieces are of such configuration that the hands are remote from the point of operation, then the safeguards should be one of the following:
146
– – – – –
Health and Safety in Welding and Allied Processes
guards or fixtures to prevent the hand entering the danger zone two-handed controls latches presence sensing devices or any similar measure.
A recent development, applicable to a wide range of machines and with little disturbance to normal operation, uses a modified air pressure system. The initial approach is with a low pressure, which cannot produce sufficient force to cause significant injury to a trapped hand or finger. When the electrode gap has been reduced to 6–8 mm, in the absence of any obstruction, the full air pressure is applied to produce the normal weld squeeze force. An alternative is a probe near or surrounding the electrode which switches off the machine if it encounters an obstruction. For those machines that have a sequence that takes 3 s or more to complete, and that have mechanical movements that could be hazardous if the guards were removed, emergency stop buttons should be provided within reach of the operator and anyone who may get trapped, unless the buttons themselves would create additional hazards to persons. Some production jobs on assemblies of a complex shape, such as used in the construction of motor vehicles, may be carried out by robots; the hazards presented are discussed in Chapter 9.
Flying particles Particles of hot or molten metal will not fly out of spot, seam or projection welds if the material and welding conditions are ideal, but such particles are frequently expelled in production work and the greatest danger is to the eyes of the operator or a passer-by. Loose metal parts should not be left in the throat of the machine because they are liable to be projected from it with some velocity by electromagnetic forces. The flash welding process inherently produces a considerable quantity of flying red hot particles which may travel up to 6 m and which may enter unprotected eyes and exposed skin with some force. Fire-resistant screens should be used to protect persons in the vicinity and precautions should be taken against an outbreak of fire. Whenever there is a risk from flying particles people entering the area should wear goggles and protective clothing.
Resistance Welding
147
Fume If the articles to be resistance welded are free from dirt, oil and other extraneous material, little fume should arise during the operation. The amount of fume is unlikely to be great and will disperse quickly in a well-ventilated shop, but in small and inadequately-ventilated workplaces it may be necessary to provide local exhaust ventilation. Paint will usually prevent the welding current flowing and so will be cleaned off the work in the weld region. It will be necessary to ascertain the nature of any fume to which workers may be exposed, and establish that it does not exceed the limits laid down (Chapter 5). The use of cadmium is now restricted, but if this material is present on the parts to be welded extra care should be taken in controlling exposure to the fume since it is extremely toxic. Noise Certain resistance welding machines, for instance some that are used for flash welding, produce excessive noise which may be harmful to hearing. The noise that arises from the machine, such as compressed air exhausts, actuator cylinders or loose transformer laminations should, wherever it is reasonably practicable, be reduced at source. If this fails to reduce the noise exposure to employees enough to comply with the standards, then ear protection will be required, see Chapter 8.