Patent GB-1232626-A (GB)
IPC: B23K11/31
Filed: 1968-05-22
Abstract: 1,232,626. Welding by pressure. WELDING INSTITUTE. 20 May, 1969 [22 May, 1968], No. 24476/68. Heading B3R. In resistance welding by holding workpieces together under pressure between electrodes and passing current between the electrodes, the load exerted by the electrodes is increased by at least 30% during at least a portion of the period of current flow as a function of the movement of separation of the electrodes due to expansion of the weld metal. The electrodes may be brought into contact with the workpieces under a light load and then looked in position mechanically, hydraulically or electrically so that by constraining the movement apart of the electrodes the expansion of the weld metal results in an increase in the load applied by the electrodes. Two workpieces 10, 11, Fig. 1, are held between two electrodes 12, 13, the lower electrode 13 being fixed and the upper electrode 12 being connected to a piston 15 in an air cylinder 16. When the initial load has been set by the piston/ cylinder 15, 16 brake shoes 19 are caused to clamp on to a plate 21 carried on the piston-rod 22 by hydraulic fluid supplied by pipes 20. Under expansion of the weld metal the frame of the machine provides compliance acting as a high rate spring and providing for increased load on the electrodes. An alternative to the brake locking system employs mechanical wedges inserted by pneumatic cylinders between the bottom of the air cylinder and a plate carried at the piston-rod. A solenoid operated valve holds the wedges in place until released by a weld timer. In another embodiment the initial load and clamping of the electrodes on the work is provided by oil supplied to a piston/cylinder arrangement through a non-return valve. When the electrodes contact the work pressure builds up in the oil until at a selected initial load a pressure switch initiates a weld timer. As expansion takes place the piston moves up and the non- return valve closes so that electrode load increases with workpiece expansion. The weld may be terminated after a pre-determined time or when the pressure reaches a pre-determined value. In a modification a double ended hydraulic ram may lock the normal air operated system. Instead of restricting expansion from the start of welding at a high rate a certain amount of expansion may be allowed with a lower increase in electrode load. For example a resilient block is initially compressed before the compliance of the frame is called upon. The electrodes may provide a high initial load which is then reduced to a low value at an early stage in the weld to increase thereafter as a function of the expansion. Expansion of the workpieces causes the upper electrode to move the piston 45, Fig. 7, in an hydraulic cylinder 46 and causes liquid to be expelled to a throttle and non- return valve 60 wherein the constriction 61 causes a pressure increase which is a function of the rate of liquid flow from the cylinder. The electrode load is related therefore to the rate of expansion of the workpieces. For clamping and unclamping the valve 60 is by-passed. In further embodiments the weld may be continued for a period after a given load has been reached or the weld may be terminated when the expansion reaches a given proportion of a predetermined maximum expansion for the workpieces in question. Larger electrodes than normal may be used and the current amplitude may be varied during welding. Gun welders or multiheaded welders with incorporated independent compliances may be used.