Push-pull welds are generally series welds which make two welds at a time. In some cases only one weld is required and there may be cosmetic reasons to keep marks off the part. In this case the second pair electrodes are flat and large. They become electrical contact pickup points. The same dual transformer arrangement is set up with the proper polarity but one set of electrodes are flat not contoured. This is an indirect push-pull weld.
Indirect welds use similar machine and power set ups to series welds. The difference is that one pole off the transformer is connected to a pick up conductor rather than an electrode. Therefore the current returns to the transformer through this location but no weld is made. This process eliminates marking at the pickup point and the need for electrodes on the other side of the part when only when weld is required.
Another variation of the series weld is the push-pull weld. In this process the electrodes are connected to separate secondary circuits on both sides of the weldment. The polarities are arranged as shown for proper current flow. Shunting current is still present but now in both workpieces. The voltage between the electrodes increases the weld current to shunting current ratio when compared to other series weld set ups. Therefore you get more power delivered to the weld.
In a series weld the current is conducted into the part by two electrodes touching the part from the same side. A weld is produced at each electrode. One electrode serves as the positive the other as the negative. The current flows into the part and through the desired weld joint and through the part and or backup mandrel to the second electrode. It makes a second weld at the second electrode. This type of set up frequently allows for all electrical components to be designed to be on one side of the part.
A direct weld is a standard spot weld where the current path is directly through the workpiece between opposing electrodes. This is the easiest weld to produce. The electrodes size and face shape can be optimized for the application. The face can be radius or flat to produce the desired spot weld.