Seam Welding
Questions and Answers
Seam welding drive shafts carry current, deliver force and rotate during the seam welding operation. Duty cycles, currents and forces are high. The shaft operates in a housing which is filled with conductive grease and has current carrying shoes which ride on the shaft to deliver current. Eventually these conductors and the shaft wear and service is required. Sometimes people ask if they can the buildup the worn shaft with new copper and machined it back to size? They are suggesting the use of plasma or some welding process to lay down this layer of copper. Unfortunately the resultant laid on copper has some amount of porosity when compared to the original copper it is replacing. This soft porous copper will not wear well or conduct the high currents experienced in seam welding.
Yes, seam welding wheels can be dressed automatically. On some machines the wheels are driven by knurled drive wheels that drive and also dress and maintain the shape of the wheel at the same time. The photo below shows a knurl drive system. The wheels are continually driven and dressed each revolution.
Intermittent and continuous seam welds are very similar. In fact they can be run on the same machine. The only difference is the spacing of the weld. A change in the machine settings could produce either weld.
A continuous seam weld is one where the individual spot welds produced overlap and normally form a fluid tight seam weld. Intermittent welds are spaced out and therefor are not fluid tight. Sometimes this type of weld is referred to as a roll spot weld. Generally the roll spot/intermittent process can produce more feet of product faster than the continuous seam weld.
Seam and spot weld are similar in many ways. They both make individual welds with the same basic process of squeeze, weld and hold. The heating principal is the same “Ohm’s Law” H = I2 rt. The electrodes are different. One employs a wheel the other a single point electrode. Their weld faces are a little different but still act to concentrate the current and force into a point contact on the part. In simple terms they produce the same individual welds.
Seam welds generally are closer together and frequently produce fluid tight seals. Spot welds normally are not that close together and are not water tight. Testing is performed on seam and spot welds in similar fashion by destructive testing of coupons or product in most shops.
Seam welding wheels are driven and move the work piece. Spot welds electrodes are stationary and the product comes to it or robots move the weld electrodes to the weld locations.
Reference: RWMA – Resistance Welding Manual Section 4
A lap seam weld is when two work pieces are slightly overlapped along their edges. Then this overlapping area is continuously seam welded. The overlapped metal is mashed down and welded at the same time. They overlapped slightly so this is not a butt joint. It is usually a continuous fluid tight joint. There will remain some raised metal at the joint.
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