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.
Seam welding especially in a continuous mode can be a major power consumer. It can really be surprising the first time you see a drum or dishwasher water tight weld being welded. It may look and sound like the machine is welding continuously. It is not. Seem Welders never exceed 50% duty cycle. The equipment is designed for a maximum 50% duty cycle. If run higher they will fail prematurely. Therefor you may not hear it but half of the time the welder could be off as it is running. The closer the welds are together and the faster the wheels turn the more power will be consumed. A simple guide to power consumption is:
Reference: RWMA - Resistance Welding Manual
Inverters and DC controls with their DC output have small inductive losses. This gives them and advantage in total power consumption and they generally can run faster than AC equipment. It is a very good system for seam welding when high energy requirements are necessary.
Reference: RWMA – Resistance Welding Manual Section 4
When making a continuous seam welds sometimes warpage becomes an issue. A lot of welds and heat are going into the material and distortion can be created. The best method is flood cooling. Another method that is sometimes practical is to weld a modest length. Then skip a similar length. Weld another length and skip another length. Repeat till you reach the end then go back and pick up the skipped welds. This has proven successful in some applications. The best method to reduce warpage is flood cooling.
Quality of the resultant weld product is important to all producers. Testing of seam welds is very important. They are tested and monitored using methods similar to spot welding. The peel test where the part is destructively pulled apart to examine the weld nugget is very common. Coupons are frequently used to represent production runs with close monitoring or weld parameters. Tension shear testing of samples is employed in some applications. Leak testing is used for fluid tight applications. Metallography is also an option.
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