Seam Welding

Questions and Answers

There are two types of drives for seam weld wheels. One is driven by the central hub, direct drive. The other is driven by knurled wheels riding on the perimeter of the seam welding wheels, indirect drive. There are pros and cons for both systems.

KNURL DRIVE
The knurl drive, indirect drive system provides a constant linear wheel speed at all wheel diameters. This allows the use of two wheels of different size. Frequently it is advantageous to use a smaller seam welding wheel on one side. The knurl driven system accommodates this easily. The knurl is also used to continuously dress the wheels on every rotation so that fresh wheel material is always being used to make the weld. The knurl imprints a knurl pattern on the wheel which enables the wheel to grip the workpiece and move it without slipping. One negative is it may leave a knurl patter on the part face. Water is frequently sprayed on the knurl drive area to remove loose material and keep the heat down. A knurl driven machine may cost up to 20% more than a direct drive machine.

DIRECT DRIVE
The alternative is a central axis shaft driven machine (direct drive). Either one or both wheels may be driven. If both wheels are driven, they must be the same diameter in order to travel at the same lineal speed. If only one wheel is driven, the size can vary. As the wheels are dressed or wear the lineal speed will change and must be compensated for to maintain weld spacing. To dress the wheels they are frequently removed and dressed off line on a lathe. The option is to design an automated dresser. At the proper time the automated dressing tool would move into position and dress each or both wheels as desired. Every time you dress the wheels, their diameter is changing and therefor the rotational speed of direct driven wheels must be adjusted to maintain the desired lineal speed and weld spacing.

Seam welding can generate large amounts of heat. Especially when making a liquid tight continuous weld. This heat is very visible at the actual weld joint. Heat is also being created in the various conductors, transformer and the control. All of these components must be cooled. The control, transformer and conductors should all be cooled per the amount specified by the supplier of that equipment. The seam weld wheels are mounted on water cooled shafts which also are water cooled. All of these components will generally be cooled with 1-1.5 gallons of water per minute of flow per device. The flow to the various components should come from the water manifold separately to insure cool water for each component.

Seam welding just like spot welding depends upon the proper application of pressure current and time. Let’s assume that the current and time are controlled properly. If the weld force varies during the welding process to lower values, the process is not stable. As the force decreases the contact between the seam weld wheel and the workpiece is not as good as it was with full force. The contact resistance (Rc) between the weld wheel and the workpiece will increase. Rc will become very large and a lot of heat will be generated on the part surface against the seam weld wheel face. This is more heat than the process was designed for with the proper force. Therefore the wheel surface will be over heated and will wear at an increased rate. Mushrooming will increase and if the wheel is being dressed larger amounts of material will be removed. The wheel will be dressed away at a faster rate and need replacement.

The seam weld wheel will fail prematurely due to low or inconsistent force application.

Seam welds are merely a line of spot welds. If their individual welds are weak it is caused by the same factors that cause spot welds to be weak. Some factors could be:
The size of the weld nugget
Indentation which indicates the wheel forging action will influence strength
Presence of cracks or inclusions in the weld nugget
Penetration of the nugget into both materials being welded
Brittleness of the material being welded. Tempering may be needed
The material being welded. Aluminum tends to form voids that need to be forged closed.
Proper wheel dressing
Proper Cooling

Burrs in seam welding might be describing weld flash or material that escaped from under the weld wheel during the weld and forms an irregular ridge along the length of the seam weld. All resistance welds seam or spot can leave some deformed material around the periphery of the weld zone. Generally this is not enough to be called a burr but in some cases the indentation would be cosmetically offensive if the surface were to be subsequently painted or were in a very visible location like the hood of a car. Product designers avoid welds in these locations.

Where seam welds generate visible indentations or slight raised areas which are not desired they can be treated as cosmetic welds. The weld schedule and electrodes, in this case weld wheels are selected to reduce the amount of heating and expulsion on the side of the part where the burr condition is not wanted. The wheel weld face might be increased. The force may be increased. This reduces contact resistance and surface heating. Adjustments to current and weld speed can be made. All of these would be made with the intent of reducing the surface heat and amount of material being expelled or pushed out during the welding process.

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