Why does the weld matter so much?
The weld becomes part of the load path, and the joint cannot be replaced afterwards. Once welded, the housing is part of the machine structure. A common field failure is welding without fixture control, which puts the pivot out of position (Q1-01; see also the weld-on ball joint guide). Before cutting any steel, confirm the housing outside diameter, width and working angles against the structure.
What about the material?
Know both steels. A carbon steel housing is the easiest to weld, but the structure you weld it to must also be a steel and a thickness your procedure covers. Ask the part maker for the material of the housing. Steels with more carbon, or more alloying elements, generally need more care, such as preheating and low-hydrogen practice, and that is why the chemistry of both steels has to be known before a procedure is chosen. Have a qualified welder or welding engineer choose the procedure for that pairing. For rod eyes on cylinders, the same advice appears in K03-Q1-27.
What does the heat do?
It expands the metal next to the weld, and when it cools the metal contracts. The heat-affected zone expands as it is welded and, because the cold metal around it holds it back, it can only yield; when it cools it contracts, and the weld ends up in tension with local plastic deformation around it (TWI has a good primer). The result is shrinkage, twisting or bowing.
For a ball socket there is a second concern: the heat can reach the parts inside, so ask the maker whether the ball may be in place during welding. A housing that distorts can pinch the ball and make the joint stiff afterwards.
The weld method and the weld type matter too. How the weld is laid out, continuous or interrupted, and the type of weld, are design decisions for a load-bearing pivot. Take them from the welding procedure document or the manufacturer, not from general advice on limiting distortion.
How do you keep distortion down?
The factors are the properties of the metal (stainless steel is more likely to distort than plain carbon steel because it expands more and conducts heat less well), how rigidly the part is held, the joint design, how well the parts fit up, and the welding procedure. The practical points:
- Fit-up should be uniform. An excessive gap increases distortion by needing more weld metal. Tack the parts so they cannot move during welding.
- Keep the weld to the minimum specified size. As a general rule weld volume should be kept to a minimum.
- Balance the welding. Weld evenly about the neutral axis where possible, and keep the time between runs short.
- Be careful with restraint. Clamping and jigs reduce distortion, but restraint raises residual stress and the risk of cracking.
What changes when the structure is a tube chassis?
The structure is thin-walled tube, and the steel may be an alloy grade, so the first rule of this article matters more: know both steels, and have a qualified person choose the procedure for that pairing. Do not carry a procedure over from mild steel to an alloy tube. Thin wall also burns through and distorts more easily than plate. For tube choice, see the chromoly tubing buyers guide.
Alignment is the other difference. Link brackets and sockets are often welded in pairs on one pin axis, and weld shrinkage can pull one socket off the line of the other. An illustration: a 0.3 mm offset between two sockets 300 mm apart tilts the common axis by 0.3 ÷ 300 = 0.001 rad, about 0.057°. The balls swivel to take that angle, but where the pin also passes through plain-bored lugs, or the sockets are pulled apart along the pin, a fraction of a millimetre is enough to make the pin bind or load one ball axially. Hold the pair as described in the next section.
Do not straighten or rework a welded chassis joint with heat unless a qualified procedure says how. Weld settings come from a qualified procedure, and a weld is acceptable only if that procedure and the vehicle maker's requirements say so. The tube adapters that hold rod ends are covered on the tube adapters page.
How do you keep it in line?
By fixing the position before the final weld. Use a fixture or jig. If two sockets share one pin, hold them on the same axis with the pin through both, tack, and check that the pin slides without forcing before you finish (K03-Q1-14). A shouldered style helps position the joint against a surface during welding; a non-shouldered one leaves the welder to set the position (Q1-01).
What do you do after welding?
Let the part cool, then check that the ball turns freely. Inspect the weld, by eye at least, for cracks and incomplete fusion; Q1-07 covers how cracks show and how they are confirmed, and K03-Q1-27 covers weld inspection in more detail. Paint, powder coating or plating go on after welding and cooling, and they are the fabricator's work.
What should you not do?
- Do not weld on an unknown steel or guess a procedure from a forum.
- Do not weld without a way to hold the position.
- Do not expect to remove it afterwards without cutting (Q1-08).