Travel speed

Travel speed is simple to define and hard to measure honestly, which is the entire practical story of the term. It is the rate the torch, wire or electrode advances along the joint, and it sits in the denominator of the arc energy formula — so unlike voltage and current, which come off calibrated meters on the power source, travel speed is often a nominal program value, a stopwatch estimate, or a figure copied from the last time the same joint was welded rather than measured on the run in question.

That matters because arc energy and heat input vary inversely with travel speed: doubling the speed halves the energy delivered per unit length, all else equal. An error of a fifth in the assumed speed therefore moves the calculated result further than many qualified bands allow, and because the error enters silently — nothing on the power source display flags a wrong travel speed — a weld can be reported as compliant while the real value sits outside its qualified window.

Weave technique compounds the problem in a way that is easy to miss. A weaving torch covers more distance side to side than the net advance along the joint, so the true instantaneous travel speed along the weave path is higher than the effective advance rate that actually governs heat input — and a wider weave lowers that effective rate at an unchanged program speed setting, because more of the motion is sideways rather than forward. Two welds run at the identical programmed travel speed can therefore receive different heat input purely because one used a wider weave than the other, with nothing in the parameter log showing why.

On robotic and mechanised equipment the estimate problem is largely solved, because the controller logs a real number rather than an estimate, and the value can be recorded against every weld rather than assumed constant across a shift. On a weaved program that real number still needs interpreting: whether the controller reports the weave-path speed or the net advance rate along the joint is a matter of how that particular control system is built, so a logged figure is not automatically the value that governs heat input. On manual and semi-mechanised work — and on any weaved program where that distinction matters — a thermal or vision-based weld monitoring system that reads travel speed from the moving arc position, tracking net advance along the joint rather than the weave path, is the more reliable route to a heat input number worth comparing against the WPS.