GaugePass

Welding math

Travel Speed

Travel speed is the one welding parameter set entirely by the welder's hand rather than the machine, and it is also the parameter that heat input is most sensitive to, which makes measuring it worth the stopwatch.

The formula

v = distance / time, or v = (V x I x 60) / (1000 x HI)

Travel speed in inches per minute is the length of weld divided by the time in minutes to make it, or, when a heat input target is set, arc volts times amperes times sixty divided by one thousand times that target.

Work it out

Measure a run with a tape and a stopwatch, and see the heat input it lands on.

Travel speed
12.00 in/min
In millimetres
304.8 mm/min
Heat input at that speed
26.40 kJ/in

The same arithmetic as the worked examples above, on your own numbers. It reports geometry and physics, never an acceptance limit: what a joint is allowed to be comes from the job's own code and contract documents.

What each symbol means

SymbolMeaningUnits
vTravel speed of the arc along the jointin/min
LLength of weld run in a timed observationin
tElapsed arc-on time for that lengths or min
HIHeat input the procedure is aiming atkJ/in

Why it works

Measuring travel speed is straightforward but rarely done well. Mark a start and stop on the joint, time the arc between them, and divide the length by the time. Convert the time to minutes and the answer is in inches per minute. The discipline is in timing arc-on time only: if the welder stops to reposition mid-run, that pause does not belong in the measurement.

Travel speed also works backwards. If a procedure caps heat input, the volts and amps are already fixed by the wire size and the transfer mode, so the only free variable left is speed. Rearranging the heat input formula gives the slowest speed that still respects the cap, which is a genuinely useful number to hand a welder before a qualification test rather than after.

The two forms answer different questions. The measured form tells you what a welder is actually doing, which is what a procedure qualification record has to capture. The back-solved form tells you what a welder ought to do, which is what a welding procedure specification has to state. Comparing the two is often how a shop discovers its production welds run hotter than its qualified procedure.

Worked examples

What you are given, what you do with it, and what the answer actually tells you. Cover the steps and try it before you read them.

  1. Timing a production pass

    Given

    Length of weld
    18 in
    Arc-on time
    90 s

    Working

    1. Convert the time to minutes: 90 / 60 = 1.5 minutes.
    2. Divide the length by the time in minutes: 18 / 1.5.

    Result

    12 in/min

    Twelve inches per minute is a brisk but ordinary semi-automatic speed. Recorded on a procedure qualification record, this is the number that later production has to be judged against.

  2. Back-solving from a heat input cap

    Given

    Arc voltage
    25 V
    Welding current
    240 A
    Heat input target
    40 kJ/in

    Working

    1. Compute arc power in joules per minute: 25 x 240 x 60 = 360,000.
    2. Divide by 1,000 to reach kilojoules per minute: 360 kJ/min.
    3. Divide by the heat input target to find how many inches that energy must be spread over: 360 / 40.

    Result

    9 in/min

    At these machine settings the welder must keep moving at nine inches per minute or faster to stay under the cap. Anything slower puts more than forty kilojoules into each inch.

In practice

  • Travel speed and wire feed speed are different things and get confused constantly. Wire feed speed is how fast filler leaves the gun; travel speed is how fast the gun moves along the joint.
  • On a heat input limited procedure, the back-solved speed is a minimum, not a target. Welders should be given a range with the fast end bounded by fusion quality rather than a single number.
  • Mechanised carriages hold travel speed far better than a hand does, which is why automated welds show much tighter heat input distributions even when the average matches.
  • If a timed measurement disagrees with the machine's readout on an automatic setup, trust the tape and the stopwatch. Carriage calibration drifts.

Related topics

Heat InputHeat input packages volts, amps, and travel speed into one number that predicts how hot the joint gets and how slowly it will cool, which is why procedures control it rather than the three parameters separately.Arc Energy and Arc EfficiencyNot all the energy an arc produces ends up in the steel, and the fraction that does depends heavily on the process, which is why the same gross heat input means different things under a flux blanket and under an open tungsten arc.Deposition RateDeposition rate measures how fast a process puts metal in the joint while the arc is actually burning, which is only half the productivity picture and often the less important half.Operating Factor and Arc TimeOperating factor is the fraction of a paid shift during which the arc is actually burning, and because it varies far more between shops than deposition rate does, it is usually where the real productivity difference lives.