Straightening and Pulling Operations
The operating principle
Straightening reverses the collision. Pull along the line of the original impact from a properly anchored vehicle, using several controlled pulls rather than one large one. Overpulling work-hardens the steel and creates new damage that then has to be pushed back, which compounds the problem.
Setup and control
- Anchor at the strongest undamaged point opposite the pull direction.
- Match pull direction to the damage direction; use a combination pull when displacement exists in more than one plane.
- Measure while pulling and again after release to quantify springback.
- Heat only where the OEM permits and only within stated limits. Most AHSS, and all press-hardened steel, must not be heated, because heat alters the microstructure and removes the strength the part was designed to provide.
- Sequence the job: rough pull, measure, replace or section damaged parts, then final alignment and welding.
Worked example
A rail shows both sway and sag. Anchor the rear of the vehicle, attach at the rail horn, and pull laterally with a slight upward component because the rail also sits low. Pull in small increments and check reference points each time. Stop when the points are inside tolerance. Continuing past center moves the rail the other way, and the extra work-hardening makes the final alignment harder to hold.
Traps that cost marks
Heating high-strength steel to make it move; pulling without measuring; anchoring on damaged metal; welding while stress remains in the structure; using one oversized pull instead of staged pulls.