 Corrective maintenance 3
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A straightening bench is an assembly of very heavy steel parts that is much stronger overall than a truck chassis. It usually forms a rectangle of steel girders with cross braces and usually provides the basis for a large
number of accessories. The body often has to be stripped a little from below, but there are also systems where it is adjusted in such a way that changes only have to be made in a few crucial areas of the underbody.
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For almost every car there is an instruction or drawing (picture below) so that the holding points of the body are also control points. If it does not fit at a certain point, then there is likely to be damage here. In addition, the
appropriate attachment points are usually so stable that you can pull on the body to be able to draw this remaining point to the right place.
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In principle, it's about ensuring that the attachments for the front and rear axles are still where they belong. This guarantees that even with an axle that no longer exists today, the connection between the wheel centers at the
front and rear is parallel, apart from very few differently designed cars. This means that the rear axle follows the front axle when driving straight ahead, either exactly on track or with the same deviation to the left and right.
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The rest of the body is not completely irrelevant, but if the air gaps are right here and, for example, the hoods can be replaced later without body work, one is satisfied. After all, the area is still often part of the crash zone.
However, if even one air gap cannot be restored through adjustment work, this is often due to a larger total distortion.
| Tolerance when working on the straightening bench: 1 to 2 mm |
So an improperly executed repair would be one that used tricks to restore air gaps. With the straightening bench you have the opportunity to draw points that are not aligned and then the air gaps are often correct again. So it
is
quite possible that botched repairs on a bench will be exposed and have to be repaired at the same time.
| Of course, the deviation in the Z direction is just as important as that in the X and Y directions. |
It has only recently become established that such a straightening bench with a universal straightening set can also be raised hydraulically, so to speak as a replacement for a lifting platform. Other companies, on the other
hand, rely on anchors in the ground to which a vehicle can be chained and pillars can be clamped for pulling. Simultaneous pulling in different directions is also possible.
The advantage is that the workshop floor can also be used for other repairs. Real vintage cars, which appeared before the first straightening benches appeared in the 1960s, also pose a problem. There are no plans.
Sometimes it wasn't worthwhile for the manufacturers to draw up such plans and the corresponding accessories, e.g. for small cars.
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Nevertheless, such vehicles can be pulled with so-called dozers (picture above). The starting point is then usually the folds under the longitudinal spars, on which with tension clamps their massive cross connections can
support such dozers. Perhaps less than 10 to 20 tons will suffice. In any case, such folds are surprisingly stable.
At this point, a word about how helpful the hydraulics are in all the attempts to pull the body back into the shape it was in before the accident. It seems to be able to transmit exactly the necessary power without this happening
too directly, so that it comes to jerks. The whole thing can be dosed, especially because you always have to overdo it a little to end up in the right position.
Not only is it important to use these great forces, but also to stabilize the result, for example by hitting it with a hammer. The kink is pulled apart and at the same time the kink itself is repaired, which also somewhat prevents
the sheet metal from returning. This also includes targeted stretching or extending of the sheet metal.
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There is one group of vehicles that could particularly benefit from a straightening bench, convertibles and others without a stabilizing roof. If, for example, work is carried out on the longitudinal beams, the entire structure is
seriously endangered. Overall, you work by using the straightening bench with the aim of restoring the body structure similar to that of the new car.
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