The U-bolts responsible for the connection of the spring to the axis are one of the simplest suspension elements of commercial vehicles. Despite this, they quite often generate problems for mechanics or drivers.

What should be paid on during the choosing of U-bolts?

The common opinion is that the hardness of the U-bolts is the main parameter. In fact, it is important, but the only area hardness has a great significance in the case of cooperating elements, where, for example, abrasion can occur. The incorrectly performed heat treatment can significantly increase the hardness and brittleness of an element. In the case of U-bolts such a phenomenon is undesirable. During their work, bending and tension can be observed. Therefore, the ultimate strength and the yield strength are important parameters. Unfortunately, to verify these, laboratory tests, such as a static tensile test, should be performed, such as a static tensile test, which will result in total damage to the yoke. Each producer has the opportunity to obtain the material certificate 3.1 for the steel of which the material is made. It determines material properties described above. Certificates are usually not delivered to recipients. So what should the customer pay attention to? First of all, he or she should look over the U-bolt and check if there are no cracks. The quality of the thread is also very important because it ensures only the connection. A well-made thread should be ‘full’ and its coils should end with sharp edges.
The bended surface clearly indicates whether the U-bolt was made during one of the few procedures. Multiple bending influences the change of material structure, what causes local hardening, and what can cause increased brittleness.

Should U-bolts be changed?

The U-bolt is responsible for spring fixing. Usually the spring is damaged and it should be replaced. In such a situation, the U-bolt is very often removed (of course, it is possible to unscrew the nut) and put on again with the new spring. This method is not appropriate because the yoke works as each suspension element. During the vehicle exploitation, the fatigue of material occurs. This influences very negatively the mechanical properties. It is common that the dismantling of the yoke is impossible. In such a situation, the U-bolt should be cut. The spring change with U-bolts and nuts is the appropriate solution.

Should U-bolts be maintained?

The U-bolt as the mechanical element does not need any maintenance. The only service that can be used is to check if the nuts have not been loosened. As the mechanical element, the U-bolt can be extended with the time duration (this is the material fatigue effect). In this situation, the nuts should be tightened. But such a situation is occasional, and the improper installation is the main cause of such a problem.

What nuts should be used for U-bolts installation?

The high nuts are the most popular nuts used during the U-bolt installation. Their detailed description can be found in the Polish normative PN-82155 or its German equivalent DIN6330B.Many shops offer yokes equipped with self-locking or fastening nuts. Using the nut with the liner for control is not the most beneficial solution, because the number of coils is much less than in the case of products described in the normative PN-82155. Because of that, the force acts on the smaller area, which is connected with friction occurrence. Friction is desirable because it prevents the nut from spontaneously unscrewing. In the case of minor U-bolts (M14, M16), self-locking nuts can be used, but in the case of bigger yokes (dedicated to bigger cars and semi-trailer), tall nuts are recommended. Using fastening nuts ensures a good fixation, but they are relatively expensive and their work conditions are identical to those of tall nuts.

The U-bolt was bent/buckled. What should I do?

Each U-bolt with the permanent deformation should be immediately changed to the new one. If the material was deformed and did not get the previous shape, it means that the elastic strength was exceeded. The straightening of the yoke will result in the changes of structure of material, which cause that the material will get hardened and its fragility to increase. Due to the above, in each case, permanent yoke deformation during exploitation should be changed.

Galvanised U-bolt or not?-What to choose.

The task of galvanizing is to protect the element from corrosion action. But different types of paints have similar effects. The choice is the matter of taste. For example, in the Czech Republic, galvanised U-bolts are the most popular. In Poland, Germany, Ukraine, Austria, France, galvanised U-bolts are met, but the percentage of their sale is small.

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