Bolt strength class: marking, GOST and tightening torque

In modern production, a lot of constructive elements are used, each of which performs a definite function in the final product, most often very important. Therefore, one of the tasks in the design and manufacture of complex composite structures is the reliable fixation of parts relative to each other.

Purpose of using the bolt

There are many options for joining two products, for example, welding, soldering, gluing, using rivets. However, they have one common and rather significant drawback - they are all-in-one. More precisely, they are detachable, but to separate the parts they will have to be deformed, cut, and so on.

It is much more convenient to use a threaded connection as a fastener, selecting the bolt strength class in advance, depending on the load imposed on the node.

Thread is used in almost all designs, which are slightly more complicated than a regular teaspoon. Just imagine a car that does not have a single screw. Yes there is a car - in a children's toy, whose strength class is much lower, you still can not do without threaded elements.

The advantages of a threaded connection

Despite the fact that sometimes a large tightening torque does not allow unscrewing the nut after some time, the bolt still has a number of advantages over other ways of fastening:

1. High connection reliability, provided by the universality of the profile of metric or any other thread. The profile is developed due to a lot of research, therefore it withstands high loads and is protected from self-unscrewing (however, for this it is necessary to choose the correct torque).
2. Ease of installation and dismantling of structures. It is provided with the use of a unified service tool - keys, wrenches, which can not be said, for example, about welding or riveting connection.
3. The possibility of creating large axial and transverse loads. The strength of the bolt is calculated both in the longitudinal and in the transverse direction. Modern materials and technologies allow reducing the number of fasteners used and reducing its size.
4. A small cost, especially if you compare it with the cost of purchasing welding materials and carrying out work.

Among the drawbacks of the joint is the concentration of stresses in the region of the valley of the thread profile. Therefore, for a certain type of construction, it is necessary to choose the correct bolt strength class in accordance with the applied load. Also, to ensure the reliability of the threaded connection, it is worth remembering the use of stop means, for example, spring washers or locknuts.

Types of threaded connection

In order for the screw connection to exist, it is necessary to make an internal thread on one part, and on the outer one. Depending on the design features, three varieties can be used:

1. Screw connection. In this case, the part of the nut fulfills the role of the nut (part with internal thread). First a hole is drilled in it. And then the thread is applied. A different one is applied to the part with a smooth round hole, and then it is attracted by a screw.

2. Bolted. Here everything is much simpler and more reliable: in both connected parts, smooth holes are drilled, a bolt is inserted in them, and a nut is attached to the back.

3. On the studs, one end of which is screwed into the part of the knot, and on the second - the nut is wound.

Strength class of bolt

As noted above, the fasteners must be properly selected. It makes no sense in the usual frame of the shelf for service tools to use parts from alloy steel. At the same time, some materials with increased mechanical characteristics are required in some flange connections (for example, pulling together the components of multiton metal structures).

In general, the bolt strength class (GOST 1759.4-87) is a whole complex of mechanical characteristics that simultaneously includes the temporary resistance, yield strength, the relative elongation after the fracture, the hardness of the material, and in some cases even the toughness. As a rule, two digits separated by a period are used to designate this parameter. The first of them, after multiplying by 10, shows the value of the minimum of the time resistance, and the second, also increased by a factor of 10, demonstrates the proportion obtained by dividing the yield strength of the material by the time resistance. The product of these two figures will indicate the value of the minimum yield stress. The bolt's strength class is contained in its marking, which looks like this: M12x1.25x60.58, where 58 is the two-digit number.

Tightening bolts

In order for the threaded connection to hold the structural elements securely, it is worth paying close attention to the torque (torque). So, imagine a situation where an inexperienced car mechanic first gets under the hood of his own new car and tries to pinch the nut or screw as much as possible. All this can end in the first case simply by inconvenient work of a chisel, and in the second - by drilling a hole in the body. This is due to incorrectly selected tightening torque. Experienced car mechanics, and simply repairmen, have a dynamometer built into the hand. But the beginner is best to use a torque wrench or a pneumatic wrench, which are adjusted for a certain tightening torque.

How to unscrew the "complex" bolt

Let's say it turned out all right. But time passes and the fasteners need to be unscrewed again. Because of corrosion, this is sometimes not very easy, and a high class of bolt strength does not save it from destruction. Therefore, it is worth keeping in mind a few simple ways that simplify the unwinding:

1. To begin with, you should use "VaDashka". The composition dissolves the rust layer.

2. Gently tap on the nut with a hammer to break the rust in the thread profile.

3. You can try to twist the nut a couple of degrees in the direction of twisting, and then unscrew it.

It is important not to rush, as there is a high probability of remaining with the nut or screw head in the key.