Bolt strength class: marking, GOST and tightening torque

2024 Author: Howard Calhoun | [email protected]. Last modified: 2023-12-17 10:16

In modern production, many structural elements are used, each of which performs a specific function in the final product, most often a very important one. 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 connecting two products, such as welding, soldering, gluing, using rivets. However, they have one common and rather significant drawback - they are all one-piece. More precisely, detachable, however, to separate the parts, you will have to deform them, cut them, and so on.

It is much more convenient to use a threaded connection as a fastener, having previously selected the strength class of the bolt, depending on the load placed on the assembly.

Thread is used in almost all designs that are a little more complicated than a regular teaspoon. Just imagine a car without a single screw. Why is there a car - in a children's toy, the structural strength class of which is much lower, you still cannot do without threaded elements.

Advantages of threaded connection

Despitethe fact that sometimes a large tightening torque does not allow unhindering the nut after a while, the bolt still has a number of advantages over other fastening methods:

1. High reliability of the connection provided by the versatility of the profile of metric or any other thread. The profile was developed thanks to a lot of research, therefore it can withstand heavy loads and is protected from self-unscrewing (although for this you will have to choose the right tightening torque).
2. Ease of installation and dismantling of structures. It is provided by the use of a unified service tool - keys, wrenches, which cannot be said, for example, about welding or riveting.
3. Ability to create 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 reduce the number of fasteners used and reduce their size.
4. Low cost, especially when compared to the cost of buying welding consumables and work.

Among the disadvantages of the connection is the concentration of stresses in the region of the root of the thread profile. Therefore, for a certain type of construction, it is necessary to correctly select the strength class of the bolt according to the applied load. Also, to ensure the reliability of the threaded connection, it is worth remembering the use of locking tools, such as spring washers or locknuts.

Types of threaded connection

To screw connectionexisted, it is necessary to make an internal thread on one part, and an external thread on the second. Depending on the design features, three varieties can be used:

1. Screw connection. In this case, the part of the assembly plays the role of a nut (parts with an internal thread). A hole is drilled in it first. And then the carving is applied. Another one with a smooth round hole is applied to the part, after which it is attracted with a screw.

2. Bolt. Everything is much simpler and more reliable here: smooth holes are drilled in both parts to be joined, a bolt is inserted into them, and a nut is inserted on the reverse side.

3. On studs, one end of which is screwed into the part of the assembly, and a nut is screwed on the second.

Bolt strength class

As noted above, fasteners must be properly selected. It makes no sense to use alloy steel parts in a conventional service tool shelf frame. At the same time, in some flanged connections (for example, tightening the components of multi-ton metal structures) it is required to use materials with improved mechanical characteristics.

In general, the strength class of a bolt (GOST 1759.4-87) is a whole complex of mechanical characteristics, which simultaneously includes tensile strength, yield strength, relative elongation after rupture, material hardness, and in some cases even impact strength. As a rule, two digits separated by a dot are used to designate this parameter. The first of them, after multiplying by10 shows the value of the minimum tensile strength, and the second, also increased by 10 times, shows the proportion obtained by dividing the yield strength of the material by the tensile strength. The product of these two numbers will indicate the value of the minimum yield strength. The strength class of the bolt is contained in its marking, which looks like this: M12x1, 25x60.58, where 58 is the same two-digit number.

Tightening bolts

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

How to unscrew the "complex" bolt

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

1. First you need to use "Vedashka". The composition dissolves the rust layer.

2. Tap the nut gently with a hammer to break up the rust in the thread profile.

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

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