High-strength nuts are high-strength bolts and belong to a type of standard parts. Generally, high-strength bolts can bear a greater load than ordinary bolts of the same specification.
Overview of high-strength nuts
The shape and connection structure of high-strength nuts are basically the same as those of ordinary bolts. The main difference between the two is that the connection of ordinary bolts relies on the compression and shear resistance of the bolt body to transfer shear force.
When tightening the nut, the pre-tension force generated by the bolt is very small and its influence is not considered. The working principle of high-strength nut connection is to intentionally apply a large pre-tension force to the bolt, causing a compressive force between the contact surfaces of the connected parts, thus generating a large friction force in the direction perpendicular to the bolt, and relying on this friction force to transfer the connection shear force.
The pre-tension force of high-strength nuts is achieved by tightening the nut. For ordinary high-strength nuts, the torque method and the angle method are generally used. For the twist-off type high-strength nuts, the pre-tension force is controlled by twisting off the tail of the bolt.
The bolts used in high-strength nut connections are made of 10.9S grade or 8.8S grade high-quality alloy structural steel and undergo heat treatment. The holes for high-strength bolts should be drilled. The hole diameter for friction-type high-strength bolts is 1.5 to 2.0 mm larger than the nominal diameter d of the bolt; the hole diameter for bearing-type high-strength bolts is 1.0 to 1.5 mm larger than the nominal diameter d of the bolt.
Differences between high-strength nuts
Generally, high-strength bolts can bear a greater load than ordinary bolts of the same specification.
The material of ordinary bolts is Q235 (i.e., A3).
The material of high-strength bolts is 45# steel, alloy steel (such as 20MnTiB, 35VB), or other high-quality materials, and they are made after heat treatment, which increases their strength.
The difference between the two lies in the material strength.
First point: From the perspective of raw materials:
High-strength bolts are made of high-strength materials. The bolts, nuts, and washers of high-strength nuts are all made of high-strength steel, commonly 45# steel, 40 boron steel, and 20 manganese titanium boron steel. Ordinary bolts are usually made of Q235 steel.
Second point: From the perspective of strength grades:
Classification of high-strength nuts
According to the force state, they are divided into: friction type and bearing type.
In fact, there is a difference in the design calculation methods. For friction-type high-strength nuts, the appearance of sliding between the plates is taken as the ultimate limit state of bearing capacity; for bearing-type high-strength nuts, the appearance of sliding between the plates is taken as the normal service limit state, and the connection failure is taken as the ultimate limit state of bearing capacity.
Friction-type high-strength nuts cannot fully utilize the potential of the bolts. In practical applications, for very important structures or structures subjected to dynamic loads, especially when the load causes reverse stress, friction-type high-strength bolts should be used.
At this time, the unutilized potential of the bolts can be used as a safety reserve. In other cases, bearing-type high-strength nut connections should be used to reduce costs.
According to the construction process, they are divided into: twist-off type high-strength nuts and large hexagon high-strength nuts.
Large hexagon high-strength nuts belong to the high-strength grade of ordinary screws, while twist-off type high-strength nuts are an improved version of large hexagon high-strength nuts for better construction.
The construction of high-strength nuts must be done in two steps: initial tightening and final tightening. For initial tightening of high-strength nuts, impact-type electric wrenches or torque-adjustable electric wrenches should be used; for final tightening of high-strength nuts, there are strict requirements.
For final tightening of twist-off type high-strength nuts, a twist-off type electric wrench must be used, and for final tightening of torque-type high-strength nuts, a torque-type electric wrench must be used.
Large hexagon high-strength bolts consist of one bolt, one nut, and two washers. Twist-off type high-strength nuts consist of one bolt, one nut, and one washer.
High-strength nuts are increasingly widely used. Common strength grades are 8.8 and 10.9, with 10.9 grade being the most common. The strength grades of ordinary bolts are lower, generally 4.4 grade, 4.8 grade, and 5.6 grade. (8.8S and 10.9S are specifically for steel structures.)
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