The Principle of Expansion Bolts

The principle of fixing expansion bolts consists of two main aspects: material expansion principle and friction principle.

First of all, expansion bolts are a kind of fasteners used to form threads in materials. Its principle is to utilize the thermal expansion property of the material, adding a section of material with high thermal expansion properties as a sealant in the screw. When the screw is subjected to heat, the sealant will expand and fill the thread gap, thus forming a tight connection.

The specific principles are as follows:
Screw: Expansion screws are usually composed of two parts, a standard threaded structure at the top for connecting to other objects, and a section of hollow tubular structure at the bottom for storing the sealant.
Sealant: The sealant of an expansion screw is usually a material with good high temperature expansion properties, such as polymer material or metal powder. At low temperatures, the sealant usually exists in a solid form, small size, can pass through the thread clearance. When subjected to heat, the sealant expands, increases in size and fills the thread gap.
Fastening process: Expansion screws are inserted into holes or assembly holes and then rotated to connect them to other objects. During the tightening process, due to the rotational force of the screw, the threads will push the sealant into the thread gap. When the screw reaches a certain tightening torque, the sealant will closely fill the thread gap and form a layer of protective gasket on the contact surface to realize the tightening and sealing effect.
Thermal expansion: When the screw is subjected to heat (e.g., in a high-temperature environment), the sealant will increase in size due to thermal expansion. Due to the tight connection between the screw and other objects, the tightening force will prevent the screw from flowing, however, due to the expansion properties of the expansion agent, it will fill the thread gap and increase the tightening force.

                  

 

Secondly, expansion screws can also be fixed by friction grip. Expansion screws consist of components such as a screw and an expansion tube. The end of the screw is conical and the inner diameter of the cone is larger than the inner diameter of the expansion tube. When the nut is tightened, the screw moves outward, and the axial movement of the thread makes the cone partially move, which in turn forms a large positive pressure on the peripheral surface of the expansion tube, coupled with the small angle of the cone, thus forming a friction self-locking between the wall, the expansion tube and the cone to achieve the fixed effect.
 
The fixing effect of the expansion screw mainly depends on the friction generated by the wedge-shaped slope. When the nut is tightened, the taper of the bolt opens up the expansion tube and increases the contact area with the substrate, thus increasing the friction and ensuring a firm fixing. This fixing method is suitable for a wide range of materials, such as concrete and brick, and is commonly used to install guardrail, awning, air conditioning and so on.
 
The spring washer on the expansion screw is a standard part, which is called spring washer because its opening is staggered and elastic. The function of the spring washer is to use the sharp point of the staggered opening to pierce into the nut and flat washer to prevent the nut from loosening. Flat washers are also standard parts, and their function is to distribute the pressure of the nut evenly over the connected parts.