PRECISION FASTENING SOLUTIONS
LEADING MANUFACTURERS & EXPORTERS
PRECISION FASTENING SOLUTIONS
LEADING MANUFACTURERS & EXPORTERS
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Though structural bolts might appear to be basic parts, they function efficiently if installed properly. In any steel structure, a bolt does not only connect two pieces of steel. The act of tightening them actually applies a certain amount of force that compresses the connected parts.
It is because of this that over-tightening and under-tightening of the bolt could lead to some problems. Under-tightening may cause looseness, while over-tightening may unnecessarily put strain on the bolt and the connected parts.
Knowledge of this would be very useful in installing high strength bolts for use in bridges, buildings, machines, towers, and other structures.
Tension is produced in the bolt when it is tightened. The tension produced in the bolt produces the clamping force between the joint members. The tension produced in the beginning is referred to as bolt preload.
The main objective of bolt preload is to ensure that the joint remains clamped during the service period.
The required preload depends on several factors, including:
Therefore, a structural bolt should not simply be tightened as much as possible. It needs to be installed according to the specified requirements.
The condition of under-tightening results in the fact that the bolt does not have the necessary clamping force. At first sight, everything looks fine, but issues may occur later due to vibrations of the structure.
Insufficient preload can allow connected components to move relative to each other. In structural connections where movement needs to be controlled, this can affect the performance of the joint.
When a joint experiences vibration or repeated loading, inadequate clamping force can increase the possibility of loosening. Once the connection loses its intended clamping condition, the load may no longer be distributed as expected.
In connections designed to resist slip, maintaining the required clamping force is particularly important. If the available friction between the connected surfaces is insufficient, relative movement can occur.
This is one reason correct installation is especially important for high-strength friction grip connections.
A properly designed connection relies on the fasteners and connected components working together. If one or more bolts are not tightened correctly, the load distribution across the connection can change.
This can place additional stress on other components of the joint.
Over-tightening occurs when a bolt is tightened beyond the specified requirement. While it may seem that a tighter bolt creates a stronger connection, this is not necessarily the case.
A bolt is designed to operate within a particular range of stress. Excessive tightening increases tensile stress in the bolt and can reduce the margin between normal operating conditions and failure.
Applying excessive force can damage the bolt or nut threads. Damaged threads can make proper assembly difficult and may affect the ability of the fastener to maintain the required connection.
Excessive tightening can permanently stretch a fastener. Once a bolt has undergone unacceptable deformation, it may no longer perform according to its intended mechanical properties.
Over-tightening does not only affect the bolt. Excessive clamping force can also damage washers, threads, joint surfaces or other connected components, depending on the application.
Both conditions can compromise a bolted connection, but they do so in different ways.
Under-tightening generally results in insufficient clamping force. This can contribute to movement, loosening and slip.
Over-tightening can create excessive stress in the fastener and may cause deformation or damage to the fastener and joint.
The objective is therefore not to make the bolt as tight as possible. The objective is to achieve the specified installation condition.
The correct tightening requirement is not the same for every structural bolt. Engineers and installers consider the design and specifications of the particular connection.
Important factors can include:
Different fastener grades have different mechanical properties. A larger or higher-strength bolt may have different installation requirements from a smaller or lower-strength fastener.
The type of connection, expected loads and role of the fastener all influence the required installation condition.
Friction between threads and between the bearing surfaces can affect the relationship between applied torque and achieved preload.
This is why a torque value should not be treated as a universal measurement of bolt tension.
Depending on the connection, installation may involve a specified torque procedure, tension-control method or another approved technique.
The method should always follow the applicable project requirements and fastener specifications.
Even a high-quality fastener can perform poorly if it is installed incorrectly. Some common installation mistakes include:
Installation procedures should be established before work begins, particularly for critical structural connections.
Bolted joint performance is not only determined by the strength of the single bolt. The bolt itself, the nut, the washer, the material and the method used in assembly make up the connection.
A good fastener assembly ensures that the joint maintains the desired clamping and loading conditions.
It is for this reason that structural fasteners must be designed as connections and not just as tightening of nuts on bolts.
A structural bolt should not be either loosely driven or excessively tightened. Loose tightening will yield inadequate clamping force, whereas over-tightening will cause excessive stressing, which may even damage the fastener or joint.
The proper way is to comply with the installation instructions of the specific fastener and connection.
In case of critical applications, one must always adhere to the design specifications, fastener specifications, and standards prior to installation.
