Introduction to fastening and prestressing devices in connections

• Hand tools: These tools include ring wrenches, spanners, and similar tools that are mostly used for fastening bolts in support connections. This tool is not applicable for prestressing friction joints.
• Mechanical tools: Since a worker does not have the necessary ability to prestress bolts in friction joints, mechanical tools must be used for this purpose. These tools include various tools, the most commonly used of which are:
• Hydraulic wrench: This tool works with oil pressure and is not widely used in steel structures and is mostly used in pressure vessels.
• Air wrench: In our country, this tool is most often used for steel structures. This tool has various types and sizes and works with high pressure. For this reason, an air compressor must be used to supply its power. In this method, the screws are tightened by using high-pressure air and impact.
• Electric wrench: As the name suggests, these devices work with electricity. This tool is not common in our country, perhaps due to the high cost of the tool and the high cost of supply and maintenance.
• In general, the tool for tightening and pre-tensioning screws is called Impactor. The important point when using this tool is not knowing the amount of torque created and the amount of pre-tension of the screw, which is very important in creating a correct connection during execution.
• Torque Meter:

Torque meter is the wrong combination of “torque” meaning torque and the unit “meter”, which is technically meaningless. Unfortunately, in Persian it is used instead of torque wrench or torque wrench. As mentioned, by using a hand or machine tool to tighten the screws of the structure, the amount of torque created and the resulting pre-tension cannot be obtained. Torque is any force or set of forces that causes rotation around an axis. To achieve preload in the bolts of a connection assembly, the amount of torque must be determined, which is done using a device called a torque meter. This device has an indicator that can be used to measure the amount of torque applied to the bolt.

Torque wrenches can be divided into several general categories:

Manual torque wrenches

Hydraulic torque wrenches

Pneumatic torque wrenches

Electric torque wrenches

Importance of controlling the torque applied to screws:

Although there are many different methods for attaching two or more parts to each other, connecting with bolts and nuts is one of the most important and practical connections in all industries. The purpose of connecting parts to each other with bolts and nuts is to attach two parts to each other with a tension greater than the force that wants to separate the two parts from each other.

In this case, the screw must remain in a state of constant tension and not suffer from fatigue or failure. If the tightening of the screw is less than the required amount, then the tensile stress applied to the screw will be less than the required amount and the connection will not be secure. If the torque applied to the screw is greater than the required amount, there will be greater stresses inside the screw than required and may first cause the screw to break and also damage the work surface and parts. Therefore, it is necessary to use a tool that can control and apply the torque applied to the screw and consequently the resulting tension in the screw and parts with a satisfactory tolerance. This tool is a torque wrench or torque wrench.

Types of manual torque wrenches:

Manual torque wrenches are divided into three general categories.

• Beam Style Shahini Torque Wrenches
• Click Type Torque Wrenches
• Dial Clock Torque Wrenches
• Digital Torque Wrenches

Torque wrenches are those types of manual torque wrenches that mechanically inform the operator of reaching the desired torque by hitting it.

This type of torque wrench is itself divided into types.

• Click type Torque Wrenches
• Breaking Torque Wrenches
• Slipper type Torque Wrench

Hourly and Shahini torque wrenches display the desired torque in analog form, as well as digital torque wrenches display the desired torque digitally.

Manual torque wrenches are generally limited to a maximum range of 3000 Nm and have an accuracy of 3% when applying torque. Of course, digital and hourly torque wrenches can also exist with an accuracy of up to 1%.

Of course, in another division, torque wrenches can also be divided into those with and without an index, where torque wrenches without an index are useful for the production line, and if torque wrenches without an index are used, it is necessary to use a torque tester.

MultiPlayer:

For bolts up to M20 (smaller sizes), it can be expected that the force of an ordinary worker can tighten it, but for sizes larger than that, where the worker’s force does not provide the tightening amount, a multi-purpose device, which is called a multiplier, should be used. This device has various types that are divided based on the relative index with the increase in the worker’s hand force. For example, a 1:2 multiplier doubles the worker’s hand force and a 1:5 multiplier five times the worker’s hand force. Typically, in construction projects, a combination of a torque meter and a multiplier is used to tighten and pre-tension the bolts of steel structure connections.

Prestressing methods in steel connection bolts:

Various standards have each introduced and recognized methods for prestressing bolts. One of the most authoritative standards in this field is the RCSC standard, which is a subcommittee of AISC and has introduced four practical methods for prestressing bolts:

1- Using a calibrated wrench: In this method, three samples of bolts, nuts and washers are selected and calibrated daily from each bolt. A device (Skidmore Wilhelm) is used to calibrate the samples, and with the help of a torque meter, the specific torque is set based on the specifications of the drawings or Table 10-4-4 of the Tenth Chapter of the Regulations and is used for all similar bolts used on the same day. A very important point is that the prestressing amount between the bolts is not equal to the amount of torque applied. That is, when the same amount of torque is applied to three bolt samples, the amount of preload force displayed on the Skidmore Wilhelm device is different. Studies have shown that about eighty percent of the applied torque is used to overcome the friction between the threads and the friction between the nut surface on the washer, and only less than twenty percent of the applied torque is used for preload. As a result, the calibration regulations recognize this method for each bolt on a daily basis, taking into account temperature, environmental conditions, and such issues.

2- Using special washers (DTI Washers): These washers have protrusions that are calibrated for all three bolts. The method is to place it in the connection and tighten the bolt, then by applying more force to the preload limit, the protrusions on the washer are flattened. After that, control is done by eye or using a feeler gauge, and the complete flattening of the washer indicates that the required preload for the bolt has been reached. This method does not require the use of a torque meter. Of course, there is another type of these washers that instead of a protrusion has a colored silicone capsule that bursts and exudes a red color when the required preload is reached, which makes it easy to distinguish preloaded bolts from non-preloaded bolts by eye. In order to ensure the quality of DTI washers, all the requirements of the ASTM-F959M standard must be applied in the manufacture, production and inspection of these parts.

3- Using special bolts (Twist off Bolt): This method is also called TC Bolt, this type of bolt has an additional lower part that, when the bolt is fully tightened with special wrenches, the nut is rotated clockwise, and the additional part is rotated counterclockwise, which causes the lower excess part of the bolt to be cut off, which is a sign of preload. This method is very accurate but impractical, because it requires sufficient space for the special wrench and only special wrenches should be used to tighten the screw.

4- Using nut rotation: In this method, first tighten the screws to the extent that they can be tightened, and then mark the nut body and the screw rod, then apply additional rotation to the nut to the extent specified in the regulations based on the length and diameter. According to Table 10-4-2 of the tenth topic, the rotation required for pre-tensioning the screws is given, which is only applicable to surfaces without a slope. The following table can be used for all surfaces.

In bolted connections, the holes are placed on a straight line in the direction of force or perpendicular to it in one or more rows. If the number of holes is large, the holes can be arranged in a zigzag pattern. The distance between the holes is chosen uniformly and in accordance with the diameter of the hole.