Common Problems with Bolted Connections

Common problems with bolted connections are divided into two main groups:

1- Problems encountered during bolting:

• Bolt breakage
• Thread stripping (scraping)
• Plastic deformation and crushing of connected parts
• Insufficient bolt tightening: parts moving apart, parts sliding relative to each other, bolts loosening and fatigue failure

2- Problems caused after bolting

• Static bolt breakage
• Bolt failure due to fatigue
• Delayed bolt failure
• Cracking due to corrosion and stress caused by it
• Parts moving apart: causing leakage and fatigue failure
• Parts sliding relative to each other:

 Rotational loosening

 Non-rotational loosening due to wear

• Fatigue failure due to bending of bolts
• Bolt loosening

Reduction of tensile stress in the bolt

• Parts moving apart until they separate
• Sliding of connected parts and Their displacement
• Noise and leakage
• Reduced connection strength and vibration
• Fatigue failure

3- Problems caused when tightening bolts

(a) Bolt breakage during tightening

Sometimes bolts break when tightening them with a wrench. This situation occurs when the tightening force (torque applied) is excessive. Also, if the threads are completely lubricated, due to reduced friction, most of the torque may be converted into longitudinal stress and the bolt breaks.

(b) Thread stripping (screw stripping)

In this case, all the threads of the bolt or nut that are engaged with each other are cut and separated from the main body of the bolt or nut. In this defect, the bolt body does not break, but due to the stripping of the threads, the bolt is separated from the nut or there is no acceptable connection between them at all. This problem may occur when the bolt or nut is made of a weaker material (for example, one is made of steel and the other is made of aluminum) or when the length of engagement between the bolt and the nut is short.

(C) Plastic deformation of the connected parts

Sometimes when a bolted connection is made, the connected parts may undergo plastic deformation, buckling, or crushing. This problem occurs when the compressive strength of the parts is low, or the shape of the parts is such that they are prone to buckling, or the force applied to the bolts and parts exceeds their resistance.

(d) Insufficient tightening of the parts

When sufficient torque is not applied when tightening the bolt, there is not enough longitudinal tensile force in it. It will not be enough to hold the connection. If for some reason the bolts and nuts stick together or if the threads are not lubricated enough (and as a result the friction between them is higher than expected), the bolted parts may not be tightened enough. This is one of the most serious problems that can cause a lot of trouble during the operation of the bolted connection. For example, the connected parts may gradually move apart. The parts may slip relative to each other, the bolts may loosen, and fatigue failure may occur in the bolts.

4- Problems caused after tightening the bolted connection

(A) Static bolt failure

This type of failure may occur suddenly and as a result of a strong impact or shock. These impacts do not occur in normal operation, but rather, for example, in accidents or other unexpected events, causing the bolts to break.

(B) Fatigue failure

Fatigue occurs as a result of repeated loads being applied to the parts, while the amount of applied load is less than the tensile strength of the parts. Therefore, this defect does not occur immediately, but rather repetitive loads (which must be greater than a certain limit) gradually cause failure in the part. In the case of bolts, both longitudinal fatigue and bending fatigue failure occur. This defect is also considered a serious problem in bolted connections.

(C) Delayed failure due to bolt brittleness

In this case, the bolt also fails immediately, but after a certain period of time when the connection is tightened and the system is used, sometimes in a bolted connection after tightening, when it is left unused in one place, such a failure also occurs in some of its bolts. The time before the bolt breaks may vary from a few hours to a few years. This defect occurs when hard bolts with a strength of, for example, higher than 1300 N/mm are used in the connection and the bolts are tightened well.

(D) Stress corrosion failure

This type of failure is almost similar to delayed failure and occurs when a bolted connection is simultaneously exposed to tensile stress and a corrosive environment. In such a situation, even if no force is applied to the bolted connection, cracks may occur in the parts due to corrosion and the stress in the bolt will lead to crack propagation.

(E) Spacing of the connecting parts

When the external load applied to the connection is greater than the tolerance of the bolts, the parts may separate. This phenomenon is likely to cause leakage in the system. Also, continuing this situation can cause fatigue failure in the bolts. Therefore, it is very important to calculate the parts and bolts in the design of bolted connections in such a way that such a situation does not occur.

(F) Slippage of parts relative to each other

When large shear loads are applied to the bolted connection, the parts secured by the bolts may slide relative to each other. This phenomenon occurs when the applied shear load is greater than the friction force between the connected parts. When parts slide relative to each other, the bolts may loosen quickly, causing wear between the parts, and as a result of the continuation and repetition of these slippages, bending fatigue failure in the bolts may occur. This defect is also considered a serious and damaging defect in bolted joints, and care must be taken during design to prevent slippage between the parts.

(G) Loosening of bolts (decrease in their longitudinal tension)

Sometimes the tensile force that is created in the bolts when they are tightened decreases and gradually causes them to loosen. This defect may occur, for example, due to slippage between the connected parts, wear between the parts, or plastic deformation. The initial tensile force created in the bolt is the most important factor affecting the optimal performance of the bolted connection. A decrease in that force for any reason causes the bolt to loosen and disrupts the reliability of the system.

Bolt loosening can be divided into two groups: fast rotational loosening and slow non-rotational loosening. Bolt loosening will cause the parts to move apart, cause slippage between them, cause shock and impact in the system, reduce the strength of the connection, vibration, fatigue failure, and leakage. Bolt loosening is the most serious problem in bolted connections.

Characteristics and causes of problems

As mentioned in the previous sections, problems in bolted connections can be divided into two groups:

Problems that occur when tightening the bolts and problems that occur after tightening the bolts (during system operation). If a problem occurs during tightening the bolts, it will be detected at the same time and can probably be solved with confidence. However, problems that occur after tightening the bolts will cause a lot of trouble and costs to the customer and user of the system. To prevent such problems, firstly, the bolted connection must be designed correctly and secondly, the assembly and tightening of the bolts and nuts must be carried out with great care and precision.

The causes of problems during tightening the bolts can be, for example, applying excessive torque to the bolts or making a mistake in estimating the amount of friction between the threads. The causes of problems after tightening bolts are usually due to failure to properly check the initial conditions or errors in calculations and design. The specifications of the bolted parts must be determined in such a way that none of the problems mentioned above occur.