Threaded fasteners work by tightening and allowing both the bolt and nut materials to elongate against each other, creating a clamping force that holds two or more parts together. However, this clamping action causes stress on the fasteners, which can lead to disastrous material failure if not properly managed. This article tackles one of the common causes of fastener failure called thread stripping.
Stripping Of Threads
Material failure in a bolt and nut assembly may occur in either of the following modes:
- Breakage at the shank or the threaded section of the bolt due to the combined tensile and torsional stress caused by the elongation strain and the torque acting on the threads
- Failure at the threads due to the shear stress caused by tightening of the threads within the engagement length
Thread stripping happens when the shear stress experienced in the thread helix surpasses the shear strength of either the bolt or the nut material used. Thus, the thread is damaged or ripped off, as shown in the illustration of a partially stripped thread below.
Modes Of Thread Stripping Failure
There are three possible scenarios for thread stripping:
- If the bolt has a higher shear strength than the nut, failure will likely occur at the root of the nut threads.
- If the nut has a higher shear strength than the bolt, failure will likely occur at the root of the bolt threads.
- If the bolt and nut have equal strength, failure will likely occur at the pitch line between the threads.
However, nuts are generally designed to have lower strength and hardness values than bolts of the same grade. For example, the hardness of a grade 8 nut would likely be lower than that of a grade 8 bolt. This is because, in a fastened assembly, it is necessary for the nut to yield first to ensure that the first thread does not carry the whole load solely; instead, it is distributed across the succeeding threads.
Causes Of Thread Stripping
Thread stripping is a mode of failure that needs to be avoided at all costs. In general, fasteners are designed so that the possibility of failure due to breakage of the bolt body is far greater than the possibility of thread stripping.
This is because, when a bolt breaks, it can be easily detected during assembly. Hence, it is easy to correct before the fastener is put in service.
On the other hand, thread stripping is a gradual type of failure and is hard to detect during assembly. It starts at the first thread and then gradually spreads throughout the entire thread engagement length until it breaks.
Once thread stripping occurs, it is like a ticking time bomb waiting to explode. This makes it a notorious source of failure which can lead to serious damage during operation.
There are many possible causes of thread stripping— all of which converge to the fact that thread stripping occurs when the stress induced exceeds the shear strength of the fastener material. These possible causes include thread bending, nut dilation, bell-mouthing, and insufficient engagement length.
As the fasteners are tightened, the bolt elongates and the nut compresses. This causes the threads to deform and the nuts to dilate because of the radial wedging action between the internal and external threads. In general, the dilation is greater for nuts with lower yield strength and thinner walls.
The problem is that dilation typically occurs at the location of the first thread, which is also the thread that bears most of the load. As the nut dilates, the length of engagement of this thread decreases, thereby reducing the shear area and increasing the stress experienced by the thread.
Nut dilation can further be intensified by thread bending. When the strength of the bolt and the nut material is approximately equal, the bending of the threads caused by the axial load can amplify the effect of nut dilation and further increase the shear stress on the threads.
Bell-mouthing is when there is a slight taper on a threaded hole, which can extend up to about half the diameter of the hole. This is primarily caused by the instability and flexibility of the drill point during the drilling process.
When tapped, bell-mouthing results in a variable thread height along the length of the tapered hole. These variations in height cause a significant reduction in the shear area, especially in short lengths of engagement and fine pitches. Thus, it also increases the induced shear stress and the possibility of thread stripping.
To minimize bell-mouthing, it is necessary to use well-aligned, sharp, and rigid drill bushes to minimize tapering.
Insufficient Thread Engagement Length
The principal reason for thread stripping is when there is not enough thread engagement length to handle the shear load. This is because the cross-sectional area through which the shear occurs is directly proportional to the thread engagement length, as shown in the equation below:
- As = cross-sectional shear area [in2]
- n = thread per inch [in-1]
- Le = length of thread engagement [in]
- Dm = minimum major diameter of external threads [in]
- Dp = maximum pitch diameter of internal threads [in]
This formula assumes shearing at the roots of the threads.
It is absolutely necessary to select the correct length of thread engagement to avoid thread stripping and provide a safety factor to ensure that, in the event a failure occurs, tensile fracture happens before thread stripping.
Shear Stripping Strength
The stripping strength of an assembly depends on the shear strength of the nut and bolt materials. Whichever has the lesser value will dictate the final shear strength of the assembly, and indicate where shear stripping is most likely to occur.
It is important to note that, unlike tensile and yield strengths, there are no published shear strength values for the different bolt and nut materials. For design purposes, the Industrial Fastener Institute states that the shear strengths of carbon steel fasteners may be assumed to be approximately 60 percent of their specified minimum tensile strengths (or proof stress for nuts).
Calculating The Thread Stripping Force
Based on the cross-sectional shear area and the shear strength of the bolt and nut materials, the maximum shear load that the fastener can handle before thread stripping occurs can be calculated using the formula:
- F = maximum shear load [lbs]
- Su = shear strength of the bolt or nut material [psi]
Based on these equations, it may appear that the maximum shear load can be increased simply by extending the length of engagement. However, it is important to note that the applied load is normally not equally distributed across the threads. Instead, most of the load is carried by the first thread, as shown in the load distribution diagram below.
Hence, there is a limit to how long the thread engagement can be extended to produce an appreciable increase in strength. For carbon steel fasteners, it is limited to approximately one nominal diameter. After that, there will be no appreciable increase in the maximum shear load the threads can withstand.