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DEBolts play a critical role in load-bearing connections for countless mechanical and structural assemblies, from simple machines to towering buildings. But behind every reliable bolted connection lies careful consideration of the forces they’ll face. Whether a bolt is holding together components in an engine or supporting beams in a bridge, understanding the mechanics behind bolt strength calculations is key.
This article explore the fundamentals of bolt strength, specifically focusing on tensile strength, shear strength, and proof load — concepts that guide engineers in designing connections that perform reliably, even under demanding conditions.
Read our previous posts about bolts:
What is Bolt Strength?
Bolt strength refers to the capacity of a bolt to withstand forces without breaking or deforming. There are different types of loading conditions that bolts may experience, including tension (pulling force) and shear (sliding force). To ensure compliance with industry standards regarding bolt strength, bolt checks are often performed using Eurocode 3 (EN1993) for European steel structures and AISC 360-10 in the USA.
These standards provide specific criteria for assessing bolt strength, making it crucial to follow them when designing bolted connections that can safely endure operational loads. Understanding the strength parameters in these conditions is crucial for designing bolted connections that can endure operational loads safely.
Tensile Strength
Tensile strength refers to the maximum amount of tensile (pulling) force a bolt can handle before it fractures. It is one of the most fundamental aspects of bolt strength and is directly related to the material properties and diameter of the bolt and should correspond to standards like AISC 360-10. Tensile strength is usually expressed in megapascals (MPa) or pounds per square inch (PSI).
To calculate the tensile strength of a bolt, the following equation is used:
P = St × As
Where:
- P= Tensile load– a direct measurement of clamp load (lbs., N).
- St= material’s tensile strength – a generic measurement of the material’s strength (psi, MPa).
- As= Tensile stress area for fastener or area of material (in2, mm2).
Note: The tensile stress area accounts for the effect of threading and is different from the nominal cross-sectional area. It can be calculated using formulas from standards like the Machinery’s Handbook.
Shear Strength
Shear strength refers to the maximum stress that a fastener can withstand before fracture when applied at a right angle to its axis.
This type of force is common in bolted joints subject to lateral (sideways) loads. A bolt typically has lower shear strength compared to its tensile strength, and this parameter is crucial for applications where bolts are subjected to shear loads, such as in structural frames or machinery joints.
Shear strength can be calculated using the following equation:
Where:
- Fs is the shear force
- A is the cross-sectional area of the bolt
- n is the number of bolts
Practical Insight: In a double-shear configuration, bolts often experience increased shear strength. When designing bolted joints, ensure the load path and bolt arrangement are evaluated to determine whether a single or double shear setup is applicable.
Proof Load
The proof load is the maximum force that a bolt can withstand without experiencing permanent deformation. It serves as a safety threshold, ensuring that the bolt will not stretch or deform under expected loading conditions. The proof load is often specified as a percentage of the bolt’s yield strength, typically ranging from 85% to 95%.
The equation for proof load is:
Fp = At x Sp
Where:
- Fp is the proof load (the maximum load that the bolt can bear without permanent deformation),
- At is the tensile stress area of the bolt (this is the effective area that resists the load, considering the threading),
- Sp is the proof strength or stress (the maximum stress the material can withstand without plastic deformation).
Important: Exceeding the proof load can lead to bolt elongation, which is especially critical to avoid in fatigue-sensitive applications, where the bolt is subjected to cyclic loading.
Bolt Grades Related to Strength
Moreover, identifying bolt grades is essential for ensuring the correct application based on strength and hardness. Here’s a brief overview:
| Grade | Tensile Strength | Yield Strength | Proof Load |
| Grade 1 | 60,000 psi (415 MPa) | 36,000 psi (250 MPa) | 33,000 psi (230 MPa) |
| Grade 2 | 74,000 psi (510 MPa) | 57,000 psi (395 MPa) | 55,000 psi (380 MPa) |
| Grade 5 | 120,000 psi (830 MPa) | 92,000 psi (635 MPa) | 85,000 psi (590 MPa) |
| Grade 8 | 150,000 psi (1,035 MPa) | 130,000 psi (895 MPa) | 120,000 psi (825 MPa) |
| Grade 8.2 | 150,000 psi (1,035 MPa) | 120,000 psi (825 MPa) | 120,000 psi (825 MPa) |
| Grade 9 | 180,000 psi (1,240 MPa) | 140,000 psi (965 MPa) | 140,000 psi (965 MPa) |
| Class 8.8 | 830 MPa (120,000 psi) | 640 MPa (93,000 psi) | 600 MPa (87,000 psi) |
| Class 10.9 | 1,040 MPa (151,000 psi) | 940 MPa (136,000 psi) | 830 MPa (120,000 psi) |
| Class 12.9 | 1,220 MPa (176,000 psi) | 1,220 MPa (176,000 psi) | 970 MPa (140,000 psi) |
Each grade reflects the bolt’s ability to handle stress under different conditions. Selecting the appropriate grade ensures safety and reliability in engineering applications.
Practical Considerations for Engineers
When performing bolt strength calculations, you should factor in not only the individual strength parameters but also the specific conditions in which the bolt will be used. Here are a few key considerations, mentioned in previous article:
- Material Selection: The material properties of the bolt significantly affect tensile, shear, and proof loads. High-strength materials like alloy steels offer higher load capacities than standard carbon steels, making them better suited for high-stress environments.
- Bolt Grade: Higher grades (e.g., 10.9, 12.9) correspond to bolts with greater tensile and yield strength, used in applications that demand high strength-to-weight ratios.
- Thread Type: Fine-thread bolts generally offer higher tensile strength than coarse-thread bolts due to their larger effective stress area. Understanding thread pitch and diameter is essential when determining bolt strength.
- Preloading: Preloading a bolt ensures that it remains in tension, reducing the risk of fatigue failure under dynamic loading conditions. Proper preload can also prevent loosening during operation. Tools like the SDC Verifier Bolt Check App help automate bolt verification in software like Ansys, Femap, and Simcenter 3D, ensuring optimal preloading.
- Torque-Tightening: Proper torque-tightening is critical for achieving the desired preload in bolted connections. Under-tightening can lead to insufficient preload, increasing the risk of fatigue, while over-tightening can lead to bolt failure due to yielding. Refer to torque-tension charts for guidance.
- Environmental Conditions: Corrosion, extreme temperatures, and vibrations can affect bolt performance over time. Selecting materials with corrosion-resistant coatings or heat-treated alloys ensures long-term reliability in harsh environments.
Conclusion
In engineering applications, accurately calculating bolt strength is critical for ensuring the safety and reliability of bolted connections. By understanding key parameters like tensile strength, shear strength, and proof load, engineers can make informed decisions when selecting and designing bolts for specific applications. Each of these parameters plays a distinct role in how a bolt behaves under different loading conditions, and together they ensure that bolts perform optimally within their intended design limits.
