Bolted Joints Explained: Preload, Torque, and Load Distribution in Tension and Shear

This video explains how bolted joints work, focusing on preload, how tightening a nut stretches a bolt to create clamping, and how load is distributed between the bolt and the joined members. It covers tension joints and shear joints, the role of friction, and the difference between slip resistant and bearing joints. It also discusses how to choose preload, the common torque based tightening method and its limitations, alternative preload control methods, and real world examples including space station berthing mechanisms. The video combines concepts of stiffness, edge distance, and failure modes to give a practical understanding of designing reliable bolted joints.

Overview of bolted joints and preload

The video introduces nuts, bolts and the concept of preload, a tensile force applied to the fastener before external loads. Tightening a nut causes the bolt to stretch, creating clamping that holds joined plates together and transmits loads through a combination of bolt stretch and plate stiffness. This clamping effect is fundamental for joint strength and fatigue life, and preload is especially beneficial under cyclic loading.

Load distribution in tension joints

In tension joints the external load tries to pull the joined members apart. Preload reduces the share taken by the bolt; most of the load goes into compressing the clamped parts. The exact distribution depends on the relative stiffness of the joined members versus the bolt itself, which is typically much less stiff. The joint can be visualized as an assembly of springs: the bolt spring is stretched by preload and the member springs are slightly compressed. Under increasing external load, the clamping force between the joined members decreases first, until the clamping force is overcome and the assembly slips apart. Beyond this point, additional load is carried largely by the bolt, which can lead to bolt fracture or other failure modes if preload is not adequate.

Shear joints and friction

For shear joints, where the load acts perpendicular to the bolt axis, preload generates a clamping force that produces friction between the clamped surfaces. If the frictional force (normal force times coefficient of friction) is larger than the applied shear, the bolt does not experience significant shear load beyond its preload. Joints designed to rely on friction are called slip resistant joints. If the applied shear exceeds the frictional force, slip occurs and bearing contact follows, possibly leading to failure depending on the design and load path.

Joint design and failure modes

Bearing joints carry load directly through bearing in the clamped parts and the bolt is loaded mainly in tension. These joints can fail by tensile fracture of the clamped material, bearing failure at the bolt holes, tear out near edges, or shear failure of the bolt. To reduce tear out, bolts should be located at least two diameters away from edges. A common improvement is using a two stage shear path called a double shear joint, which effectively halves the bolt shear stress and increases the allowable load compared to a single shear plane.

Combined loading and eccentricity

Often joints experience both tensile and shear loads, or eccentric loads that introduce bending moments. These conditions reduce clamping force and can add extra loads on bolts and joined members. For example, a pattern of six bolts under a pattern with eccentric loading experiences distributed shear plus a moment that increases the load on bolts away from the centroid. Such considerations must be included in design checks for safety margins and fatigue life.

How much preload to apply

The preload is typically chosen to be as high as possible without damaging parts, often around 70% of the bolt yield strength. The preload corresponding to a desired tensile stress depends on the bolt yield strength and the bolt’s tensile stress area, which accounts for thread reduction of cross section. For example, an M12 structural bolt might target a preload around 38 kN, but specific values depend on material properties and geometry. Increasing preload can be achieved by using larger bolts or higher yield materials.

Controlling preload: torque, turn of nut, and elongation methods

The most common method is tightening with a torque wrench, which is convenient but not highly precise due to friction, lubrication, and head-nut interface variations. The nominal relation between torque, bolt diameter, and preload uses a nut factor parameter, typically around 0.2, but actual values vary with lubrication, fit, and coating. A second method is the turn of nut, where the nut is tightened to seat the mating surfaces and then rotated through a defined angle to achieve the desired preload, offering better control than torque alone but still with limited accuracy. The most accurate method measures bolt elongation to infer preload, either through caliper-based measurements if accessible or ultrasonic elongation measurements in practice. Ultrasonic elongation can achieve preload accuracy within a few percent of the target value and is used in critical applications like space docking mechanisms.

Preload maintenance and embedment

Preload decays over time due to embedment, where microscopic peaks flatten and gaps shrink after initial tightening. In-service vibration and temperature can further reduce preload through creep and loosening. To minimize preload loss, engineers use locking features such as adhesives or special washers and perform re-torqueing after embedment has occurred to restore the intended preload.

Useful design tools and real-world example

Joint diagrams are a helpful visualization for understanding force distribution and deformations in bolt joints. While this video does not cover the joint diagram in depth, it mentions a bonus video on Nebula that explores joint diagrams in detail. A real-world example highlighted is the berthing mechanism used on the International Space Station, which uses 16 bolts and torque motors to apply a preload of about 90 kN per bolt, with load cells and strain gauges integrated to precisely monitor bolt elongation and preload, demonstrating the importance of accurate preload control and measurement in critical systems.

Summary of key takeaways

Preload improves joint strength, fatigue life and leak sealing, but its optimal value depends on bolt and component strengths. The distribution of load between the bolt and the clamped members depends on stiffness; in tension joints clamping dominates until clamping fails, while in shear joints friction matters more. Different preload control methods offer trade-offs between ease of use and accuracy, with ultrasonic elongation providing the highest precision. Edge distance and joint plane design influence failure modes like tear-out and bolt shear. These concepts form the foundation for robust bolted joint design across a wide range of engineering applications.