Bolted joints

  

Bolted joints 

 

Bolted joints are used to connect parts which combine to form a mechanical structure. In comparison to other joining methods such as welded joints and riveted joints, bolted joints have been widely used due to their ease of assembly and disassembly, load bearing capacity and low cost. Bolted joints also serve easy part replacement, maintenance, and repair. Bolted joint consists of a threaded fastener called bolt, a nut that mates with the threaded fastener and the connecting parts. The frictional force between the threads of nut and bolts holds the connecting parts firmly.  

 

How is the bolt tightening  taking place? 

When the operator applies tightening torque on the bolt nut assembly, a compressive load is acting on the connecting parts. So, here in the diagram, compressive force acts on the plates and firmly holds the assembly. The situation is entirely different inside the bolt fastener, a tensile force is acted on the bolt inside the A-B region. This tensile force causes an elastic deformation on the bolt. This tension force acting on the bolt that causes elastic deformation and thus holds the assembly firmly is termed as bolt preload. The higher the value of bolt preload, the higher is the clamping force and there is less chance of bolt loosening. But there is an upper limit value for this preload. That is what I am explaining in the coming section.  

What is proof load? 

 The bolt preload value at which the bolt shank undergoes plastic deformation and undergo failure is termed as proof load. In simple terms, the deformation taking place inside the bolt shank should never enter plastic deformation, that is the stress value inside the bolt because of bolt preload should never reach the yield stress value for that bolt material.  

What are the types of bolted joints? Bolted joints can be classified in terms of the load acting on them. These are tension joints and shear joints. The tension joints are intended to bear the tensile load acting on them, whereas shear load in case of shear joint. The transfer of force, clamping action and failure criteria differs in both the cases of joints. Let us have a closer look at both the types of joints.  

Tension joint.  

 

 

Tension joint is used to transfer tensile force between the clamped parts. Here the tension force acting on the bolt fastener acts in the same direction as the preload on the bolt. So, the effective load acting on the bolt is the sum of bolt preload and the external tensile force. So, the greater concern over tension joint is to avoid the stress inside the bolt to reach elastic limit. The Factor of Safety (FOS) can be calculated as Proof Load / Bolt load. 

To avoid failure condition in tension joint, Bolt load (F) + Preload (P) ≤ Proof load. 

FOS = Proof load / Bolt Load 

Proof load = (Sigma y x π (diameter)^2 /4)  

Bolt Load = P + F 

 

 

 



 

Shear joint 

 

In shear joint, the connected parts tend to separates each other in lateral direction. The bolt preload acts longitudinally of bolt as same as in tension joint. Here in the shear joint, friction between the bolt head surface and clamped parts and nut surface and clamped parts plays a significant role in clamping the bolted assembly. The failure of the shear bolted joint occurs only if the lateral force exceeds the frictional force in the assembly. That is,  

Lateral shear force (F) ≤ u x Preload, where u is coefficient of friction. 

How to determine the torque required to achieve a particular value of bolt preload? 

As the necessary preload to be applied to the bolted joint is found out, the required torque ca be estimated from the following equation. 

Torque = K x D x P, where K = nut factor or tightening factor, D = diameter of bolt and P = bolt preload. The nut factor “K” depends on friction between threads of nut and bolt and their material. The significance of nut factor can be seen in several examples.  

Another important thing to be taken care of while choosing threaded fastener for shear bolted joint is the shear plane. The plane of shear should not lie at the threaded region of fastener, rather it should lie at the bolt shank region.  

Why do we lubricate threads of bolted joints prior to tightening?  

This can be explained from the above formula containing nut factor. During lubrication, the frictional coefficient between the threads of nut and bolts reduces significantly. As a result, the nut factor “K” also reduces. So, on looking at the formula, torque applied to achieve a particular preload value reduces. So, while applying a specific value of torque, high value of preload and thus high clamping force is achieved with lubricant. 

Why is it difficult to clamp rusted bolted joints?  

This can be clearly explained by the same formula. The rusted joints have significantly high value of coefficient of friction, so does the “K” value. So, a significant amount of torque is required to achieve that value of preload required for clamping. This makes the rusted bolted joints difficult to handle. 

What is a pressure cone?  

There is a three-dimension conical region around the bolt shank inside the clamped parts where the stress distribution is higher than the rest of the parts. This is due to the compressive force acting on the parts from the clamping force. So, care should be taken to avoid these pressure cones of adjacent bolted assemblies to interfere each other. Interference of pressure cones leads to unnecessary increased stress states in the assembly and may lead to failure. The shape of pressure cones differs both in fully threaded bolted joint and nut-bolt joint.