THE INTERPLAY OF FORCES IN A BOLTED JOINT

Naturally, it is desirable for threaded joints to be reliable and without risk of loosening on their own. For a high degree of reliability, it is extremely important to understand the forces present in a bolted connection because every element in the connection influences the final result.

Figure 1	Figure 2

In simple terms, there are two possible types of static load in a bolted connection:

Without a clamping force – the force is transmitted between the plates by bearing forces and shear forces in the body of the bolt or the thread. The plates being connected move relative to each other until the bores bear against the body of the bolt or against the thread. In this case, the bolts are loaded in shear through transverse loading; see figure 1.
With a high clamping force – the clamping force prevents the clamped parts from being displaced. The force is transferred by friction, and the bolts are loaded in tension through an axial load; see figure 2. For stainless steel joints, see our guide on preventing the seizing of stainless steel fasteners.

Typically, mutual displacement of the parts being connected is undesirable.

Sufficient clamping force must therefore be applied in the bolted connection. This force is the preload achieved after the nut or bolt has been tightened.

The preload that can safely be applied depends partly on the bolt property class and proof load. Bolt strength is determined by heat treatment — learn more about fastener heat treatment.

If the forces on the structure regularly change direction or are not constant, the connection is subjected to a dynamic load. As explained below, dynamic loading can cause bolted connections to loosen or bolts to break.

For a joint to fulfil its function, especially under dynamic loading, the clamping force must be maintained.

ELASTIC RESILIENCE OF A THREADED JOINT

When designing and making a threaded joint, it is very important to understand the following points:

The bolts and connected parts function as an elastically resilient unit. The clamped elements are compressed elastically, while the bolt stretches during assembly. If the bolt stretches further due to an external load, the clamped parts spring back.
The tensile force in the bolt is equal to the compressive force acting on the clamped elements, as illustrated in figure 3.

Figure 3

Force equilibrium in a bolted joint — diagram showing tensile force in bolt equals compressive force on clamped elements

The interplay between force and deformation is presented in a force-deformation triangle, as shown in graph A below. Line 1 in the graph shows the deformation that a bolt undergoes due to tensile force. Line 2 relates to the clamped set, which deforms under the compressive force created by the bolt.

Force-deformation triangle (Graph A) — showing bolt elongation curve and clamped element compression curve at clamping force Fm

fsm = elongation of the bolt due to clamping force Fm

fpm = compression of the clamped elements due to clamping force Fm

From the graph above, it is evident that at a clamping force Fm, the elongation of the bolt is equal to fsm and the compression of the clamped parts is fpm. Because the materials and designs used for the bolt and clamped parts differ, fsm and fpm are usually not equal.

An external load Fa is then applied to the bolted connection; see figure 4.

Figure 4

External axial load Fa applied to a bolted joint — showing how external force acts on the pre-loaded connection

To plot this external tensile force Fa on graph A, it must be fitted between the two deformation characteristics. If the bolt stretches due to the external force, the clamped material springs back by the same amount; see graph B.

Force-deformation diagram with external load (Graph B) — showing bolt load increase Fsa, clamping force reduction Fpa, and residual clamping force Fkr

Effect of elastic bolt on force distribution (Graph C) — showing flatter bolt deformation curve reduces bolt load increase under external force

Fm = original clamping force in the connection

Fa = external axial load

Fpa = reduction in clamping force due to Fa

Fsa = increase in bolt load due to Fa

Fkr = residual clamping force in the connection

Fs = total load on the bolt

A MORE ELASTIC BOLT CAUSES A SMALLER INCREASE IN BOLT LOAD

On the one hand, Fa causes a reduction in clamping force, identified as Fpa. On the other hand, it causes an increase in the load acting on the bolt, identified as Fsa.

It is desirable to keep the increase in bolt load as small as possible, not only to prevent the bolt from becoming overloaded. If the external load is dynamic, the bolt experiences the fluctuations in Fsa. A high amplitude of Fsa can quickly cause a fatigue fracture. The residual clamping force Fkr must also never reach zero. If it does, the joint separates and can no longer function correctly.

The increase in bolt load Fsa can be limited by using a more elastic bolt. This makes the bolt deformation curve less steep, allowing more of the external force to be absorbed through a reduction in clamping force; see graph C.

The same effect can be achieved by using very rigid clamped materials. The deformation curve of the clamped materials becomes steeper, and most of the external force is absorbed through a reduction in clamping force; see graph D.

Effect of rigid clamped materials on force distribution (Graph D) — showing steeper clamped material curve absorbs external force through clamping reduction

More rigid clamped materials cause a smaller increase in bolt load.

IN SHORT

Particularly under dynamic loading, it is extremely important to keep any additional load acting on the bolt as low as possible because sudden fatigue fracture can occur.

When an external load is applied, there are several ways to limit the additional force acting on the bolt:

The structural members should be as rigid as possible.
The clamping force should be as high as safely possible and should remain greater than the external separating load.
More elastic bolts can be used. Choose a high clamping length-to-diameter ratio of at least 5×D, select a longer threaded length where appropriate and, when necessary, use a reduced-shank bolt. For dynamic connections, see our guide to fastener locking methods.

BOLT JOINT DESIGN PARAMETERS

Design Parameter	Effect on Bolt Load	Effect on Clamping Force	Recommendation
More elastic bolt with a longer L/D ratio	↓ Reduces additional bolt load Fsa	↓ Greater clamping force reduction Fpa	Use a clamping length of at least 5×D.
Rigid clamped materials	↓ Reduces additional bolt load Fsa	↓ Greater clamping force reduction Fpa	Use rigid materials such as steel or cast iron and avoid soft gaskets where possible.
Higher preload Fm	No change to the Fsa amplitude for a given joint stiffness	↑ Higher residual clamping force Fkr	Where the joint design and tightening method allow, tighten to approximately 90% of the bolt proof load.
Reduced shank diameter	↓ Makes the bolt more elastic	See “more elastic bolt”	Use purpose-designed reduced-shank bolts. Where appropriate, a partially threaded bolt such as DIN 931 can provide a longer unthreaded shank than DIN 933.
Longer threaded length within the clamping length	↓ Makes the bolt more elastic	See “more elastic bolt”	Select a longer threaded length where the joint design and thread engagement requirements allow.
Soft gaskets or washers	↑ Increases bolt load fluctuation	↑ Greater preload loss over time	Avoid soft elements where possible. If unavoidable, consider disc springs and an appropriate locking method.
Dynamic or vibration loading	↑ Fatigue risk increases	↓ Risk of complete clamping-force loss	Apply a suitable locking solution, such as Nord-Lock wedge-locking washers or Spiralock thread forms. See our guide to locking methods.

FREQUENTLY ASKED QUESTIONS ABOUT BOLTED JOINT FORCES

What is preload in a bolted joint?

Preload, identified as Fm, is the initial clamping force created when a bolt is tightened. It compresses the clamped parts and stretches the bolt, creating an elastic system that resists external loads. Without sufficient preload, external forces act more directly on the bolt, increasing the risk of fatigue failure. Insufficient or lost preload is associated with approximately 80% of bolt fractures in service.

Why do bolted joints loosen under vibration?

Vibration can cause micro-slip between the clamped surfaces, gradually reducing the clamping force or preload. As the residual clamping force Fkr decreases, more of each load cycle is transferred directly to the bolt as fluctuating stress. Once Fkr reaches zero, the joint can separate. Locking devices such as Nord-Lock wedge-locking washers or Spiralock thread forms help maintain preload under dynamic loading. See our guide to fastener locking methods.

How does bolt elasticity affect fatigue life?

A more elastic bolt can be achieved through a higher clamping length-to-diameter ratio of at least 5×D, a longer threaded section within the clamping length or a reduced shank. In the force-deformation diagram, the flatter bolt curve means that less of the external force Fa translates into an increase in bolt load Fsa. Because fluctuations in Fsa contribute to fatigue, reducing this increase can extend the fatigue life of the bolt.

What is the clamping length-to-diameter ratio and why does it matter?

The clamping length-to-diameter ratio, or L/D ratio, is the total thickness of the clamped material divided by the nominal bolt diameter. A ratio of at least 5:1 is recommended because it makes the bolt sufficiently elastic to accommodate external loading without excessive stress fluctuation. Short bolts with a low L/D ratio are comparatively stiff and transfer more of a dynamic load directly to the bolt.

How do I choose between shear-loaded and tension-loaded bolted joints?

In shear-loaded joints with transverse loading, force is transferred through the bolt body or thread bearing against the bore. This may require fitted bolts, close-tolerance holes or reamed holes. In tension-loaded joints with axial loading, the clamping force creates friction that prevents the plates from moving. Highly preloaded tension joints are generally more predictable, easier to design for fatigue and compatible with standard clearance holes. For corrosive environments, see our stainless steel fastener guide.

Last updated: 12 July 2026