HOW TO DESIGN A BOLTED JOINT TO OPTIMIZE FUNCTION AND ASSEMBLY.

 

It is crucial that the bolted joints are strong enough to withstand the axial loads and the shear loads to which they are exposed. This is a short and simplified guide to the design of different bolted joints, including correct tightening techniques to secure your application.

 

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Introduction, how to design a bolted joint

 
When designing a bolted joint, it is important to complete two missions:

1) Make sure that the fastened parts will not separate.
Risk of separation occurs when the bolted joint is exposed to an axial load.

2) Stop the parts from sliding.
The risk of sliding is present when the joint is exposed to shear loads.
 

 
In this guideline, an example bolted joint application is used to illustrate the different aspects that you need to be aware of when designing a bolted joint.

The example application application consists of three parts, an aluminium block, a steel lifting eye and a fastener (see picture below).
 

 
The bolted joint should be designed to handle both axial and shear loads.
 

 

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1. The bolt must be able to handle the load

 
First of all, select a fastener that can handle the load – in the standard joint above; 5,000 kg × 9.82 ≈ 50,000 N = 50 kN.

To find out which fastener dimension and property class that can handle the load on the bolted joint, we recommend using BuProx, torqued tensile load, see figure below.

In this case, an M10 property class 10.9 would be suitable. Other solutions are possible, e.g. two M8 property class 8.8.

Calculation of ultimate tensile load using BuProx.
 

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If the nut member is an unstandardized threaded part, the thread engagement length needs to be calculated.
The length of the needed thread engagement depends on:

• The dimension of the fastener; a larger bolt diameter needs more thread engagement length.
   
• The property class of the bolt; a property class 10.9 bolt is harder and stronger compared with an 8.8 bolt and therefore needs more thread engagement.
   
• The material that the female threads are made of; threads made of soft aluminium need more thread engagement compared with threads made of steel.
   

 

 

To ensure that the bolt breaks first, the minimum thread engagement length must be determined.
To calculate the minimum thread engagement length the formula below can be used:
 

 

Or use BuProx for calculating the minimum thread engagement.

 

In the standard bolted joint application, the female threads are made of aluminium (hardness 100 HB) and the bolt is an M10, property class 10.9.

Inserting the above data in BuProx results in a calculated minimum thread engagement of: 21 mm.

Adding a safety factor to this value, e.g. 20 percent results in a thread engagement of: 25 mm.

If information regarding the hardness of female threads is missing, the guidelines below can be used.
 

Estimation of thread engagement depending on the thread diameter (d) and materials.

 

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3. The surface under the bolt head needs to be strong enough to handle the pressure

 
When a bolted joint is exposed to an axial force (in the example joint when an aluminium part is lifted in the lifting eye), the pressure under the head will rise. If the material under the head is not strong enough and/or the surface area under the head is too small, the bolt will deform the underlying material.
 

 

The deformation can have a negative effect on the bolted joint (loss of clamping force, called settling). It can also damage/break the clamped part. A good rule is to design for surface pressure < clamped parts minimum yield strength.

To calculate the surface pressure, use the formula below, or use BuProx surface pressure calculation.
 

Calculation of the surface pressure, Ph = surface pressure, F = force on the bolt joint, dw = outer diameter, dh = inner diameter.

In the example application with the lifting eye made of steel, calculation of the surface pressure may not be needed because the yield strength of steel generally is higher than the surface pressure. However, if the lifting eye in the example application was made of aluminium instead of steel, the surface pressure definitely needs to be calculated.

Input to BuProx:
Preload: 50 kN (the weight of the aluminium block)
Hardness of the aluminium: HB 110
Yield strength of the aluminium: 220 N/mm²
 

Hardness and yield strength of cast-aluminium alloys.
 

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Dimension of the bolt head according to the drawings below.

Hexagon head with flange

 

 

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Hexagon head without flange

 

 

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Design to handle the shear load

 
When the 5,000 kg heavy aluminium part is lifted in the lifting eye (see picture below), the bolted joint is affected by a shear load and the clamped parts risk starting to slide.
 

 

If the clamped parts in the bolted joint start sliding, there is a great risk that the bolted joint will fail.

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Learn more – take the full training

 
This is a shortened version of the Bulten Academy training course Design Guidelines. To take part of the full version, please contact Bulten Academy – academy@bulten.com.