BY USING A MORE ADVANCED ASSEMBLY METHOD AND SMALLER BOLTS, IT IS POSSIBLE TO AVOID BOLTED JOINT FAILURE AND SAVE WEIGHT.
The most common way of tightening a bolt is by using a standard torque. The standard torque is usually depending on the dimension and property class of the bolt itself. To avoid the risk of fracture during assembly, the target clamping force is about 55 % of the ultimate tensile force. By applying a smart assembly strategy, it is possible to use up to 80 % of the tensile force.
The target for the bolted joint assembly is:
– Creating high enough clamping force to minimize the risk of fatigue breakage or selfloosening when the bolt joint is exposed to an external load.
– Securing that the clamping force is not too high, risking to break the bolt/threads during assembly.
What clamping force is needed is depending on the external loads and friction between the clamped parts (if the load is in a transverse direction). What torque is needed to break the bolt is depending on the assembly friction and the strength of the bolt.
A torque and angle curve are usually recorded when assembling a bolt using an electric torque wrench. This torque and angle curve could be very useful for determining if the bolt is assembled inside the clamping force window or not (high enough to handle the loads and low enough to not fracture during assembly).
How to read the torque and angle curve?
The most common cause to bolted joint failure is a deviation in friction. By comparing the slope of the torque and angle curve of a “problem” assembly and a “normal” assembly, it is possible to identify if the assembly friction is higher or lower than usual. If the friction is higher than usual, higher torque is needed to rotate the bolt, and the torque and angle curve is steeper.
Why is friction such an important factor when using a torquecontrolled assembly?
As mentioned before, just about 10 % of the assembly torque is converted into clamping force.
Small deviations in friction have a massive impact on the clamping force. For example, if the friction beneath the head is increased by 10 %, half of the clamping force will be lost.
Torque and anglecontrolled assembly
To be able to use more than 55 % of the tensile force in the bolt, an angle-controlled assembly is recommended. The first two phases (1A and 1B) are controlled by torque. The angle controls the third phase.
The angle-controlled part of the assembly is not sensitive to friction. Therefore, the deviation in clamping force using torque and angle control is much smaller compared to torque controlled assembly. By using torque and angle-controlled assembly, it is possible to assemble the bolts to yield and by that reaching almost 80 % of the ultimate tensile force of the bolt (dependin on the torsional loss and the yield strength of the bolt).
Gradient controlled assembly
Instead of controlling the last phase of the assembly by torque or angle it is possible to control the assembly process by the gradient of the torque and angle curve. When the gradient is changed, the assembly tool automatically knows that it has passed the yield point of the bolt or/ and the clamped part.
Why is the bolted joint controlled?
The control can be performed:
– In production before the joints leave the factory.
– During service, when the joints have been exposed to external loads.
The purpose of the inspection in the factory is to:
– Identify if the bolted joints have been assembled with the correct torque.
– Check the level of the relaxation.
The Purpose of the inspection in service is to:
– Check the level of the settlings after the bolt joints have been exposed to external loads, high temperatures, etc.
Different methods can be used to check the torque, here are the most common control methods presented.
Residual torque is measured by continuing the tightening of the bolt. If there is no relaxation in the joint the residual torque is higher than the assembly torque. Make sure the entire bolt rotates and not just the head.
Break away torque
Breakaway torque, or loosening torque, is measured by slightly loosening the bolt. The breakaway torque is about 20 % lower than the tightening torque (due to downhill instead of uphill).
Return to mark
The return to mark torque is very similar to the assembly torque (if the lost in clamping force is close to zero). The methods is performed according to instruction below.
Summary torque control methods
All control methods is depending on the operator. The result is also depending on the which control methods is used.
It is not possible to identify if the clamping force was correct after assembly when using a torque control method.
What is possible to control is what has happened with the screw joint after assembly and also if the bolt has been assembled with the wrong torque.
SUMMARY CONTROL METHODS
Which control method to use, depends on the assembly method.
Below is a suggestion from Bulten which method is recommended to use:
If the bolt is assembled beyond yield, it is not recommended to use the residual torque method as a control method because then, there is a significant risk for overtightening.
Learn more – take the full training
This is a shortened version of the Bulten Academy training course Assembly. To take part of the full version, please contact Bulten Academy – firstname.lastname@example.org.