Relaxtion of a tightened assembly
One of the most important factors of influence in the quality of a tightened assembly is « relaxation ». Per definition, it is the decrease in tension once the assembly is finished. It is shown by the difference between:
- The torque applied during tightening
- The torque measured after a while
A tightening curve of a bolted joint has following pattern:
The first zone is the assembly zone: it is the area where torque is built up (torsion-, tension- & shear forces) and where the tools shuts off at a predefined value. Once the the tool is taken away from the joint, the torsion forces disappear and the tension in the bolt has to prevent that the assembled parts stay together. This point is called “the initial pre-load” in the joint.
Just after the initial preload, a certain “loss” called “relaxation” occurs. All bolted assemblies show relaxation, but not all at the same level.
The largest part of relaxation happens in the first 10 to 50 milliseconds after assembly before a certain stabilisation takes place; this zone is called “the static zone”. To minimize the amount of loss due to relaxation, It is highly important to manage the speed in the last part of the tightening sequence.
A high speed nutrunner will come very quickly to its end torque.
A slow nutrunner or a controlled nutrunner will reach the end torque with a better controlled end speed that will bring equal compression in the joint. Therefore, there will be more time to reduce tension losses in the assembly caused by relaxation or setting. Relaxation is present whatever assembly and is linked to the surfaces in contact or is linked to deformable parts like cylinder head that settles under the effect of tension. When assemblies are correctly engineered, relaxation is small and can be ignored. However, this phenomenon is a particular problem for assemblies including washers. Other examples of « soft » components are e.g. paints. On this type of assembly; it can take quite some time before the reduction of tension resulting from the deformation of parts stabilizes.
For these assemblies, there are several techniques that can be used to reduce this setting effect:
- The assembly can be re-tightened after the setting has stabilized
- The assembly can be tightened to a higher torque, then untightened and re-tightened to the required torque.
- The assembly can be tightened, untightened and re-tightened in one step
- The assembly can be achieved in two (or more) steps like e.g. pre-tighten, pause and tighten, to the target torque
Whatever way is chosen, the best solution to such problem is to define assemblies which are finalized correctly at once: e.g. replace the cylinder head soft joints by a harder metallic joint.
Once the bolted joints are submitted to dynamic loads (e.g. when the car engine is started) all bolts are submitted also to dynamic loads. Also in this zone called the “clamp load zone” a certain setting due the usage creates another amount of loss in the bolted joint. Moreover, when several bolts are applied on one piece, the problem of “elastic interaction” between the bolts can create differences in tension (or differences in clamping forces) in each joint, especially in soft joints.
Imagine tightening a top cover using 6 screws and one tool. The first bolt will be tightened to final torque and will have its full capacity at the time of its individual tightening. A clamping force will be created and the intention is to keep it like that during its entire life. But when the second bolt is tightened, the first one will lose a part of its clamping force. Then the tightening of the third screw will influence further losses of clamping force in the two first screws, etc …
The elastic interaction is caused by the relaxation between the different joints and is hard to be managed using only one tool that tightens in one step to final torque. Available solutions are e.g. tighten in a first step all 6 screws to 50% of final torque, in a well defined sequence (e.g. in case of a round top cover, the first screw on top, the second one down, the third one left, etc …) then tighten to 75% in the same sequence and last but not least tighten to 100% using a shut off torque key. This is of course a long procedure and using a hand tool to tighten to final torque can be very tiring for the operator. This is not a ideal method for large production series. An alternative solution is to apply a multi-spindle. This implicates in our case study to tighten the 6 screws at the same time with 6 tools to a pre-torque; the then wait for each other before going on to final torque all together again. To even better guarantee correct tension in the 6 screws, hold power on the screws at the end of the cycle for some tenths of milliseconds! This will guarantee a better clamping force for the life time of the assemblies. If cost is an issue, an alternative could be appiyng a two-spindel.