Tightening is all about acheiving the right clamp force in a joint for it to work and function properly. Understanding of tightening technique is essential to choose the right tightening solution that will fulfill your specific needs.
About screw joint
Screw joints are the most common way of joining components due the advantages in simplicitiy of design and assembly, easy disassembly, productivity and in the end - cost. A screw is exposed to tensile load, to torsion and sometimes also to a shear load. The tensile load corresponds to the force that clamps the joint members togheter. But due to difficulties measuring clamp force the tightening torque is often used as they correspond to each other.
The Tightening Process
The tightening process has a major influence on the quality of the screw joint. The goal for the tightening process is to achieve enough clamp force to hold the joint together. A process which achieves the required clamp force with a minimum of scatter will give a high quality joint. The process is determined by:
• Torque application method; hand tightening, continuous drive and discontinuous drive.
• Tightening control method; torque, angle or gradient.
• Monitoring capabilities; A tool or system which can monitor the controlled and non-controlled parameters
• will give a higher confidence in the process and quality of the joint.
Different torque application methods
Hand tightening is often carried out using a torque wrench. The wrench is set to a predefined torque value and the operator will be notified when the correct torque is reached. Advantages with this solution is the ease of use and setup. Disadvantages are low productivity and limited tightening control capabilites even though there are advanced types of wrenches which can measure the angle thus allowing the use of angle control.
Tools with continuous drive/direct driven tools can both be electric and pneumaticly driven. In general pneumatic tools due shut off via a mechanical clutch when the set torque is reached and electric tools can be equipped with sensors to control and monitor torque, angle and gradient. Controling and measuring more parameters will increase the confidence in the process.
The discontinuous drive method use inertia which is released intermittently during tightening. This is what is found in impact and pulse tools. Advantages are fast tightening with low reaction force even a high torques. Disadvantages are that in general results scatter more than with a direct driven tool. Most common tools of this type does not have any type of sensors to but there are products available with sensors.
In general the scatter in clamp force is lower for direct driven tools and when using gradient control and angle control rather than when using impact/pulse tools and torque control. Using more advanced control methods and monitoring methods allow downsizing of the fastener with weight reduction as result. However, in this case the choice of tool have to be set in the design phase of the product.
Want to know even more about tightening technique or have any questions? Get in contact with your Atlas Copco representative for assistance.