~ Article courtesy of Accu Components
Torque, a pivotal concept recognized by engineers globally, is the twisting or rotational force causing movement about a central point. In simple terms, it’s the capacity of an object to counteract resistance to rotation. In fastener installations, torque defines the peak twisting force a fastener can withstand.
Tightening torquing is a critical factor to consider, ensuring the stability of fastener installations and the joint’s ability to withstand external forces and vibrations. Proper torquing reduces loosening risks and upholds the integrity of an assembly.
Maximum tightening torques indicate the extreme boundaries for fastener use. Any force beyond these limits might introduce undue stress, risking a potential malfunction. Attaining this exact force is generally discouraged, as it’s the brink of failure. As such, approaching this point typically leads to damage to the components.
It’s worth noting that maximum tightening torques are not constants and vary based on the specific use case. Over-tightening a fastener usually results in going beyond the suggested limits.
Recommended tightening torques
Understanding the maximum rotational torque a component can tolerate can determine its recommended tightening torque.
Recommended tightening torques differ from maximum tightening torques in ensuring fasteners reach the appropriate tightness for assembly without exceeding the limit — where failure becomes a concern.
The link between preload and torque
Preload, like torque, is a force exerted on a component during installation. But rather than being rotational, preload represents the axial force exerted on a fastener when tightened. Applying torque to install a fastener results in a force termed preload.
Upon installation, minor elongation occurs in the fastener. This generates tension within the assembly and leads to elastic deformation. Once the tightening force is removed, fasteners will typically return to their initial dimensions.
In specific scenarios, elastic deformation can be exceeded by using unique parts called torque-to-yield fasteners. The preload’s tightening force is vital for preserving the cohesion of a joint, uniformly distributing the load a fastener carries across the connected materials.
When a fastener reaches adequate “tightness,” it compresses the adjacent material, typically the bolt and nut. This compression affects both the assembly and the fastener, solidifying their connection and forming a reliable joint. Such compression enables a jointed fastener to withstand a stronger force than one under no tensile load.
Torque-to-yield fasteners
Commonly found in automotive sectors and mechanical setups, torque-to-yield fasteners are crafted to be tightened past their elastic limit.
Once a component exceeds its yield strength, it undergoes permanent deformation. This unique characteristic enables a fastener to sustain tension beyond the typical limits for its size — paving the way for lighter, cost-effective alternatives.
Torque-to-yield fasteners are crafted to slightly and permanently elongate when tightened past their elastic threshold. Such deliberate stretching bestows accurate and uniform preload in the joint, ensuring even clamping force distribution among the connected parts.
Torque-to-yield fasteners are often in critical areas where strong, durable, and lightweight components are ideal for performance, reliability, and safety. For example, they’re often used in critical engine applications like cylinder head bolts, main bearing cap bolts, and other high-stress areas.
Given these components can stretch permanently, they’re usually intended for single use and should be substituted upon removal. However, their even load distribution permits a greater clamping force even with a reduced fastener size.
The significance of materials
The recommended tightening torque for fasteners is influenced by multiple factors, including a fastener’s material, size, type, and the specific demands of the job.
Different materials possess different properties in regard to resilience, toughness, and flexibility, which determine their reaction to tightening. For instance, when fastening two metal components together, the recommended tightening torque values will vary if the metals are aluminum versus steel.
Polycarbonate screws, having a lower tensile strength, will have a lower recommended tightening torque when compared to a similarly sized hardened steel one. Likewise, fastening varied materials together, like metal to wood or plastic, will have different torque requirements.
Although several factors determine the required tightening torque, general torque guidelines exist, typically derived from the fastener’s dimensions and material.
Torque influencers
Just as recommended tightening torques can be influenced by the fastener’s size and material, external factors can also sway the behavior of a fastener upon tightening.
However, misconceptions persist about which aspects truly influence the suggested torque.
- The tool chosen will not alter the required tightening torque, provided it’s suitable for the fastener in question. However, certain drives, like Torx, enhance torque transmission, resisting higher torques and preventing slippage.
- Fastener head and drive types have a marginal effect on torque. Nonetheless, specialized components exist, like captive screws that might necessitate a lesser torque. Similarly, shear bolts typically have lower driving torques due to their design.
- Lubrication does influence the necessary torque by decreasing friction between threaded surfaces, hence decreasing the driving torque needed. This can lead to a significant increase in the joint’s tensile stress at the same torque and can lead to failure. Lubricants can significantly lower the driving torque required and are rarely specified within installation guidelines.
Filed Under: Fastening + joining, Screws • nuts • bolts • rivets