Clamping force in plastic assemblies
What is clamping force?
The clamping force in a bolted joint is the load that acts on the joint elements to keep them together and resist any external loads they may experience. These loads are usually vibrations and/or tensions and can be static or dynamic.
When assembling a bolted joint, we convert rotation into displacement. The applied torque rotates the screw, which, due to the helix of its thread, advances axially in the joint until the screw head is seated. From that moment on, the applied torque generates tension in the axial direction (preload). This tension is transmitted to the parts, clamping them.
How much clamping force do I need?
It is important to note that the clamping force must be adequate to ensure a safe and strong joint. If the clamping force is insufficient, a sufficiently high external load could separate the parts, resulting in a weak and unstable joint. This could lead to premature fractures or failures due to the loss of tightness. On the other hand, if the clamping force is excessive, plastic deformations can occur in the parts, or even their rupture, eventually resulting in breaks or leaks. (See Fig.2)
Usually, we consider joints in terms of energy, so a force applied over time, such as the torque during tightening, introduces energy into the joint. This energy is stored in the form of clamping and is what allows the joint to resist external loads.
It is worth noting that the energy transfer in bolted joints is quite inefficient, as a significant amount of energy is lost due to friction between the sliding components. Control of friction conditions is important to determine the final clamping transmitted to the joint.
Applying a higher torque will not always result in a higher clamping force. There are many other factors that determine the clamping capacity of a particular screw class:
- Tightening torque: this is the maximum torque applied on the screw, and the main responsible for its clamping force.
- Coating friction coefficient: greater the friction coefficient, the lower the clamping force resulting from the input torque. (See Fig. 3).
- Recess efficiency: the recess transmits the input torque from the screwdriver to the screw. Highly efficient recess will allow better torque transmission and consequently, a higher clamp load of the assembly. TORX PLUS® recess guarantees an optimal assembly torque transmission, providing higher clamping force than Philips or POZI drive recesses.
- Thread geometry: thread design also affects the ratio between torque and clamping force. Generally, a finer pitch increases clamping force if the resulting rise of friction doesn’t overcome that increase.
How do we measure the clamping force?
There are accurate techniques to measure clamping force in a joint but it can be complicated since most sensors must be located inside the joint. Retrieving the sensor in this case is a serious inconvenience in industrial processes. For this reason, clamping force is usually characterized indirectly by measuring torque. The torque is proportional to the applied clamping force and can be measured externally to the joint. That is why, to achieve optimal clamping force, it is necessary to use appropriate screw tightening techniques.
CELO recommends using suitable tightening tools, such as torque wrenches or electronic control screwdrivers, to ensure the right tightening torque.
Our application engineers can assist you in estimating the clamping force in your joint and the appropriate tightening methods to achieve it. Do not hesitate to contact us to receive our technical support.