Clamping force between the parts assures the assemly and it's the main parameter which determines the mechanical properties of the application.
Applying higher torque will not always results in higher clamping force. There are many other factors determining the clamping capability of a screw:
Tightening torque: torsion force applied on the screw is the main responsible of clamping force generated.
Coating friction coefficient: the greater the friction coefficient is, the less the torque will be profited to obtain clamping force in the assembly.
Recess efficiency: the recess transmits the torque from the screwdriver to the screw. High efficient recess will allow a better torque transmission and consequently, higher campling of the assembly.
TORX Plus® recess guarantees an optimal assembly torque transmission, providing higher clamping force than Philips or Pozidrive recess.
Thread geometry: thread design also affects the relationship between torque and clamping force. Generally, a finer pitch increases clamping force if the resulting rise of friction doesn’t overcome that increase.
The following curves show the effect of thread geometry (pitch) and recess type on Pre-Load force during the assembly process of screws diameter 4mm on plastic part PA66 + 15%GF, length of engagement of 6 mm.
Methods to check clamping force are complicated, so normally we check torque values to verify the application. Torque doesn’t give a quantitative indication of the assembly’s properties, but a qualitative approach to the tightening of the parts.
Aside from these problems, new developed technologies to measure clamping load are improving and getting easier and more accurate by the day. CELO technical laboratory is well equiped with adequate clamp load cells which allows us to relate torque and clamping force and also to evaluate the effect of creeping in plastic assemblies.
Please, contact us in case you need further information.