EngineerBenchEstimating bolt torque and preload
The relationship T = K × F × d links tightening torque, preload, nominal bolt diameter, and a nut factor K. It is a simplified torque-preload model. Most of the applied torque is affected by friction, so using the same torque with a different finish or lubricant can produce a different preload.
Use a nut factor for the actual fastener condition
Enter a factor appropriate to the coating, lubricant, mating surfaces, and installation process. The proof-load fraction is a chosen target, not a universal tightening recommendation. Check that the strength data and diameter match the fastener being installed.
Read the range as well as the target
The uncertainty calculation combines the entered nut-factor range and torque-tool variation to show possible preload extremes. It does not capture every installation effect. For a critical joint, use a specified tightening procedure and an appropriate means of verifying preload rather than relying on a single estimated torque.
Checking the torque units
With a nominal diameter of 10 mm, a target preload of 10,000 N, and an assumed nut factor of 0.2, the simplified relationship gives 20 N·m. Diameter must be 0.01 m when torque is expressed in newton-meters. Entering 10 as though millimeters were meters would create a thousandfold error. The unit controls handle this conversion, but the original measurement still needs the correct label.
When solving for preload from a fixed torque, a smaller nut factor gives a larger estimated preload. This is why changing lubrication without changing the tightening procedure can matter. The uncertainty endpoints should be read alongside the nominal result: low friction with high applied torque can govern maximum preload, while high friction with low torque can govern minimum preload. Confirm which endpoint is important for bolt stress, separation, or slip. A narrow torque-tool tolerance does not eliminate friction uncertainty, and repeated tightening can alter conditions. Use qualified fastener and assembly data for a critical joint.
Formula
T = KFd. Preload extremes divide torque extremes by the opposite nut-factor extremes.