Why Correct Torque Specifications Matter in Industrial Bolting

What torque actually does

Most of the torque you apply is used up in friction under the head and in the threads. Only a small fraction becomes useful stretch in the bolt. That is why the same torque can give quite different clamp force if the threads are dry, oiled or plated differently.

Where the right value comes from

Use the torque given by the equipment maker or the joint designer. Where none is given, use a standard table for the bolt size, grade and lubrication condition, and note that the table's assumption about friction must match your bolts. Flanged joints with gaskets have their own procedures.

Factors that change the result

• Lubrication: oiled or anti-seize threads need less torque for the same clamp load than dry ones.
• Coating: zinc plating, phosphate and other finishes change friction.
• Dirty or damaged threads raise friction and give false torque readings.
• Reused bolts, particularly high-tensile ones, may have yielded; replace them where the design says so.

Tightening practice

Use a calibrated torque wrench, applied smoothly. Tighten in stages and in a cross pattern on flanges and multi-bolt covers, so the load builds up evenly. Check the wrench calibration at regular intervals.

Consequences of getting it wrong

Under-torque leads to loosening, fretting and leaking gaskets. Over-torque leads to stretched or broken bolts, stripped threads and distorted flanges. In both cases the joint can fail while the machine runs.