🔧 Bolts, torque and fits

Bolt grade markings

Metric class numbers read like this: the first number × 100 is the nominal tensile strength in MPa, and the number after the point × 10 is the yield as a percentage of tensile. So 10.9 means about 1000 MPa nominal tensile, with yield at 90% of that (about 900 MPa). Use nuts of a matching or higher grade or class.

Torque basics

Torque is only a way to get the bolt stretched to the right clamp load (preload). The short-form relationship is T = K × D × F: torque = nut factor × nominal diameter × target clamp load.

• K is often taken as about 0.20 for dry, plain steel. Lubricated, plated or coated threads have a lower K. The same torque then gives much more preload, so a dry figure can overload a lubed bolt.
• Always use the equipment maker's torque spec and sequence, including whether threads are dry or lubed. Values here are typical, for estimating only. Verify against the manufacturer.
• Tighten in a cross pattern in stages. Use a calibrated wrench.

Fits and tolerances basics

• Tolerance: the total amount a size may vary (upper limit − lower limit).
• Allowance: the intended difference between mating parts at maximum material (tightest fit).
• Clearance fit: the shaft is always smaller than the hole. Interference (press) fit: the shaft is always larger. Transition fit: it could be either.
• ISO hole-basis examples: H7/g6 is a close running or sliding fit, H7/h6 a locational clearance fit, H7/k6 a transition fit, and H7/p6 a locational interference (press) fit. The letter sets the position of the tolerance zone and the number sets its width (grade).
• Bearings: the ring that rotates relative to the load usually gets the interference fit. Use the bearing maker's fit tables.