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Bolted joints - How tight is tight?

Assembly, installation, and servicing of industrial machinery all involve bolted joints. To ensure joint integrity, bolts must be properly tightened.

That produces an initial tension or reload in the bolt, as well as compressive or clamping forces within joint members. Any external tensile load applied to the joint will only act to reduce the clamping forces, without significantly increasing the force within bolts themselves, until that external load exceeds the initial reload.

Such external loads are often cyclic, leading to premature fatigue failure of bolls. Proper reload will prevent that, and is achieved by applying the right amount of tightening torque. A torque wrench is often used. Another way to tighten bolts, common in structural joints, is the "turn-of-nut" method. After applying a small initial torque, the nut is rotated a specified additional amount, such as 90 degrees, or a half turn. Because this can easily result in over stressing the boll, it must be done carefully with full knowledge of the materials and stress limits involved.

However, the exact relationship between torque and bolt tension will vary with thread condition; the material (including plating); and lubrication. The only true indication of reload, but the most difficult to measure accurately, is the elongation of the tightened bolt.

Joint integrity is also dependent upon several conditions in the joint members themselves. Mating surfaces must be flat and at right angles to the bolts. Otherwise, the joint may act as a spring, leading to destructive resonant vibration. In electrical joints, poor contact between the surfaces results in overheating.

Also, the bolt head and nut must uniformly bear against surfaces with which they are in contact. Otherwise, the bolt will undergo bending stress that greatly reduces fatigue life (as much as 80% with only one degree of surface angularity).

Regardless of how well the joint is tightened initially, vibration or cyclic loading can cause fastener loosening. Tightening force creates a small loosening torque within threads. Thai, plus the air space always existing within the mating threads, inevitably leads to loosening. Preventive measures include sealants that fill the voids; locking washers; or special fastener designs that inhibit rotation.

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