The resulting compressive stresses provide substantially increased fatigue strength at the thread root, which is usually the weakest point. The thread root is the weakest point because it is the smallest cross-sectional area in the bolt. The stressed area A of a bolt is computed from
|Comparison of proof strength of fine and coarse threads, SAE Grade 5, ³⁄₄-in bolts|
Securing and Tightening. For the average low- and medium-pressure installations, bolts are made in staggered sequence with wrenches which will usually result in adequately tight joints. For the high-pressure and -temperature joints, it becomes increasingly important to make up each stud to a definite tension. Torque wrenches are sometimes used for this purpose.
In exceptional cases where a more positive method is desired, the studs may be tightened until a definite elongation has been attained. For this condition, an initial cold tension of 30,000 psi (2100 kg/cm2) to 35,000 psi (2460 kg/cm2) in each stud is recommended. Since the modulus of elasticity of stud material is about 30 106 psi (2.1 x 106 kg/cm2), a tension of 30,000 psi (2100 kg/cm2) would result in an elongation 0.1% of effective length. The effective length is the distance between nut faces plus one nut thickness. Special studs with ground ends are required to make micrometer measurements for this purpose. After the joint has been in service, periodic checks of the actual cold lengths as compared with the tabulated lengths will detect any permanent elongation of the studs. Permanent elongation will indicate over-stressing, relaxation, and creep. When these conditions become severe, new studs may be required to maintain the joint properly. Special thread lubricants are available for temperatures both below 500 F (260 C) and from 500 F (260 C) to 1000 F (540 C). Such lubricants not only facilitate initial tightening but also permit easier dis-assembly after service.
|Turning Efforts to Tighten Eight-Pitch-Thread Bolts|