How long should be left after the bolts are tightened?

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In the field of general machinery, bolted connections are one of the common structural joining methods, and their safety and reliability are directly related to the stability of the entire mechanical system. How long should be left after the bolts are tightened? Firstly, the “Technical Specification for High-Strength Bolt Connections in Steel Structures JGJ82-2011” states: “6.4.1 The length ‘l’ of high-strength bolts shall be such that after final tightening, there remain 2 to 3 threads exposed outside the bolt.” ”This standard merely specifies the requirements for high-strength bolts in steel structures. If the bolt protrudes too far, it not only wastes material but also poses a risk of being scratched; moreover, more steps are required for disassembly ； If the bolt protrudes too short, its strength may not be sufficient, which also fails to meet the requirements. Secondly, the construction standard widely used in the chemical industry, the \"General Specifications for the Construction and Acceptance of Mechanical Equipment Installation Projects GB 50231-2009\", also specifies clear requirements regarding the tightening length of anchor bolts, ordinary bolts or screws, as well as pipe flange connections: 4.1.1 Anchor bolts installed in the pre-drilled holes shall comply with the following provisions: ……6 After the nuts are tightened, the bolts should protrude from the nuts, with the protrusion length preferably being 2 to 3 thread pitches ； 5.2.1 When tightening bolt or screw connections, the following requirements shall be met: ┅┅ After the bolt and nut are tightened, the bolt should protrude from the nut by 2 to 3 pitches. 6.2.2 When pipes are connected using flanges, the following requirements shall be met: 6┅┅ The length of the thread that is exposed after tightening should not be more than 2 to 3 times the pitch. This standard sets clear requirements for the length of bolts to be left for equipment mounting foot bolts, equipment assembly, and pipe flange connections. Third, the standard commonly used for pipeline installation in the chemical industry, the \"Code for Construction of Industrial Metal Piping Projects GB 50235-2010\", states: \"7.3.4 Flange connections shall use bolts of the same specification, and the installation direction shall be consistent.\" The bolts should be tightened symmetrically. After the bolts are tightened, they should be in close contact with the flange, with no wedging gaps. When washers need to be added, no more than one should be used per bolt. All nuts should be fully tightened onto the bolts, and the bolts and nuts after tightening should be at the same level. It is obvious that the requirements of this standard are not consistent with those of the previous one. Then, the \"Technical Specification for Construction of Equipment and Pipeline Installation in Natural Gas Purification Plants SY/T 0460-2018\" states: \"4.1.4 The foundation bolts installed in the pre-drilled holes shall comply with the following requirements: ┅┅4 After tightening the nuts, the bolts shall protrude from the nuts by 0 to 3 pitch lengths.\" ”7.2.4 “Uniformly specified bolts are used for flange connections; the installation direction of the single-headed bolts should be consistent” ； After the bolts are tightened, the nuts should be in close contact with the flange. If washers are to be used, no more than one washer per nut is allowed. The bolts, after tightening, should protrude from the nuts by 0 to 3 thread pitches. ”7.2.8 The installation of valves shall comply with the following requirements: The flanges connected to the valves must remain coaxial to ensure that the bolts can move in and out freely; after tightening, the exposed length of the bolts should be 0 to 3 thread pitches. “This standard is somewhat similar to the previous one; it is acceptable as long as it is level, even if it isn’t visible. In addition, the \"Code for Acceptance of Construction Quality of Metal Piping in Petrochemical Projects GB 50517-2010\" states: \"8.1.10 The installation direction of the flange connection bolts shall be consistent, and after the bolts are tightened, they shall be in close contact with the flange.\" When washers are required, no more than one washer should be used per bolt; after tightening, the bolts and nuts should be at the same level or protrude by 1–2 pitch lengths, and the protruding parts of bolts on the same side should be at the same level. After reviewing the standards, no mandatory requirements were found; what is provided are merely empirical guidance methods, and these are not entirely consistent with each other. A correct understanding of these standards and their implementation in accordance with actual conditions is essential to ensure the safe operation of mechanical equipment.