THE DIFFERENCE BETWEEN BOLTS AND SCREWS

Judging from the bolts and screws listed in national standards, bolts are characterized by three types of heads (except special bolts such as anchor bolts and union bolts), hexagonal heads, square heads (including T-bolts) and round heads. , Round head bolts are generally countersunk head bolts with tenons or necks. The head feature of the screw is basically round, with slots, cross recesses and hexagonal sockets (including hexagonal holes) on the end face of the circle. Use one sentence to determine whether it is a bolt or a screw. The round head without a tenon and neck is a screw, and the rest is a bolt. This is the most intuitive way for us to determine whether it is a bolt or a screw.

Bolts and screws (excluding self-tapping screws and wood screws, because the thread profile of self-tapping screws and wood screws are different from that of machine screws, while the thread profile of machine screws is the same as that of bolts) are all threaded fasteners , the connections composed of fasteners include bolt connections, double-head stud connections, and screw connections (including set screw connections).

The characteristic of bolt connection is that the connecting part of the connected parts is a through hole, and the bolt connection pair is generally composed of bolts, nuts, and washers.

The characteristic of double-ended stud connection is that one of the connected parts needs to be processed with a threaded hole, and the other connecting parts are processed into through holes. The double-ended stud connection pair generally includes double-ended studs, nuts, and washers. This kind of connection is suitable for situations where bolt connection cannot be used structurally. For example, one of the connected parts is too thick and should not be made into a through hole.

The characteristic of screw connection (excluding set screws) is that one of the connected parts also needs to be processed with a threaded hole, and the other connecting parts are processed into through holes. The screws are directly screwed into the threaded holes of the connecting parts, but the screw connection pair does not need to use nuts. , only screws and washers. This type of connection is structurally simpler and more compact than studs. Its purpose is similar to that of stud bolt connection.

The set screw connection uses the end of the screw screwed into the threaded hole of the part to push against the surface of another part or into the corresponding pit to fix the relative position of the two parts and transmit small force or rotation. moment.

From the concept of fastener connection, the significant difference between screws and bolts is whether nuts are used for connection.

Currently, there are 57 national standard bolt types in use (including 15 bolts for assemblies and bolt connection pairs, and 2 bolt bolt standards are summarized in the 5th edition of the "Mechanical Design Manual", but have been abolished). There are 68 types of standard screws (including 10 screws used in assemblies, and 2 standards are shared with bolts).

The national standard catalog of bolts and screws is attached.

Generally speaking, the bolts of 57 national standards are used in bolted connections, and the screws of 68 national standards are used in screw connections. But in some cases, bolts are used in screw connection situations (usually hexagonal head bolts), and screws are used in bolt connection situations (rarely used).