The thread is an ornate spiral with a constant pitch, applied to a conical or cylindrical surface. It is the main element of the connection of two types of fasteners, as well as helical and gear-helical gears. The main parameters of threads on modern fasteners are defined in GOST 11708-82: this is the thread pitch between adjacent turns, the outer and inner diameters of the fastener element, the angle of the apex of the thread turn.
Types of threads on fasteners are divided into:
- internal - are applied to nuts, anchor sleeves, couplings, pipes;
- external - are located on self-tapping screws, screws, screws, bolts, studs.
The following types of threads according to the relevant GOST:
- metric - profile in the form of an equilateral triangle (it happens with small, medium and large steps);
- inch - triangular or trapezoidal shape;
- metric conical - has a triangular profile;
- round - the protrusion of the top of a round shape;
- trapezoidal - protrusion of the top of a trapezoidal shape;
- persistent - trapezoidal asymmetric profile;
- modular - trapezoidal shape of the profile;
- conical and cylindrical pipe (the profile has the form of an isosceles triangle of an inch size with a rounded (or flat-cut) apex and a hollow of a turn);
- conical inch - a profile with a flat-cut top of a triangular shape.
There are also such types of threads as square and rectangular. They are applied to fasteners with screw-cutting machines according to individual drawings. Such types of threads do not allow to obtain a high degree of accuracy, therefore, they are used quite rarely. There are no GOSTs on them.
Today, for general engineering applications, the main external and internal thread is considered metric. In the drawings, it is indicated by the capital letter "M" indicating the outer diameter in millimeters. Pipe threads are used to connect various pipes. Their nominal diameter is the inner diameter of the pipes on the outside of which a thread is cut. In the drawings, it is indicated by a large letter "G" indicating the inner diameter of the pipe in inches.
The main dimensions of the threads of all types are reference data. They can be found in any directory of the machine builder. For metric threads, reference data are described in detail in accordance with GOST 9150-81, 24705-81 and 8724-81. For a cylindrical pipe thread, the dimensions are specified in GOST 6357-81.
One common metric thread fastener is a bolt. It is a metal rod with a head on the end. A helical groove is applied along the length of the shaft. The purpose of the bolts is to connect various parts of mechanisms and structures using a nut. The head of the bolt is hexagonal and shaped (countersunk, semicircular, butt, embedded, terminal).
The following types of bolts are used for their intended purpose:
- furniture - are used to connect products in furniture production;
- machine-building - are used in industries of industrial importance;
- ploughshare - designed for coupling machinery equipment in the agricultural industry;
- road - are used to connect various metal structures and road fences.