What is a thread?
A thread is a shape with uniform spiral lines on the section of the outer or inner surface of a solid.
1. Shape of section
Screw threads according to its cross section shape (tooth type) into triangular thread, rectangular thread, trapezoidal thread and serrated thread, etc.
Triangular thread is mainly used for connection (see thread connection), rectangular, trapezoidal and serrated thread is mainly used for transmission. Pipe threads are used for tight connection of pipe fittings. Rectangular thread has high efficiency, but because it is not easy to grind, and the inner and outer thread rotation and centring is difficult, so it is often replaced by trapezoidal thread. The working edge of the serrated thread is close to the rectangular straight edge, which is used to bear unidirectional axial force. The tooth shape of conical thread is triangular, which mainly depends on the deformation of the tooth to ensure the tightness of the thread pair, and is mostly used for pipe fittings.
Note: The triangle thread has good self-locking performance. It is divided into two kinds of coarse teeth and fine teeth, generally connected with coarse teeth thread. (mostly used for fastening wood) fine tooth pitch is small, small Angle of rise, better self-locking performance, often used for small parts in thin-walled tubes, vibration or variable load connection, and fine-tuning device.
2. Internal and external distribution
The thread distribution in the external surface of the mother is called the external thread, the inside surface of the mother is called the internal thread.
Eg: The thread on the surface of a bolt is an external thread, while the thread on a nut is an internal thread.
The internal thread of the same nominal diameter is larger than the external thread. Because the outer thread is screwed into the inner thread.
The left is internal thread and the right is external thread
3. Other categories
The thread formed on the cylindrical mother is called cylindrical thread, and the thread formed on the conical mother is called conical thread.
4. Helix direction
Screw thread according to the direction of the helix is divided into two kinds of left-handed and right-handed, generally with right-handed thread.
Left and right rotation is based on the needs of the mechanism. The thread on the rotating part, considering the need to prevent loosening, the thread direction is opposite to the rotation direction of the part.
Judgment method
1. In the picture on the right, imagine placing your finger in the position of the black arrow, and turning the thread counterclockwise to change to the yellow position. Using the arrow as a reference, the thread moves downward. So it is a left screw thread. 2. Place the thread vertically (either front or back). The left high thread is the left screw.
5, single line and multi line
Threads can be divided into single thread and multi – thread.
Single thread thread:
A thread with only one spiral slot on a cylinder is called a single thread thread
Double thread thread:
Double thread, also called double thread, refers to a cylinder with two spirals.
The connection is mostly single line; For transmission requirements of fast or high efficiency, the use of double or multiple lines, but generally not more than 4 lines.
Single thread due to its spiral Angle is small (not easy to slide), screw and nut screw forming greater friction (with self-locking ability), used in the thread locking, such as fixed ceiling fan screws and nuts, gas cylinder joints and mechanical equipment parts between the fixed connection; The multi-thread thread due to its larger thread Angle (easy to slide), screw and nut screw forming less friction, used for transmission of power and movement, such as a jack for wheel maintenance, used to clamp the workpiece for locksmithing processing table vice and lathe lead rod for thread processing, etc.
The important parameters
The main parameters of the thread are outer diameter (D), inner diameter (D1), middle diameter (D2), pitch (t), line number (n), lead (s= N t), Angle (λ) and conodont Angle (α), etc. Except for the pipe thread, the pipe inner diameter is the nominal diameter, and the other threads are the nominal diameter.
Post time: Aug-26-2022