Threads

Threads are applied for interconnection of machine parts and for transmitting motion from one part to another. When a thread is cut on the outside of a part it is known as an "external" or "male thread". A thread is called an "internal "or "female thread" when cut inside a part. Depending on the shape of the threading tool different profiles of thread are obtained, such as triangular, square or trapezoidal, shown in Fig. 33.

triangular thread, or V-thread
square thread, or flat thread
buttress thread
trapezoidal thread or acme thread
round thread

Fig. 33. Profiles of Threads

In practice triangular threads are most widely used. The main elements of a thread are: the angle of the thread, the major, minor and pitch diameters, the depth and the pitch. These elements are shown in Fig. 34.

Fig. 34. Main Elements of a Thread:

d₁ - minor diameter; d₀ - major diameter; d₂ - pitch diameter; S - pitch; t₂ - depth

The angle of a thread is the angle included between the sides of the thread and measured in an axial plane. The major, or outside, diameter of a thread d₀, (sometimes referred to as "full diameter") is the distance between the two extreme outside points of the thread in the direction square to the axis¹.

The major diameter is the largest diameter of the thread of a screw or a nut.

The minor diameter d_b being the smallest diameter of the thread, is the distance between the two extreme inside points of the thread measured at the right angle to the axis. The minor diameter is also called the "core diameter" or "root diameter".

The pitch diameter d2 is the distance between the two opposite parallel sides of the thread profile perpendicular to the thread axis. The depth t₂ of the thread is the distance between the crest and the base of the thread measured normal to the axis, or

t₂=(d₀-d₁)/2.

A crest is the top surface joining the two sides of a thread, while a base of a thread is the bottom surface joining the two adjacent threads. The pitch of a thread is the distance from a point of thread to the corresponding point of the next thread measured parallel to the axis.

Screw threads are of both right-hand and left-hand types. In right-hand threads the direction of the thread is from the right to the left. Right-hand threaded screws are turned clockwise to be screwed into a nut, while left-handed screws should be turned counterclockwise to do that.

Fig. 35. Screw Thread:

I - single thread screw; II - double thread screw; III - triple thread screw; a, b, с – pitch

In screw fastenings threads are made of various shapes, but always of the triangular type, such as: single thread, double thread and triple thread (Fig. 35). On the type of the latter depends the lead of the thread which is the distance a screw thread advances axially in one turn. On a single thread screw the lead and the pitch are the same; on the double thread screw the lead is twice the pitch, while on a triple thread screw the lead is three times the pitch.

The most widely used systems of triangular threads in machine-building are: metric, inch and pipe threads. Each thread has its own angle and application. A metric thread profile resembles a triangle with an angle of 60° at its apex. Such a thread is widely used for bolts and nuts. An inch tread profile has an angle of 55°. This type of thread may be used when making spare parts for foreign-made machines. An angle of 55° is also used with pipe threads. Pipe threads are applied for gas and water pipes, as well as for clutches connecting such pipes.

1. in the direction square to the axis - в направлении, перпендикулярном к оси

Дата добавления: 2014-12-27