Screw Threads (螺纹)

Screw threads manufacturing(螺纹的形成)

Screw threads are continuous protrusions or grooves found along a helix on a revolving component with the same cross sections. Screw threads on an external surface are called external threads,and those found on the internal surface are called internal threads,as shown in Fig.7- 1.  


Various methods are employed in thread manufacturing, such as turning, roll-pressing, and using tools like thread chasers and spiral taps. In the process of lathe manufacturing, the workpi,ece,clamped with a chuck, rotates at a constant speed. After getting to a given distance around the radius, the lathe blade moves along the axial direction and thread curves are thus manufactured on the surface of the workpiece. For workpieces of small diameters, external threads and internal threads can be manufactured manually with the help of a thread chaser or a spiral tap, as shown in Fig. 7-2.


Thread structure(螺纹结构)

(1) Thread head  For installation purposes, to prevent damageand to provide operational safety, thread heads are shaped with certain features and the commonly used features are chamfer,round head and flat head as shown in Fig. 7-3.

(1) 螺纹端部  为了便于安装、防止损坏和安全操作, 通常在螺纹的起始处加工成一定形状,常见的形式如倒角、圆头及平头,如图7-3所示。

(2) Vanishthread and undercut Vanish thread is the incomplete segment at the end of a thread,originated when the tooth shape gradually becomes low for the lathe blade to gradually get out of the workpiece. In order to avoid the formation of vanish thread and to make it easier for the lathe blade to complete its work, a small groove,called undercut, can be lathed in advance at the end of the screw termination as shown in Fig. 7-4.

(2) 螺尾及退刀槽  当车削螺纹终了时,在车刀逐渐离开工件处出现的一段牙型逐渐变浅的不完整螺纹,称为螺尾。为了避免产生螺尾和便于退刀,可在螺纹终止处预先车出一个小槽,称为退刀槽,如图7-4所示。

Thread elements(螺纹要素)

(1) Form of thread  The outline of the thread section cutting through the thread axis shows the form of thread. The common form of thread are triangular,trapezoidal(acme), buttress and square profiles as shown in Fig. 7-5.

(1) 螺纹牙型  通过螺纹轴线剖切时,螺纹断面的轮廓形状称为螺纹的牙型。常用螺纹的牙型有三角形、梯形、锯齿形和矩形,如图7-5所示。

(2) Thread diameter   Thread diameters include the major diameter (d,D), the minor diameter(d1,D1) and the pitch diameter(d2, D2).External thread diameters are indicated with lowercase letters, while internal thread diameters are indicated with capital letters, as shown in Fig. 7-6.

(2) 螺纹直径  螺纹的直径包括大径(d, D)、小径(d1, D1)、中径(d2,D2)。外螺纹的直径用小写字母表示,内螺纹的直径用大写字母表示,如图7-6所示。

The thread’s nominal diameter is generally the major diameter of the thread. The major diameter is the diameter of the column which passes the crest of the external thread or the root of the internal thread. The minor diameter is the diameter of the column which passes the root of the external thread or the crest of the internal thread. The pitch diameter is the diameter of the column that the thread groove width of which is equal to that of the thread ridge thickness.


(3) Number ofthreadsn) A screw can be a single or multiple thread. If a screw has only one helix, it is called a single thread screw. While if it has two or more helix(equidi stant distribution of the axial), it is called a multiple thread screw. The number of threads as shown in Fig.7-7 is specified with letter “n”.

(3) 螺纹线数(n)  螺纹有单线和多线之分。沿一条螺旋线所形成的螺纹,称为单线螺纹。沿两条或两条以上(在轴向等距分布的)螺旋线所形成的螺纹,称为多线螺纹。螺纹线数用n表示,如图7-7所示。

(4) PitchP The axial distance between two points of adjacent teeth on the central diameter line is called thread pitch, symbolized as P (Fig. 7-7).

(4) 螺距(P) 相邻两牙在中径线上对应两点间的轴向距离,称为螺距,以P表示(图7-7)。

(5) Lead (Ph) The axial distance between two points of adjacent teeth of the same spiral line on the central diameter line is called lead, denoted as Ph(Ph = nP), shown in Fig.7-7.

(5) 导程(Ph 同一条螺旋线上相邻两牙在中径线上对应两点间的轴向距离,称为导程,用Ph(Ph = nP)表示(图7-7)。

(6) Revolving direction A thread is called a right-hand thread if it follows clockwise turning when entering, otherwise it is known as a left-hand thread. A thread can be classified following the right or left spire rule, according to the methods shown in Fig.7-8. Left-hand threads are generally used for devices with special requirement,such as a gas bottle switch.

(6) 旋向 顺时针旋转时旋入的螺纹称为右旋螺纹,反之为左旋螺纹。可用右手或左手螺旋规则,按图7-8所示的方法判断。左旋螺纹一般用于有特殊要求之处,如煤气罐开关处等。

Types of threads(螺纹的种类)

(1) Classification following thread standardization  

According to the classification following thread standardization, there are three types of threads:

1)Standard thread. The thread profile, diameters and pitch are standardized.

2)Special thread. Threads manufactured following thread profile norms of national standards, but their diameters and pitch do not necessarily have to meet such standard.

3)Nonstandardthread. Threads with thread profiles not specified in national standards.

(1)按标准化程度分类   可分为三大类:

1) 标准螺纹。 牙型、直径和螺距均符合国家标准的螺纹。

2) 特殊螺纹。 牙型符合国家标准,而直径和螺距不符合国家标准的螺纹。

3) 非标准螺纹。 牙型不符合国家标准的螺纹。

(2) Classification according to application   There are four types of thread.

1) Thread for fastening joining, or fastening thread for short.Typical examples include general thread,mini thread, transition fitting thread and interference fitting thread.

2) Thread for transmission, or transmission thread for short.  Typical examples include trapezoidal (acme) thread, buttress thread and square thread.

3) Thread for pipes, or pipe thread for short.   Typical examples include:Pipe threads with 55 degree thread angle where pressure-tight joints are notmade on the threads, Pipe threads with 55 degree thread angle where pressure-tight joints are made on the threads, Pipe threads with 60 degree thread angle where pressure-tight joints are made on the threads and metric taper thread.

4)Special application thread,or special thread for short.   Typical examples include self-drilling screw thread, wooden nail thread and thread for gas tanks. 

(2) 按螺纹用途分类    可分为四大类,如表7-1所示。

1)紧固联接用螺纹,简称紧固螺纹。  例如,普通螺纹、小螺纹、过渡配合螺纹和过盈配合螺纹。

2)传动用螺纹,简称传动螺纹。 例如,梯形螺纹、锯齿形螺纹和矩形螺纹。



Commonly used standardthreads are shown in Table 7-1.