How to determine the pitch of a metric thread

Operational purpose of the thread

The fastening thread provides a complete and reliable connection of parts under various loads and at various temperature conditions. This type includes metric .

The fastening and sealing thread is designed to ensure the tightness and impermeability of threaded connections (without taking into account shock loads). This type includes metric fine pitch, cylindrical and conical pipe threads, and conical inch threads.

Leading thread is used to convert rotational motion into translational. It perceives great efforts at relatively low speeds. This type includes threads: trapezoidal , thrust , rectangular , round .

Special thread has a special purpose and is used in certain specialized industries. These include the following:

– metric tight thread – a thread made on a rod (on a stud) and in a hole (in a socket) according to the largest limiting dimensions; designed to form threaded connections with an interference fit;

– metric thread with gaps – thread with necessary to ensure easy screwing and loosening of threaded connections of parts operating at high temperatures, when conditions are created for setting (joining) oxide films that cover the surface of the thread;

– watch thread (metric) – thread used in the watch industry (diameters from 0.25 to 0.9 mm);

– thread for microscopes – thread designed to connect the tube with the lens; has two sizes: 1) inch – diameter 4/5 І (20.270 mm) and pitch 0.705 mm (36 threads per 1І); 2) metric – diameter 27 mm, pitch 0.75 mm;

– ocular multiple thread – recommended for optical instruments; the thread profile is an isosceles trapezoid with an angle of 60 0 .

Figure 104 – Classification of threads

Advantages and disadvantages of threaded connections
Advantages of threaded connections:
– high load capacity and reliability;
– interchangeability of threaded parts in connection with the standardization of threads;
– ease of assembly and disassembly of threaded connections;
– centralized production of threaded connections;
– the possibility of creating large axial compressive forces of parts with a small force applied to the key.

Disadvantages of threaded connections:
– the main disadvantage of threaded connections is the presence of a large number of stress concentrators on the surfaces of threaded parts, which reduce their fatigue resistance under variable loads.

Axial load distribution over threads

The axial load on the nut threads is unevenly distributed due to the unfavorable combination of screw and nut deformations (the threads in the most stretched part of the screw interact with the threads in the most compressed part of the nut).
The statically indeterminate problem of load distribution over the turns of a rectangular thread of a nut with 10 turns was solved by Professor N. E. Zhukovsky in 1902.

The first turn transmits about 34% of the total load, the second – about 23%, and the tenth – less than 1%. It follows that it makes no sense to use too high nuts in the fastening connection. The standard provides for a nut height of 0.8d for normal and 0.5d for low nuts used in lightly loaded connections.

To equalize the load in the thread, special nuts are used, which is especially important in joints operating under cyclic loads.

Thread metric

Metric thread (Fig. 120). The main type of fastening thread in Russia is a metric thread with a triangular profile angle a equal to 60°. The dimensions of its elements are given in millimeters.

This is the main type of fastening thread designed to connect parts directly to each other or using standard products that have metric threads, such as bolts, screws, studs, nuts.

According to GOST 8724-81, metric threads are made with coarse and fine pitch on surfaces with diameters from 1 to 68 mm – over 68 mm, the thread has only a fine pitch, while fine thread pitch can be different for the same diameter, and large thread has only one value. The coarse pitch in the thread symbol is not indicated. For example: for a thread with a diameter of 10 mm, the coarse thread pitch is 1.5 mm, the fine pitch is 1.25; one; 0.75; 0.5 mm.

Rice. 120

According to GOST 8724-81, metric threads for diameters from 1 to 600 mm are divided into two types: with a large pitch (for diameters from 1 to 68 mm) and with a fine pitch (for diameters from 1 to 600 mm).

Coarse pitch threads are used in connections subjected to impact loads. Thread with a fine pitch – in the connections of parts with thin walls and to obtain a tight connection. In addition, fine threads are widely used in adjusting and set screws and nuts, as they make it easier to make fine adjustments.

When designing new machines, only metric threads are used.

The metric thread is denoted by the letter M:

M16, M42, M64 – with a large pitch

M16×0.5; M42×2; M64×3 – with fine pitch

M42 × 3 (P1) – this means that the thread is multi-start with a diameter of 42 mm, a pitch of 1 mm and its stroke is 3 mm (three-start)

M14LH, M40 × 2LH, M42 × 3 (P1) LH – if you need to designate a left-hand thread, then put the letters LH after the symbol

How to determine the pitch of a metric thread

The easiest way is to measure the length of ten turns and divide by 10.

You can use a special tool – a metric thread gauge.

Thread inch

Currently, there is no standard that regulates the main dimensions of an inch thread. The previously existing OST NKTP 1260 was canceled, and the use of inch threads in new developments is not allowed.

This is a thread of a triangular profile with an angle at the top of 55 ° (a equal to 55 °). The nominal diameter of an inch thread (the outer diameter of the thread on the shank) is indicated in inches. In Russia, inch threads are allowed only in the manufacture of spare parts for old or imported equipment and are not used in the design of new parts.

As mentioned earlier, the UK with its English system of measures can be considered the birthplace of standardized carving. The most prominent English engineer-inventor who took care of putting things in order with threaded parts is Joseph Whitworth , or Joseph Whitworth, that’s also correct. Whitworth proved to be a talented and very active engineer; so active and enterprising that the first BSW thread standard he developed in 1841 was approved for general use at the state level in 1881. By this point, the BSW thread has become the most common inch thread, not only in the UK, but also in Europe. The fruitful J. Whitworth developed a number of other standards for inch threads for special applications; some of them are widely used to this day.

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