Permanent connections: technological process and classification

2024 Author: Howard Calhoun | [email protected]. Last modified: 2023-12-17 10:16

Docking of elements and structures can be divided into two main groups: detachable and permanent connections. The first include those that can be disassembled without violating the integrity of the fastening elements. These are fasteners with nuts, bolts, studs, screws, all connections with and without threads. One-piece are those, when disassembling which will have to break the fasteners.

These include: welded, glued, riveted, stitched and soldered. Detachable and permanent connections are widely used in certain industries. Below we will consider each of the species in more detail.

Plug connections

Their execution consists in drilling holes of a slightly larger diameter than the fastener (screw or bolt). This is done so that there are accurate holes in both fastened parts. An error of a fraction of a millimeter is compensated, especially for elements with a large number of fasteners. When using bolts and screws for the reliability of the joint on themput on the nut and washer.

The first one is placed under the second one for the immobility of the connection, it does not allow the parts to rotate. There is also a spring ring that has two sharp teeth. With them, it rests against the workpiece and the part, thereby preventing spontaneous unwinding of the nut.

Screws tighten parts by cutting threads on their own. When using them, nuts and washers are not needed. Studs are used if another is attached to a massive part. It has a thread at both ends, a hole is drilled under it in the workpiece more than the thread length of the stud.

Permanent connections

They come in:

• welded;
• rivet;
• brazed;
• glutinous.

Such types of permanent connections have found application in certain areas of production. Let's consider each of them separately.

Welding

A joint adopted by interatomic bonds between parts of parts when heated is called welded.

Permanent joints, welded correctly, achieve the necessary strength, cost savings, and part weight.

Sources of element heating can be:

• molten slag;
• gas flame;
• electric arc;
• plasma;
• laser beam.

The metal to be welded is called the base metal. And the one used in the bath is filler.

A section tacked in this way is called a weld.

Obtaining permanent connections in this way can be of the following types:

• contact welding;
• electric arc manual;
• automatic submerged arc and semi-automatic;
• arc.

The seam is also subdivided into:

• butt;
• lapped;
• angular;
• tee.

Any of them can be either one-sided or two-sided.

They are divided into discontinuous and continuous. There are also differences in cross-sectional shape: normal seam, convex or concave.

Benefits:

1. Low cost for such one-piece connections, due to the simplicity of the seam and low labor intensity.
2. Relatively light weight compared to other working methods.
3. No need to make holes in the part, which gives strength in its section.
4. Automation of the welding process implies its tightness.

Flaws:

1. The appearance of deformation and warping after the work performed, as well as the occurrence of residual stresses.
2. Withstands mild vibration and shock.
3. Difficulty in quality control.
4. Workers who make permanent connections of parts by welding must be trained and confirm their qualifications.

Soldering

Parts in soldering method are bonded by introductionadditional solder metal. Moreover, the melting temperature of the solder should be less than that of the parts to be joined. According to this criterion, solders are distinguished:

• especially fusible. Their required melting point is only 145 degrees;
• soft or fusible. Operating temperature not higher than 450 degrees Celsius;
• hard or medium melting. Their melting point ranges from 450 to 600 degrees;
• high-temperature or high-melting. Such metals melt at temperatures above 600 degrees Celsius.

Solders

Depending on the component, they are divided into:

• tin-lead (PIC);
• tin (PO);
• zinc (PC);
• silver (PSr);
• copper-zinc (PMC, brass).

Most soldering work is done with POS grade tin-lead material. As a rule, they are produced in the form of wire, ribbons or twigs.

Before soldering, the surfaces are well cleaned. So that they do not oxidize, a special soldering flux is used. This substance prevents the formation of oxides and cleans the surfaces of parts from them, contributes to a better spreading of the solder. A certain type of flux is suitable for a specific temperature, above which it stops working and burns out.

Riveting

These are connections that are created using a special part - a rivet. It has a stem and a head. The process of obtaining permanent joints occurs due to the formation ofthe other end of the part of the closing head, it is obtained by compressing the end of the rod. Such a design is completely motionless and at the same time one-piece. It does not have the ability to shift parts relative to each other.

Use this fastening for parts of small thickness, mainly sheet materials, or where the use of high temperatures is unacceptable due to possible deformation of the parts. When the rivets are side by side, they form a rivet seam.

The material of the elements must match the material of the fastened parts, otherwise electrochemical corrosion may occur due to the difference in thermal expansion coefficients. Rivet heads are available as round, countersunk, semi-sunk and flat.

Pros

The benefits of this compound:

1. The ability to withstand high vibration and impact loads, which is beyond the power of welding.
2. Use possible in materials that are not weldable or the process is very long.
3. No application of high temperatures when joining.

Cons

Among them are the following points:

1. High metal consumption for the work done.
2. Increasing the weight of the structure.
3. High labor intensity.
4. Manufacturability of the process is low.

Adhesive

To get strong one-piece connections, it is enough to connect the parts with an adhesive. The action occurs by the formation of bonds at the intermolecular level of the surface of the bonded part and the filmglue.

The use of this method can be found in structures made of various materials. Glue-based fastening is used even in bridge building and aviation. The durability of such a connection and its quality will depend on the preparation of the surfaces of the parts and the type of load that will affect them. It is necessary to clean the surfaces from rust and grease stains, then treat the places with sandpaper.

It is not necessary to glue parts that will be subject to a shear or rotation load with a small joint area. This will result in a loss of strength. It is better to glue those parts that are subject to displacement relative to each other or tensile load.

Advantages of the adhesive method:

1. You can connect any blanks and structures in this way, regardless of their shape, weight or materials.
2. High corrosion resistance.
3. Tightness, which allows you to work with pipelines.
4. Does not cause deformation of parts.
5. No stress concentration is created.
6. Reliable performance under vibration loads.
7. Low cost consumables.
8. Adhesive one-piece connections do not make the structure heavier.

Cons:

1. Low strength, especially under pull-off load.
2. Fragile, some adhesives may age.
3. Low resistance to thermal stress.
4. Many compounds must undergo long exposure before use.
5. Compulsory observance of security measures.

Permanent polyethylene-steel connection

One-piece polyethylene-steel connection has been widely used for joining steel and modern polyethylene pipes.

It allows you to securely fasten plastic and metal pipes together, as well as install the necessary fittings for constipation. To make an inextricable structure, polyethylene pipes made according to a certain standard are used.

A one-piece steel connection (PE-steel adapter) is obtained by welding a branch pipe of a metal section with a polyethylene one. This method can be used as plugs on gas and water pipelines of main networks.

Such permanent pipe connections are mounted to the gas pipelines of residential buildings. You can often find them in boiler plants. The use of steel pipelines in our time is increasingly being replaced by an analogue of polyethylene. This is due to the obvious advantage of plastic pipes over metal ones. Therefore, they are being used more and more. The one-piece polyethylene-steel connection is so reliable that it does not require much maintenance.

It is installed directly into the ground without the use of wells. Installation is carried out using butt welding or thermistor. One-piece polyethylene-steel connection can be with or without a reinforcing sleeve. This part gives the adapter the ability to withstand high pressure and a continuous load of 1 MPa. An adapter without a clutch can withstand a load of no more than 0.6MPa. The connection of metal with polyethylene can occur using threads or using various flanges.

So, we have considered the main types of connections, their advantages and disadvantages.