High Strength Bolt Friction Couplings

Friction joints have a high bearing capacity and are less labor-intensive in manufacturing than welded ones. Thanks to this, they are widely used in the production of building metal structures. Obtaining the necessary friction forces in the joint is achieved in various ways - by abrasive blasting and flame treatment, as well as using adhesive compositions.

Description and Application

In mechanical engineering, 2 types of bolted connections are distinguished by the nature of the transfer of force in them:

• Non-shear resistant. Most often, they are designed on bolts of coarse, normal and high accuracy (less often high strength). Tightening force is not controlled. The calculations take into account the internal tensile, shear and shear stresses, but do not take into account the friction forces.
• Friction (shear-resistant). The external force is counteracted by friction forces arising in the contact planes of the parts to be joined. Friction is due to the preliminary tension of the fixing bolts, which should be maximum. Therefore, in such compounds, high-strength hardware with heat treatment is used.

The latter type is divided into 2 subcategories: friction and friction-shear joints, in which one part of the force is transmitted through friction, and the other through shearing.

The disadvantage of these compounds is the high cost of fasteners. On the other hand, the use of friction joints on high-strength bolts can increase reliability and reduce the number of assembly welds. Accordingly, the complexity of the assembly is reduced by almost 3 times. This type of connection is used in the construction of industrial buildings, in the construction of bridges, cranes, and other lattice structures that experience vibration or dynamic loads.

Promising directions in the development of this design solution is the use of removable preservation coatings and the use of “block” manufacturing technology, when the assembly and painting of enlarged units are carried out at the plant, and only the final installation is done at the construction site.

Roughness coefficient

The necessary surface roughness, providing the calculated friction force, is achieved by abrasive, flame treatment of mating surfaces or by using special coatings. The coefficient of friction for calculations is taken according to the table indicated below.

The type of processing should be indicated on the drawings. The mating surfaces are completely cleaned from ice, snow, oil, scale, rust and other contaminants before assembly with bolts.

Sandblasting

The following requirements are imposed for cleaning metal structures with friction joints using sandblasting technology:

• roughness of the prepared surface - not more than Ra 6.3;
• complete elimination of oxides and rust (2nd degree of purification according to GOST 9.402-2004);
• the degree of surface degreasing is the first (the time for tearing a film of water is more than 1 min., the absence of oil stains on filter paper);
• preliminary cleaning of compressed air entering the sandblasting apparatus from oil and moisture (this is controlled at least 1 time per shift); drying quartz sand to a moisture content of not more than 2%.

After sandblasting, dust residues should be removed from surfaces by blowing with air or wiping with a clean cloth.

Flame treatment

When cleaning parts with a gas torch, an oxygen-acetylene flame is used. Combustion products (oxides) are subsequently removed using wire brushes. In this case, you can not bring the metal to a shine. Gas-flame cleaning can only be used for parts with a thickness of at least 5 mm, in order to avoid their temperature warping. Processing is performed in the following modes:

• oxygen pressure - 0.6 MPa, acetylene in cylinders - 0.05 MPa;
• oxygen supply - maximum (the flame core should be torn off the heating pad nozzle, but not go out);
• burner moving speed - 1 m / min (for thin-walled metal structures of 5-10 mm - 1.5-2 m / min);
• the next passage should overlap the previous one by 15-20 mm;
• burner tilt angle - up to 45 °.

During this process operation, special burners with a wide flame are used.

Shot and brush cleaning

Before processing with metal brushes (painting), paint is removed from surfaces with solvents or in a gas-flame method. It is impossible to clean parts to a metallic luster, as this prevents the formation of the proper coefficient of friction. The work is carried out using a mechanized pneumatic or electric tool. The remaining dust is removed by blowing air or a hair brush.

For shot blasting, steel or cast-iron chipped (chopped) fraction of a fraction of 0.8-1.2 mm is used. In shot blasting plants , cast iron is used.

Coverings

To increase the friction forces in the flange and friction joints, adhesive substances - adhesives are also used. As a friction coating, compositions are used whose main component is epoxy resin, and hardener, solvent, accelerator or isopropanol are additional components.

Assembly work is carried out in the following order:

1. Oversized structural elements are cleaned using one of the above methods, except for flame, and also degrease. The time interval between cleaning and gluing should not exceed 0.5 days. Storage at the same time is carried out in conditions of humidity not more than 80%.
2. Glue is prepared immediately before use.
3. An adhesive composition is applied to the surface of the smaller elements and is filled with 2 mm thick silicon carbide powder by rolling with a metal roller. It is allowed to glue both contacting parts with glue. Within 1-2 hours, freshly prepared glue should be fully used.
4. Remove excess powder by turning the part and making a few taps.
5. Withstand time until complete curing of the epoxy resin.
6. Friction joints are assembled on high-strength bolts.

Quality of hardware

Fasteners for friction joints must be accompanied by a certificate of quality. Bolts, washers and nuts are prepared as follows:

• cleaning of preservation grease in an alkaline solution (holding in a container for 15-20 minutes);
• drying, blowing with compressed air;
• threading with a screwdriver or on a lathe;
• mineral oil lubrication;
• the complete set of bolts with twin nuts that were used during the run;
• storage until assembly (no more than 10 days.).

Friction joint calculation

The most important parameters of compounds of this type are:

• coefficient of friction μ on contact surfaces, determined according to the table above, depending on the type of treatment;
• coefficient of twisting of bolts;
• bolt tension force;
• torque required to obtain the required value of the previous indicator.

The calculated value of the torque is found by the formula:

M = K ∙ N ∙ d _nom,

where K is the twisting coefficient determined according to GOST 22356-77;

N is the tension force of the bolt, kN;

d _nom - its nominal diameter, mm.

The value of N is determined by the formula:

N = σ _p ∙ S _n ∙ k,

where σ _p - tensile strength for the material of the bolt, N / mm ² ;

S _n - the cross-sectional area of ​​the bolt, net, mm ² ;

k is the coefficient of working conditions (for steel metal structures and road bridges, it is 1).

The force created in one bolt VKontakte is found by the formula:

N ₁ = N ∙ μ / ɣ,

where ɣ - coefficient, reliability, selected depending on the number of bolts in the connection.

The minimum required number of high-strength bolts is determined as follows:

n = P / (k ∙ N ₁ ∙ s),

where P is the acting longitudinal load, kN;

s is the number of contacts in the connection.

Assembly

The rules for performing friction joints are to comply with the following technical requirements:

• Before assembly, it is necessary to prepare the surface using one of the above methods (according to the design documentation), remove bumps and burrs that impede the snug fit of parts.
• During transportation and intermediate storage of parts, oiling or contamination of prepared surfaces should be excluded. If this could not be avoided, then a repeated cleaning procedure is necessary.
• At the first stage of assembly, the parts are combined with holes using mounting plugs.
• Install bolts with washers (no more than one under the bolt head and nut), tighten them with nuts by 50-90% of the calculated force and check the tightness of the connection.
• The calculated tightening force is adjusted using torque wrenches.
• Putty or primer mixed with cement, white clay, chalk. This is done to seal the connection against moisture.

Quality checking

Quality control is carried out at all stages of preparation and assembly. The results of interoperational checks are recorded in the assembly assembly manufacturing journal.

The complex of such works includes the following operations:

• incoming quality control of raw materials, components, purchased products;
• checking the status of the tool, calibration of torque wrenches;
• control of surface cleaning and preparation of hardware;
• checking the tightness of joints tightening (using probes);
• selective control of torques by a drag method;
• sealing control;
• testing of samples (at the request of the customer of construction works).