Despite the fact that all the fixing thread self-locking, in most threaded connections provide locking fasteners from unscrewing at variable load and shock due to lower friction force in the thread and on the end support surface of the nut or bolt (screw) during vibrations. All methods of locking can be divided into three groups: an additional friction locking; locking additional details; locking tightly. Locking additional friction — the most widely used method for fixing fasteners where vibration affects their reliability. To exclude samootkruchivaniya threaded connections using washers of various designs, special bearing surface profiling and other techniques. One of the most effective methods for locking threaded connections — is the use of aerobic sealants.
Machinery are separable compounds which in operation must be stationary and are flush with each other. This couples with numerous threaded metric pipe or a special connection thread. Their stiffness is provided by the friction occurring in threaded surfaces. Under the influence of vibration and friction forces pressed the crystal lattice of the surface layers under cyclic tangential displacement shaken and destroyed. The process of destruction is rubbing the surface without removing the products of wear. Come off the metal particles are rapidly oxidized. Oxides accumulate in the contact area, accelerating the wear. This type of vibration wear called fretting corrosion. For its appearance enough of a reciprocating displacement of at least 0,025 m. Because of the gradual wear of the surface of the thread changes its geometrical dimensions, clearances are increased and as a consequence increases the amplitude of the vibration displacement is lost functionality of the threaded connection. This leads to the failure of units and machinery, reduced structural rigidity and other undesirable consequences. Development fretting causes fretting fatigue. On the surface of the compound microcracks appear, the growth of which leads to the destruction of the connection.
One effective way to combat fretting corrosion is to fix the fixed polymer sealing compounds anaerobic sealants whose main purpose — to exclude the possibility of tangential displacement portions of a threaded joint when subjected to vibration. The sealant applied to the threads in liquid form and then polymerized Xia under anaerobic processes.
In the presence of the polymer layer tangential shear force pressed against the surface of T, that is, the frictional force caused by mechanical compression force F (twisting force) and the force of adhesion (molecular) interaction of the polymer with the surface (Figure 1), which are connected tribotechnical addiction. Its value depends on the physical and chemical nature of the polymer and the surface roughness of the thread, which determines the actual area of contact. Molecular component perpendicular to the surface.
Effects of vibration in the tangential direction causes a shear deformation of the polymer, which takes energy costs, thereby reducing the effects of vibration on the fasteners. This phenomenon is widely used in modern engineering for vibration protection of machines and mechanisms.
The mechanical component of the force depends not only on the pressing force, and determined macroroughnesses surface, ie its deviation from the ideal forms.
In the presence of a solid polymeric layer contour deviation, the inaccuracy of the angle profiling give the effect of propping, which underlies the fixing screw connections for rigid circuit.
The role of molecular components also determines a significant role in the reliability of locking threaded joint minimizing the impact of such adverse factors such as cleanliness of the surfaces and the presence of these small contaminants.
Macroroughnesses surfaces arise because of the relative accuracy of manufacturing fasteners, and depend on the equipment on which the manufacture, installation errors preforms power and thermal deformations of the system "machine — workpiece — tool", Tool wear. On the creation of micro and macroroughnesses no additional cost, since they are present on any real surface.
Sealants used in mechanical engineering and the automotive industry to reduce the effect of vibrational loosening fasteners are anaerobic reactive polymers. They are resistant to fuels and lubricants oil-based fluids and atmospheric moisture, as well as many organic solvents, acids and alkalis. Sealants have a high fluidity, good wetting of metal, plastic and ceramic surfaces. They polymerise by reaction of the anaerobic type, when there is no contact with oxygen and thus cause reaction of polymer formation in the monomer linkages. The reaction occurs at room temperature. Liquid sealant remains inactive as long as it is in contact with atmospheric oxygen. If the sealant is deprived of oxygen, such as when connecting two surfaces sufficiently developed, a rapid cure. Contact with the metal accelerates the polymerization process. FLOW anaerobic polymerization reaction can be represented as follows (Fig. 2a, 26, 2b).
With continued exposure to oxygen sealant remains in the liquid state (Fig. 2a). When it enters into the nip terminates the supply of oxygen (Fig. 26), the peroxides are converted to free radicals. The metal ions accelerate the process of disintegration of peroxides and free radical formation. The free radicals stimulate the formation of polymer bonds. In the polymerized state the sealant is a solid structure with the cross-linked polymer chains (Fig. 2c). The polymerization rate at the anaerobic reaction also depends on the type of the substrates, the gap between the components and the temperature. Table. 1 shows the types of materials by impact on the activity of anaerobic polymerization.
The greatest impact on the rate of polymerization of the anaerobic sealant has copper and its alloys. Relatively high polymerization rates are observed on the surface of carbon and carbon structural steels. Aluminium alloys with a low content of copper and alloy steels inactive. The rate of crosslinking of the polymer on the surfaces of these materials is much lower than on steel surfaces (Fig. 3).
Great influence on the rate of polymerization of the sealant has a fever. Lowering the temperature of parts below 15 ° C slows down the polymerization process, and its increase above 30 ° C significantly accelerates the synthesis of the polymer.
By increasing the gap between the surfaces increases the access of atmospheric oxygen to the liquid polymer, which also slows its solidification.
To accelerate anaerobic reactions on the surface inactive materials or working at low temperatures may be used promoters, representing the active metal salt solution in a volatile solvent. After evaporation of the solvent remains on the surface of the active metal ions that are 2-3 times accelerate the process of formation of the polymer and increase its cohesive strength.
Devices to prevent loosening must comply with rather stringent technical requirements of fixing the thread. Typically, the liquid thread clamps meet these requirements. Liquid threaded fasteners are single component liquid adhesives which completely fill microscopic gaps between the planes of the threaded connections and then, in contact with the metal and the absence of oxygen to polymerize strong, hard, thermosetting plastic. Threaded clamp screw creates a connection boundary planes pin roughness of their surfaces, to prevent any movement of the threaded parts. Thus, the problem is solved where it arises, namely in the thread. Liquid anaerobic clamps are applied to the threaded portion manually or using special dispensers.
The optimum amount of product applied depends on the following parameters: thread size, viscosity, and locking configuration details. If the parts are large in size, it is necessary to apply adhesive to both surfaces. The blind tapped holes adhesive must be applied to the bottom of the hole in such an amount that after assembly extruded threaded retainer distributed over the entire length of the threaded contact. Some anaerobic clips (liquid state) have a positive effect on the coefficient of friction in the threads during installation, with a lubricant comparable bolts.
With mass production of fasteners use liquid retainers impossible. Solve the problem of applying the lock on the thread in the manufacturing process have allowed kapsupirovannye anaerobic thread sealant.
Threaded fasteners for pre-application on fasteners
For pre-coat retainer (sealant) to the threads during manufacture fastener using special adhesives. They perfectly fit into the high-performance production lines, do not require a complex process and can be used with fasteners that have external as well as internal threads.
These products are called encapsulated threaded fasteners (sealing). During assembly microcapsules are crushed and poured activator whereby anaerobic polymerization reaction occurs, like the one that occurs with liquid anaerobic sealant, with the same strength, chemical resistance and anti-vibration properties. Application CBSA-lated anaerobic clamps for pre-application on any fasteners ensures high quality performance properties of the threaded joint.
Most anaerobic threaded fasteners intended for pre-coat, are water-based solvent without turning, so they are non-toxic and non-flammability. For ease of recognition, they are painted in bright colors. Anaerobic latches for pre-application have a long shelf life in the state applied to the threads. When removing the threaded connections with encapsulated threaded clamp for assembly can be used anaerobic liquid locks.
Tests anaerobic thread fasteners
One way to assess the reliability of fixing rezbovago compounds is to conduct tests on a special installation determine the effect of cyclic loading on the residual compressive force, which provides a threaded connection. The smaller the loss of details provided by compression force effort tightening a threaded connection with a particular fastener thread, the more reliable connections and provide an anchor for higher efficiency of the method of fixing.
To build a conservation effort compression curves for different methods of fixing used a special test stand. The fixed bolt subjected to vertical pressure on the installation displaceable force application point by the cam. Among all types of fixing thread, the tested liquid anaerobic retainer 1_osMe showed the best characteristics of the conservation efforts of compression.
Most mechanical methods of fixing the threaded connections could not stand this test (Fig. 4). This does not mean that, to a certain extent, these methods are unacceptable.
Application of anaerobic sealants in a technological process of fixing
In the context of mass production of fasteners used lubrication technology. Typically, the process operation is complete lubricant removal from the surface of the product is associated with high production costs. Remains of grease on the surface of the product before the step of applying anaerobic sealants are not a technological obstacle. It may be applied to the threads of the retainer fixture in the presence on the surface of grease. The sealant composition administered additives which dissolve petroleum based oils. It should be remembered that the increase in the oil dissolved in the sealant reduces its strength after polymerization. Therefore, the amount of lubricant should not be excessive.
Synthetic lubricants based on silicone is not recommended, since they are not soluble in the sealant and prevent reliable fixation under dynamic loads.
A variety of anaerobic locking compound
Market sealants for fixing thread now offers quite a wide range of products as the well-known foreign companies and Russian producers. At its core, the proposed product, as a rule, meets the technical requirements of fixing the thread with anaerobic sealants, and the choice is the customer.