Threaded connection are conventionally divided into normal and special.
In normal cases, the coupling of parts is carried out using bolts, screws, studs and nuts; in special – using threads cut on the connected parts.
The degree of thread accuracy of fasteners is selected depending on their working conditions:
1)-Threaded connections and thread accuracy:
Working under vibration conditions and experiencing dynamic loads, with a long screw length and the need for precise alignment of the connected parts: (thread accuracy 4-5).
2)-High precision with small thread diameters and screw lengths, subject to dynamic loads and made of brittle materials: (thread accuracy 6).
3)-Other, which do not have special strength and accuracy requirements:(thread accuracy 7.8).
The Assembly quality of threaded connections is determined by the correct tightening of bolts and nuts, the achievement of the necessary fit, the absence of distortion in the connections, the absence of curvature of bolts and studs, and the reliability of locking devices.
a) – bolts; b) – studs; C) – screws; d) – threaded bushings; d) – special nuts.
Assembly quality and reliability of assembled parts can only be ensured if the following technical requirements are met;
1)-Bolts are installed only of the types and sizes specified in the drawing, the number of bolt connections corresponds to the drawing;
2)-The heads of bolts and nuts are symmetrical, the same height and size;
3)-The faces of the heads of bolts and nuts do not have nicks, the length of the bolts or studs is selected so that the bolts, studs and screws protrude over the end of the nut more than 2-3 threads, the outer diameter and thickness are the same for all bolts;
4)-The locknuts are the same, the turnkey dimensions correspond to the dimensions of the nuts;
5)-Bonded parts do not have sharp edges, mating surfaces are carefully processed;
6)-When assembling, bolts and studs are installed perpendicular to the mating surfaces, the support surfaces of bolts and nuts are tightly in contact with the parts, when bolting parts with inclined surfaces, oblique washers are installed;
7)-All nuts are evenly tightened to the point of failure, if necessary, they are stopped to prevent self-unscrewing;
8)-The gap between the connected parts after tightening the nuts is the same;
9)-The roughness of the end surfaces of the nut and the surface of the part meet the drawing requirements; if the bolted connection is valid for variable loads, the smallest scallops rough edges deform and the bolts weakens.
When assembling, do not crumple and break the slot of the screw heads. An important condition for the normal operation of the threaded connection is the absence of bending stresses in the body of the bolt or stud. In this regard, loose fitting of the nut to the end of the part is unacceptable.
The order in which the wrappings of screws
The nuts are tightened in three steps; first, until the nut comes into contact with the part or washer, then the nuts are tightened with a small effort and finally the final tightening is performed.
In order to improve the reliability of threaded connections, it is recommended that for critical connections, the diameter of the rod of the fastener is smaller than the diameter of its threaded part, soft metals are used for galvanic coating of threads, and for the manufacture of nuts – a softer material than for rod fasteners.
For uniform tightening of all nuts, special limit keys are used; single-spindle and multi-spindle, manual and mechanized, which are adjusted to a certain force, when reaching which they are automatically disabled.
There is also another method of uniform tightening – this is the use of torque wrenches with a pointer to the value of the applied torque when tightening.
For wrapping hairpins
For the purpose of locking nuts from self-unscrewing at the variable loads arising as a result of work of a product (pushes, vibrations), lock nuts, adjustable splints, wires, spring and toothed washers, figured plates are applied.
Special keys allow you to capture the hairpin for a smooth part. Keys of this type have a high performance, because when they are used, there is no need to screw and screw the keys from the studs.