Yaroslavl ???-236, Руководство по обслуживанию, Страница: 31 / 270

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- Snugging up of the part (screw or nut) - screw driving into the threaded hole of the joining
part for 2-3 threads by hand or screwing on of the nut on the bolt for 2-3 threads by hand;
- choosing, preparation or approach of the tool for assembling of the threaded joint;
- Previous snugging and screwing of the threaded parts till the contacting of the head or
nut with the joining part by the ratch or nut runner;
- Previous tightening of the threaded joint with application of fixed timing (torque wrench
or nut runner controlling of the tightening torque);
- Final tightening with application of the fixed timing;
- Control of the tightening of the threaded joint;
- Lock of the threaded joint (using special washers with the bend on the edge or other
constructions);
- Tool withdrawal;
- Control of the assembling quality of the threaded joint.
2.8.9 Controlled tightening torques of the threaded joints are described in the
chapter Engine assembling.
2.8.10 To provide security of the threaded joint it is necessary to lubricate the thread and
butt end of the bolt (screw) head with diesel oil before the tightening. It is necessary in order to
decrease the friction in the thread and under the bolt head while tightening that will provide greater
tension in the thread because of the bolt (extension of the body) stretch.
Attention! Dints and other defects can lead to the friction increase in the thread, decrease
of the bolt (screw) stretch that will cause self-turning of the threaded part during the operation
under the impact of the external load (for example: gas forces, belt tension, vibration and others).
Required tightening torque of the specified joint is determined by the following variables:
- coefficient of friction between the nut and the bolt (pin);
- coefficient of friction between the surface of the nut and surface of the joining part;
- quality and geometry of the thread, strength class of the bolt.
The most significant is the friction in the thread between the nut and the bolt (pin) as well
as the nut and the part. With rubber friction, coarse surface and shrinkage factor losses for friction
can be so great that while tightening direct at strain of the joint bolt stretch will be less than 10% of
torque. The other 90% are spent for friction resistance overcoming and shrinkage. Thus, though the