The connecting rod bolts of the engine are made of medium carbon alloy steel or chrome-nickel-molybdenum alloy steel with high yield strength and good impact toughness, such as 35CrMo, 42Mn2V, 40MnB and 30Ni3Cr2Mo.
The connecting rod bolt is subjected to variable stress, so various measures are taken to increase its fatigue strength and to avoid additional stress on the bolt.
Ways to improve the fatigue strength of connecting rod bolts are:
(1) Reduce the rigidity of the bolt
The method of reducing the rigidity of the screw bolt is to reduce the diameter of the bolt portion of the bolt and increase the length of the bolt.
The stress concentration at the root of the thread is the weak link of the bolt. From the strength of the shaft and the thread segment, the diameter of the shaft can be smaller than the diameter of the root of the thread (generally the diameter of the shaft is approximately equal to 0.8 times the diameter of the thread). This is appropriate in view of reducing the bolt stiffness and increasing the fatigue strength of the bolt.
It is pointed out here that in order to increase the rigidity of the connected parts and thereby improve the fatigue strength of the connecting rod bolts, soft washers should be avoided between the connected parts , and spring washers are not allowed.
(ii) Improve the load distribution of the thread
Both theory and practice have proved that in the bolt-and-nut connection structure, due to the tensile deformation of the bolt body material during operation , the nut body material is compressed and deformed, resulting in extremely uneven load distribution on the coils. The data shows that about 65% is fatigue fracture at the first and second turns of the thread on the nut support surface.
In order to improve the load distribution of the thread, one of the methods is to cut the first few turns of the thread into a chamfer of 10-15°. Due to the easy deformation of these threads, a part of the load is transferred to the threads of the rear rings, so that the entire load distribution is uniform. Another way to improve the thread load distribution is to use a tension nut. After the tension nut is used, the deformation direction of the nut is the same as that of the bolt, and the load of each thread is relatively uniform.
(3) Reduce stress concentration
In the variation of the cross section of stress concentration easily occurs, resulting in fatigue fracture, and therefore, the change in the cross section of the chamfered transition should be smooth. The radius of the transitional chamfer should generally not be less than 0.2 times the diameter of the transitional outer rod.
(four) increase the thread strength
Fine thread of the screw small extent weakened, stress concentration is better than the coarse thread, of the connecting rod bolt generally use a fine thread.
(5) Improve the surface quality of the thread
The roughness of the bolt shaft and the transition fillet should generally be below 0.04~0.08um, and the thread should also be polished to
0.04~0.08um.
The root of the thread should be rolled, which creates a chill layer on the metal surface and produces compressive stress. This improves the machine's performance and eases the tensile stress concentration at the root of the thread, improving the fatigue strength of the bolt.
(6) Replace the cutting thread with a rolled thread
With the thread turned by the bar, the inner metal fiber is cut and the strength is lowered. And a processing thread rolling method, the continuity and strength of the metal fiber is much improved. Similarly, the bolt head is constructed to maintain its metal fiber continuity and strength.
 To reduce the additional bending moment of the bolt , a special requirement for non-perpendicularity is proposed for the bolt head support surface with respect to the thread center line and the connecting rod nut support surface with respect to the bolt hole center line. It has been shown that the bolt's fatigue limit drops by 40% when the non-perpendicularity increases from zero to 2°. For special bolt heads that prevent bolt rotation when tightening the nut, appropriate methods should be used to reduce the additional bending moment.
When the bolt is tightened, due to the pre-tightening torque, additional torsional shear stress is caused on the thread and the rod body. Its value can sometimes reach 30~80% of the pre-tightening stress, so it is avoided as much as possible. It is usually possible to eliminate the additional stress by reversing the wrench back a small angle.
After the bolt is tightened, it will work under alternating load for a long time, and it may loosen. The pre-tightening force is reduced. Therefore, it is necessary to take measures to prevent loosening. Commonly used are: anti-loose washers, split pins, slotted nuts, etc. Copper-plated anti-loosening method has also been widely used this year, which is coated with a layer of copper of 0.008 mm to 0.012 mm thick on the threaded portion of the bolt. This layer of copper is plastically deformed after the bolt is tightened, so that the thread surfaces are engaged with each other to prevent the nut from being loosened.
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