The structure selection of automobile connecting rod bolts used in fasteners

The connecting rod bolts of the engine are made of medium-carbon alloy steel or chromium-nickel-molybdenum alloy steel with high yield strength and good impact toughness, such as 35CrMo, 42Mn2V, 40MnB and 30Ni3Cr2Mo.

The connecting rod bolt bears variable stress, so various measures must be taken to improve its fatigue strength and avoid additional stress on the bolt.

Methods to improve the fatigue strength of connecting rod bolts are:

(1) Reduce the stiffness of the bolt

The way to reduce the stiffness of the screw bolt is to reduce the diameter of the polished part of the bolt and increase the length of the bolt.

There is stress concentration at the thread root, which is the weak link of the bolt. From the perspective of the strength of the shaft and thread section, the shaft diameter can be smaller than the thread root diameter (generally, the shaft diameter is approximately equal to 0.8 times the thread diameter). From the perspective of reducing the stiffness of the bolt and increasing the fatigue strength of the bolt, it is appropriate to do so.

It is pointed out here that 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 should not be used.

(2) Improve the load distribution of the thread

The theory and practice have proved that in the bolt-nut connection structure, the main body material of the bolt undergoes tensile deformation and the main body material of the nut undergoes compression deformation during work. As a result, the load distribution on each circle of the thread is extremely different. Evenly. The data shows that about 65% is the fatigue fracture at the first and second round threads from the nut supporting 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 10~15° chamfer. Since these turns of threads are easy to deform, part of the load is transferred to the threads of each subsequent turn, so that the entire load is evenly distributed. 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 on each circle of the thread is more uniform.

(3) Reduce stress concentration

It is easy to produce stress concentration at the change of the section and cause fatigue fracture. Therefore, a smooth chamfer transition should be adopted at the change of the section. The radius of the transition round chamfer should generally not be less than 0.2 times the diameter of the transition outer rod.

(4) Improve thread strength

Fine thread has less weakening to the screw, and the stress concentration is better than that of coarse thread. Therefore, connecting rod bolts generally use fine thread.

(5) Improve the surface quality of threads

The roughness of the bolt shaft and 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, because the metal surface produces a chilled layer and generates compressive stress. In this way, the machine type performance of the material is improved, the tensile stress concentration at the root of the thread is relaxed, and the fatigue strength of the bolt is improved.

(6) Use rolling thread instead of cutting thread

In the thread that is turned with bar stock, the inner metal fiber is cut and the strength is reduced. And the thread processed by rolling method, the fiber continuity and strength of the metal are much improved. Similarly, the bolt head adopts the upsetting method, which can maintain its metal fiber continuity and high strength.

The additional bending moment of the bolt is reduced, and special requirements for non-perpendicularity are set for the supporting surface of the bolt head relative to the centerline of the thread, and the supporting surface of the connecting rod nut relative to the centerline of the bolt hole. Some data show that when the non-perpendicularity increases from zero to 2°, the fatigue limit of the bolt decreases by 40%. For the special bolt head that prevents the bolt from rotating when tightening the nut, appropriate methods should be adopted to achieve the purpose of reducing the additional bending moment.

When the bolt is tightened, due to the pre-tightening torque, additional torsional shear stress will be caused on the thread and the rod. Its value can sometimes reach 30~80% of the pre-tightening stress, so it should be avoided as much as possible. Usually, after tightening, the wrench can be reversed to release a small angle to eliminate this additional stress.

After the bolts are tightened, if they work under alternating loads for a long time, they may loosen, because the pre-tightening force is reduced. For this reason, measures must be taken to prevent loosening. Commonly used are: lock washers, split pins, slotted nuts, etc. The copper plating anti-loosening method has been widely used this year, which is to plate a layer of copper with a thickness of 0.008mm~0.012mm on the threaded part of the bolt. This layer of copper is plastically deformed after the bolt is tightened, so that the threaded surfaces engage with each other and achieve the purpose of preventing the nut from loosening.