Wheels are an essential part of a car. The quality of the wheels, including the quality of the wheels and their installation, directly affects the performance and safety of the car. There are news reports that the wheels of a truck are flying out of the dead during the driving process. During the development and test of a certain type of vehicle, there are times when the wheel bolts are loose and the breaking event occurs. After analyzing these events, the direct cause of bolt loosening is the insufficient clamping force of the wheel thread joint; the cause of bolt breakage is mostly caused by fatigue, and the cause of fatigue is often also due to insufficient bolt preload, resulting in bolts being externally changed. The load is too large, causing fatigue fracture. The clamping force of the wheel thread joint can be equivalent to the tensile force of the wheel bolt. Li Jing et al. studied the optimal design of wheel bolts and wheel nuts to solve some problems that occurred during the installation of some wheel bolts or nuts. Zhou Hua et al. studied the abnormal fracture of a high-strength automobile wheel bolt and found that the cause is the special condition caused by the mismatch of the heat treatment process. In this paper, the influence of materials, structure and number of uses on the tensile force of wheel bolts will be analyzed.
The material of the early wheel hubs was steel. As the demand for car performance and energy efficiency increased, the weight of the wheels was reduced, and the material of the wheels began to develop into aluminum. Due to changes in the material of the hub, such as: aluminum is softer than iron, the friction pair of the original rigid wheel and steel bolts changed to the friction pair of aluminum hub and steel bolts. These changes will inevitably affect the assembly effect of the wheel, that is, the bolt tensile force used to clamp the hub, brake disc and bearing flange will be affected, then it may cause the clamping force of the wheel thread joint to be insufficient, and in some Under the harsh working conditions, the wheel bolts are loose, which leads to a series of problems such as bolt breakage. In order to solve this problem, major auto manufacturers have their own solutions according to their respective conditions.
At present, the structural form of the wheel thread joint on the market can be classified according to different methods. If the structure of the contact surface between the fastener and the wheel is divided, it can be divided into a cone type, a ball type, and a flat type. Can be divided into one piece and two pieces. This paper will analyze and compare the effects of these types of structures on the preload of the wheeled joints.
1 Theoretical analysis
In the real world, the surface of any part is not absolutely flat, and the extruded surface between the fastener and the clamped part is also not absolutely flat. Therefore, although it is theoretically calculated that the pressing force of the fastener on the clamped part does not exceed the compressive strength of the part, the clamped part is just after the fastener is installed (for the threaded joint of the wheel, here) More importantly, the local area of ​​the pressing surface will exceed the yield strength of the material, causing the material to be permanently deformed by extrusion, thereby shortening the amount of tension of the bolt and reducing the tensile force of the bolt. One of the short-term relaxations, referred to herein as subsidence relaxation.
For a tapered structure, this sagging relaxation is exacerbated. Taking a tapered structure as an example, if there is a given amount of compression deformation e on the surface of the part, the slack generated in the axial direction of the fastener is amplified, as follows
r = e/sin θ( 1)
Where e is the amount of compression deformation (mm) on the surface of the part; r is the amount of slack (mm) in the axial direction of the fastener; θ is half (degree) of the angle of the cone. For a spherical structure, it will be better, but there will be no amplification for a planar structure. Therefore, for the tapered structure, it needs to be considered in the design when replacing the hub material.
One problem that automakers face when replacing the hub material is aluminum is that the friction factor between the fastener and the hub changes due to material changes, and the corresponding friction factor also changes. It is generally believed that the friction coefficient of steel and steel will be less than the friction coefficient of steel and aluminum, then under the same installation torque, the resulting bolt tensile force will become smaller, which will affect the clamping performance of the wheel thread joint. Moreover, the friction coefficient dispersion between steel and aluminum is generally greater than the friction coefficient dispersion between steel and steel, which affects the consistency of the bolt tensile force. Therefore, a two-piece fastener has appeared, one of the important functions of which is to effectively control the friction factor.
From the above analysis, there are two important problems when the steel wheel hub develops into an aluminum wheel hub. One is that the short-term slack may be intensified, especially for the cone-shaped structure; the other is that the friction factor control will change. It is even more difficult. The following will be analyzed through some experiments.
2 test analysis
Based on the results of the previous theoretical analysis, the following will be based on the influencing factors of the study (whether the hub is used for the first time, the hub material, the contact surface structure, and whether the fasteners are two-piece). The test method used in this paper is to use ultrasonic method to measure the tensile force generated on the bolts installed with a certain torque, and the tensile force of the bolts is used as an index to evaluate the characteristics of the threaded joints of the wheels. This test method is collectively referred to below as a bolt tensile force test.
2. 1 Steel and aluminum wheel comparison test
Select a certain type of wheel hub: one is made of steel and the other is made of aluminum. The bolt tension test is now applied to both types of hubs.
The average tensile force of the bolts of the steel hub is 39.5 kN, and the tensile force of the bolts of the aluminum hub is 31.3 kN, so that the friction pair matched with the wheel bolts is known, and the friction coefficient of the steel hub is higher than that of the aluminum. The friction coefficient of the wheel hub is relatively small, and the bolt tension generated by the aluminum hub is smaller, and attention should be paid to the mounting torque of the wheel bolt. If the defined dispersion is the standard deviation divided by the average value, the friction pair that matches the wheel bolt, the steel tube hub produces a bolt tensile force dispersion ( 16.7%) that is more than the bolt tension force generated by the aluminum hub. ( 19.9%) To be small.
2. 2 Comparison test between new hub and used hub
Select a model of aluminum wheel hub for bolt tensile test, install the new wheel hub twice according to the torque specification (115 Nm), and measure the initial tensile force of the bolt just installed and the bolt tension for 24 hours after installation.
The bolt tension force attenuation rate of the first use of the hub is 6.7%, and the bolt tensile force attenuation rate of the second use of the hub is 4.3%. Repeated use of the hub can make the wheel bolt tensile force decline better. .
2. 3 Cone, spherical and flat contrast tests
Conical, spherical and flat wheel threaded joints were selected as test objects. The hubs were made of aluminum and tested for bolt tensile force (installation torque 115 Nm).
The tapered structure of the three types of bolts, which are tapered, spherical, and flat, is a cone-shaped structure, and the average attenuation rate is 10. 2%; according to the dispersion defined by 3.1, then the initial The dispersion of the tensile force is 15.7%, which is also the poorer of the three structures. Therefore, we should try to avoid using this tapered structure.
2. 4 One-piece and two-piece comparison test of fasteners
A certain type of aluminum wheel hub was selected, and a one-piece fastener and a two-piece fastener were used for the bolt tensile force comparison test.
0%。 The tension of the two pieces of the dispersion is 5. 0%. Moreover, the use of a two-piece fastener on an aluminum hub produces a tensile force greater than a one-piece fastener. It can be seen that the two-piece fastener is superior to the one-piece control coefficient of friction.
3 Conclusion
In summary, this paper can obtain the following general conclusions through theoretical analysis and experimental analysis:
1) The change of the hub from steel to aluminum will increase the risk of bolt tension reduction and dispersion increase, which should be taken seriously.
2) The bolt tension of the aluminum hub will be significantly reduced when used for the first time.
3) When the contact surface structure of the hub and the fastener is tapered, the attenuation rate of the tensile force of the bolt is increased, and the dispersion of the tensile force of the bolt is also increased. This type of structure should be avoided as much as possible. .
4) In the choice of fastener type, the two-piece fastener is superior to the one-piece fastener in the dispersion and size of the bolt tensile force generated by the control.
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