1 Introduction
The brake caliper is a key component of the wheel braking system. The function of the caliper guide pin is to connect the caliper with the bracket so that the caliper can move axially. When the vehicle is braking, the caliper moves along the guide pin with the action of hydraulic pressure. , to clamp the brake disc to form a brake, and at the same time when the pedal is released, the caliper can be returned to achieve the effect of releasing the brake. When the vehicle brakes at high speed, the rotational inertia of the brake disc will be transmitted to the caliper guide pin, so the caliper guide pin needs to have good shear resistance, and at the same time ensure excellent noise performance during braking. The above technical capabilities can pass the test Or measured by CAE analysis.
2 Explanation of the broken fault of the guide pin of the caliper
During the 24-channel test of the test vehicle, the guide pin of the left front caliper fractured during the 467th cycle. After the guide pin was replaced, the fracture occurred again at the 500th cycle. According to the standard requirements, the brake system component test needs Meet the 480 cycle without exception. The number of broken cycles of the front guide pin does not meet the standard requirements. The fracture part of the guide pin is the transition point of the diameter of the pin shaft. According to the fracture analysis, the fracture belongs to the fatigue fracture under the action of shear force. Another phenomenon is that the rear caliper has no abnormality at the guide pin after 800 cycle tests;
3 Cause Analysis of Guide Pin Breakage
Under the conventional design, manufacture and assembly state of the guide pin, the shearing force is relatively small. At the same time, the guide pin is formed by the cold heading process, and the product itself has a strong shear resistance. Normal state will not fatigue fracture. The abnormal fracture failure of the guide pin did not occur in the actual road test. In order to find out the root cause of the abnormal fracture of the guide pin, this paper conducts research from the perspectives of design, manufacturing and test methods, and formulates improvement measures to avoid similar failures in the market. question.
3.1 Analysis of the influence of product design factors on guide pins
The guide pin connects the caliper with the caliper body bracket. When braking, the caliper moves to the side of the brake disc under the push of the oil. When the friction plate contacts the brake disc, the caliper moves along the guide pin as the axis. During the moving process, it will be affected. The shear force along the radial direction of the brake disc and the Z-direction gravity brought by the self-weight of the caliper, the combined force of the two forces will cause a certain impact on the guide pin on bumpy roads. Whether the impact force will cause the guide pin to break requires theoretical CAE analysis. In view of this fault, a CAE comparative analysis of the stiffness and strength of the front and rear guide pins was carried out: 1) Constraint conditions: 1-6 degrees of freedom at the clamping point, see Figure 1;
Figure 1 Schematic diagram of the analytical load and constraint loading of the front and rear guide pins 2) Loading conditions: the radial load force on the head of the guide pin is 5000N; after analysis, the deformations of the head of the front and rear guide pins when the load is 5000N are respectively 0.5mm and 0.48mm, the shear stress meets the allowable stress of the material. The same type of guide pin structure was tested, and the guide pin design of other models is consistent with the structure of the faulty part. It shows that the guide pin has no design defect and will not cause problems such as fracture failure.
3.2 Analysis of the influence of test conditions on guide pin fracture
The vehicle 24-channel test bench is a road simulation test bench, which adopts the road simulation test method and can reproduce 90% of road failures in the laboratory. This test method can quickly test whether there are design defects in the product structure. At present, it is also the main way for mainstream OEMs to verify structural parts. The test conditions for braking in this test method are:
Fig. 2 Deformation and stress analysis results of the front and rear guide pins 1) The brake oil pressure is set to 20Mpa; 2) The test cycle is 480, and the number of brakes per cycle is 32; 3) The test fluid storage tank is placed under the caliper. During emergency braking, the oil pressure of the pipeline is generally 8-10MPa, and the brake oil pressure does not exceed 16MPa during the design verification of parts. The oil pressure setting of the road simulation test exceeds the design verification range. , the deformation of the brake disc, etc. exceeds the design expectation, and the force model of the guide pin changes. The liquid storage tank is placed under the caliper, which will cause the brake fluid of the caliper piston to flow back after the brake is pressurized and released, and the caliper piston will retreat without pre-pressure. In a stable state, it is easy to cause the force change of the guide pin of the caliper, and at the same time, a metal knocking sound is generated at the caliper during the test, and the knocking sound is generated 3s after braking. It shows that after braking, the oil fluid returns to the fluid storage tank, the gap between the disc and the gap between the piston and the plate increases, and the caliper has been working in a non-design state, resulting in an increase in the shear force of the guide pin.
3.3 Analysis of the influence of the front and rear caliper structure on the fracture of the guide pin
The guide pins that broke in the test were all front calipers, and the structure and size of the guide pins of the rear calipers were similar to those of the front calipers, but there was no failure. There are differences in the weight and structure of the front and rear calipers. The front calipers are 2kg heavier than the rear calipers. At the same time, the rear calipers integrate the parking mechanism. The gap is only 0.55mm. In order to verify whether the gap and weight will have a negative effect on the guide pin under the test conditions, this paper conducts CAE analysis on the guide pin under different gaps. 1) Analysis purpose: the force difference of the guide pin under the initial position of the front and rear brake calipers and the maximum retraction of the caliper piston; 2) Constraint conditions: restrict the caliper mounting bracket 3) Loading load: 30g weight acceleration mass is loaded at the center of gravity of the brake caliper .
Fig. 3 Schematic analysis of force analysis load and constraint loading of guide pin The analysis results show that the stress of the front guide pin under the above conditions is 184.72MPa and 209.932MPa, indicating that the increase of the retraction amount of the caliper piston will affect the stress state of the guide pin. At the same time, the stresses of the rear guide pins under the above conditions are 107.796MPa and 108.960MPa respectively, which are quite different from the front guide pins, which also verifies why the rear caliper guide pins did not fail.
Figure 4 Stress state of the guide pin at the initial position of the front caliper
Figure 5 The stress state of the lower guide pin when the front caliper piston retracts 4.4mm
Figure 6 The stress state of the lower guide pin when the rear caliper piston retracts 0.55mm
Figure 7 The stress state of the lower guide pin when the rear caliper retreats 0.55mm
4 Risk Analysis of Guide Pin Breakage
The unreasonable test method led to the abnormal fracture of the guide pin. Will it exist in actual working conditions? According to the statistics of an OEM, 98% of the vehicle's braking deceleration is below 0.3g, and the maximum braking strength of this model under extreme working conditions is 1g. To achieve a pressure of 20Mpa, a pedal force of 1000N is required, and the driver cannot step on it. Therefore, although the guide pin fracture fault occurred on the simulation platform, this working condition will not occur in reality, and the risk is extremely low. At the same time, the vehicle has undergone a three-month structural durability test at the proving ground, and there is no report of abnormal guide pins, indicating that the product meets the requirements in terms of design and quality control.
5 Conclusion
Brake calipers are safety components, product design and verification are very important. In this paper, the product design is reorganized through troubleshooting of fracture faults, and the reliability of the product design is determined. At the same time, the unreasonable part of the test method is also improved. For example, the test oil pressure is set at the maximum lock-up pressure, which is consistent with the actual worst working condition, and the liquid storage tank is placed on the caliper, which will ensure the verification. Reasonability makes the verification results more reasonable.