This paper provides a detailed interpretation of the relevant requirements of national standards, discusses the methods and approaches for flexural fatigue testing of motor vehicle brake hoses, and offers implementation strategies for the test methods. It covers the mechanical component construction scheme, measurement and control system construction scheme, and corresponding improvement schemes for the brake hose flexural fatigue testing equipment. This paper establishes a digital prototype of the brake hose flexural fatigue testing equipment and illustrates the actual performance of the equipment currently used in the China Automotive Certification Center laboratory. Regarding the mechanical component as the actuator, this paper also proposes an optimization scheme to address the shortcomings of similar equipment and analyzes its feasibility. Furthermore, this paper introduces the key components and construction methods of the equipment's measurement and control system, providing a reference for the research and development of similar equipment in the industry.
Keywords: brake hose; flexural fatigue test; brake fluid; gear mechanism
introduction
Motor vehicle brake hoses, commonly known as brake pipes, are widely used in motor vehicle braking systems. Their main function is to transmit the braking medium during motor vehicle braking, ensuring that the braking force is transmitted to the vehicle's brake shoes or brake calipers to generate braking force, so that the braking is effective at all times. Brake hoses are one of the key components of the braking system, and their performance directly affects the personal safety of motor vehicle occupants. In view of this, my country has included motor vehicle brake hoses in the CCC certification product catalog. The currently effective certification implementation rules [1] promulgated by the Certification and Accreditation Administration of the People's Republic of China are CNCA-C11-06:2014 "Implementation Rules for Compulsory Certification of Motor Vehicle Brake Hose Assemblies", which involve relevant requirements for factory quality assurance capabilities and product consistency. In addition, my country's national standard [2] GB16897-2010 "Structure, Performance Requirements and Test Methods of Brake Hose" has clear provisions on the inspection items and test requirements for motor vehicle brake hoses.
The flexural fatigue test of motor vehicle brake hoses is an important performance test stipulated by national standards for hydraulic brake hoses. This test is of great significance for judging the flexural fatigue performance of hydraulic brake hoses and helps improve driving safety. Brake hose flexural fatigue testing equipment is a specialized device used for testing the flexural fatigue performance of hoses. According to the test conditions specified by national standards, the flexural fatigue testing equipment operates at high speeds and may involve brake fluid pressure loading, delivery, and pressure maintenance. Careless design and use of the equipment can easily lead to a series of problems. Through visits to some industry laboratories and brake hose manufacturers in China, it was found that such equipment generally suffers from "high noise, high vibration, significant hidden dangers, and high pollution." If not improved, this may adversely affect operators, the testing process, and the on-site environment. In response to the above situation, and in accordance with national standard requirements, the author attempted to establish different implementation and optimization schemes for flexural fatigue testing, and has built a digital prototype of the equipment based on 3D CAD product design technology. The physical equipment has also been upgraded in the China Automotive Certification Center laboratory and has completed more than fifty flexural fatigue tests. Currently, the vibration and noise are within acceptable ranges. The equipment is designed with a "slow acceleration, two-stage start" method, requiring personnel to evacuate the site before the equipment reaches the second-stage high-speed operation. A pressure loss protection function is also included in the PLC program, ensuring timely shutdown before a large amount of brake fluid leaks in the event of sample failure and leakage, thus largely preventing environmental pollution.
1. Test methods and requirements specified in national standards
GB16897-2010 "Structure, Performance Requirements and Test Methods of Brake Hoses" [2] clearly stipulates the test equipment for hydraulic brake hoses, requiring that the equipment should mainly consist of a rotating part and a fixed part, as shown in Figure 1. The rotating part consists of a movable horizontal connecting rod and a turntable. The two ends of the movable horizontal connecting rod are vertically installed on the turntable through bearings, and the center of the turntable is 101.6 mm away from the center of the bearing. The fixed part is an adjustable non-moving horizontal connecting rod, which is parallel to the movable horizontal connecting rod. Several joints for parallel installation of the brake hose assembly are installed on both horizontal connecting rods. When the turntable rotates at a speed of 800 r/min ± 10 r/min, the end of the brake hose fixed on the movable horizontal connecting rod also rotates at this speed, forming a circular trajectory of 203.20 mm ± 0.25 mm, while the other end of the brake hose remains fixed. The joints on the movable horizontal connecting rod are closed, and the joints on the non-moving horizontal connecting rod are connected to the hydraulic source. The volume of the hydraulic source and the pipeline settings of the equipment are not allowed to affect the test results. During the test, when the brake hose is damaged and the pressure drops to the set value, the equipment should be able to automatically stop, and at the same time record the running time and the system pressure in the pipeline at the time of shutdown.
1- Turntable 2- Movable horizontal link 3- Fixed horizontal link
Figure 1. Actuator of hydraulic brake hose flexure fatigue testing equipment
The test preparation process includes removing hose accessories, measuring the free length of the assembly, installing the assembly, and adjusting the slack. During the test, the static pressure of the piping system should be 1.62 MPa ± 0.10 MPa, and the test medium should be distilled water or HZY3 grade brake fluid. All gases should be purged from the system piping before the test begins.
Experimental design
Mechanical Actuation System
In the flexural fatigue test of motor vehicle brake hoses, the actuator's movement is simple, but the overall requirements for the equipment are high. The equipment's pressure supply system should be able to generate stable and reliable pipeline static pressure, and the pressure should have a certain adjustment range to meet the needs of expanding test conditions. Furthermore, different test media have different chemical and physical properties, which will have different effects on the initial sample installation, the flexural execution process, post-test cleaning, and equipment maintenance. HZY3 grade brake fluid, compared to distilled water, has good viscosity-temperature characteristics, a low freezing point, and better low-temperature fluidity; it has a high boiling point, making it less prone to vapor lock at high temperatures; and it is similar to distilled water, with minimal quality changes during use, making it less likely to cause corrosion or deterioration of metal and rubber parts. Using HZY3 grade brake fluid as the test medium is consistent with the actual vehicle conditions, better reflecting the actual performance of the brake hose. The overall principle of the pressure supply system is relatively simple, similar to a hydraulic jack, providing pressure to the system pipeline by pressing the oil supply handle. Pressure is acquired through a pressure transmitter, the signal is transmitted to the PLC, processed and converted, and then displayed on the host computer touch screen.
The rotary actuator mainly consists of a servo motor and three reducers. The motion transmitted from the servo motor is reduced by the central reducer and then transmitted to the two reducers on both sides in a 1:1 ratio. The output shafts of the two reducers move in the same direction and synchronously, driving the two turntables, which in turn drive the movable horizontal connecting rod to produce the expected motion trajectory.
The relaxation adjustment mechanism mainly includes a lead screw and a guide rail. The relaxation amount of the hose sample is changed by turning the handwheel and rotating the lead screw to change the position of the slide on the guide rail.
Based on the above, and after selecting components such as motors and reducers, a three-dimensional digital prototype model of the brake hose flexure fatigue test equipment was established [3], as shown in Figure 2. After the design, processing, procurement and assembly of the components, the equipment operates normally, and the noise and vibration are within acceptable ranges, and its functions meet the requirements of national standards.
Figure 2. Three-dimensional digital prototype of brake hose flexure fatigue testing equipment
Measurement and Control Systems
The main hardware and communication relationships of the equipment measurement and control system are shown in Figure 3. The host computer is a touch screen, which can read the system pressure, turntable speed and test duration, and can set functions such as motor start, stop, pressure and speed alarm and clear.
Since there are no real-time requirements and both speed and position can be strictly controlled, the servo system adopts a position control mode. In position control mode, the Yaskawa servo motor's soft start cannot be set by the driver. The rotating parts have high speeds and large moments of inertia; if they cannot be started and stopped slowly, the impact on the overall equipment will be significant, easily leading to wear on components and bearings, and creating safety hazards. In speed control and torque control modes, if precise positioning is required, the external wiring is slightly more complex than in position control mode. Therefore, a specific soft start/stop anti-shock program was written. By setting a target speed, and gradually increasing the pulse frequency to the target frequency during program execution, the servo motor starts slowly until the target speed is reached. When the equipment stops, to minimize impact, an inertial stopping method is set, relying on the resistance of the equipment and the sample itself to stop slowly. Thus, a slow and shock-free ideal state is achieved during both starting and stopping.
Figure 3. Main hardware communication relationship diagram of the measurement and control system. Figure 4. Wiring diagram of the pressure transmitter.
The system's oil supply pressure is measured using a pressure transmitter. Changes in the pressure signal are directly converted into changes in current, as shown in Figure 4. The oil supply pressure during the pressurization process can be displayed in real time on the touchscreen. A leakage protection program is implemented to ensure timely shutdown in case of a sudden pressure drop due to sample failure or machine damage, preventing excessive brake fluid leakage and environmental pollution. The test duration can also be recorded in real time via the touchscreen. This is achieved using a timer composed of the CNTR and 0.1S pulse instructions of a CP1H PLC.
The main logical functions of the equipment are implemented through PLC programming, and the PLC program flow is shown in Figure 5.
Experimental scheme optimization and analysis
3.1 Optimization of Mechanical Actuation System Scheme
Given that the equipment described herein is an upgrade and renovation of existing resources at the China Automotive Certification Center laboratory, while meeting national standards and usage requirements, it is not optimal due to objective constraints and the fact that some aspects, though optimized, are not yet fully effective. If the current approach is abandoned and a new approach is developed, the following solution should be superior. Details are provided below to stimulate further discussion and offer reference for equipment development by laboratories or hose manufacturers in the industry.
The principle of the existing equipment is shown in Figure 6. It is a double crank mechanism. The crank part has a large moment of inertia and a relatively fast speed. It requires high precision in processing and assembly. Otherwise, it is easy to increase the load on the bearing of the hinge part, resulting in rapid wear. If other measures are taken to avoid the "dead point" [4], such as using redundant constraints, or rotating the flywheel at an angle and installing another connecting rod parallel to the existing connecting rod, it will undoubtedly increase the overall moment of inertia, increase vibration and noise, or cause excessive stress in some parts due to low processing and assembly precision. In order to avoid the disadvantages of the existing scheme, the author believes that a gear mechanism [4] can be used to replace the existing double crank mechanism, as shown in Figure 7, which is one of the feasible alternatives. This scheme is more convenient for adjusting the counterweight, optimizing dynamic balance, and reducing vibration. According to the size ratio of 1:1, a comparison diagram of the output end of the two schemes is established. See Figures 8 and 9 for details. Obviously, the latter has a small moment of inertia, and as long as the gear set is assembled correctly, the requirement for processing precision is lower than that of the former.
Figure 5 PLC program flowchart
Figure 6 Schematic diagram of the existing equipment Figure 7 Schematic diagram of the optimized equipment
Figure 8. Schematic diagram of the output end of the double-crank type (1:1 scale). Figure 9. Schematic diagram of the output end of the gear mechanism type (1:1 scale).
Using UG software, motion simulation was performed on the output ends of the two schemes [5]. The motion trajectory of the output end of the gear mechanism is exactly the same as the motion trajectory of the connecting rod in the double crank mechanism. The relationship between the amplitude and time of the movable connecting rod in the execution part of the two schemes is shown in Figures 10 and 11. It can be seen that the amplitude and period of the simple harmonic wave in the two relationship diagrams are exactly the same, that is, the motion trajectories of the two schemes are equivalent. The value of the center distance can be achieved by gear displacement. Since the output end has only one output point, the movable horizontal connecting rod will not bear additional load due to the processing and assembly accuracy problem. Therefore, the movable horizontal connecting rod part in this scheme can be designed as aluminum alloy material and the weight can be reduced as much as possible. Under the premise of meeting the function, the best vibration reduction and noise reduction effect can be achieved.
Figure 10. Relationship between amplitude and time of the movable connecting rod at the output end of the double-crank type.
Figure 11. Relationship between amplitude and time of the movable connecting rod at the output end of the gear mechanism.
If the pressure supply section only needs to meet national standards, it can be further simplified. The hydraulic components below the platform of the prototype shown in Figure 2 can be completely omitted. Only a medium inlet and a pressurizing device need to be added above the platform. The pressurizing device is simple to implement and can be achieved in many ways, such as using a small-capacity manual hydraulic cylinder. After pressurizing to the set value, the hydraulic cylinder is disconnected from the sample oil circuit. This simplified mechanical structure will significantly reduce the complexity of the equipment, facilitate miniaturization, and avoid problems such as excessive medium filling, rapid consumption, and large-scale leakage causing contamination when the sample is damaged.
Analysis of Measurement and Control System
The equipment control system described in this paper uses a PLC as the lower-level machine and a touch screen as the upper-level machine for human-machine interaction. Pressure transmitters and encoders serve as pressure and speed measurement and feedback devices, respectively. The overall implementation is relatively simple, but there is still room for optimization. Using a three-phase asynchronous motor with a frequency converter as the power unit, communicating with the PLC, would still meet the relevant national standards for this equipment and save costs. If higher position and speed accuracy are required, it is recommended to use a servo motor with an encoder.
in conclusion
This article provides a detailed interpretation of the national standards for the flexural fatigue test of motor vehicle brake hoses, and addresses relevant regulations:
(1) A scheme for constructing a flexural fatigue testing device is presented, including specific schemes for the mechanical actuator and the measurement and control system. The device is currently operating normally and continuously, which fully proves the feasibility of the device scheme.
(2) An optimized scheme for the mechanical actuator of the flexural fatigue test equipment was proposed. This scheme belongs to the first category. The gear speed ratio can be adjusted and matched by itself. As long as the output part meets the expected trajectory, it can be used. Compared with the above scheme, it has the advantages of small rotational inertia, low requirements for machining accuracy, miniaturization, low vibration, and low noise. However, it is recommended to consider the design of lubrication of the gear mechanism in this scheme [6].
The flexural fatigue test of brake hoses thoroughly examines the flexural fatigue performance of hose samples. The national standard currently requires a rotational speed of 800 rpm for 35 hours of continuous operation, resulting in a total flexural fatigue test of 1.68 million revolutions. Whether increasing the rotational speed and reducing the operating time (to expedite the test) or decreasing the rotational speed and increasing the operating time (to reduce equipment vibration, noise, and safety risks) while maintaining the total number of revolutions effectively tests the flexural fatigue performance of the samples is a question worthy of discussion within the industry. Besides hoses, other automotive component products also undergo similar durability tests. It is recommended that qualified laboratories, hose manufacturers, and industry technical experts consider research and discussion in this area, which would also benefit the formulation, revision, and improvement of technical standards.
[References]
[1] Certification and Accreditation Administration of the People's Republic of China, CNCA-C11-06:2014. Implementation Rules for Compulsory Certification of Motor Vehicle Brake Hoses [Z]. Beijing: Certification and Accreditation Administration of the People's Republic of China, 2014.
[2] General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China, Standardization Administration of China, GB16897-2010. Structure, performance requirements and test methods of brake hoses [S]. Beijing: China Standards Press, 2011.
[3] Beijing Zhongqi Huanyu Motor Vehicle Inspection Center Co., Ltd. A motor vehicle brake hose flexure fatigue testing machine [P]. People's Republic of China CN203658184U, 2014-06-18.
[4] Sun Huan et al. Principles of Machinery [M]. Beijing: Higher Education Press, 2006.
[5] Zhan Yougang. UGNX8.0 Example Handbook [M]. Beijing: China Machine Press, 2012.
[6] Pu Liangui et al. Mechanical Design [M]. Beijing: Higher Education Press, 2013.
