Simulation of the Mechanical Subsystem of an Air Brake towards its Diagnosis Siddharth Murugan 1, S. Sudhakar 2, and * Shankar C.

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1 Simulation of the Mechanical Subsystem of an Air Brake towards its Diagnosis Siddharth Murugan 1, S. Sudhakar 2, and * Shankar C. Subramanian 2 1 Institute of Road and Transport Technology/Department of Automobile Engineering, Erode, India sid91mugan@gmail.com 2 Indian Institute of Technology Madras/Department of Engineering Design, Chennai, India shankarram@iitm.ac.in Abstract A brake is one of the most important systems used in a vehicle to regulate its motion by decelerating the vehicle and stopping it when necessary. Most commercial heavy vehicles, such as trucks and buses, are equipped with an air brake. From the perspective of vehicle safety, it is very important to ensure that the air brake is in proper operating condition. One of the parameters that are indicative of the health of the air brake is the pushrod stroke. The pushrod stroke increases with regular use of the brake due to the wear of the brake friction linings. The maximum pushrod stroke must be kept within a manufacturer recommended limit, which if exceeded, will severely affect the brake force available to decelerate the vehicle and thus compromise the operational safety of the vehicle. Hence, the pushrod stroke must be periodically monitored and corrective action taken if it exceeds the safe limit. In this paper, a simulation-based method is proposed to predict the pushrod stroke. A model of the mechanical subsystem of an air brake is developed in a simulation software (MD ADAMS). The brake chamber air pressure is provided as the input to this model and the pushrod stroke is obtained as the output from this model. The model prediction is compared with experimental data and it is found that the model predicted the pushrod stroke to within an error of 6 % for a wide range of test runs. Thus, this method can be used to diagnose the air brake. Index Terms Air brake, vehicle safety, pushrod stroke, mechanical subsystem, simulation, MD ADAMS I. INTRODUCTION Ensuring the safety of the vehicles operating on roadways and the passengers travelling in the same are two important requirements in automotive design. The brake system is one of the important systems in a vehicle that ensure its safe operation. A brake system must work properly to ensure vehicle and passenger safety. A well designed brake system must not only stop the vehicle with in the shortest possible distance, but also dissipate the heat energy released during braking in an efficient manner. A typical road vehicle brake system may be actuated by three means, namely, mechanically actuated, hydraulically actuated and pneumatically actuated. Mechanical brakes are commonly used in bicycles and light two wheelers, while hydraulic brakes are commonly used in passenger cars. Air brakes are commonly used in heavy commercial vehicles such as buses, trucks and tractor-trailers. Air brakes use compressed air as the * Corresponding author. Tel: , Fax: , shankarram@iitm.ac.in 9 energy transmitting medium. They may use drum and/or disc brakes at the wheels. In addition to these foundation brakes (i.e., brakes at the wheels), retarders may also be used in the transmission shaft to generate the braking force. But retarders are typically used to assist the friction brakes at the wheels and they do not provide the complete braking force. An air brake is an effective way of stopping large and heavy vehicles, but must be maintained properly to ensure its safe operation. Air brakes are sensitive to maintenance and they need periodic maintenance and enforcement inspections. Defects in air brakes increase the chances of accidents involving heavy vehicles. The most common brake defects are due to worn out brake linings, out-of-adjustment pushrods (a pushrod is said to be out-of-adjustment when its stroke exceeds a safe limit) and leakages in the system [1]. The most commonly used foundation brake in heavy vehicles in India is the S-cam drum brake (it has a cam shaped like an S and hence the name). The simulation of this S-cam brake is performed in this paper with the objective of predicting the pushrod stroke. The pushrod stroke has been predicted for a wide range of test runs and the results compared with the corresponding experimental data. A. Mechanical Subsystem of an Air Brake The air brake system may be classified into two subsystems: the pneumatic subsystem and the mechanical subsystem. The pneumatic subsystem consists of the air reservoir tanks, drain valves, foot valve, brake lines and brake chambers [2]. The mechanical subsystem starts from the brake chambers and includes the pushrods, slack adjusters, S-cams, brake shoes and brake drums [2]. When the driver presses the brake pedal, the compressed air is metered out from the reservoir to the brake chambers in the axles. Due to increasing air pressure, the pushrod is stroked out against the slack adjuster. The other end of the slack adjuster is mounted on a splined shaft that also has an S-cam mounted over it. The translational motion of the pushrod causes the rotation of the slack adjuster. This, in turn, rotates the splined shaft and thus the S-cam. One end of the brake pad rests on the profile of the S-cam through rollers. The rotation of the S-cam results in the motion of the brake pads to consequently contact the brake drum. This results in the generation of the brake force and subsequently in vehicle deceleration. Figure 1 illustrates the mechanical subsystem of an air brake.

2 Figure1. Operation of the mechanical subsystem [1] B. Importance of Pushrod Stroke The pushrod stroke is of great importance in heavy vehicle air brake systems. The braking force available from the brake chamber decreases rapidly once the pushrod stroke exceeds a recommended re-adjustment limit. Every manufacturer provides this limiting value of the pushrod stroke, within which, the brake system would provide sufficient braking force to stop or decelerate the vehicle [1]. This maximum limit is generally in the range of 5.1 cm to 6.4 cm. The motion of the pushrod beyond this re-adjustment limit is indicative of excessive brake wear. This will result in degradation in the performance of the brake system and hence the pushrod stroke should be kept within the re-adjustment limit. Thus, it is important to monitor the pushrod stroke continuously and take corrective measures if it approaches the re-adjustment limit. C. MD ADAMS ADAMS (Advanced Dynamic Analysis of Multi-body Systems) is a simulation software created by MSC SOFTWARE. Dynamic systems can be modeled and simulated using MD ADAMS [3]. The system models can be created and simulated using MD ADAMS/View [4] and their corresponding graphs can be plotted in MD ADAMS/Post processor [5]. II. METHODOLOGY The proposed model takes the force on the pushrod (due to the air pressure) as the input and provides the pushrod stroke as the output. The geometric and dynamic properties for building the model of the mechanical subsystem of the brake were obtained from a commonly used air brake in India. The different components of the mechanical subsystem were modeled and assembled in MD ADAMS/View [6]. Various properties such as mass, modulus of elasticity and material type were assigned to the components. The brake friction lining model was created and its coefficient of friction, modu 10 lus of elasticity and Poisson ratio were assigned [7]. The stiffness of the springs were measured and assigned to the springs in the model. The different joints, contacts and motion constraints were specified in the model. The brake drum, the brake chamber, the camshaft and the brake pads were assigned fixed joints. The S-cam, the slack adjuster and the brake pad pivot were assigned hinged joints. The contacts were assigned at the brake pads, the brake drum, the S-cam and the rollers. The brake chamber pressure values measured during brake application were converted to the corresponding forces on the pushrod and given as the input to the model. The simulation results were then compared with the experimental data. The data obtained from the experiments were filtered using the Butterworth filter to reduce the effects of measurement noise present in the same. A. Experimental Setup The experimental setup consisted of a commonly used air brake in a four-wheeled commercial vehicle with one front axle and one rear axle. A compressor, along with a storage reservoir, was used to provide the compressed air to the air brake. A data acquisition system was interfaced with the air brake to record the experimental data. A pressure sensor was used to measure the brake chamber pressure and a linear potentiometer was used to measure the pushrod stroke. In addition, a rotary potentiometer was used to measure the rotation of the S-cam shaft. When the brake was applied, the pushrod would move due to the increase in pressure in the brake chamber and this subsequently leads to the application of the brakes. The data from the sensors were collected by the data acquisition system and stored in a computer. Experiments were carried out for a wide range of test runs at two different supply pressures, namely 650 kpa and 720 kpa. B. Relation between Pushrod Force and Brake Chamber Pressure The relation between the pushrod force (F p ) and the brake chamber pressure (P b ) is given by, (1) where A b is the area of the brake chamber and P th is the threshold pressure. This equation was used to obtain the values of the force on the pushrod for the corresponding experimentally measured brake chamber pressure at different intervals of time. A spline was fit through the different force values using the AKISPL function (akima spline fitting) in MD ADAMS [8]. These force transients were given as the input for the developed model. The simulation time interval was fixed as 5 s for all the test runs. III. MECHANICAL SUBSYSTEM LAYOUT AND DEVELOPED MODEL IN ADAMS A layout of the components of the mechanical subsystem of an air brake is shown in Fig. 2.

3 Figure 2. A layout of the mechanical subsystem The model developed in MD ADAMS is illustrated in Figs The various components included in this model are as follows: 1. Brake drum 2. Brake friction lining 3. Back plates 4. S-cam 5. Shaft 6. Slack adjuster 7. Pushrod 8. Retracting springs 9. Brake chamber return springs The components were assembled and the corresponding constraints were given to each component of the subsystem. In addition, mass and material properties were given to each component as discussed in section II. Figure 4. Isometric view of the model Figure 5. Rendered view of the developed model Figure 3. Front view of the developed model 11 IV. COMPARISON OF EXPERIMENTAL AND SIMULATED PUSHROD STROKES The push rod stroke predicted by the model and the corresponding experimental data for a wide range of test runs are shown in Figs The supply pressure is the pressure given from the compressor/storage reservoir to the air brake

4 system and the brake chamber pressure is the pressure generated in the brake chamber to move the pushrod. The pushrod starts to move once the threshold pressure is reached in the brake chamber. This threshold pressure is required to overcome the pre-loads present on the pushrod. Figure 11. Test run at supply pressure of 720 kpa and chamber pressure of 460 kpa brake Figure 6. Test run at supply pressure of 650 kpa and brake chamber pressure of 370 kpa Figure 12. Test run at supply pressure of 720 kpa and brake chamber pressure of 550 kpa Figure 7. Test run at supply pressure of 650 kpa and brake chamber pressure of 460 kpa Figure 13. Test run at supply pressure of 720 kpa and brake chamber pressure of 640 kpa V. OBSERVATIONS Figure 8. Test run at supply pressure of 650 kpa and brake chamber pressure of 550 kpa Figure 9. Test run at supply pressure of 650 kpa and brake chamber pressure of 640 kpa It can be observed from Figs that the proposed model was able to predict the push rod stroke very well. The model was able to predict both the transients and the steady state values consistently well. The steady state value of the push rod stroke was predicted within an error of 6 % in all these test runs. Thus, the proposed model can be used to diagnose the air brake for its push rod stroke. This model can be used as part of a diagnostic system that will predict the pushrod stroke when the brake is applied and provide a warning if the predicted value exceeds the re-adjustment limit. The main advantage of this method is that information on the pushrod stroke can be obtained without using a sensor for measuring the same. CONCLUSIONS Figure 10. Test run at supply pressure of 720 kpa and brake chamber pressure of 370 kpa 12 The work presented in this paper focused on the simulation of the mechanical subsystem of an air brake to predict its pushrod stroke. The model of the mechanical subsystem of the air brake was developed using MD ADAMS. This model was simulated for various test runs and its predictions were compared with the corresponding experimental data. It was observed that the model was able to

5 predict the pushrod stroke to within an error of 6 % and hence can be used to monitor the air brake for excessive pushrod stroke. ACKNOWLEDGMENT This research work was done when the first author was a summer intern in the Department of Engineering Design, IIT Madras. He thanks IIT Madras for providing him with the 2011 IIT Madras summer fellowship that allowed him to carry out this research work. The authors thank Prof. R. Krishnakumar, Department of Engineering Design, IIT Madras, for his valuable help during the course of this work. REFERENCES [1] S. D. Bhagawant, An Experimentally Corroborated Dynamic Model for the Mechanical Subsystem of an Air Brake, M.S. Thesis, IIT Madras, Chennai, Tamilnadu, India, June [2] S. C. Subramanian, S. Darbha, and K. R. Rajagopal, Modeling the Pneumatic Subsystem of a S cam Air Brake, Proceedings of the 2003 American Control conference, 2003, pp [3] Basic ADAMS full simulation training guide, Version 12.0, [4] Getting started using ADAMS/View, MD ADAMS 2010 supplement, [5] Getting started using ADAMS/Postprocessor, MD ADAMS 2010 supplement, [6] J. Engstrom, E. Richloow, and A. Wickstrom, Modeling the Robotic Hand for Dynamic Simulation, Bachelors Thesis, KTH Industriell Teknik och Management, Sweden, [7] J. Day, P. R. J. Hardring, and T. P. Newcomb, A Finite Element Approach to Drum Brake Analysis, Proceedings of the Institution of Mechanical Engineers, vol. 193, no. 1, pp , [8] J. F. S. Alacid, Parameter Studies of a Machine Feed Axis in Time Domain by Application of Multibody Simulation, Diploma Thesis, Institut fur Produktionstechnik, Universtat Kalsruhe, Germany,