Browse Topic: Brake pads
In an earlier publication, it was reported that the pad compressibility measured under 160 bars on NAO formulas keeps decreasing with increasing number of repeated measurements due to unrecoverable residual deformation of the friction material combined with increasing moisture adsorption, which increases the hardness of the friction material. This current investigation was undertaken to find out if this same phenomenon occurs for NAOs under a low pressure of 100 bars during compressibility measurements and under 700N during dynamic modulus measurements. In all cases, it is found that the same phenomenon occurs, meaning that friction materials become permanently compressed without full recovery, making them harder to compress and raising up the modulus. The dynamic modulus of friction material attached to a backplate is found to be lower as compared with the friction material without the backplate, which is caused by more rapid moisture adsorption of friction material pads without a backplate. As pad properties are continuously changing under pressure at temperature during usage, compressibilities and dynamic moduli measured for the initial quality must not be used for predicting brake performance/NVH – an important issue for AI databases.
As Lowmet pad porosity increases, pad hardness decreases; pad ISO compressibility increases; the nominal friction coefficient increases (SAE J2522); and the disc wear/pad wear decreases. Brake squeal occurrence is affected by the total wear of disc and pads; the wear differential between the inboard pad and outboard pad; pad tangential taper; and pad hardness/material damping. Also, pad chamfer shape has a strong influence on brake squeal occurrence.
Many performance sport passenger vehicles use drilled or grooved cast iron brake rotors for a better braking performance or a cosmetic reason. Such brake rotors would unfortunately cause more brake dust emission, appearing with dirty wheel rims. To better understand the effects of such brake rotors on particle emission, a pin-on-disc tribometer with two particle emission measurement devices was used to monitor and collect the emitted airborne particles. The first device was an aerodynamic particle sizer, which is capable of measuring particles ranging from 0.5 to 20 μm. The second device was a condensation particle counter, which measures and collects particles from 4 nm to 3 μm. The testing samples were scaled-down brake discs (100 mm in diameter) against low-metallic brake pads. Two machined surface conditions (plain and grooved) with uncoated or ceramic-coated friction surfaces were selected for the investigation. The results showed that the grooved friction surface led to a higher particle emission than a plain friction surface finish. The ceramic coating can indeed reduce the negative effect of the grooving through reductions of both the brake wear and particle emission.
The influence of moisture adsorption, prior braking, and deceleration rate on the low-speed braking noise has been investigated, using copper-free disc pads on a passenger car. With increasing moisture adsorption time, decreasing severity of prior braking or increasing deceleration rate, the noise sound level increases for the air-borne exterior noise as well as for the structure-borne interior noise. The near-end stop noise and the zero-speed start-to-move noise show a good correlation. Also, a good correlation is found between the noise measured on a noise dynamometer and on a vehicle for the air-borne noise. All the variables need to be precisely controlled to achieve repeatable and reliable results for dynamometer and vehicle braking groan noise tests. It appears that the zero-speed start-to-move vehicle interior noise is caused by the pre-slip vibration of the brake: further research is needed.
This paper’s aim is to explain alternative friction lining formulations based on inorganic polymer binders for the production of new, future-proof brake friction materials. The aspects of high-temperature stability in the fading tests of the AKM- and AMS tests, as well as the reduction in PM10 emissions compared to classic organic friction materials, make these materials particularly fascinating for future use. Additionally, the energy savings potential of this type of friction lining could be of particular importance when sustainability considerations further influence our development activities in friction brake related applications.
Moisture adsorption and compression deformation behaviors of Semimet and Non-Asbestos Organic brake pads were studied and compared for the pads cured at 120, 180 and 240 0C. The 2 types of pads were very similar in moisture adsorption behavior despite significant differences in composition. After being subjected to humidity and repeated compression to 160 bars, they all deform via the poroviscoelastoplastic mechanism, become harder to compress, and do not fully recover the original thickness after the pressure is released for 24 hours. In the case of the Semimet pads, the highest deformation occurs with the 240 °C-cure pads. In the case of the NAO pads, the highest deformation occurs with the 120 0C-cure pads. In addition, the effect of pad cure temperatures and moisture adsorption on low-speed friction was investigated. As pad properties change all the time in storage and in service because of continuously changing humidity, brake temperature and pressure, one must question any approach trying to relate unused virgin pad properties to brake friction and noise in service, including any attempt to model or simulate brake friction and noise using virgin pad properties.
Abrasion of the Electromechanical brake (EMB) brake pad during the braking process leads to an increase in brake gap, which adversely affects braking performance. Therefore, it is imperative to promptly detect brake pad abrasion and adjust the brake gap accordingly. However, the addition of extra gap adjustment or sensor detection devices will bring extra size and cost to the brake system. In this study, we propose an innovative EMB gap active adjustment strategy by employing modeling and analysis of the braking process. This strategy involves identifying the contact and separation points of the braking process based on the differential current signal. Theoretical analysis and simulation results demonstrate that this gap adjustment strategy can effectively regulate the brake gap, mitigate the adverse effects of brake disk abrasion, and notably reduce the response time of the braking force output. Monitoring is critical to accurately control EMB clamping force. Pressure transducers are often expensive and have limited accuracy in high-temperature environments, so an estimate of the clamping force is required. In this research, the clamping force is estimated based on the identified contact points and the stiffness profile of the EMB. This method performs exceptionally well under low stiffness conditions and maintains a narrow error range even in high-stiffness scenarios.
Brake-based park systems, where an electric parking brake system becomes fully responsible for vehicle immobilization and enables elimination of the traditional driveline-based parking pawl, has increased in popularity, especially in full Electric Vehicles. At face value, the promise of saving mass, cost, and critical packaging space in an electric drive unit is compelling. However, this must be weighed carefully against less obvious impacts, which include engineering in added redundancy, significant changes in “real world” duty cycle of EPB components, risk of brake pad and rotor crevice corrosion, and perhaps most acutely because it affects every drive cycle, the impact to residual drag and therefore vehicle energy use. The present work endeavors to present a balanced view of the considerations, both advantages and tradeoffs, for brake-based park systems, with a special focus on the residual drag behavior because it is perhaps the most difficult to characterize, most variable in its behavior, but most impactful to the vehicle effect.
Shim bond coverage analysis is a common practice in brake and pad manufacturing during brake pad development. This analysis is used to assess the quality of a shim bond and quantify it in case of any quality or de-bond issues during production and warranty returns. Currently, the analysis is carried out manually in the industry using a 1:1 template printed on tracing paper, which is placed on the deboned shim to identify bad bonded regions. The bond coverage is then calculated manually based on the data obtained from the template, which is a time-consuming process taking around 15 minutes per pad/shim analysis. To minimize manual work and increase accuracy, artificial intelligence is being used to estimate the shim bonding quality and coverage. The idea is to feed the deboned shim and pad picture to the model and predict the following: Whether the bond coverage is good or bad. Identify the good/bad and unnecessary regions on the shim/pad for bond coverage analysis. Finally, provide a bond coverage percentage with a pass/fail criterion. The training dataset was prepared with good and bad coverage information, and a convolutional neural network model was selected to predict the bond coverage. This paper discusses in detail how the model was trained and deployed to analyze shim bond coverage using visual AI.
Despite efforts to reduce disc brake noise occurrence, it remains a significant concern in the automotive industry, particularly in the current era of electric vehicles, where it can be an intermittent issue. There is no standard solution available for every noise frequency, as it depends on various conditions and parameters that need to be experimentally identified and addressed. This paper specifically focuses on addressing low-frequency noise. During dynamic conditions, the contact pressure becomes uneven, leading to uneven pad wear and making the disc brake system susceptible to noise. In noise rigs, the paper selects the most suitable shim and pad geometry based on trials that analyze the interaction between the shim and pad. In conventional practice, shim modification was performed using computer-aided engineering, but obtaining accurate pressure patterns in dynamic conditions with CAE is challenging due to certain assumptions. Through dynamometer trials, the paper identifies that the critical frequency is caused by the coupling of the disc and pad mode shapes. Wear analysis reveals greater wear on the leading side of the piston, which can contribute to noise at critical frequencies. Pressure patterns were examined using Tekscan™ across different pressure ranges from 10 bar to 50 bar to understand the cause of uneven wear, confirming the bias in the caliper loading pattern. Consequently, the details of piston contact pressure were investigated, indicating higher pressure distribution on the leading end of the piston compared to the trialing side. Further analysis using finite element analysis (FEA) confirms a similar bias towards the caliper on the leading side. To modify the pressure pattern and reduce noise, a half-moon cut profile was introduced in the shim, resulting in the elimination of occurrences at 3.9 kHz.
The moisture adsorption kinetics of copper-free brake pads was studied to confirm an earlier finding that the adsorption weight gain follows a logarithmic relationship with respect to the square root of humidity exposure time and the relationship is linear in the beginning. When the pad cure temperature was raised from 120 to 180 and 240 °C, the adsorption rate increased. The 180 °C cure produced the highest pad modulus. With increasing moisture adsorption, the pad compression modulus increased just like the pad dynamic modulus, meaning decreasing compression/compressibility while the ISO ‘compressibility’ determined after 3 compressions under 160 bars increased in contradiction. It is concluded that the ISO ‘compressibility’ is a destructive hardness measurement like the Gogan or Rockwell hardness: the key difference is the indenter covers the entire surface of the pad. The true compressibility must be determined as an inverse function of bulk modulus. It is recommended that the pad compression modulus should be measured under low pressures like 10 bars for the purpose of determining the initial quality of virgin pads if the current ‘compressibility’ machines are to be used. Compression tangent modulus and dynamic modulus measurements produce the same results, meaning that one could replace the other. Pad cure temperature affects friction coefficient and moisture adsorption influences friction coefficient in opposite direction for moderate-speed snubs of 80 km/h vs. low-speed stops of 5 km/h.
The pending Euro 7 vehicle-emissions regulations include a significant new sustainability wrinkle: first-ever restrictions for PM emissions from brakes. In a proposal submitted in November of 2022, the European Commission detailed its new Euro 7 vehicle emissions standard, which is widely expected to be approved by the European Parliament and Council and begin phase-in starting on July 1, 2025. Another phase of emissions legislation is nothing new, but one critical element of Euro 7 is new to the regulation chessboard: first-ever limits on how much particulate matter (PM) can be generated by a vehicle's brakes. This element of Euro 7 has auto and commercial-vehicle brake-component suppliers scurrying. Commercial vehicles are subject to their own compliance levels as they interpret how the new regulations will impact their existing technologies and what new solutions will be required. The proposed Euro 7 regulations also address the emissions of fine microplastic particles created by tire friction on the road. As of late Spring 2023, however, the EC had yet to assign permissible values for tire particulates.
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