A Review of Automotive Brake Squeal Mechanisms
1Automotive and Tractor Eng. Dept., College of Engineering, Minia University, El-Minia, Egypt
2Department of Automotive Technology, Faculty of Industrial Education, Helwan University, Cairo, Egypt
In the last few decades, a considerable amount of research has been done by many researchers around the world on the possibility of understanding mechanisms of brake squeal noise in order to improve vehicle users’ comfort and reduce the overall environmental noise level. Despite these efforts, still no general solution exists. Therefore, it is one of the most significant issues that require a detailed and in-depth study for realizing mechanisms of squeal generation in order to design silent brakes. The aim of this review paper is to gain insight into the disc brake squeal mechanisms that lead to audible noise. The principal mechanisms will be introduced to explain the brake squeal phenomenon namely; stick-slip, sprag-slip, modal coupling and hammering excitation mechanism. As more experimental evidence and simulation results become available, it is found that none of the above mechanisms alone can explain all events related to the squeal noise.
At a glance: Figures
Keywords: brake squeal, stick-slip, sprag-slip, modal coupling, hammering excitation mechanism
Journal of Mechanical Design and Vibration, 2014 1 (1),
Received November 21, 2013; Revised January 13, 2014; Accepted January 24, 2014Copyright: © 2014 Science and Education Publishing. All Rights Reserved.
Cite this article:
- Ghazaly, Nouby M., Mohamed El-Sharkawy, and Ibrahim Ahmed. "A Review of Automotive Brake Squeal Mechanisms." Journal of Mechanical Design and Vibration 1.1 (2014): 5-9.
- Ghazaly, N. M. , El-Sharkawy, M. , & Ahmed, I. (2014). A Review of Automotive Brake Squeal Mechanisms. Journal of Mechanical Design and Vibration, 1(1), 5-9.
- Ghazaly, Nouby M., Mohamed El-Sharkawy, and Ibrahim Ahmed. "A Review of Automotive Brake Squeal Mechanisms." Journal of Mechanical Design and Vibration 1, no. 1 (2014): 5-9.
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A brake is a mechanical device to be used to decelerate a moving body in a controlled manner. Because of its frictional mechanism, brake noise can occur easily, and the high-frequency noise known as brake squeal makes the driver uncomfortable. Therefore, it is regarded as one of the most important rating items in the automobile initial quality standard test, and automobile companies as well as brake manufacturers have tried to solve the problem. The noise and vibration generated by the braking system in passenger cars are important technical and economic problems in the automotive industry .
Despite brake noise is not a safety issue and has little impact on braking performance, it gives customers the impression of underlying quality problems of the vehicle. In addition, the customers view that the noise emitted from the brake system is indicator of malfunctioning condition and consequently lose confidence on the quality of the vehicles . In order to solve the problems of brake noise and vibrations, the mechanisms and theories must be known for improving the quiet and comfortable performance of vehicles. Among the different types of brake noise, squeal noise, because of its higher frequency contents between 1 kHz and 16 kHz , is the most troublesome and irritant one to car passengers and the environment, and is expensive to the brakes and car manufacturers in terms of warranty costs. It is predominantly generated at low vehicle speeds (below 30 km/hr) and at low brake pressures (brake line pressure below 2 MPa).
During the earlier years, researches were oriented towards understanding, identifying critical factors and possibly in reducing the effect of squeal noise. Increasing the knowledge of the mechanisms generating squeal is one important contribution to the extensive research and development work being performed in order to solve this problem. Up to now, there are a rich number of publications on the subject in the literature [4-12]. From the literature review, it is found that there are six mechanisms of squeal formation namely: stick-slip, negative friction – velocity slope, sprag-slip, modal coupling, splitting the doublet modes and hammering. These mechanisms are indispensable for better understanding of squeal. Despite much progress has been made in gaining physical insight into brake squeal mechanisms and causes in recent years and present brakes have become quieter. However, squeal still occurs frequently and therefore much work still needs to carry out. This is due to the fact that disc brake squeal has been a challenging problem due to its immense complexity which is very sensitive to variables including corner component design, component interaction, usage history and many operating and environmental conditions .
The objective of this review paper is to present the main mechanisms of the squeal phenomenon in order to explain the brake squeal occurrence. To this end, the brief introduction on the importance of brake noise and vibrations problem is first reviewed then construction of the complete disc brake system is described. After that, the main mechanisms of the brake squeal occurrence are discussed in details. Finally, the observation on squeal causes is concluded with some future recommendations in order to understand, study and elimination of brake squeal noise.
2. Construction of a Disc Brake Corner
A car disc brake corner typically consists of steering knuckle assembly, wheel hub, and the actual disc brake assembly. The disc brake assembly consists of a ventilated rotor, a floating caliper with a single piston, an anchor bracket, two bolts, two guide pins, and two brake pads. The pads are loosely housed in the caliper and located by the anchor bracket. The brake pad mounted on the piston is often referred to as the piston-pad, and the pad on the opposite side is called the finger-pad. The caliper itself is allowed to slide freely along the two mounting guide pins in a floating caliper design. The actual and FE model of the whole disc brake corner is shown in Figure 1. The brake corner is connected to the car suspension system through the steering knuckle, which is mounted on the vehicle chassis. The wheel hub is connected to the drive line, and the brake cylinder in the caliper is connected to the hydraulic brake line system, where the piston slides inside the caliper. Hence, the brake corner can be looked upon as a subsystem consisting of a number of components interrelated to each other and to other sub-systems in the vehicle. When hydraulic pressure is applied, the piston is pushed forward to press the inner pad against the disc and simultaneously the outer pad is pressed by the caliper against the disc. Most of the kinetic energy of the travelling car is converted to heat through friction between the disc and pads. Also, a small part of it is converted into sound energy and generates noise.
3. Mechanisms of Brake Squeal
Investigations into brake squeal may be traced back to 1930s. However, there are not any technology and method available to completely eliminate brake squeal up to now . Understanding mechanisms of squeal generation is essential in designing silent brakes and for dealing with noisy brakes. A number of mechanisms of brake squeal have been presented in the research articles namely; stick-slip, sprag-slip, modal coupling and hammering excitation [1-13]. In this literature review, the main mechanisms will be introduced to explain the brake squeal phenomenon in details.3.1. Stick-Slip Mechanism
The first mechanism states that brake squeal is a result of a stick – slip mechanism. The mechanism was believed to cause friction-induced vibrations in the brake at low speeds. These audible oscillations are produced as a result of the negative slope characteristics of dynamic friction coefficient against the sliding velocity in the contact interface. Consequently, energy is fed into the system, which leads to large vibration and negative damping induced limit cycle. As cited by Kinkaid et al. , in his review paper that, Mills , hypothesised that brake squeal originated because the dynamic friction coefficient is decreasing with increasing slipping velocity and leads to the steady state sliding becomes unstable and caused friction-induced vibrations. A finite element model with multiple degrees of freedom to simulate a brake system and analyze friction-velocity negative slope effect on squeal is used . It reached the conclusion that (1) squeal propensity increases as the friction level increase when negative friction-velocity slope is absent. When it is present friction level increase may not have much influence on squeal propensity; and (2) for certain brake systems, negative friction-velocity slope may have a dominate influence on brake squeal propensity. Ouyang and Mottershead  studied negative friction-velocity slope effect on brake squeal via parametric resonance. In addition, negative friction-velocity slope may exist after brake standing at low temperature and high humidity environment . There is quite a few more research papers regarding negative friction-velocity slope influence on brake squeal [19, 21].
To give a basic picture on the stick – slip mechanism, a SDOF system consists of the brake pad as a mass (m) that rests on a rotating disc with a constant speed (v) and connects to a linear spring at a fixed end, as shown in Figure 2 (a). Initially, the spring force is smaller than the static friction force so that the mass moves together with the disc. As the deformation of the spring increases, the spring force increases to a value that equals or is larger than the static friction force, and the mass starts to slide relatively to the disc. As the mass slides, the motion is now governed by the dynamic friction force (which is smaller than the static friction force), and the deformation of the spring and spring force decrease. This causes the mass to gradually stop sliding, and the above cycle repeats to generate the stick slip.
Mathematically, if the coefficient of friction µ between the pad and the disc in Figure 2 (b) is assumed to decrease linearly with the sliding velocity, i.e., µ = µs - αv, the equation of motion of the pad will be:
where; k is the spring stiffness, m mass and F the friction force. The coordinate x is a measure of the displacement of the mass from the equilibrium position. The damping coefficient (c-αF) may thus be negative if (αF >c) results in oscillation of the pad with increasing amplitude and could lead to squeal, rather than decreasing oscillations associated with positive damping.
It was later realized that the stick–slip mechanism was not the only reason for a brake squeal, and that self-excited vibrations could be produced under constant friction coefficient. Sprag-slip is geometrically or kinematic constraint induced instability to reach a limit cycle. As reported by Papinniemi , sprag-slip mechanism was first defined by Spurr . He explained the contact behavior of internal and external drum and disc brakes as the form of locking a body in contact with a sliding surface, followed by a slip due to a displacement of the fixed end of the body. This is known as geometrically-induced or kinematic constraint instability, which occurs even though the coefficient of friction is constant. In describing this mechanism, Spurr presented a semi-rigid strut that was inclined at an angle θ to a rubbing surface and pushed horizontal to the surface as shown in Figure 3. Assume Ff = μFN and consider the equilibrium of the system; the following equation is derived:
where μ is the coefficient of friction and L is the load. It can be seen that the friction force (Ff) will approach infinity as μ approaches cot θ. When μ = cot θ the strut ‘sprags’ or locks and the surface motion become impossible. Due to the flexibility within the assembly the strut releases itself from the spragging condition and returns to its first state to repeat the cycle which could lead to a sprag-slip limit cycle.
The sprag-slip mechanism has been extended by Earles , in an attempt to model a brake system more completely. A SDOF model is developed to present the importance of non-linear coupling within the combined system. An experimental investigation based on the pin-on-disc model was carried out. It was observed that within a certain range of angles of orientation of the pin, instability of the self induced vibration motion existed. This was due to the non-linearity in the system. They also concluded that generation of squeal was dependent on the mean coefficient of friction, direction of disc rotation and the presence of a torsion vibration mode of the pin subsystem.3.3. Mode Coupling Mechanism
Brake squeal noise is the result of vibration created by coupling of two vibration modes of brake component such as pads, rotor, caliper, suspension links etc., As reported by Balvedi et al. . In mode coupling mechanism, two modes of vibration geometrically matched (same wavelength) and close resonances can induce more energy into the system than it can dissipate . Figure 4 show couple between the rotor and the pad at the same frequency. The mode coupling is often locked depending on operational conditions such as (speed, pressure and temperature) and interface characteristics such as contact stiffness, roughness, adhesive force, etc. There are other names used to define this mechanism namely; binary flutter, mode lock-in and non conservative displacement dependant forces. Modal coupling of the structure involved sliding parts and the coupling results in changes of friction forces which is necessary for self-excited vibration.
North's , presented a significant advance in the analysis of brake noise by proposing a binary flutter model for a disc brake. He published the first experimental work on a real brake apparatus and correlated his measurements with 8-DOFs model that included two brake pads, a disc and caliper. The distinctive features of this theory were the presence of the disc and the friction forces produced by pressure of brake pads, and the presence of two independent disc modes. The contribution of this model was the friction forces between the disc and the layer of brake pad were incorporated as follower forces and a possible instability condition was met by using only a constant friction coefficient. Millner  developed 6-DOFs lumped parameter model of fixed caliper disc brake that coupled by a kinematic constraint. The main feature of this model is that it can examine the effect of centre of pressure between the piston and brake pad. He found that for a constant friction value, the occurrence of squeal and squeal frequency depends on the stiffness of the disc brake assembly, and the contact configuration of the pad and the piston. Among others approaches, the modal coupling (mode lock-in) between two system modes is one of the most accepted and the complex eigenvalues analysis of the brake system is a popular numerical tool for squeal instability prediction [29, 30, 31].3.4. Hammering Excitation Mechanism
Hammering excitation vibration induced by uneven rotor surface variation during disc rotation. Rhee et al. in 1989 , presented a simple mechanical impact model (hammering) to explain the excitation mechanism of brake noise without having to deal directly with the frictional force. They noted that the noise frequencies identified in a vehicle test with a typical brake are similar to those observed from modal analysis with a hammer of the same brake components, hence hypothesized that the brake noise and vibration might be activated by a “hammering” type of mechanism at the contact surface that excites a mode of the brake system. The hammering during braking may be initiated by the rocking action of the disc pads when they slide across the rotor surface against the “hills and valleys” which are formed by thermal distortion (or by any other mechanical distortion such as uneven rotor wear or massive friction material transfer to the rotor).
Hammering during braking might occur between the brake pad and the rotor, or between the brake pad and the caliper, and thus start a chain reaction of hammering among the brake components. During this process one or more components might be excited into natural modes of vibration or resonance which results in noise and vibration, the order as to which component would go into resonance first probably depends on the specific system design and its operational conditions. Chen et al.  explained the “local hammering” as material detachments or asperity formations and deformations. This tribological behaviour would be reached in the advanced phase of braking, when the friction coefficient reaches a larger value than the starting value. The critical friction coefficient found by Bergman et al. , is also related to local contact phenomena appearing after a consistent number of braking events. In addition, Chen et al.  in their experimental analysis showed also a coincidence between the squeal frequency and a natural frequency of the system.
4. Comments on the Brake Mechanisms
Disc brake noise and vibration generation during braking has been one of the most important issues and definitely worrying problem to automotive manufacturers worldwide. Understanding the mechanism of brake squeal is a challenging task. It involves many design variables in a complex brake system and there are many operational and environmental conditions under which squeal may occur. It is a challenge task to develop and design a brake system that is broad stable for all operational conditions. As reported by Oberst et al.  and Ibrahim,  that brake squeal has become of increasing concern to the automotive industry but guidelines on how to confidently predict squeal propensity are yet to be established. In addition, Soh and Yoo  reported that squeal is very difficult to express the phenomenon exactly, since the origin of squeal noise is physically complex.
In 1984, Murakami et al.  used a lump stiffness-mass model to analyze stick-slip and sprag-slip effect on brake squeal and it concluded that both stick-slip and sparg-slip contributes to an increase in squeal based both simulation and test results. They reported that the stick-slip was reasoned to add an energy source for the squeal, and the sparg-slip provides the pathway for squeal occurrence. In addition, Rhee et al. , found both the stick-slip and sprag-slip theories lacking in that they only described the conditions under which brake noise might occur, but they do not clearly define the physical phenomenon which causes brake noise. Later, Chen et al.  used a ball against a block mechanism to conduct experiments on squeal under reciprocating sliding contact. They found that squeal can occur in regions with both negative and positive friction – velocity slopes. Hence, there is no correlation between negative friction velocity slope and the generation of squeal. As more experimental evidence and simulation results become available, it was found that none of the above mechanisms alone can explain all events related to the squeal noise. In spite of all of this, mode coupling is generally recognized to be one of the most significant mechanisms leading to self-excited vibration in relative sliding systems with friction .
One may find all four mechanisms mentioned above can be applied only to explain the initiation of squeal. That is the main limitation of these mechanisms. However, what limits infinite increase of squealing vibration is seldom studied and so also is the question what results in disappearance of squealing vibration. Since friction between two sliding surfaces is very complicated and some friction feature characteristics probably remain unknown, the current understanding of squeal has not led to an acceptable solution closure to the squeal problem.
Squeal noise that occurs in disc brakes for automobiles has been one of the major concerns in the automotive industry due to the persistent complaint that reduces customers’ satisfaction with their vehicles. It is observed that it is commonly accepted by researchers working in the field of brake noise and vibration that squeal noise in a disc brake may be generated by either one of the above mechanisms, or the combined depending on the brake design and operational conditions. The authors concluded that there is still much work needed to be done for understanding the mechanism of brake squeal noise. In addition, the authors think that Further numerical and experimental investigations should be carried out using optimization methods in order to find the optimal design of the brake system.
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