Troy, Mich. — This is the first article in a series of four by Narcís Molina, Project Manager, Braking Systems in Applus IDIADA, about an examination of the relationship between brake creep groan and brake squeal in disc brakes.
Brake creep groan and brake squeal are very distinct vibration and noise problems exhibited by brakes under certain braking conditions. Creep groan is a low frequency (< 100 Hz) vibration induced by a stick-slip condition associated with the friction/velocity characteristic of the friction material. Brake creep groan can evolve into a resonant condition when a dominant vehicle resonance is excited. This situation can induce high vibration levels, leading to a comfort annoyance.
On the other hand, squeal noise is a phenomenon that occurs at high frequencies (>1 kHz) and typically involves complex modes of the brakes. Squeal noise transmission is basically airborne. In general, creep groan and squeal are studied as independent events with no mutual interaction.
This work investigates how these two distinct brake phenomena can be related to each other during the operational use of a disc brake, as they appear concatenated in time.
The vehicle under study is a medium-duty commercial vehicle with identical front and rear air disc brakes (Table 1):
2.1. Creep Groan
Table 2 lists the main tests that are conducted for the investigation of the creep groan. The study includes operational and impulse-response measurements of the front axle —with special interest to the brake system—, together with modal data of the brake assembly under free-free conditions.
Operational measurements are analyzed with advanced post-processing methods, as detailed in Table 3. The investigation of the vibration energy evolution as the groan phenomenon develops allows the different creep groan phases to be identified. Meanwhile, the point trajectory analysis is aimed at characterizing the relative displacements between the different components that make up the brake system under the actual groan conditions.
2.1.1 Vibration Characterization
Firstly, a subjective test matrix is conducted in order to identify the contribution of the following parameters to the vibration that shakes the truck:
- Hill slope: 15 percent, 18 percent and 20 percent.
- Vehicle direction: forwards and backwards.
- Brake lining temperature: cold (~50 ºC) and hot (~250 ºC).
- Vehicle load: unladen (5,625 kg), half load (9,762 kg) and laden (13,415 kg).
The truck, initially stationary, starts moving backwards or forwards once the brake is slowly released. During the test, the brake pedal travel is modulated to lengthen the creep groan over enough time to assure a proper recording. Each condition is repeated several times along the descent, to give a score mark of the perception of a statistically relevant amount of samples. Each score mark is agreed among the driver and a passenger.
The subjective evaluation quantifies the noise level perception, duration and susceptibility of occurrence of the noise phenomenon; the vibration is assessed, basically, in the cabin.
Preliminary tests are also monitored objectively with the array of sensors detailed in Table 4 and shown in Figure 1. The worst-case scenario is found to happen with the vehicle rolling forwards (downhill) on a 20 percent slope, in neutral and with cold brakes. Hereinafter, all operational measurements are conducted under these test conditions.
2.1.2. Operational Deflection Shapes
The aim of the operational measurements is to understand the actual (on-vehicle) vibration generation and transmission mechanisms associated to creep groan.
The Operational Deflection Shapes (ODS) represent the harmonic movements exhibited by the system, at the frequencies of interest, as they occur during creep groan. They show the relative amplitude and phase between all measured points. This information is useful to determine, from a vibrational point of view, what parts are active and how they can interact to reinforce the response at a certain point in the structure.
Figures 2 and 3 show the set-up of accelerometers that is instrumented to obtain the vibration response of the FL quarter-vehicle. 42 locations are identified: 14 tri-axial accelerometers are glued over the FL brake caliper (including housing, anchor and pads), while the 28 remaining sensors are distributed over multiple components of the front axle, suspension and steering systems:
2.1.3. EMA (Brake-Suspension Assy, On-Vehicle, Static)
Impulse-response measurements are performed to identify the in-vehicle modal shapes of the front axle, with special interest in the FL caliper, which does participate in the generation of groan.
The modal analysis is carried out with the vehicle stationary in the workshop; the engine is off. The manoeuver experienced during the operational tests is reproduced by artificially setting a stable brake pressure of 2.6 bar.
The measurement points for the impact tests belong to the same virtual geometry as the ODS (Figures 2 and 3). The system is excited by hammering the FL wheel center, as shown in Figure 4, in all 3 independent directions (x, y, z).
2.1.4. EMA (Brake Assembly, Free-Free)
The EMA of the isolated brake assembly, under free-free conditions, is conducted in order to understand how the brake structure behaves when it is unconstrained.
As seen in Figure 5, a shaker excites the assembly, which is supported by elastic rubbers. The shaker is positioned forming an angle with the 3 different planes, so to excite x, y and z directions, thus introducing a certain momentum into the brake system.
Figure 6 and Table 6 detail the virtual geometry used for this EMA. Some measurement points are shared with previous tests.
Two different scenarios are considered for the modal tests:
- Without brake pressure.
- With a constant brake pressure of 2.6 bar.
Table 7 lists the main tests that are conducted for the study of squeal. The investigation includes operational measurements of the brake assembly, together with impulse-response data for the individual brake components under free-free conditions.
The study is also backed with the conclusions inferred from the already-presented in-vehicle EMA of the entire front axle (Section 2.1.3) and the free-free EMA of the brake assembly (2.1.4).
2.2.1. Noise Characterization
As seen in Figure 7, the brake assembly under study is fitted to a single-end brake dynamometer; its configuration is detailed in Table 8.
The acquisition system of the dynamometer inherently captures the rotational speed, the hydraulic pressure and the braking torque. Rubbing and embedded thermocouples are also installed in order to monitor, respectively, the disc and pad temperatures.
A bespoke noise research matrix is conducted in order to identify the main characteristics of the squeal noise —in terms of frequency, SPL and occurrence—, along with the variables that contribute to its generation.
2.2.2. Operational Deflection Shapes
Again, the ODS are investigated in order to characterize the dynamic operation of the squealing brake assembly. For this task, measurements are conducted by means of a Scanning Laser Doppler Vibrometry (SLDV), since it allows the measurement of rotating elements —such as the brake disc.
The strength of the SLDV is that it can measure many points of vibration very quickly, thus generating the movement of a whole surface. The weakness, though, is that each measurement is captured sequentially and, consequently, the same excitation has to be maintained throughout the test. To that end, the system uses a ‘trigger’, so measurements uniquely occur during the squeal frequency of interest.
In this particular case, the ODS investigates the relative movement of the following brake components under squealing conditions: pad springs (1), inner and outer pads (2), pressure plate (3), caliper housing (4), caliper anchor (5) and disc (omitted).
Figure 8 depicts the experimental set-up, with the Polytec PSV-500 3D laser vibrometers scanning the brake assembly. Figure 9, on the other hand, shows views of the measurement grid (left) and the final virtual geometry (right).
2.2.3. EMA (Brake Parts, Free-Free)
The following components are independently measured under free-free conditions: caliper housing —including the pressure plate—, inner/outer pads and disc. Figure 10 shows the different test set-ups.
The caliper, suspended from shock cords, is left to settle during 48 hours to ensure its stability. The vibration comes from a shaker, mounted at a certain angle to excite a wide range of modes. The excitation signal is quantified by a force transducer bonded to the caliper. The response of the caliper is measured at 130 points on the caliper surface using a SLDV. Recorded data are in the form of Velocity / Force —technically referred to as ‘mobility’. A frequency range of 3.125 kHz is used —the nature and mass of the structure makes it difficult to excite anything higher.
Both inner and outer pads, on the other hand, are mounted on a compliant piece of foam to allow them to respond freely to the excitation —which, for simplicity, is generated by means of an impact hammer. A metal tip is used on the hammer to try and ensure the highest possible frequencies. Since the pad generally moves during testing, the vibration response is measured with an accelerometer —rather than an optical technique such as the SLDV. FRFs, thus, are in the typical form of Acceleration / Force: ‘inertance’. The response of the pad is measured at 26 locations, with a high frequency range of 12.5kHz.
Finally, the disc is mounted on three pins on the hub mounting face due to the impossibility of using cords or foam. A shaker, mounted at 45° to the underside of the braking and attached through a force gauge bonded to the disc, excites the disc with a white noise approach —thus maximizing the frequency range up to 12.5 kHz. The response of the disc is measured at 584 points on the surface of the disc using a SLDV.
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