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Introduction
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The purpose of the BSQL module in the CDH/VAO software is to provide a code to be used by engineers for numerical simulation of the phenomenon known as disk brake squeal. The program will predict the onset of brake squeal for a given set of loads, velocity, and frictional properties. The physics of the brake squeal problem are simulated by step-by-step transient integration of the equations of motion for high frequency nonlinear dynamics. The basic structure is represented by matrices produced by an NASTRAN finite element model. All structural components may be modelled with fine detail, and may include sliding joints and other linear contact.
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However, the pad contact forces and nonlinear friction effects are modelled in the BSQL program specifically for the brake pad and rotor friction effects.
The advantage of the BSQL system is that it will simulate the more important physics of the brake dynamics with a minimum of a-priori assumptions.
- Realistic simulations will be assured by allowing the use of detailed models with more accurate higher eigenfrequencies.
- Efficient nonlinear solutions are obtained by using a reduced model.
- Design changes may be quickly evaluated by the use of superelements in the linear model.
- A general friction law is provided for the pad/rotor interface, using empirical tables obtained from actual experiments.
- With the complex eignenvalue analysis an estimate of the system stability can easily be obtained.
Capabilities
CDH/BSQL provides the following specific capabilities:
- Sliding friction effects are represented by tabular functions and polynomial surfaces. Friction forces may be functions of pressure, velocity, and temperature.
- Friction forces are calculated on an element-by-element basis using local instantaneousvalues of pressure, velocity, and temperature.
- Coefficients for the distribution of the brake dust effects are optional inputs.
- Damping for the pads, rubber seals, and other components may be modeled with NASTRAN.
- Transient analysis is an option to be used for determining the existence of squeal for a specific condition. This is a direct simulation including nonlinear effects.
- Complex Eigenvalue analysis is an option to be used for predicting potential squeal frequencies. This is a linearized method to show critical frequencies.
With both methods the contribution of each normal mode to the unstable behavior is available. This capability will aid the designer in selecting design changes to reduce the response.
Ease of use
A primary advantage to the BSQL system is the use of NASTRAN to generate the finite element data and the normal modes. No special mesh generators or visualization tools are required. The only additional input data requirements for BSQL are the friction curves and a few control parameters.
Efficiency is also a user-friendly feature of BSQL. Since the typical application will use a large-order model and require many analysis runs for various combinations of brake velocity and petal force, the turn-around time is important to the hurried Engineer. Each new run starts the analysis at the reduced model stage.
How it works
The steps performed for a complete brake squeal analysis are:
| 1. |
NASTRAN finite element model for the entire brake assembly is generated by the user. The special modeling tools required to represent the brake fluid and the sliding connections are available in MSC/NASTRAN. |
| 2. |
NASTRAN is used to produce the stiffness, mass, and damping matrices, along with tables used to build the contact surface connections. |
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The user supplies the friction tables or functions along with control parameters such as initial pressures and velocity as well as error tolerances and time step limits. |
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A steady state solution is obtained for the user-selected initial velocity and pressure state. This is a static nonlinear solution since the sliding forces are nonlinear functions of the perturbed pressure distribution. The run may be terminated here and "tangent matrices" are saved for a Complex Eigenvalue run in NASTRAN.
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Transient analysis is performed as a perturbation from the steady-state conditions. The iterations are controlled by a powerful system based on the higher order implicit integration algorithm. |
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Outputs consist of gap forces and contact point displacements, velocities, and accelerations as well as overall energy measures for selected time intervals. All outputs are formatted as flat ASCII tables, suitable for many available postprocessors. |
Hard-/Software Requirements
| Software: |
MSC.NASTRAN or NX-NASTRAN
MSC.Nastran is a proprietary product of MSC.Software Corporation.
NX-NASTRAN is a proprietary product of UGS Corporation. |
| Platforms: |
IBM AIX
Linux |
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