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01/11/2004
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For years designers of pneumatic disc brakes for commercial vehicles have dreamed of doing away with the massive, and heavy, ‘floating calliper’ design needed for pad wear adjustment. Roger Bishop reports on the mechatronic solution to the problem.
A leading European truck technology supplier has designed the world’s first pneumatic disc brake with a fixed calliper system, similar to those found in cars. It saves around 14kg per wheel – perhaps up to 150kg on a complete vehicle – and includes a mechatronic wear adjustment system that effectively recalibrates the pads after every braking cycle.
Normally the callipers of pneumatic disc brakes for trucks and trailers slide on a frame to accommodate wear of the friction material, which can be up to 25mm. However, designs based on this approach, the industry standard for many years, require the use of a large amount of material to transfer the braking forces to the disc. There are no issues with the performance of such systems but they are extremely bulky and heavy.
Bolting the calliper to the frame would solve the weight problem but because of the large amount of pad wear take-up necessary, it would be impossible to transfer forces evenly to the disc.
In response to these challenges, Germany’s Knorr-Bremse has fundamentally revised the design concept for its third generation of disc brakes. The development was described to European Automotive Design by Claus Beyer, vice president, CoC Electronics. The first system could be on the road on a trailer by 2007, closely followed by its application to a truck tractor unit.
Fixed calliper system
The new system is based on a fixed calliper with single-side operation and internal mechanical transmission. The calliper has an independent wear adjustment system on each side of the disc and it is this that is the key to single-side operation without the calliper or disc having to move to compensate for wear. Only very slight relative movements between calliper and disc are required to bring the discs and pads together.
The adjustment systems are driven by electric motors installed in the brake housing on both sides of the disc, controlled by an electronics package integrated into the housing that communicates with the main brake system controller and, if necessary, the vehicle’s central control unit via a databus. The motors are both drivers and sensors, providing feedback on wear and allowing the brakes to be recalibrated at any time.
Brake clearance can be increased or reduced at will. In practice, the system allows clearance to be set to much lower values than normal because the adjustment system automatically compensates for thermal expansion of both brake pads and disc.
The mechatronic system also performs a range of additional functions. When pads need replacing the brakes are opened and the clearance reset automatically. In wet conditions or when the vehicle is operating off-road or on a construction site, the pads can be set to remain just in contact with the disc to keep the contact surface clean. The position control of the motors also provides precise indication of pad wear. And when the brakes are not being used, clearance is automatically increased to ensure that there is no unintentional contact with the disc.
Very slight axial movement of the brake disc provides the necessary relative movement between the calliper and Knorr-Bremse’s splined disc (above). The disc is installed on the hub splines with thermally insulated intermediate pre-stressed spring elements that allow the required axial movement. Only when the brake is operated is the disc pushed slightly out of its fixed position. Knorr-Bremse calls this concept micro-adjustment. It also protects the system against vibration.
The new brake was designed by a team at the company’s Schwieberdingen facilities led by Hans Baumgartner, head of brake system development.
The benefits to truck designers go beyond weight and braking performance issues. For example, the optimised elasticity of the system means that less stroke length is required for operation which has design benefits right through the pneumatic system. And because the complete assembly is 80 to 100mm narrower than conventional designs, more steering angle can be applied giving vehicles an improved turning circle.
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Author
Roger Bishop
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