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Scania EBS with disc brakes all-round

Scania is the first heavy truck manufacturer in the world to offer an electronically controlled brake system with disc brakes all-round. Compared to conventional pneumatic brakes, the new system provides faster response, higher retardation and easier maintenance. This new technology shows the way to the brake system of the future.

From October 1996, EBS (electronically controlled brake system) and discs all-round will be offered on high-output Scania tractor units for long-haul operation. The production version of this new state-of-the-art brake system makes its first public appearance at the IAA international truck exhibition in Hanover in September.

The gross weights of heavy vehicle combinations are rising and so are axle weights. In addition average speeds are higher, schedules are tighter, traffic density is growing and air and rolling resistance is being reduced. This is the setting for the demands imposed on present and future braking systems and how these should be adapted to suit individual vehicles and road trains.

Braking by wire

In a conventional system compressed air controls the braking process. Electronics and electrics perform this function in Scania's advanced new braking system. "Bytes and amperes instead of bars" is therefore the maxim for the EBS brake control system. Compressed air is only used as a medium for applying the brake at the wheel.

The joint development project between Bosch and Scania began in the early 1980s. It was shown publicly for the first time at the 1984 IAA in Frankfurt. In mid-1994 a field test began with haulage companies in Germany, and around 65 vehicles are currently in operation - "showing good results" according to the continuous follow-up surveys.

EBS components

The EBS system now going into production, combined with disc brakes all-round, is characterised by a small number of components, simplified wiring, improved functions and generally optimised layout.

EBS control unit

Service brake valve with pedal travel sensor

Load sensor

CAN data bus (CAN - Controller Area Network)

Trailer control valve

One pressure-regulating module per wheel, including a control unit for ABS- and

TC-functions

One lining wear sensor per wheel

One wire harness per wheel to the central control unit

Wheel speed sensors at each wheel 

The mechanical components are disc brakes of sliding caliper type on all wheels, the pneumatic components for pressure generation and supply and the mechanical parking brake.

Quick, precise and uniform reactions

The truck as a workplace: The driver needs to slow down his vehicle - as part of normal driving or as an emergency stop.

The driver communicates his intention via the brake pedal and service brake valve electronically to the control unit. Pedal feel with the electronic control system is similar to that of a passenger car, enabling the driver to exercise precise brake control, without the slight vagueness of a conventional pneumatic system.

The control unit also receives signals from the pressure-regulating modules (one per wheel) and the load sensor measuring the weight of the vehicle. It processes the information and transmits braking pressure signals to the pressure-regulating modules, as well as to the EBS circuit for the semi-trailer via the data bus.

The pressure-regulating modules in turn give the command to pressurise the service brake cylinders with compressed air from the storage reservoir, thereby engaging the wheel brakes. Precise knowledge of the actuating pressure also improves the ABS function.

All this happens considerably faster than with a conventional pneumatic system. The electronic control system also allows the brakes to be applied simultaneously on all wheels fitted with EBS. The slight delay associated with conventional air brakes is reduced - both when the brakes are applied and released.

Since only a few metres may separate a near-accident from a disaster, every metre gained in stopping distance will benefit safety. If a fully loaded truck travelling at 80 km/h can stop in 38 metres instead of 44, this represents more than the length of a passenger car. These six metres result from the faster response time of the EBS, which is shorter by 300 milliseconds. The disc brakes offer additional safety benefits.

In summary, Scania EBS stands for:

Rapid and simultaneous brake response from the first to the rearmost axle.

Good brake pedal response, similar to a passenger car.

Possibility of varying the brake forces dynamically between the front and rear

axle, depending on the load and the degree of retardation.

Standardised communication interface (ISO11992) between the EBS functions

on tractor and semi-trailer. 

Electronics plus discs

The improvements achieved with EBS are not only noticeable in emergencies, which thankfully do not happen every day. Scania's engineers also focused on economy, service friendliness, and not least driving comfort in an effort to optimise the braking and wear behaviour in everyday braking. The result is the combination of EBS and disc brakes all-round.

Simple design, easy maintenance

Scania's disc brakes are ventilated and of the sliding caliper type, with brake torque supported by the lining carrier. The design is simple. The brake chambers are directly flanged to the brake calipers, eliminating the need for external shafts, levers and slack adjusters, and is similar on both front and rear axles.

The braking force of the service brake is generated by pneumatic actuation of the brake chamber, whereas the parking brake is spring-actuated. The brake disc is bolted to the huband withstands temperatures of up to 900 degrees. The disc is manufactured by Scania from a high-strength grey cast iron alloy, which provides good cooling properties and efficient thermal discharge. The brake chamber is installed to act axially, except for air-suspended front axles, where it acts radially.

Wear is continuously monitored via an indicator in each brake lining, a sensor built into the caliper and a warning light on the dashboard. The system is also connected to Scania Diagnos, which covers all electronic systems on the vehicle.

The system is designed to achieve similar disc and lining wear characteristics front and rear. Service life is at least equivalent to drum brakes and lining replacement is very convenient. The wheel brake is a sealed unit requiring no maintenance or lubrication.

A conventional pneumatic brake system is retained as a back-up if something happens to the electronics.

Identical components front and rear

Identical components are used front and rear to simplify parts supply - one disc and lining size, one set of seals, one adjuster and one type of brake chamber.

Disc brakes provide more uniform loading of the lining surface and better use of the friction volume available in the lining. Thermal efficiency is higher and the brakes are thus less susceptible to fading under extended high-speed braking. Mechanical efficiency is high (95%) and response benefits from the low number of moving parts and low-friction bearings. Shorter stopping distances can be obtained during emergency braking from high speed.

Disc brakes are lighter than drum brakes - approximately 30 kg per vehicle - giving lower unsprung weight. They are less prone to noise and vibration and the contact area between rim and hub is less sensitive to irregularities and uneven tightening. The hot parts are well ventilated, leading to lower rim and tyre temperatures.

Overall benefits of Scania's new brake systemScania's new system with EBS and disc brakes provides numerous advantages on long-haul tractor units:

Operational factors:

High safety reserves due to less fading and power variation, as well as the effi-

cient distribution of braking power to all axles.

Simple servicing - helped by Scania Diagnos.

Uniform brake wear on all axles, automatic wear monitoring - via Scania Diagnos.

Simple design with few components.

Sealed wheel brake unit requires no maintenance or greasing.

Rims and tyres run at lower temperatures.

Disc brakes are not so sensitive to a rim, which is not in perfect contact with the

hub. 

Driver benefits

Good response and pedal feel for precise brake application.

Fast brake release.

Less noise and vibration.

Easy system supervision. 

EBS on the trailer, too

Maximum braking efficiency is obtained when both truck and trailer are equipped with EBS. Stability is then at its best and the risk of jack-knifing or trailer slides is minimised due to simultaneous brake actuation on all wheels and axles.

The modular design of Scania's EBS system enables the relevant components on both vehicle units to be used to their full potential. Vehicles with EBS are fully compatible with conventional trailers and semi-trailers (with or without ABS).

Scania Retarder

Scania's retarder, which is integrated with the gearbox and the engine's cooling system, can naturally be combined with Scania EBS and disc brakes, as well as with Scania drum brakes.

Control of the retarder is integrated in the brake pedal, leading to wear-free braking for light applications. Correctly used, the retarder will handle up to 80 percent of all brake applications, leaving the wheel brakes cool and in fine shape for occasional heavy braking.

A vehicle equipped with a Scania retarder will thus have substantial safety margins also with a conventional, pneumatic brake system.

Future possibilities

Scania EBS, disc brakes and retarder is not the "end of the story", but a new platform for the integration of other functions such as:

Even more sophisticated diagnostics can be built into the vehicle.

Automatic matching of the brake force between truck and trailer will be possible on future

systems. Every unit will then automatically brake its own weight.

Future systems will also be fully integrated with the other control systems of the vehicle,

for example the Scania Retarder and Scania Opticruise. 

Technical data - Scania disc brakes

Rim size: 22.5"

Disc diameter: 430 mm

Disc thickness: 45 mm

Permissible disc wear: 8 mm

Lining thickness: 21 mm

Wear limit of lining: 2 mm thickness

Mechanical efficiency: 95 percent

Weight of wheel brake without disc: approx. 50 kg

Weight of disc: approx. 30 kg