Master cylinder

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Master brake cylinder with expansion tank (passenger car)

When the master cylinder is called in the hydraulic brake system, the pressure-generating cylinder. In motor vehicles, for example , it is actuated by the brake pedal or brake lever via a piston rod. In today's cars, a tandem master brake cylinder is used, in the housing of which two brake cylinders are arranged one behind the other. This operates a dual-circuit brake system, as required by law. The first master cylinder was developed and patented by Malcolm Loughead in 1920 .

construction

Sectional
drawing of the master brake cylinder, passenger car (dual-circuit system) Pushrod = piston rod
Circuit 1 = brake circuit 1
Circuit 2 = brake circuit 2

The master brake cylinder consists of the following components:

Plunger

The pressure piston is moved by the piston rod in the cylinder. The primary and secondary cuffs are attached to it. It also has follow-up bores behind the primary collar.

Primary cuff

It is the pressure-building cuff . It also enables additional brake fluid to flow in when the brake is pumping.

Secondary cuff

It is usually a cup seal, which has the task of sealing the cylinder from the outside.

Refill container

It ensures the supply of brake fluid. He's sitting on top of the cylinder. The container screw connections always have a ventilation hole. This hole must always be open. This hole is kept small to prevent dirt from getting into the brake system. The layer of dirt that settles on the container screw connection must be removed by cleaning with spirit during the inspection or when refilling the brake fluid. The ventilation hole must be closed when the engine is washed, otherwise water will get into the system and there is a risk of vapor bubbles forming.

Compensation and follow-up bore

They establish the connection between the refill container and the interior of the cylinder. The smaller compensating bore serves to compensate for lining wear, since pressure can only be built up when the piston or the primary sleeve has bypassed the compensating bore. It also compensates for temperature- related volume fluctuations in the brake lines and ensures that the brake pressure is reduced quickly. The much larger follow-up bore enables the brake fluid to run on when pumping and at the same time ensures that no negative pressure is created in the cylinder.

Filling disc

The filling disc sits between the primary sleeve and filling holes in the pressure piston. It prevents the cuff from being pushed through the filling holes by the brake pressure. This would lead to the failure of the brake, since no more pressure can be built up.

Bottom valve

The bottom valve used to have the task of maintaining a low residual pressure (around 0.4–1.7 bar) in the brake line in order to compensate for mechanical play, especially on the drum brake , and to prevent cavitation (gas formation due to rapid pressure reduction, which leads to corrosion) to prevent. It was attached to the main brake cylinder and installed in vehicles with cup collars on the wheel brake cylinder of the drum brake. Nowadays, pre-pressure valves are used instead of bottom valves. They are screwed onto the master cylinder in the brake circuit for the drum brakes. The pre-pressure also serves to enable a better sealing of the sleeves in the wheel brake cylinder. There are also drum brakes that do not have a pre-pressure, so so-called "lip spreaders" do the sealing.

The special bottom valve is installed in disc brake systems so that the brake fluid pressure is reduced with a delay when there is a residual pressure of approx. 2 bar, which prevents cavitation. However, a complete pressure reduction through the throttle bore in the valve is possible and also makes sense, since the brake lining would otherwise be unnecessarily in contact and would lead to high wear.

function

Functioning of the single circuit brake
Sectional view of the master brake cylinder for a motorcycle (single circuit system)

If the driver presses the brake pedal, the tappet transfers the foot force to the pressure piston. This is moved towards the brake line. The primary sleeve moves over the compensating bore. As a result, the pressure chamber is closed and braking pressure can be generated. If the brake pedal is released, the brake pressure - supported by a spring - pushes the pressure piston back into its starting position. The primary sleeve folds forward and allows the brake fluid to flow over the filling holes of the pressure piston. If the brake pedal is pressed again immediately (brake pumping), there is a larger amount of fluid that can be shifted. A higher brake pressure is therefore generated.

With the tandem master cylinder, two individual pressure pistons are attached one behind the other. The primary piston is actuated by the tappet. The secondary piston or floating piston is actuated by the pressure it generates and also generates pressure in the second brake circuit. If one of the two brake circuits fails, the other brake circuit is still functional, with the failure of one circuit being mechanically noticeable through an extended pedal travel.

The use of tandem master brake cylinders also makes it possible to generate different pressures in the two brake circuits. This is done through the different diameters of the two pistons. These master cylinders are then called stepped or stepped master cylinders.

Special feature for vehicles with an anti-lock braking system

Vehicles with ABS have special requirements for the master brake cylinder. Since the pressure in the brake line can fluctuate greatly during ABS braking, the pressure piston is made to vibrate, which is noticeable on the brake pedal as a strong pulsation. However, this means that the primary sleeve is very often moved over the compensating bore. With the resulting vibrations, it is exposed to a greatly increased load and is at risk of leaking prematurely. This is why a so-called central valve is installed in vehicles with ABS. This completely replaces the compensating bore, filling disc and filling bores. A movable pin is attached in the middle of the pressure piston, which closes when the piston is actuated and is only pushed back on when it reaches its end position via a mechanical stop.

literature

  • Jan Drummans: The car and its technology. 1st edition, Motorbuchverlag, Stuttgart, 1992, ISBN 3-613-01288-X
  • Karl-Heinz Dietsche, Thomas Jäger, Robert Bosch GmbH: Automotive pocket book. 25th edition, Friedr. Vieweg & Sohn Verlag, Wiesbaden, 2003, ISBN 3-528-23876-3

Web links

Commons : Brake Master Cylinder  - Collection of Images, Videos and Audio Files

Individual evidence

  1. U.S. Patent 1430746