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1. Stop signal switch. 2. Brake pedal. 3. Vacuum amplifier. 4. Front brake hydraulic piston. 5. Master brake cylinder. 6. Rear brake hydraulic piston. 7. The pipeline of the rear brake hydraulic circuit. 8. Wheel cylinders (indoor and outdoor) front brakes. 9. The piston of the front brake cylinders. 10. Brake pads. 11. Brake disc. 12. The pipeline of the hydraulic circuit of the front brakes. 13. Nutrient tank VAZ-2103. 14. Pressure regulator. 15. The pipeline to the wheel cylinders of the rear brakes. 16. Regulator piston. 17. Brake shoe. 18. Rear brake wheel cylinder. 19. Wheel cylinder pistons. 20. Brake pad. 21. Short lever arm.
The above diagram shows the interaction of components and parts of the brake system of a VAZ-2103 car, complicated by the introduction of a vacuum booster, in comparison with the brake system of the VAZ-2101, 2102. The operation of the VAZ-2101, 2102 brakes is otherwise similar to the VAZ-2103 scheme.
I. Starting position. the system is disinhibited
In the initial position, the brake pedal 2 is retracted by the spring until it stops against the switch 1 of the brake signal. The pusher of the vacuum booster, and with it the valve body with the stem, are pressed by the return spring and are in the rearmost position.
The access of outside air to the atmospheric cavity B is blocked, and the vacuum cavity A freely communicates with the cavity B.
When the engine is running, a vacuum is created in both cavities, since the vacuum cavity A is connected to the engine intake pipe.
Pistons 4 and 6 of the main brake cylinder 5 under the action of the return springs are pressed into the rearmost position and rest against the locking screws. The spacer rings, resting against the locking screws, press the seals into the forward position, opening the passages for the brake fluid.
The cavities of the main cylinder freely communicate with the cavities of the nutrient tank 13 and with pipelines 7 and 12.
The pistons 9 of the wheel cylinders of the front brakes are pressed from the brake pads 10 by about 0.1 mm due to the elastic deformation of the sealing rings. The brake pads 10, not being pressured by the pistons, remain only in light contact with the friction surfaces of the brake disc.
When the car is moving without braking, the piston 16 of the regulator is raised to its uppermost position until it stops with the protrusion of the head into the regulator plug. The brake fluid can freely pass from the pipeline 7 to the pipeline 15 to the wheel cylinders 18 of the rear brakes. By the force of the coupling spring, the brake shoes 17 rest against the adjusting eccentrics (VAZ-2101, 2102) and bushings in the axle (VAZ-2103). Pistons 19 are pushed inside the wheel cylinders, but remain pressed against the brake shoes by a spacer spring that is weaker than the coupling spring. Between the brake drum and the brake pads there is a gap of 0.1-0.15 mm (VAZ-2101, 2102) and 0.8 mm (VAZ-2103).
II. Start of braking
When a VAZ-2103 car moves with the engine running, a vacuum is created in cavities A and B of the vacuum booster, which is transmitted from the engine intake pipe.
The driver, pressing the brake pedal 2, moves the booster valve pusher. The valve shoulder, having reached the hole in the valve body, will close the annular gap and separate the atmospheric cavity B and the vacuum cavity A of the amplifier.
The booster valve piston moves forward, a gap appears between the piston and the valve collar, which connects cavity B with the atmosphere. Filling the vacuum, outside air enters through the amplifier filter, passes between the front end of the pusher and the valve shoulder and then enters cavity B through the channel, creating pressure on the valve body through the diaphragm that separates the amplifier into cavities, and thereby reduces the required force on the brake pedal.
The pressure developed on the valve body depends on the degree of vacuum in the engine intake pipe and on the force of pressing the foot on the brake pedal.
The stem moves along with the valve body. Having chosen the gap between the tip and the piston 4, the rod moves the piston forward.
When moving away from the locking screw, the rear edge of the annular groove of the piston is pressed against the seal and separates the cavity of the hydraulic drive of the front brakes from the cavity of the supply tank 13. From this moment, with further advancement of the piston 4 in the cavity of the hydraulic drive of the front brakes, the pressure of the brake fluid is transmitted through pipelines 12 to the inner wheel cylinders and further along the connecting tubes - into the outer wheel cylinders of the front brakes.
The pressure of the brake fluid, overcoming the elastic deformation of the sealing rings, pushes the pistons 9 out of the wheel cylinders of the front brakes until they come into contact with the brake pads. With a further increase in pressure, the brake pads clamp the brake discs 11, slowing down the rotation of the front wheels.
If at this moment the driver stops pressing the pedal, but, without removing his foot, leaves it depressed, then the valve body of the vacuum booster will move forward under the pressure of atmospheric air by the amount of the gap between the thrust plate and the groove of the valve piston. In this case, the released valve collar, having reached the piston collar, will block the air supply to cavity B, and the excess air pressure in cavity B will pass into the vacuum cavity of the amplifier through the newly opened annular slot, the channel in the amplifier housing and further into the engine intake pipe. The pressure in both cavities will be balanced and the servo action of the amplifier will stop.
If you resume pressing the brake pedal, then the increasing pressure of the brake fluid in the cavity of the hydraulic drive of the front brakes will begin to move the piston 6 of the hydraulic drive of the rear brakes. With the beginning of the movement of the piston 6, the same sequence of interaction of the parts associated with this piston will be repeated.
The increasing pressure of the brake fluid in the cavity of the hydraulic drive of the rear brakes will be transmitted through pipeline 7, through the regulator 14 and pipelines 15 to the wheel cylinders 18 of the rear brakes. Moving under the pressure of the brake fluid, the pistons 19 move apart and press the brake pads 17 to the friction surface of the brake drum.
From the beginning of braking, the mass of the car tends to move forward, the load on the front suspension increases, and the load on the rear decreases. Due to this, the rear of the body will begin to rise, the short arm 21 of the regulator drive lever will go down, as the other (long) the lever arm connected to the rear axle beam copies the movement of the suspension. The piston 16 under the pressure of the brake fluid, overcoming the resistance of the spring, begins to fall.
III. Full braking
At the moment of full braking of a moving car, there is a maximum transfer of the load from the rear suspension to the front and the greatest rise in the back of the body. The grip of the rear wheels with the road drops. The piston 16 of the regulator, descending lower, comes into contact with its head with the seal and shuts off the fluid supply to the wheel cylinders of the rear brakes. Further braking of the rear wheels is stopped, possible slippage of the wheels relative to the road and skidding of the car are prevented.
Until the back of the body is lowered and the short arm 21 raises the piston 16, it will remain pressed against the seal by the difference of forces applied to the head from both sides: from above, the pressure of the brake fluid in the wheel cylinders, multiplied by the area of the head; from below, the pressure of the brake fluid from the main brake cylinder, multiplied by the area of the lower belt of the head, as well as the compression force of the piston spring and the twisting force of the drive lever.
In the event of a failure of the rear brake hydraulic drive circuit, the brake fluid pressure developed when the piston 4 advances will move the piston 6 until it stops in the plug. In this case, the holes connecting the cavity of the hydraulic drive of the rear brakes with the supply reservoir and with the pipeline 7 will be blocked, there will be no leakage of brake fluid, and the circuit of the hydraulic drive of the front brakes will remain fully operational.
In this case, only the free play of the brake pedal and the braking distance will increase.
If the front brake hydraulic circuit fails, piston 4 will move forward until it stops against piston 6, close the compensation hole connecting the cavity with the supply reservoir, and when the brake pedal is pressed further, the rear brake hydraulic circuit will be actuated. Pedal free play and braking distance will also increase.
IV. release
If you release the pedal, it will return to its original position by the force of the release spring until it stops (VAZ-2101, VAZ-2102) to the brake light switch 1. The return of the brake pedal to its original position in the VAZ-2103 proceeds somewhat differently, since the pedal is connected to the piston of the booster valve using a pusher. The released pedal pulls the booster valve lifter and piston along with it. The piston, in contact with the shoulder of the booster valve, closes the access of outside air to the cavity B. The shoulder moves away from the edges of the hole for the piston and opens an annular gap that communicates the atmospheric cavity B with the vacuum cavity A through the channels. The servo action of the amplifier stops, and the brake pedal, housing, valves, together with the stem, return to their original position.
No stem pressure (at VAZ-2103), and for VAZ-2101 and VAZ-2102 from the side of the pusher, pistons 4 and 6 under the action of return springs move back to their original position until they stop against the locking screws. When returning, the spacer rings, resting against the locking screws, take the seals to the front wall of the annular groove of the pistons and communicate the cavities of the main cylinder with the feed tank.
The pistons 9 of the rear brake are retracted to the non-working position by reducing the sealing rings, elastically deformed by the exit of the pistons from the cylinders at the beginning of braking, and the pistons 19 of the rear brake - by reducing the coupling spring. Excess brake fluid flows through the compensation holes into the reservoir 13.