Ural vehicles are equipped with three independent brakes: a service brake with hydropneumatic drive on all wheels, a parking brake with mechanical drive acting on the transmission, and an auxiliary compression-type brake installed on the exhaust system pipelines.
Service brake system
The service brake system is designed to reduce speed and stop the vehicle regardless of its speed, load, and road gradients for which it is designed.
The drive of the brake mechanisms is mixed (pneumohydraulic) dual-circuit, with separate braking of the front axle and two rear axles.
Control is carried out by a pedal in the driver's cab, connected by levers and rods to the two-section brake valve.
The service brakes of the vehicles have a hydropneumatic drive consisting of the following main components: compressor, brake valve, pressure regulator, two pneumatic boosters with master brake cylinders, inter-tank pressure reducing valve, air tanks, combined into a single system using pipelines, hoses and connecting fittings (Fig. 1).
A dual-needle pressure gauge and a fault signalling system are used to monitor the components of the vehicle's service brakes.
The lower scale of the pressure gauge shows the air pressure in the section air tank — brake valve, the upper scale shows the pressure in the pneumatic boosters at the moment of braking.
The service brake fault signalling system consists of sensors installed in the service brake components (assemblies) and red warning lights located on the instrument panel.
The minimum air pressure warning light «Air», when the ignition is switched on, gives a signal when the pressure in the air tank drops below 4 kgf/cm².
The brake emergency state warning light «Brake» comes on when braking with the service brake if there are faults in the mechanical part of the service brake (large clearances between the brake shoe linings and the brake drum, etc.) or in the hydraulic brake system as a whole (fluid leakage, air entering the system, etc.).
The service brake mechanism is of the drum type with internal shoes 4 (Fig. 2), interchangeable for all wheels.
Each brake mechanism has two hydraulic cylinders 7 made in one housing.
The brake shoes are mounted on support axles 6.
The service brake mechanism is adjusted as the linings wear by reducing the clearance between the lining and the drum using eccentrics 2.
Brake adjustment procedure:
- - using a 22 mm wrench, turn the adjusting eccentrics of the shoes until they stop, turning the right eccentric (on the shield side) clockwise, the left one counterclockwise;
- - release the eccentrics by turning back approximately 30°, which corresponds to turning the head of the eccentric axle by half a face.
After performing these operations on all wheels, check whether the brake drums heat up while the vehicle is moving.
When adjusting the brakes, do not disturb the factory setting of the brake shoe axles 6.
Adjust the clearances between the shoe and the brake drum using the shoe axles only when replacing the friction linings or the complete shoes.
Initially install the shoe axles with the marks on the ends facing each other.
Through the opening in the brake drum, insert a feeler gauge 0.2 mm thick and 200 mm long between the drum and the shoe at a distance of 30 mm from the lower edge of the lining.
By turning the shoe axle 6, lightly clamp the feeler gauge.
Remove the feeler gauge, turn the drum and, using another feeler gauge and the adjusting eccentric 2, set a clearance of 0.35 mm between the shoe and the drum at a distance of 30 mm from the upper edge of the lining.
Secure the shoe axles and recheck the clearances between the shoe and the drum.
When the linings are worn down to the plane of the rivet head, replace them.
If the brake linings are oily, wash them with petrol.
If the brake drum is worn and has annular grooves deeper than 2 mm, bore the working surface of the drum with alignment based on the outer rings of the hub bearings.
The runout of the working surface of the drum should not exceed 0.25 mm, and the drum diameter should not exceed 424.38 mm.
For the front axles of vehicles with increased load capacity, only linings of code 143–63 (grey) should be installed.
Mixed (pneumohydraulic) drive of the service brakes
The vehicle is equipped with a dual-circuit brake drive with a combined trailer drive and with additional brake devices.
The drive allows the connection of the brake systems of towed vehicles having single-line or double-line brake drives.
The schematic diagram of the brake drive is shown in Fig. 4.
Compressor 29 supplies compressed air through pressure regulator 4 to the protection valve block.
The block consists of a triple 5 and a single 7 protection valve, which distribute and fill the air tanks 3, 9 and 27 of independent circuits:
- - drive of the brake mechanisms of the front wheels;
- - drive of the brake mechanisms of the middle and rear wheels;
- - combined drive of the brake mechanisms of the trailer wheels.
The first main circuit consists of air tank 3, the upper section of brake valve 28, pneumatic booster 24 and wheel cylinders 23, and the second main circuit consists of air tank 27, the lower section of brake valve 28, brake force regulator 20, pneumatic booster 24, wheel cylinders 23.
The third circuit consists of air tank 9, trailer brake control valves: 19 — with single-line drive and 15 — with double-line drive, type «А» coupling head 18 for connecting trailers with single-line drive, automatic coupling heads 16, 17 for connecting trailers with double-line brake drive.
From air tanks 3, 27, through the triple protection valve 5, air is taken to operate the windshield wipers, air horn, pneumatic booster of the clutch drive and other consumers.
To monitor the air pressure in each circuit, control outlet valves 25 are installed, to which a portable pressure gauge can be connected.
When the vehicle is moving with a trailer that has a single-line brake drive, the connection of the vehicle to the trailer is made by coupling head 18; for a double-line drive, by coupling heads 16, 17.
When the brake pedal is pressed, the first and second circuits of the vehicle's brake drive are activated, as well as the third circuit of the trailer brake drive.
If one of the circuits fails, the others remain operational.
To brake the vehicle with a trailer while parked, set the parking brake lever to the upper fixed position: the control valve 10 then releases compressed air from the outlet "II" of valve 15 (see Fig. 4) and actuates the brake mechanisms of the trailer.
The vehicle is equipped with a system for signalling and monitoring the condition of the brakes. Minimum air pressure sensors are installed in the air tanks.
Sensors for signalling a malfunction of the service brake system (brake fluid leakage or large clearances between the shoes and the drum) are installed in the pneumatic boosters.
