Howo Off-Highway Mining Truck Brake System | Dual Circuit Air Drum Brake

Applicable models: ZZ5707 series 6×4 mining dump trucks (70-ton Mine King). All wheels adopt S-cam drum brakes, equipped with large-size brake drums and widened brake shoes specially for heavy-load mining conditions.

I. Air Source and Pneumatic Circuit Core Assemblies (Power source of the whole air brake system)

1. Air compressor: Driven by the engine to generate compressed air.

2. Air dryer: Filters moisture and oil contaminants in compressed air, integrated with pressure regulator. Rated working pressure of the system is 0.8 MPa (8 bar), cut-out pressure approx. 0.85 MPa. When pressure drops below 0.55 MPa, dashboard low air pressure warning light illuminates; heavy-duty driving is prohibited.

3. Four-circuit protection valve (core safety component)It divides the air circuit into 4 independent branches:

• Circuit 1: Service brake for middle and rear axles

• Circuit 2: Service brake for front axle

• Circuit 3: Spring energy-storage parking brake circuit

• Circuit 4: Auxiliary air consumption (horn, differential lock, clutch booster, etc.)

If any single circuit leaks and fails, the remaining circuits can still maintain air pressure to guarantee basic braking capacity, which is the safety foundation of the dual-circuit service brake.

4. Air reservoirs: Multiple independent air reservoirs, including front axle reservoir, middle-rear axle reservoir and parking brake reservoir, to prevent all air pressure depletion caused by leakage of one single circuit.

5. Valve components: Brake master valve (foot brake valve), relay valve, quick release valve, parking brake control valve

   1. Relay valve: Mounted close to vehicle axles. The foot brake valve only outputs signal air. The relay valve directly draws high-pressure air from air reservoirs and quickly feeds it into brake chambers to reduce braking delay of long pipelines, which is mandatory for mining trucks with heavy load and large chassis.

   2. Quick release valve: Rapidly exhausts air inside brake chambers when releasing the brake pedal for fast brake shoe return.

II. Service Brake (Foot brake, primary brake, dual-circuit air-operated drum brake)

Structure

• Actuator: Diaphragm brake chamber + S-cam drum brake. Front, middle and rear axles all use drum brakes.

• Power transmission chain: Brake chamber pushrod → brake slack adjuster → S-camshaft → cam expands two brake shoes → brake shoes press inner surface of rotating brake drum to generate friction braking force.

• Brake slack adjuster: Automatic or manual type, used to compensate clearance from brake lining wear.

Working Principle

1. Circuit 1: Controls service brake of middle and rear axles

2. Circuit 2: Controls service brake of front axleThe two circuits work independently. If pipeline rupture or air leakage occurs in one circuit, the other circuit can still operate and retain partial braking force to avoid complete loss of braking.

Features: Large braking force, high temperature resistance and good resistance to mine dust, suitable for off-highway mining trucks with dozens of tons gross weight. Disadvantage: Prone to brake fade during continuous long downhill, so auxiliary brake must be equipped.

III. Parking Brake + Emergency Brake (Spring energy-storage fail-safe brake)

The handbrake of mining truck serves as emergency brake, which is spring energy-storage fail-safe brake. It is integrated into brake chambers of middle and rear axles (combined spring brake chamber: front diaphragm chamber for service brake, rear heavy-duty spring chamber for parking/emergency brake). New high-end models support spring braking on both front and rear axles.

Working Principle

1. Normal driving state: High-pressure air from air reservoir enters spring chamber to compress the powerful internal energy storage spring. The spring is held compressed, parking brake released, vehicle ready to travel.

2. Apply parking brake: Exhaust air from spring chamber via cab parking valve, the energy storage spring extends and pushes the pushrod directly to clamp brake drum and achieve parking. Continuous air supply is not required.

3. Emergency protection: In case of severe insufficient air pressure or pipeline rupture and air leakage during driving, pressure inside spring chamber automatically disappears, spring pops out to lock wheels and force braking to prevent vehicle runaway, which is the most critical safety design for mining trucks.

Note: Spring braking force is limited. It cannot be used for speed reduction during travelling, only for parking and emergency braking when air pressure fails. When air pressure is zero, manual emergency release bolt can be used to release spring brake for towing.

IV. Auxiliary Brake System (Protection for long downhill, reduce thermal fade of service drum brake)

1) Engine exhaust brake (standard for all models)

• Structure: Exhaust pipe butterfly valve, ECU-controlled, switched by combination switch.

• Working principle: Block exhaust passage to increase exhaust resistance of engine. The engine acts as a load to consume vehicle kinetic energy and realize continuous deceleration.

• Operating conditions: Gear engaged, rotational speed ≥900 rpm, throttle released. Automatically disengages when pressing accelerator or clutch. Partial models support brake linkage: slight press on foot brake will activate exhaust brake synchronously to reduce load of drum brake. It is recommended to turn off linkage on slippery road to avoid wheel lock and skidding.

2) Hydraulic retarder (optional, recommended for heavy-load large tonnage mining trucks)

• It is an auxiliary retarder brake, installed at the rear end of gearbox. It consumes kinetic energy by fluid damping, providing stable continuous deceleration on long downhill. Its braking capacity is much higher than exhaust brake, greatly reducing wear of brake drums and linings, suitable for mines with continuous long downhill.

• Multi-stage adjustable braking force, constant-speed downhill available; hydraulic retarder is only auxiliary and cannot replace service brake for emergency stopping.

V. System Protection and Alarm

1. Dual air pressure gauges: Monitor air pressure of front axle circuit and middle-rear axle circuit separately.

2. Low-pressure alarm: When pressure below 0.55 MPa, dashboard red warning light and buzzer alarm. Insufficient air pressure will lock spring parking brake and prevent vehicle starting.

3. Brake lining wear alarm: Newer partial models are fitted with brake lining wear sensors.

VI. Common Fault Points (For maintenance reference)

1. Water ingress in pipelines: Failure of air dryer causes pipeline water accumulation and icing in winter, valve sticking.

2. Overheated brake drum: Long downhill without using exhaust brake or hydraulic retarder, resulting in brake fade and spongy brake feel.

3. Fault of spring brake chamber: Spring fatigue or diaphragm rupture, causing dragging brake or failed parking.

4. Stuck relay valve: Slow brake release, dragging brake and burnt brake linings.

VII. Summary of the whole system