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How Air Brakes Work in Trucks Full 3D Animation Explained - 3D Casual Learning
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00:00When a truck driver steps on the brake pedal, they're not directly stopping the wheels.
00:05They're releasing a powerful wave of compressed air, over 100 psi of pressure.
00:11That force travels through tanks, valves, and chambers, translating a simple pedal press,
00:17into thousands of newtons of stopping force. This is the air brake.
00:22And today we'll uncover how this system is designed not just to stop, but to stop safely,
00:28reliably, and under full control.
00:31Trucks have an engine-driven air compressor that continuously pumps air into the system.
00:37The compressor's crankshaft and connecting rod convert the engine's rotational force into a
00:43vertical linear motion, driving the pistons up and down, compressing air with every stroke and
00:48feeding it into the brake system's reservoirs. However, if the compressor operates continuously,
00:58what mechanism prevents the system from exceeding safe pressure limits?
01:03This is controlled by the air governor. A pressure-actuated control device,
01:08that continuously monitors the pressure within the reservoir tanks,
01:11and regulates the compressor's cut-in and cut-out cycles accordingly.
01:16It connects to the supply reservoir via an air line, allowing reservoir pressure to enter the
01:21governor through its reservoir port. This same pressure also interacts with the air dryer,
01:26which we'll cover shortly. But first, let's take a closer look at how the governor actually works.
01:35The air governor has several key parts.
01:38As the air compressor runs and pressure builds in the reservoir, air pushes against the governor's internal piston.
02:02This piston, along with the attached valve, moves against the resistance of the pressure-setting spring.
02:09When the pressure reaches the governor's cut-out setting, typically around 120 psi,
02:15the piston moves far enough to close the exhaust valve and open the inlet valve.
02:20At this point, reservoir air flows through a passage inside the piston and exits via the governor's unloader port,
02:26heading straight to the compressor's unloader mechanism.
02:30This air pressure activates the unloader pistons inside the compressor,
02:34forcing the inlet valves to stay open, and as a result, air compression stops.
02:39Once the system's pressure drops to the cut-in point, typically around 100 psi,
02:45the governor spring pushes the piston back.
02:47This closes the inlet valve and opens the exhaust, allowing the unloader air to vent out through the governor.
03:00With the pressure gone, springs inside the compressor,
03:14return the unloader pistons to their normal position,
03:17seating the inlet valves and compression resumes.
03:19To safeguard the air system in the event of a failure,
03:28in either the governor or the compressor unloader mechanism,
03:31a safety valve is installed on the supply reservoir.
03:37This valve is usually set to open at 150 psi.
03:43It operates using a spring-loaded stem that presses against a seated ball valve.
03:49When the pressure in the reservoir exceeds the set limit of 150 psi,
03:54the pressure forces the ball off its seat,
03:56allowing excess air to escape through the exhaust port.
04:09At the heart of a dual-circuit brake system are three key reservoirs.
04:14The supply reservoir.
04:15The primary reservoir for the rear axle brakes.
04:23And the secondary reservoir for the front axle brakes.
04:27These reservoirs are interconnected but protected from one another by a single check valve.
04:33The supply reservoir is the main source of compressed air.
04:36From here, air flows through the check valve into the primary and secondary service reservoirs.
04:43The check valve allows air to move in only one direction,
04:46from the supply reservoir to the service reservoirs,
04:49while preventing any reverse flow.
04:51This is crucial in case the supply reservoir pressure drops due to a malfunction or leak.
04:56The check valve design is simple but very effective.
05:01It contains a supply inlet port,
05:04a valve seat,
05:07a spring,
05:09and a delivery port.
05:11When air pressure from the supply reservoir exceeds the spring force,
05:17the valve opens,
05:18letting air into the service reservoirs.
05:20If the pressure falls,
05:24the valve closes,
05:25sealing off the service reservoirs to retain braking capability.
05:29To ensure compliance with safety regulations,
05:33the air pressure in both service reservoirs is monitored and displayed on a dual-pressure gauge
05:37mounted on the dashboard,
05:39providing the driver with real-time information on brake system readiness.
05:42The air produced by a vehicle's compressor is 100% saturated with water vapor.
05:49When temperatures drop,
05:50this vapor condenses into liquid water,
05:53which can damage components and affect brake performance.
05:56To address this issue,
05:58a desiccant air dryer is installed between the compressor and the supply reservoir.
06:03Its job is to remove 100% of solid and liquid contaminants,
06:07and approximately 95% of the water vapor,
06:10before the air enters the brake system.
06:13During normal operation,
06:15known as the charge cycle,
06:16compressed air from the compressor enters the supply port located on the air dryer's end cover.
06:22As the air flows through the end cover,
06:24it begins to cool,
06:25causing some contaminants to condense and collect in the end cover sump.
06:30The partially cleaned air then enters the oil separator,
06:33where additional liquid and solid contaminants are removed.
06:37Still saturated with water vapor,
06:39the air moves into the desiccant cartridge,
06:42passing through the desiccant drying bed.
06:44Here,
06:45the water vapor is removed through a process called adsorption,
06:48where moisture clings to the desiccant material.
06:51Most of the dry air exits the cartridge through the integrated check valve,
06:56filling the purge volume between the desiccant cartridge and the outer shell.
06:59From there, air travels to the delivery port and on to the supply reservoir.
07:11The air dryer stays in this charging state until system pressure reaches the governor cutout pressure,
07:17typically around 120 psi.
07:19When system pressure hits 120 psi,
07:24the governor sends a signal to both the compressor and the air dryer.
07:28This signal causes the compressor to unload and triggers the purge cycle in the air dryer.
07:42Air from the governor enters the control port, moving the purge valve piston.
07:46This action does two things,
07:50closes the turbo cutoff valve,
07:52sealing the dryer's inlet port to prevent turbo pressure loss,
07:56important if the compressor is turbo fed,
07:58and opens the purge valve,
08:00releasing contaminants collected in the end cover sump through the exhaust port.
08:09Simultaneously,
08:10the check valve closes to protect the supply reservoir from pressure loss.
08:13Dry air stored in the purge volume reverses direction and flows back through the desiccant cartridge,
08:23passing through a small orifice near the check valve.
08:26As it expands,
08:27it becomes super dry,
08:29stripping away the accumulated moisture in the desiccant material,
08:32a process known as regeneration.
08:35Any remaining contaminants in the oil separator are also expelled during this purge.
08:40The entire purge cycle lasts around 25 seconds,
08:43and continues until system pressure drops to the governor cut-in level,
08:47typically 100 PSI.
08:50When the air pressure drops below the governor's cut-in threshold,
08:54the governor vents air from both the compressor unloader and the air dryer's control port.
08:59With the control pressure removed,
09:01the purge piston is pushed back by its return spring,
09:04causing the purge valve to close.
09:06This action resets the air dryer,
09:09and the system transitions back into the charging cycle.
09:13This charge-purge sequence repeats continuously,
09:16maintaining a consistent supply of clean, dry air to the brake system at all times.
09:21The service brake operation begins at the two service reservoirs,
09:26which serve as the starting point for a dual or split brake system.
09:31To utilize these separate reservoirs effectively,
09:34the system relies on a dual brake valve,
09:36a single unit that contains two independent valves housed together,
09:39both operated simultaneously by one foot pedal.
09:42The actuation components include the pedal,
09:52the plunger,
09:55the roller,
09:58boot,
10:00and fulcrum pin,
10:05all working together to transmit the driver's input into braking force.
10:12Internally, the valve consists of several critical components.
10:20The spring seat.
10:24Graduating spring.
10:27Primary piston.
10:30Primary inlet and exhaust valves.
10:33Secondary piston.
10:35And secondary inlet and exhaust valves.
10:42Each reservoir delivers compressed air to its designated supply port on the dual brake valve.
10:49The primary reservoir supplies air for the rear service brakes and the parking brake,
10:53while the secondary reservoir serves the front service brakes.
10:59In the valve's default, unactuated state,
11:02both circuits are closed,
11:04and the delivery ports are open to the atmosphere,
11:07venting through the exhaust.
11:08When the brake is applied,
11:13pressing the treadle pushes the plunger down onto the spring seat.
11:17This compresses the graduating spring,
11:20causing the primary piston to move.
11:22As it travels,
11:30the piston first closes the primary exhaust valve,
11:33then unseats the primary inlet valve,
11:35allowing pressurized air from the primary service reservoir to flow into the primary delivery port.
11:42Some of this air also flows through an internal bleed passage into the secondary piston cavity,
11:47where the pressure buildup pushes the relay piston forward.
11:52As the relay piston moves,
11:56it closes the secondary exhaust valve and opens the secondary inlet valve,
12:00allowing air from the secondary reservoir to flow into its delivery port.
12:08Now let's explore where the air from the brake valve goes,
12:11and how it activates different parts of the truck's braking system.
12:14Most manufacturers configure the braking system using a front-rear axle split.
12:24The primary circuit, shown in green,
12:27controls the rear axle service brakes and the spring brake service portion.
12:31The secondary circuit, shown in orange,
12:36operates the front axle service brakes.
12:38Once the driver presses the brake pedal,
12:54air pressure flows through the dual brake valve,
12:57sending compressed air into both circuits simultaneously.
13:01Air from the secondary circuit,
13:03enters the front brake chamber,
13:05which operates similarly to a piston in a cylinder.
13:07This chamber consists of a pressure plate,
13:11non-pressure plate,
13:13and a flexible diaphragm sandwiched between them.
13:17A return spring holds the push rod and plate against the diaphragm's non-pressure side.
13:21When air enters through the inlet port,
13:27it pushes against the diaphragm,
13:29forcing the push rod outward.
13:31This mechanical force is transmitted to the slack adjuster,
13:35which rotates the S-cam,
13:36spreading the brake shoes against the brake drum to slow the wheel.
13:41The braking force depends directly on the air pressure applied to the diaphragm.
13:45When the driver releases the brake,
13:48air exits the same inlet port via the exhaust passage,
13:52and the return spring retracts the push rod,
13:54releasing the brakes.
13:57While the front axle uses standard service brake chambers,
14:01the rear axle is typically equipped with spring brake chambers,
14:05which provide both service braking and emergency or parking brake functions.
14:09Internally, the service side of a spring brake chamber works like the front chamber.
14:15A large mechanical spring inside the chamber applies the brake if air pressure is lost,
14:20making it fail-safe.
14:22We'll cover that in just a moment.
14:24The first component to consider is the double-check valve.
14:31This valve serves two purposes.
14:33It directs airflow to specific functions and selects the higher pressure from two possible sources.
14:39For example, parking brakes can be controlled using air from either the primary or secondary reservoirs.
14:45The most common design uses a shuttle housed in a guide inside the valve body.
14:55It has two inlet ports and one delivery port.
14:58When air enters one of the inlets, the shuttle moves in response to the pressure,
15:04sealing off the port with lower pressure while allowing air to flow through the delivery port.
15:12If pressure levels change, the shuttle automatically reverses its position without obstructing airflow.
15:17The blue line indicates the parking or emergency air circuit.
15:30The spring brake system serves dual functions on the rear axle.
15:34It acts as the service brake and also provides emergency and parking braking.
15:38The service brake chamber contains a pressure plate and a non-pressure plate,
15:45separated by a flexible rubber diaphragm.
15:47A return spring holds the push plate and push rod assembly firmly against the non-pressure side of the diaphragm.
15:59The rear section of the spring brake assembly, often referred to as the piggyback,
16:04includes several key components.
16:06When the vehicle is started, air pressure is directed into the diaphragm,
16:21compressing the spring and holding the brakes in a released position.
16:24The spring brake chambers are designed so that air pressure releases the brakes,
16:37while the removal of air pressure applies them.
16:40This is the opposite of how service brakes function.
16:45With strong spring lifted, a service brake chamber can function without obstacle while vehicle is moving.
16:51When a brake application occurs, compressed air enters the chamber, expanding the diaphragm.
16:59This expansion drives the push rod and push plate outward, overcoming the resistance of the return spring.
17:06As a result, the brake is applied.
17:14That concludes our video for today.
17:17Thanks for watching, and we'll see you in the next one.
17:21We'll see you in the next one.
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