Before the spring brakes, parking brakes were firstly supplied by a disk brake mounted on the drive shaft and rear service brakes were the best option. However, there was a problem when air pressure loss occurs. That’s why spring brake system became a must have option in today’s trucks.
But firstly, I will write about different brake systems and some specific braking processes. After that I’ll go to the details about spring brakes.
To understand the process, a driver should know the basic things about air pressure in braking. By pushing the brake pedal you control the air pressure. The air brake chambers convert air pressure force into mechanical push rod force.
Air-over-Hydraulic Brake System
This system is combination of compressed air and hydraulic pressure use for brake operation. An air-over-hydraulic power cylinder contains both air and hydraulic cylinder, and they are fitted with a piston and common rod.
Because the air piston is greater than hydraulic, there is higher hydraulic pressure admitted to the air cylinder. So what this means for brake pedaling?
Since the hydraulic pressure cause greater valve movement, the valve admits higher air pressure in the power cylinder that results in stronger braking action.
In this system, when brakes are applied, air gets exhausted from the cylinder head, while air pressure acts on the pistons rear side to exert a pull on the rod attached to the piston.
If the brake valve is closed, the pressure is equal on both sides of the piston and no pull is exerted.
Controlled, Stab and Downhill Braking
Also referred as squeeze braking, controlled braking method means applying the brakes maximally to the point of not locking the wheels. When using control method, try to avoid turning the steering wheel. If steering is absolutely necessary, release the brakes until the tires gain traction. You should do this if the wheels begin to slide.
Stab braking means using the brakes hardly and releasing them the moment the wheels lock up. After this releasing, it will pass a second before the wheels start to roll, so you should stay off the brakes long enough. When the wheels start rolling, apply the brakes again.
The right way to go down long grades is using a low gear at slow speed which will allow you steady use of brakes. This way you prevent speeding up, release the pressure and brakes are cooling down and working correctly. Otherwise, overuse of the brakes can get them overheated and eventually faded.
If the vehicle is on manual transmission, you shouldn’t push the clutch before the engine rpm is down to idle speed. After smooth and safe stop, select a starting gear.
How Spring Brakes Work?
To understand how brakes system works, let’s firstly remind ourselves how is that working in cars.
Pressing the brake pedal is forcing brake fluid through the master cylinder that passes through the ABS control module.The fluid reaches the calipers and pressure squeezes pads against the rotors.
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Now, unlike passenger cars, heavy trucks use air pressure to apply braking power. Except that, the main difference is in parking brakes. Usually, spring brakes are used as parking brake. They can be found on the rear axle of heavy trucks.
The spring brakes works like this: The pressure is applied to the spring side, which allows the parking brake to disengage. When the spring is decompressed (at 20 lbs), parking brakes have been applied. Air pressure hold back the the springs when you are driving. In this system, a leak is lowering air pressure and causes springs to engage the brakes.
Important to remember: Due the lack of braking power of the full service brake application, vehicles can be driven with spring brakes applied. Therefore, to avoid this situation, ensure that brake system has enough air pressure – 60 psi.
This system was firstly implemented for safety concerns, because trucks that had lost air pressure would struggle to reach controlled stop. So how did spring brake solve the problem?
Basically, the driver have more time, because the spring decompresses as the air pressure drops. Alarm is triggered whenever the pressure drops below the 60 lbs.
There are two chambers in spring brakes. One release spring, while other apply braking power. The spring is strong enough which means, that in case of sudden loss of pressure, it will apply full braking power to the axle and keep vehicle under control.
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Finally, when there is no air pressure, and you need to move your vehicle, use a cage bolt. After the vehicle is at desired location, remove the bolt.
How do I know if I have spring brakes?
Look at the driver’s control panel and the rear axle. On the spring brake control valve there is usually a label. The valve is probably opened, which means it is in off position. By closing the valve you will allow the air off the chambers and springs will engage the rear brakes. On the rear axle, you will have brake chamber at sides of rear axle.
Spring Brake Chamber
I’ve already explained that service brakes and spring brakes are not applied in the same way. Spring are applied when air pressure leaves the brake chamber.
So, what is a spring brake chamber? It’s a brake chamber that includes both service brake and spring brake sections.
People often ask, what is the difference between spring brake actuator and brake chamber? Both are used in air braking system in trucks, but unlike brake chamber, spring brake actuator has hand brake function. Also, brake chambers are used in front axles while brake actuators are used in rear axles.
Spring brake chamber
Large coil spring provides sufficient force that hold brakes in the applied position. They are not using air for that matter. This large coil spring in accommodated in section added to the service brake chamber.
Remember, the large coil spring is compressed under high tension which generates various hazards and risks such as spring brake chambers explosions.