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What Are Spring Brakes? Pros, Cons & FAQ

Courier in blue uniform changing gear when driving his truck

Stopping over 40 tons of weight on the highway is no easy task, but tractor-trailer drivers confidently manage their speeds thanks to the reliability of air brakes. Like any brake system, air brakes aren’t immune to malfunctions and failure. When that happens, spring brakes explode into action to provide the stopping power necessary to avert disaster. These backup brakes serve several functions, operating off cleverly simple principles to deliver efficiency and security. Spring brakes are essential emergency features of heavy trucks. Let’s look at the basics of what spring brakes are and how they work to protect truck drivers and their fellow motorists.

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How Do Spring Brakes Work?

They include an air brake chamber (service brake), which is the primary stopping mechanism for the vehicle that engages via the brake pedal. An attached rear chamber houses the spring brake mechanism.

The spring brake serves a dual function as a parking brake that you can engage manually and as an emergency brake when the service brake system fails. The parking brake control is usually a four-sided yellow knob near the steering column. Pulling it out engages the brake, and pushing it in supplies the spring brake with air to release it. Some vehicles reverse this function.

Closeup view of a truck's front disc brake with visible suspension system.
Image Credit: Chettaprinn P, Shutterstock

Service Brake Basics

A service brake uses compressed air to generate mechanical force against a pushrod. The service brake chamber is a squat, fat metal cylinder with a push rod inserted into a hole in the center of one end. Part of the pushrod sticks into the brake chamber, while most of it extends outside it to connect with the slack adjuster. The slack adjuster engages the S-cam at the wheel, which expands the brake shoes in a drum brake and slows the vehicle.

Along with a portion of the pushrod, the interior of the service brake chamber includes a diaphragm and a return spring wrapped around the pushrod. The diaphragm is a rubber disk that bisects the chamber, forming two sealed chambers within the service brake. On one side of the disk is the pushrod, and on the other is an empty chamber.

Air brakes draw compressed air into the empty side. The filling cavity builds pressure that forces the diaphragm to flex to the other side of the chamber. When the diaphragm flexes out to the pushrod side, it presses the pushrod out, thus activating the brake. Air compressor governors on semi-trailer trucks typically maintain around 100–120 psi to ensure the air is powerful enough to push out and hold the pushrod in a braked position.

A return spring wraps around the pushrod portion that slides within the brake chamber. The spring creates constant return force, but not enough to overcome all of the compressed air in the chamber when the brake is engaged.

When you release the brake, it dumps the air from the chamber. The unchallenged return spring can now decompress. The diaphragm flexes back to the empty cavity side, the spring pulls the pushrod back into the brake chamber, and the slack adjuster disengages the S-cam and brake shoes.

Spring Brake Construction

brake chamber of a truck
Image By: Ake Apichai Chumsri, Shutterstock

A spring brake is a single unit containing a standard service brake chamber and an additional chamber holding the spring brake emergency/parking mechanism. The spring brake system “piggybacks” on the service brake, sitting on the end of the service brake opposite the pushrod end. With the service brake, the entire spring brake housing is about two to three times larger than a typical service brake.

The service brake portion of the spring brake works like any other service brake in regular operation, utilizing a compressed air intake to activate when slowing down. Another air intake leads to the emergency spring brake section. The interior of the spring brake portion looks similar to the service brake, with a few essential differences.

Again, a rubber diaphragm splits the chamber through the middle. A coil-wrapped pushrod sits on the side of the diaphragm closest to the service brake chamber, and the pushrod slides in and out of an air-sealed O-ring leading to the service brake side when engaged and disengaged.

When engaged, the spring brake pushrod forces the service brake pushrod out and hits the brake. When disengaged, the return spring wrapped around the pushrod keeps it completely inside the spring brake chamber, preventing it from affecting the standard operation of the service brake in front of it.

A massive power spring is on the other side of the spring brake diaphragm (the side furthest from the service brake).

Spring Brake Operation

The compressed air intake for the spring brake side leads to the section holding the coil-wrapped pushrod. Unlike a service brake, this section is constantly under pressure from compressed air. That pressure flexes the diaphragm to the back of the spring brake chamber, compressing the power spring and stopping the brake from engaging.

It takes about 414 kPa, or 60 psi, of air pressure to compress the emergency power spring. When the chamber depressurizes, either from the driver popping out the parking brake or the braking system losing compression due to failure, the power spring decompresses with up to 2,000 pounds of force.

Once the pressure drops below 60 psi, the emergency brake will begin to engage and may drag. Below 20–45 psi, the emergency brake will engage fully and stop the vehicle.

A gauge on the dash indicates the air pressure in the systems. Vehicles with dual air brakes will have two gauges or a single gauge with two needles for the separate braking systems. When the pressure drops, the vehicle will tell you with a red light and buzzing sound. Some devices drop an arm called a wig wag into your view to warn you.

Brake Spring
Image By: .xiao gyou, Pixabay

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What Are the Different Types of Spring Brakes?

Spring brakes come in various styles and sizes. The most common size for heavy-duty applications is a Type 30/30. The numbers refer to the space upon which the compressed air acts, or the area of the diaphragm, noted in square inches. The first number indicates the service brake chamber size, and the second shows the spring brake chamber size.

Air brake types can range from six to 36, each getting larger as the number increases. The numbers can vary within a single piece. For example, a Type 30/36 chamber means the spring brake section’s diaphragm is wider than the service chamber’s diaphragm. A higher number indicates a more intense braking force. Larger diaphragms correspond to more air pressure in the service chamber or a more powerful spring in the spring brake chamber.

To find the correct type of spring brake for your vehicle, you can reference the owner’s manual or check the brake for markings indicating the size. Alternatively, you can measure the size of the clamp around the chamber.

Long-Stroke Spring Brakes

Pushrod stroke length will also vary. You’ll commonly see standard or long-stroke options. Long-stroke chambers move the pushrod further than a standard stroke chamber, typically by about ½ inch.

The chamber will have markings to identify it, including:
  • Trapezoid-shaped tag with rated stroke
  • Marking such as an “L” or “LS” (e.g., “Type 20L”) with service data and ID information on the chamber body
  • Square-shaped air-fitting ports
  • Square-shaped embossing on the pressure cap portion of the housing

A longer stroke allows for extra stroke capacity that can activate the slack adjuster more. Having the added braking threshold is especially helpful when brakes heat up. As drum brakes heat and expand, they require the shoes to travel further to apply braking friction.

With an added reserve stroke, the S-cam can move more freely and provide the added motion to create a tight lock against the brake at all times. Long-stroke spring brakes can significantly improve braking efficiency and performance.

Where Is It Used?

Spring brakes can appear anywhere that you’ll find air brakes. They’re in tractor-trailers, buses, straight trucks, and fire trucks. The rear axles hold the spring brakes to give the driver control over the vehicle when the emergency brake is engaged.

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Advantages of Spring Brakes

Spring brakes use a clever bit of simple engineering. The emergency mechanism uses one of the system’s greatest threats against itself to provide intense braking power if the compressed air system is compromised. The harder it is for your service brakes to operate, the easier it is for the emergency brake to engage.

Disadvantages of Spring Brakes

Spring brakes may leave room for human error that could cause damage to the system. Compounding the brakes occurs when the driver applies the service brake while the parking brake is engaged. By pumping the service brake full of compressed air alongside the spring brake pushrod, the driver risks damaging the chamber, slack adjuster, or several other components along the brake system.

Fortunately, modern air brake systems prevent compounding with an anti-compounding valve. The system stops the simultaneous actuation of the two braking mechanisms by releasing the parking brake as air fills the service brake.

The power spring’s high tension is another potential hazard that has become safer over time. With the spring’s explosive force, opening the chamber could have deadly consequences for anyone standing in front of it. New brakes make service safer with anti-tampering features.

commercial driver driving his truck
Image By: Virrage Images, Shutterstock

divider 4 Frequently Asked Questions (FAQs)

How Do You Test Spring Brakes?

Spring brakes are part of the multi-point air brake test. The spring brake test occurs with the vehicle on and the engine off. After testing low-pressure sensing for the main service brake, you keep pumping the brakes until the air pressure reaches the emergency brake activation threshold. The limit is generally 20–45 psi. At this point, the emergency brake knob should pop out to show that the spring brake is working.

How Do You Manually Disengage a Spring Brake?

If the service brakes fail and the spring brakes engage, you’ll need to pull over to arrange a fix. But if you need to move the truck to a safe location after the fact, the activated emergency brakes will give you issues. You’ll need to cage the power spring to release them and make the truck movable again.

Spring brakes include a caging tool mounted on the chamber. It uses a washer, nut, and bolt. The bolt enters a hole at the end of the spring brake to cage the spring. With a ¼ turn, the bolt locks into a slot in the chamber. After inserting and locking the bolt, you add a washer and nut. By tightening the nut with a ratchet, the bolt pulls on the spring to slowly collapse it and lock it in place. With the spring collapsed, the brake is ready for repairs, and you can move the vehicle.

How Do You Maintain Spring Brakes?

View of underside of Truck for maintenance pit
Image By: I Tooley, Shutterstock

Corrosion and broken power springs are common culprits in a non-functioning spring brake. Proper spring brake maintenance and inspection are crucial to prevent accidents on the road. Some of the essential checks include:

Some of the essential checks include:
  • Chamber housing inspection for rust and damage
  • Brake cycling to ensure slack adjusters extend equally
  • Releasing air brakes to listen for diaphragm leaks
  • Inspecting caging bolt plug seals and internal chamber for spring damage
  • Tapping the housing to listen for rattling that might indicate a broken spring

You should also check for issues such as incorrect pushrod length or mounting arrangements that can cause misalignments. Improper setups can lead to premature wear on the braking system and diaphragm damage.

With newer sealed spring brake chambers, air venting holes are no longer an issue. Vent holes on the exterior of older spring brakes would allow moisture and dirt to enter the part and speed up corrosion. Maintenance and component checks are still vital on sealed spring brakes, but broken spring issues and failure are less frequent now that venting occurs through water-sealed breather tubes and ports.

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Spring brakes have been around since the 1950s, but the simple mechanics still see frequent safety and performance advancements. Traffic authorities have tightened standards and regulations on brakes in recent decades. By buying quality parts and appreciating proper spring brake maintenance, you can avoid violations and guarantee the safety of everyone sharing the road.

Featured Image Credit: Dragon Images, Shutterstock


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