Air and Hydraulic Brakes: Similarities and Differences

Written by Godfrey on . Posted in Blog

Many people, including a large proportion of laymen, understand that semitrucks and other large vehicles use air brake systems. However, people don’t always understand how air brakes function, especially in comparison to hydraulic braking systems found on smaller vehicles.

Below is an explanation of air brake and hydraulic brake systems and what you should know about each, including the similarities and differences between the two types.

Hydraulic Braking Systems

In smaller vehicles, including almost all passenger cars and light trucks, hydraulic systems are used to provide braking power. Hydraulic systems utilize liquids to apply force to objects; for example, hydraulics provide the necessary force to raise or lower forklift arms. Brake fluid is the key medium for transferring the force that stops a moving vehicle equipped with hydraulic brakes.

In a car, brake fluid is stored in a reservoir located in the engine compartment. Fluid is drawn into the master cylinder as needed, where it is pushed out into the brake lines whenever force is applied to the brake pedal. In vehicles with power brake systems, a pump provides additional assistance and makes braking action easy and light.

As brake fluid is pushed through the lines, it transfers force to the wheel cylinders located in each wheel. Brake fluid is not compressible, so the wheel cylinders reflect any movement in the master cylinder. Fluid pressing against the wheel cylinder forces brake shoes or pads to engage with the wheel’s drum or rotor, depending on what type of braking mechanism is in place.

Hydraulic braking systems work well with automobiles because the vehicles have relatively light construction and weight. In addition, the minimal amount of room needed for hydraulic systems when compared to air brake components makes hydraulics a popular choice for smaller vehicles.

However, hydraulic brakes are far from perfect, especially in heavy-duty applications, and air brake systems can help overcome shortfalls.

Air Braking Systems

As opposed to hydraulic braking systems found in light passenger vehicles, air brakes are prevalent in heavy vehicles. For example, tractor trailers, railroad locomotives, buses, and other large vehicles are dependent upon air braking systems.

One reason air brakes are widely used in heavy-duty applications is the need to provide absolute stopping power. Even though the actual braking components at the wheel level — pads, calipers, brake shoes, and rotors — are similar to hydraulic systems, the key difference is in how the power is applied to stop an air brake-equipped vehicle.

To ensure stopping power is always available, air brakes operate as the default status, meaning they are always engaged. Strong springs push the braking mechanisms into position and hold them there until a sufficient amount of air pressure is available to disengage them. Pressure holds the braking mechanisms back away from the wheels and allows for vehicle movement.

That means when the brake pedal is depressed, air brakes function by releasing air from the system and allowing the spring to push the mechanism back into a stopping position. Likewise, whenever the operator lets off the brake pedal in an air brake-controlled vehicle, air pressure builds again and pushes the braking mechanism back out of the way.

Advantages of Air Brakes in Heavy Trucks

The chief advantage of air brakes over hydraulic brakes is their combination of stopping power and inherent safety. As noted above, air brakes are engaged automatically, and deliberate action must be taken to disengage them from their positions.

That provides a huge safety advantage over hydraulic brakes since fluid leaks or other failures in hydraulic breaks mean they will be unable to engage. An air brake failure will cause the mechanisms to engage and cause the vehicle to stop.

Another advantage of air brakes is their strength of stopping power. Hydraulic brakes can be powerful, but their lack of mechanical backing to provide stopping power means they are secondary to air brakes. The springs used to hold down braking mechanisms in an air brake system are superior for halting a moving truck or another large vehicle.

If you have questions about air brakes, be sure to contact Godfrey Brake Service for assistance. We are available to help you with all of your heavy-duty truck service needs.

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