Everything You Need to Know About Air Over Hydraulic Brake Lines
Air over hydraulic brakes are typically used in medium to large commercial vehicles. They were originally invented for the military in the 1940s. At the time, the military needed to be able to move vehicles quickly in emergencies without having the air build up, and they had to be able to tow them quickly. These brakes were ideal in combat environments at the time, as they are very damage resistant. So, what are they and how do they work?
What Are Air Over Hydraulic Brakes?
As the name suggests, this type of braking system is a combination of parts of an air brake system and a hydraulic brake system. It uses both air and hydraulic compression to operate the brakes. This type of braking system was created with the hopes of increasing the braking power compared to the power in a hydraulic braking system. This system is not the most common, but it can often be found in trucks, trailers, cranes, and other industrial equipment. Because of all of their parts and components, these systems must be inspected often and maintained by a professional.
Not to be confused with air brakes, this system has hydraulic drum or disc brakes. This brake system works by converting control line air pressure from the foot brake valve into hydraulic pressure to operate wheel cylinders or calipers and applying brakes. Foot brake valve supplies control line pressure directly to the pneumatic piston.
How Do Air Over Hydraulic Brakes Work?
Air over hydraulic brakes work by using air compression to provide power to the braking system, but has a hydraulically controlled system with a compressor, air dryer, and reserve tanks lines. A typical air-over-hydraulic (A/H) anti-lock brake system (ABS brakes) consists of wheel speed sensors, an ABS relay valve, an integral electronic control unit (ECU) and an A/H booster. The ECU monitors wheel speeds through the use of two-wheel speed sensors mounted on the hubs of the trailer axle. An exciter, also referred to as a tone ring, is a required component of the wheel speed sensing system and is usually included on the axle as received from the axle manufacturer.
When the ECU detects that a sensed wheel has locked up during brake application, the air pressure in the brake chamber on the A/H booster is reduced via the ABS relay valve, allowing the wheel to recover. The ECU then commands the ABS relay valve to apply additional pressure until maximum braking potential is produced. This ABS “cycle” is repeated approximately five times per second as required, or until the trailer is traveling at less than 6 MPH.
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