Arvin Meritor and Meritor WABCO Vehicle Control Systems have announced their new “Premium Hydraulic Braking System” for medium-duty truck and bus applications. It mates Meritor WABCO’s Hydraulic Power Brake (HPB) with the well-proven Meritor Quadraulic hydraulic disc brake. Combined, they’re said to offer “unparalleled stopping and performance capabilities.” The package is available to OEMs now.
The new HPB system’s benefits are said to include shorter stopping distances, longer lining life, and a pedal feel that’s similar to that of a passenger car. As well, the system offers fewer aftermarket parts and single-source shopping. It provides ABS, automatic traction control (ATC), and electronic brake force distribution, with optional auto-apply electronic parking brake control.
Significantly, it doesn’t need an external source to provide power assist to achieve low pedal effort as required in a conventional hydraulic braking system. The hydraulic energy required to actuate the brakes and control their functions is stored in hydraulic accumulators. When the vehicle’s ignition is turned on, internal pumps are activated and fill both accumulators with pressurized hydraulic brake fluid. When the brake pedal is applied, the small master cylinder provides a hydraulic signal to the relay valve. Proportional to that signal, the accumulators provide pressure to the brake calipers. When the pedal is released, brake fluid returns to the reservoir from the brake calipers. For ABS and ATC activation, pressure at the wheel is individually modulated by ABS and ATC solenoid valves in the hydraulic modulator assembly. When required, hydraulic pressure is supplied and released to the appropriate foundation brake to achieve the desired functional effect.
In addition to ATC, the system can accommodate stability enhancement capability which hasn’t previously been available to the medium-duty market.
Electronic brake force distribution is another key development of the HPB system. It provides constant review of the vehicle’s load status and monitoring of each wheel end. Based on wheel slip observed by the HPB system during braking, braking forces at the axles are automatically adjusted to achieve a balanced, efficient use of the brakes.
With a fixed-caliper opposed four-piston brake, the Quadraulic disc brake is designed to package within 19.5-x-6.75- in and 22.5-in wheel sizes. It’s available in two piston sizes-4 x 64 mm and 4 x 70 mm. The fixed caliper eliminates a main concern often associated with rail slider or pin slider designs-external environment factors. And when lining changes are required, only the lining retention bolt needs to be removed and the new linings can be dropped into the caliper. The caliper does not swing out so the brake hose remains untouched during this process. It was introduced several years ago.
Having tried the HPB system in a brief road test, side-by-side with an older truck using conventional brakes, we can confirm its car-like pedal feel, low pedal effort, and comfortingly sure stops. There was no science to this test, but panic stops from 50 km/h were much shorter with the HPB system and the truck remained completely stable-and pointed in the right direction.