The disc brake system has been applied on heavy vehicles for both the purpose of weight reduction and enhanced brake performance. The conventional disc brake applied to the wheel end module is simply assembled with hub and rotor. It can be easy for this type to be manufactured; however, there are some durability problems with heat crack because of the difference of temperatures on each side of the rotor caused by the brake heating.
The splined disc brake system is one of the solutions for this thermal damage. This system has the separated rotor, which is the friction part of the disc brake, and the adapter, which is the mounting part of the wheel hub and disc rotor. The separated rotor and the adapter are assembled with elastic fasteners to absorb the distortion of the rotor by the brake heating.
Recently, truck manufacturers have tried the integration of the hub and the adapter to reduce the weight of rotation parts. The concept of the integrated hub is designed to add the splined rotor mounting parts on the conventional hub that is made up of the wheel mounting part and hub bearing part. Therefore, the integrated hub reduces the weight of the adapter and lightens the wheel end module, which is part of the unsprung mass.
The integrated hub must be designed to avoid the heat damage over the lifetime of the hub bearing, because the direct-mounted rotor on the hub is a heat source when braking since it produces friction with the lining. When the hub unit bearing is exposed to much higher temperature than the heat-resisting temperature, the heat will change the properties of the grease that is inside the hub unit bearing. Therefore, the lifetime of the hub unit bearing will be shortened because the lubricating performance will decrease remarkably.
The integrated-hub design concept includes penetrated radiation holes, air gaps, and radiation fins. The penetrated radiation holes are formed with the same circular interval from each rotor mounting tooth part to wheel mounting flange. The holes radiate the heated air from inside the vehicle to the outside.
The air gap is formed between the rotor mounting part and hub bearing part to avoid the heat conduction caused by braking. The ribs are formed at the opposite side of the rotor mounting teeth to expand the surface of the heating area up to 15%, which plays the role of the radiation fin.
The fins make high-strength structures against the braking force. The stress by braking force is reduced up to 9.8%, compared with the simple annular shaped air gap.
This designed integrated hub was evaluated by the temperature measurement test and the fatigue acceleration test according to SAE J1095 and SAE J2562.
For performance testing, the hub and bearing assembly was assembled on the test rig to measure the temperature that was conducted to the hub bearing. The test results showed that the hub only with the penetrated radiation holes had a 19.7% higher temperature than that of the benchmark. The maximum temperature was measured at the outer ring of the inner hub bearing that is the closest point of the rotor.
The hub with both the penetrated radiation holes and air gap was measured to have 15.5% lower temperature compared with the benchmark. The maximum temperature was measured at the outer ring of the outer hub bearing because the heat is conducted from outside of the hub, to the flange part, and then to the hub bearing.
For the on-vehicle test, temperature measurement was performed on a severe-conditioned road with regard to brake heating (driving down an asphalt-paved mountain road). The determined hub was formed with the penetrated radiation holes and air gap. The GVW of each tested vehicle was the same 40 ton, but the difference between the 27-ton test-vehicle dumper’s front axle load and the 25.5-ton benchmark dumper’s was 9.8%. The maximum temperature was measured at the outer ring of the outer hub bearing, and the temperature was 4.8% higher than that of the benchmark dumper’s. The measured temperature at the rotor was 28.5% higher than at the benchmark’s rotor.
The performance of the newly designed integrated hub with regards to the avoidance of the brake heat was better than the performance of the benchmark. Therefore, the new hub design is suitable to be applied on the vehicle.
Computational analysis of both static load and fatigue was also performed. The results of the analysis showed that the new hub is robustly designed. The fatigue life of the hub is predicted to have 200% of the test running life that the acceleration test requires.
The biaxial fatigue test according to the SAE specification showed that the hub conformed to the required test run and the 200% endurance test as predicted by the computational analysis. Therefore, the hub has an on-road life of 1 million km (620,000 mi) or off-road life of 500,000 km (310,000 mi) or more.
This article is based on SAE International technical paper 2011-01-2170 presented by EunHo Lee and YongSoo Lee of Hyundai-Kia Motors at the 2011 SAE Commercial Vehicle Engineering Congress.