To achieve weight savings in vehicles, OEMs and component suppliers are increasingly using ultra-high-strength steel, aluminum, magnesium, plastics, and composites. One strategy is to develop components using a multi-material concept. Ashok Leyland researchers used this approach when developing a bimetal brake drum with cast-iron inner ring and aluminum outer shell. Two different design configurations were proposed and made, both resulting in significant weight savings compared to a conventional gray cast iron brake drum, with similar or better performance.
The researchers will present their study as part of the “New Design Concepts for Medium and Heavy Truck Steering and Braking Systems” technical session at the SAE 2014 Commercial Vehicle Engineering Congress, taking place October 7-9 in Rosemont, IL (http://www.sae.org/events/cve/).
One of the major drawbacks of drum brakes is that they are more sensitive to brake fade because they are not capable of dissipating the generated heat. During braking, friction between the brake drum and lining increases the temperature to 315°C (600°F). The excessive temperature rise damages the lining material severely.
To determine the thickness requirement of the cast-iron inner shell and Al outer shell, a detailed CAE analysis was carried out for the two designs. Model 1 consisted of a braking surface in cast iron and outer shell with fins (for better heat dissipation) and clamping face in Al. In Model 2, the braking surface and clamping face were cast iron and the outer shell was Al. The 3D models of the brake drum were created using Dassault Systèmes’ CATIA.
A detailed contact model was developed between the liner and brake drum cast-iron surface. The second-order tetrahedron element was selected for fine meshing. The brake drum hub seating face was clamped in vertical direction, and variable pressure was applied in the leading and trailing contact area. Based on the simulation, high stress was observed in the fillet radius of the inner surface and clamping face of Model 1. The thickness was increased slightly to meet the stress limit. In Model 2, high stress was not observed in high-pressure conditions.
The bimetal brake drum was then developed via a conventional casting process and validated via dynamometer test rig. The test was conducted at three different brake lining temperatures. The drum was rotated at a speed of 60 km/h (37 mph). The 0.2 g braking torque was applied by brake lining to stop the vehicle. The test was conducted for 1000 stops.
The final chemical composition was analyzed using a spectrometer. The hardness measurement revealed that the secondary phases were not formed in both the inner and outer shells—an indication that the inner ring material property was like that of the conventional brake drum material.
Mechanical properties such as proof strength and Young’s modulus were measured as per ASTM E8 standard, and thermal properties and density were also measured. Based on the mechanical properties analysis, the bimetal brake drum material inner-surface properties were observed as similar to conventional brake drum properties. The lining and drum wear pattern also were similar to the existing system design.
Furthermore, the weight was reduced by 42% in Model 1 and by 26% in Model 2 compared to a conventional brake drum. The weight reduction directly as well as indirectly improves performance of the axle system; it improves the tire life, and unsprang mass reduction marginally improves fuel efficiency. In addition, it reduces braking distances with less brake pedal pressure.
Three optimized locations were identified near the brake lining and drum contact zone to measure heat dissipation. In three different temperature conditions—200, 250, and 300°C (392, 482, and 572°F)—it was clearly shown that the model 1 brake-drum temperature was 50% lower than the conventional cast-iron brake drum. Model 1’s outer surface fins increase the cooling rate; the fin design itself improves the heat dissipation rate 33%.
Model 2’s heat dissipation rate also was higher than the gray cast iron brake drum. The better heat dissipation rate in the bimetal brake drums reduces the thermal stress and reduces the tire temperature because of high thermal conductivity aluminum in the outer surface.
The initial and final thicknesses of the brake drum and the lining material were measured after completing the tests at three different temperatures. The bimetal brake drum wear was equal to the conventional cast-iron brake drum wear at 200 and 250°C. In both cases, the wear rate of lining and bimetal brake drum was equal to wear rate of conventional brake drum/lining.
In addition, based on the microstructure analysis, the secondary phases were not observed after the aluminum outer surface castings. In 300°C test conditions, a crack was observed in the Model 1 bimetal brake drum mounting area. The crack propagates throughout the thickness of the brake drum in radial direction; the drum distorted and eccentricity of the brake drum was changed to increase the wear rate in both brake drum and brake lining material.
In visual observations in the brake drum surface, minor scoring marks were observed, indicating that wear severity was low after the 1000 stops. In brake lining materials, the worn surface wear track depth was very low and wear debris was not observed.
As a result of this study, development of the bimetal brake drum has been taken forward for vehicle-level performance analysis and implementation.
This article is based on SAE International technical paper 2014-01-2284 written by Sunil raj of Ashok Leyland Technical Center and S Ravi Shankar of Ashok Leyland Ltd.Continue reading »