FIELD OF INVENTION

The invention relates to a brake drum used in vehicles, in particular to a brake drum having a cast iron braking surface with an outer surface made of high strength aluminum.

BACKGROUND OF THE INVENTION

Brake drum is one of the most important part in vehicles as it is used to stop the vehicle which is in motion. The safety of the vehicle is closely linked to the efficiency of the braking system where the brake drum is used to decelerate the vehicle by friction, converting kinetic energy into thermal energy generated during the braking process. During braking enormous heat, even up to approximately 500°C, is generated in the drum braking surface and it cools to ambient temperature. This cyclic stress due to continuous use of the brake causes fatigue and thermal crack which reduces the life of the brake pad. Since the brake pad is a poor conductor of heat energy the heat generated during braking has to be dissipated through the brake drum.

Conventionally, brake drum is manufactured with grey cast iron which has better thermal conductivity, self-lubricant property, widely available material and easy to cast. Due to increase in market demand, like the need for high speed, reduced pad wear, etc. the brake drum material should possess properties such as good thermal conductivity, light weight, good corrosion resistance, good durability, stable friction, and low pad wear rate.

US 4,262,407 (Petersen et al.) disclosed a composite brake drum comprising a substantially biplanar stamped steel drum back joined to a cast iron braking ring by means of die cast aluminum or other lightweight alloy casting. The lightweight metal fills a plurality of holes and voids in its outermost edge in one plane of the drum back and interlocks with the rough outer surface of the braking ring to provide a unitary brake drum. Since the mounting area of the brake drum comprises of stamped steel plate, the weight of the brake drum increases. Moreover, such a construction does not avoid displacement of the cast iron liner from the aluminum housing due to rotational and/or lateral load on the brake drum.
Therefore, there exists a need for a bimetallic brake drum that is lightweight and capable of handling high loads with efficient heat dissipation without displacement of the brake liner from the housing.

SUMMARY OF THE INVENTION
The present invention describes a brake drum having an inner surface and an outer housing. In a preferred embodiment, the inner surface is made of cast iron and, keeping the cast iron liner as core, a light metal, such as aluminum, is poured to obtain the brake drum assembly with aluminum outer housing. The cast iron liner is uniformly ribbed throughout its exterior surface to mechanically interlock with the aluminum housing thereby preventing relative rotary and linear movement between the inner surface and the outer housing. Aluminum is used because it is light in weight and has better heat dissipation property. The external surface of the aluminum housing has reinforcement fins to provide better heat dissipation and structural strength to the assembly.

The cast iron liner, which will be in contact with the brake pad during braking operation, has an increased end wall thickness to reduce displacement during braking. The thickness of the reinforcement provided on the aluminum housing gradually increases toward its end to provide adequate stability and strength to the assembly.

By means of the invention described above, a better heat dissipation is achieved together with a weight reduction of more than 40% making the brake drum assembly cooler during operation and lighter in weight. Consequently, the brake pad life is improved.

BRIEF DESCRIPTION OF THE DRAWINGS
Referring now to the drawings wherein the showings are for the purpose of illustrating a preferred embodiment of the invention only, and not for the purpose of limiting the same,

Fig. 1 shows the cast iron liner of the brake drum assembly according to the invention;

Fig. 2 shows the sectional view A-A of the cast iron liner;

Fig. 3 shows the aluminum cast brake drum assembly according to the invention;

Fig. 4 shows the sectional view B-B of the brake drum assembly according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described with reference to the illustrations shown in figures 1-4. The brake drum assembly according to the present invention comprises an inner surface (1) over which a light metal cast is poured to obtain the outer housing (2) of the brake drum assembly. The inner surface (1) is preferably made of cast iron and the outer housing (2) is made of a lighter metal, preferably high strength aluminum.

The mounting area of the brake drum is thus made completely of the light metal.

The inner surface (1) will be in contact with the brake pad during braking operation. The inner surface (1) is uniformly ribbed throughout its circumference. The inner surface (1) and the outer housing (2) are mechanically interlocked due to the ribs (3) formed on the outer circumference of the inner housing (1).

The ribs (3) form a mechanical interlock such that both rotary and linear relative movement between the inner surface and the outer housing is restricted and/or fully arrested. This avoids any slips or displacement of the inner surface from the outer housing during braking operation.

In a preferred embodiment, the ribs (3) are formed in the same direction along the circumference of the inner surface (1) and the ribs (3) are positioned equidistant from one another. In another preferred arrangement of the ribs (3), at least three bands of ribs are formed on the circumference, each band having a plurality of ribs formed in the same direction along the circumference of the inner surface (1), equidistant from one another. Each band of ribs may be positioned at an equal space apart from each other. Figure 1 shows such an embodiment, wherein three bands of ribs are provided around the circumference of the inner surface (1).

The end wall thickness of the inner surface (1), in portion (5) is increased. Such an increased end wall thickness of the inner surface (1) further improves the reinforcement thereby reducing the displacement of the liner during braking. Section A-A shown in figure 2 clearly illustrates the increased end wall thickness of the inner surface (1).

Keeping the cast iron liner (1) as the core, a light metal cast, such as aluminum, is poured to obtain the brake drum assembly. The aluminum outer housing (2) is designed to have optimum strength and better heat dissipation properties. In an embodiment, the aluminum outer housing (2) has plurality of reinforcement strips (4), each strip extending from one end of the housing to the other in a plane perpendicular to the housing (2). The thickness of the reinforcement strips (4) gradually increases from the closed to the open end of the assembly. The Section B-B illustrated in figure 4 clearly shows the gradual upward tapering of the reinforcement strip (4). Such a housing structure provides better heat dissipation and structural strength to the brake drum assembly.

The brake drum assembly is machined as per the drawing requirement once the outer housing is cast on the liner surface.

In a preferred embodiment the inner surface (1) is made of cast iron of the grade FG 250 and the aluminum cast is of the grade LM 25. The property comparison of these materials and the resulting brake drum assembly is provided below.

While the above paragraphs explain the various embodiments of the invention, it is apparent that numerous modifications and variations can be made without departing from the scope and spirit of this invention. It is therefore intended that this invention be limited only as indicated in the appended claims.

WE CLAIM

1. A brake drum assembly comprising

an inner surface made of a first material for contact with a brake pad,

an outer housing cast on the inner surface, said outer housing made of a lighter and better heat dissipating material than the first material,

wherein the inner surface and outer housing are mechanically interlocked by means of ribs formed on the circumference of the inner surface, said ribs being formed so as to arrest both rotational and lateral movement between the inner surface and the outer housing.

2. The brake drum assembly according to claim 1, wherein an end wall thickness of the inner surface is increased to reduce displacement during braking

3. The brake drum assembly according to claim 1, wherein reinforcement strips are provided on the outer housing for heat dissipation.

4. The brake drum assembly according to claim 3, wherein thickness of the reinforcement strips gradually increases from one end to the other.

5. The brake drum assembly according to claim 1, wherein the ribs are formed in the same direction along the circumference of the inner surface and each rib is positioned equidistant from each other.

6. The brake drum assembly according to claim 5, wherein at least three bands of said ribs are formed, equidistant from one another, on the circumference of the inner surface.

7. The brake drum assembly according to claim 1, wherein the first material is cast iron and the lighter metal is aluminum.