Drum brake systems
The simplex brake:
- Simple design
- 1 x leading brake shoe
- 1 x trailing brake shoe
- Shared actuating unit
- (e.g. wheel brake cylinder, brake cams, brake lever)
- Each brake shoe has a fulcrum or support point
- The same braking performance when moving forward or reversing
Disadvantage: with floating application (hyd. or expanding wedge)
High Tbb wear on the leading brake shoe.
The simplex brake is the brake that is fitted most frequently because of its low C* value – which makes more precise control via the brake pedal possible – and its simple design.
In the simplest design, the simplex brake, there is one leading and one trailing brake shoe which ensure consistent brake performance when moving forward or reversing. An actuating unit expands the brake shoes, while on the other side the brake shoes are fitted with a bearing around a fulcrum (support bearing).
The duplex brake:
- One actuating unit per brake shoe
- Both shoes are leading during forward movement (high self-reinforcement)
Disadvantage: poor braking performance when reversing (both brake shoes now trailing).
In the duplex brake, each brake shoe has its own actuating unit, which however can only be moved to one side. As a result, both shoes are leading during forward movement. The advantage over a simplex brake: the brake performance is approximately 50% higher with the same actuating force. The disadvantage: higher manufacturing and maintenance costs and a significantly lower braking performance when reversing, as the brake shoes are trailing in this case.
The duo duplex brake:
- Two actuating units working in both directions
- When moving forward or in reverse brake shoes are always leading
Disadvantage: High production costs.
In this design, two brake cylinders that act on both sides are used. Two leading (self-reinforcing) shoes are actuated when moving forward and reversing. This means: the two shoes apply the same force. This brake type is easy to actuate but must be operated sensitively. It is no longer fitted today because installation is extremely complex and is associated with high production costs.
The servo brake:
- Floating support bearing
- Both shoes leading during forward movement
- Like simplex brake when reversing
This design has the highest internal reinforcement. With this brake there is a brake cylinder that applies force on both sides, for example, and the lower fulcrum of the brake shoe moves around a fixed support. During braking, the supporting force of the primary (leading) shoe is transferred to the secondary (trailing) shoe, which results in self-reinforcement in the direction of travel. The self-reinforcement does not apply when reversing.
The duo servo brake:
In the duo servo brake, there are two leading (self-reinforcing) brake shoes in both directions, as the pressure pin can be supported by a bearing according to the direction of movement.