Parking Brake
The parking brake is a braking system designed to prevent a vehicle from moving while parked or on a slope. It is typically operated manually, though some vehicles use a foot pedal. Additionally, some vehicles adopt an electric parking brake.
Figure 1: Configuration of the Parking Brake
Its mechanism is divided into two parts: the operating mechanism, which transmits the manual operating force via cables and similar means, and the braking mechanism, which receives that force and locks the wheels.
Operating mechanisms include lever type, stick type, pedal type, and electric type. Braking mechanisms are classified into drum (wheel) type and disc type.
Among these, the drum (wheel) type must apply braking force to both rear wheels simultaneously, so a distribution mechanism (equalizer) is required to evenly distribute the operating force from the brake lever.
Table of Contents
- Parking Brake Body (Drum/Wheel Type)
- About the Automatic Adjustment Mechanism
- Parking Brake Lever Operating Mechanism
Parking Brake Body (Drum/Wheel Type)
The drum/wheel type is generally mounted on the rear axle and comes in two varieties: drum type and disc type.
Figure 2: Types of Parking Brakes
Drum-Type Parking Brake
Figure 2(1): Drum Brake Used as Both Foot Brake and Parking Brake
In the case of a drum-type parking brake, as shown in Figure 2(1), the flexible wire transmitting the force from the brake lever pulls the brake shoe lever, which presses both brake shoes against the drum, locking the wheel.
Figure 2(2): Operating Principle of the Drum-Type Parking Brake
- Pulling the side brake lever in the driver’s seat also pulls the flexible wire at ➀. This pulls the brake shoe lever at ➁, pressing the brake shoe on the right side of the diagram against the drum.
- When the brake shoe lever at ➁ is pulled, it also presses the brake shoe at ➃ on the left side of the diagram against the drum via the shoe strut at ➂.
- The brake shoe at ➃ and the one on the opposite side are pressed against the inner surface of the drum at ➄, locking the wheel.
In other words, the operating stroke of the parking brake is determined by the length of this shoe strut, which in turn depends on the clearance between the brake shoe (lining) and the drum.
Drum-in-Disc Type Parking Brake
Figure 3(1): Parking Brake Integrated into the Disc Brake
The configuration shown in Figure 3(1) is known as the drum-in-disc type, used as a parking brake in vehicles equipped with rear disc brakes. The parking brake is integrated into a small drum brake mounted on the inner side of the disc rotor. The operating principle is essentially the same as the drum type.
Figure 3(2): Operating Principle of the Drum-in-Disc Type Parking Brake
- Pulling the side brake lever in the driver’s seat also pulls the flexible wire at ➀. This pulls the brake shoe lever at ➁, pressing the brake shoe on the right side of the diagram against the drum.
- When the brake shoe lever at ➁ is pulled, it also presses the brake shoe at ➃ on the left side of the diagram against the drum via the shoe strut at ➂.
- The brake shoe at ➃ and the one on the opposite side are pressed against the inner surface of the drum at ➄, locking the wheel.
Furthermore, the automatic adjustment mechanism of this brake device is not located at the shoe strut but is incorporated as a clearance adjustment mechanism at the pivot point (anchor) at the lower end of the brake shoe. It maintains a constant clearance between the brake shoe and the lining in response to wear, allowing adjustment of the side brake lever stroke. Although it resembles an automatic adjustment mechanism in shape, in practice it can be manually adjusted periodically through an adjustment hole on the back of the drum-in-disc unit.
About the Automatic Adjustment Mechanism
When the brake shoe (lining) wears, the clearance between the brake shoe and drum increases, which increases the remaining stroke of the brake pedal and the operating stroke of the parking brake. To resolve this issue, periodic clearance adjustment becomes necessary. The automatic adjustment mechanism eliminates this maintenance task and keeps the clearance between the brake shoe and drum at an appropriate level.
Figure 4: RT-Type Automatic Adjustment Device
Here, we explain the type of automatic adjustment mechanism that is operated and adjusted by applying the parking brake.
Figure 5: Movement of the Adjust Lever
From Figure 5:
- When the brake lever is pulled, the adjust lever in Figure 5(1) is raised upward, pivoting on the pin, and climbs over the teeth of the shoe adjuster.
- When the brake lever is released, the spring force of the tension spring causes the adjust lever to descend as shown in Figure 5(2), rotating the shoe adjuster and expanding the shoe strut outward to bring the clearance between the brake shoe and drum within the specified value.
- When the clearance between the brake shoe and drum is within the specified value, pulling the brake lever produces only a small movement of the adjust lever, which cannot climb over the teeth of the shoe adjuster. Therefore, the length of the shoe strut remains unchanged and the adjustment mechanism does not operate.
※: Maintenance Inspection
Since the length of the shoe strut is automatically adjusted in response to brake shoe wear, the parking brake operating stroke remains constant. However, if wear progresses beyond the adjustment range of the automatic adjustment mechanism, the parking brake stroke may increase and the required operating force may grow. At this point, an inspection should be performed. If the brake shoe is worn, replacement is necessary. If not, the cause may be elongation or deterioration of the flexible wire, requiring further inspection.
Parking Brake Lever Operating Mechanism
Types of parking brake lever operating mechanisms operated from the driver’s seat include lever type, stick type, pedal type, and electric type.
The most common lever type is described below.
Lever Type
Figure 6: Operating Mechanism of the Lever-Type Parking Brake
This method raises a brake lever located on the cabin floor. The ratchet lever pawl engages with the teeth of the built-in ratchet gear to hold the lever in the raised position.
To release the brake, the release button is pressed while slightly pulling the brake lever, which disengages the ratchet lever pawl from the teeth, allowing the lever to return to its original position.
As one example, in a stick-type lever used in trucks and similar vehicles, prolonged use can cause the ratchet gear pawl to wear down, resulting in a half-engagement condition. This creates a risk that the brake lever cannot be held in the raised position and the brake releases on its own. In such cases, the ratchet gear pawl must be replaced. For safety reasons, periodic inspection of such components is necessary.