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Brake braking torque improvement measures
Insufficient braking torque in a vehicle can lead to a dangerous sliding phenomenon, especially when parking on slopes, which significantly compromises the vehicle's overall braking performance and safety. Therefore, it is essential to implement effective measures to address this issue.
When an off-road vehicle is parked on a prepared slope, insufficient braking torque can cause the vehicle to slide downward. Based on calculations of the parking slope and vehicle parameters, the required parking brake torque for a single rear wheel should be no less than 10,034.5 N·m. However, the measured maximum parking brake torque of the rear wheel was only 5,400 N·m, far below the required standard.
This paper investigates the root causes of the insufficient braking torque in the rear wheel brake system of an off-road vehicle. While the main factor is the unreasonable design of the brake’s structural parameters, insufficient braking force also contributes significantly to the problem. To resolve this, several improvement measures were implemented, including increasing the output push rod thrust of the brake chamber, extending the length of the brake adjusting arm, reducing the base circle radius of the involute cam, and enhancing the strength of the brake shoes. These modifications successfully increased the braking torque to meet the required specifications.
**Brake Structure**
The rear wheel brake of the off-road vehicle is shown in Figure 1. It is a centering involute cam-actuated shoe drum brake, driven by pneumatic pressure. The brake chamber uses a compound energy storage spring for both service and parking brakes. When the brake is applied, the cam mechanism ensures that both brake shoes move equally, resulting in balanced normal reaction forces and equal braking torque. The opening force of the shoe is inversely proportional to its performance factor.
*Figure 1: Rear Wheel Brake Structure*
1. Brake drum
2. Brake shoe and roller
3. Camshaft
4. Brake bottom plate
5. Camshaft support
6. Brake adjustment arm
7. Brake chamber
**Cause Analysis**
1. **Physical Reinspection**
The brake was disassembled and each component was re-inspected. All parts met the required specifications, ruling out manufacturing defects as the cause.
2. **Calculation of Braking Torque**
The braking torque $ M $ generated by a single brake is calculated using the following formula:
$$ M = \frac{Q \cdot L \cdot \eta \cdot f' \cdot m}{R_b} \cdot (K_1 + K_2) $$
Where:
- $ Q $: Thrust of the spring brake chamber output push rod (N)
- $ L $: Length of the brake adjustment arm (mm)
- $ \eta $: Transmission efficiency of the cam support (0.7 in calculation)
- $ R_b $: Radius of the involute cam base circle (mm)
- $ f' $: Friction coefficient at the contact point between the cam and roller
- $ m $: Force arm of tangential force (mm)
- $ K_1, K_2 $: Performance factors of the two brake shoes
- $ R $: Radius of the brake drum (mm)
Using the main structural parameters: $ Q = 7500\, \text{N}, L = 132\, \text{mm}, \eta = 0.7, R_b = 13\, \text{mm}, f' = 0.15, m = 13.62\, \text{mm}, R = 190\, \text{mm}, K_1 = 1.01, K_2 = 0.45 $, the calculated braking torque is $ M = 5449.58\, \text{N}\cdot\text{m} $, closely matching the measured value of 5400 N·m. This confirms that the primary cause of the insufficient braking torque is due to an unreasonable design, and improvements are necessary.