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Braking Performance Standards for Rubber-Tired Self-Propelled Cranes (SAE J1977:2019)
Understanding the Braking Systems
According to SAE J1977:2019, rubber-tired self-propelled cranes must be equipped with three independent braking systems: service, secondary, and parking. The service brake is the primary means for stopping and holding the machine, while the secondary brake provides stopping capability in the event of any single failure in the service brake system. The parking brake is designed to hold the machine in a stationary position for extended periods and must not rely on an exhaustible energy source.
Common components are allowed between systems, but a failure of any single component must still allow the machine to meet secondary braking performance. This design requirement ensures redundancy and safety.
Braking systems consist of a supply, control, power transmission means, and the brake itself. For air systems, reservoirs must have a combined volume at least 12 times the volume of all service brake chambers at maximum travel, and the air compressor must be capable of raising pressure from 85 to 100 psi in 25 seconds at the engine’s maximum recommended rpm.
Performance Criteria and Test Conditions
Performance tests are conducted under strictly defined conditions. The test course must be a hard, dry surface with a well-compacted base, grade ≤1% in the direction of travel and ≤3% cross slope. The machine must be at normal operating temperature (66–93 °C) before testing, and brakes should be cold according to the definition (not actuated in the previous hour, or disc/drum ≤100 °C, or housing ≤50 °C).
Stopping distance is measured from the point of initial brake control movement to the full stop. Deceleration is calculated using the formula a = V² / (2L). The table below summarizes the required instrumentation accuracy:
| Parameter | Instrument Accuracy |
|---|---|
| Braking System Pressure | ±3.0% |
| Machine Speed | ±3.0% |
| Machine Mass | ±2.5% |
| Stopping Distance | ±1.0% |
| Brake Control Actuating Force | ±3.0% |
| Grade | ±1.0% |
Control actuating forces must not exceed the limits specified in Table 2 of the standard, and all braking controls must be operable from the driver’s seat. The secondary and parking brake controls must be arranged so they cannot be released unless immediate reapplication is possible.
Design Insights and Frequently Asked Questions
Engineering design should prioritize redundancy and independence of braking systems. The secondary braking system must be capable of stopping the machine after any single failure in the service brake system, which often requires careful analysis of common components. The parking brake must be mechanically applied or use a source that does not deplete over time.
A common mistake is failing to ensure secondary braking performance after a component failure. Always verify that the system meets the secondary stopping requirements even when a common component fails.
🔍 Cold Brakes Definition: Brakes are considered cold if they have not been actuated in the previous 1 hour, or if the disc/drum temperature is ≤100 °C, or (for enclosed brakes) the housing temperature is ≤50 °C. This ensures consistent test results.
⚠️ Stored Energy Warning: If stored energy is used, a warning device (visual or audible) must activate before energy drops below 50% of maximum or the secondary stopping level, whichever is higher. Simple pressure gauges do not meet this requirement.
Additionally, the test course should have a hard, dry surface with grade constraints: ≤1% in the direction of travel and ≤3% cross slope. Grade measurement accuracy must be ±1%. Burnishing the brakes before testing is recommended to ensure proper lining-to-drum contact.
What is the definition of stopping distance in SAE J1977?
Stopping distance is the distance traveled from the point at which the brake control movement begins to the point where the machine fully stops.
What are the requirements for a warning device in stored energy braking systems?
If stored energy is used, a warning device must provide a visual or audible signal before the energy falls below the higher of 50% of the maximum operating level or the level needed to meet secondary stopping performance. Gauges alone do not suffice.
How often should the brake reservoirs be sized for air systems?
The combined volume of all service and supply reservoirs must be at least twelve times the combined volume of all service brake chambers at maximum piston or diaphragm travel.
What is the permissible grade for the test course?
The test course grade in the direction of travel must not exceed 1.0%, and the cross slope must not exceed 3.0%.
