Wednesday, February 12, 2025

ONRSR Safety Message: Rolling stock net braking ratio testing

NBR is defined as the total brake block force against the wheels of a wagon divided by the weight of the wagon.

This brake block force is transmitted from brake pistons to wheels by a series of mechanical levers, rods, beams, pins and guides collectively called the brake rigging.

With wear and corrosion over time, the clearances and friction between these various mechanical components varies, which can result in changes to the rigging’s efficiency in transmitting force to the wheels. Similarly, components of the rigging that have been replaced or modified, might affect its efficiency.

Rolling stock operators must consider appropriate periodic vehicle NBR testing programs to effectively manage the risks of runaways and/or derailments. Such programs should be designed by a competent rolling stock engineer based on their assessment of the risk of changed rigging efficiency.

As a minimum, NBR testing must be done when:

• New wagons are purchased or leased
• Previously untested used wagons are purchased or leased
• Brake equipment is modified
• Vehicle tare mass or allowable payload is modified
• An incident has occurred related to braking performance.

* For hauled rolling stock, Australian Standard 7510.2 stipulates that the automatic air brake of a vehicle fitted with high friction composite brake blocks shall be designed to achieve a net braking ratio of at least 13% with a brake pipe pressure reduction of 150 kPa.

For further information please contact your local ONRSR Office.