Background
The department has received reports of potentially serious incidents involving graders when either a front steering wheel or a driving wheel experienced catastrophic failure of its stub axle, resulting in the wheel suddenly detaching during operation. Recent incidents include:
- seven steering wheels detaching
- a driving wheel detaching.
These incidents were injury free. In all cases there was temporary loss of control of the machine and the detached wheel rolled away in an uncontrolled manner. The modes of failure, the physical root causes and contributory factors in all these incidents were found to be similar.
Summary of hazard
- The mass of a detached uncontrolled wheel assembly can typically range from 500 kg to 1,600 kg.
- Loss of grader steering control and rapid deceleration
- Interference to operations from the disabled grader, detached wheel and recovery operation/s.
Contributory factors
- Non-genuine parts fitted to shaft group assemblies.
- Inadequate management of fatigue limited components by poor tracking of operating hours. When parts are swapped between different machines or used stub axles are installed, information about operating hours should be transferred.
- Failure of programmed maintenance practices to adequately inspect stub axles at appropriate periodic intervals as defined in original equipment manufacturer (OEM) manuals.
- Inspection and crack testing of stub axles is not always undertaken by suitably trained and competent personnel using OEM approved methods.
- Support from the OEM dealership is not always obtained when expert technical guidance is required.
- Poor hazard awareness and reporting - some sites fail to report such incidents to the Department.
- Poor operating practices that cause excessive loading of the stub axles can also contribute to their premature failure. These include:
- mounting one front tyre on a dump pile and the other on the ground
- running the front end directly into the pile
- grading road verges and windrows - tyres mounting on windrows
- operating on sharp turns and tight switch-backs
- exceeding the oscillation stop limits of axles in rough terrain (total range of 32 degrees)
- imposing excessive speed and shock loads to the grader front end (e.g. from excessive or sudden braking, or impacts with rocks and depressions in the road).