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New wheel safety technology

By Rob Coates
DuPont Engineering Polymers, UK

Wheel nut relaxation is a serious and recognized global issue in the commercial transport industry. The typical annual frequency in the UK of wheel fixing defects is between 7,500 and 11,000, resulting in between 150 and 400 wheel detachments. Although the exact cause of wheel nut relaxation and losses is not yet known, it is clear that if not managed correctly, operators and management teams around the world could face severe penalties and even prosecution.

The VisiLok road wheel safety nut is the culmination of four years of research and development, testing and trials, which addresses both the need for nut retention and relaxation indication. It consists of a steel nut incorporating a spring-driven lock and flag mechanism (housed in a 50% glass-reinforced grade of Zytel® HTN) and a modified, slotted stud. In the event of clamping force being lost, the nut starts to rotate. At this point the VisiLok system arrests further rotation in order to prevent the nut spinning off. At the same time, highly visible, yellow flags are displayed to alert the operator that the wheel requires attention. While achieving the dual function of arresting the movement and flagging the problem, VisiLok behaves in all other respects as a standard road wheel nut. No special tools are required and there is no time loss in fitting or removing a VisiLok nut.

Reasons for the selection of the particular grade of Zytel® HTN, 53G50, for the housing of the lock and flag mechanism were numerous. The polymer’s high stiffness prevents deformation of the housing during use, while its dimensional stability is fundamental to the long-term, reliable operation of the safety device. Moreover, despite being partially embedded within the steel nut, the upper section of the housing is exposed to road conditions, requiring the polymer to be resistant to chemicals such as road salts, oil and fuel, as well as forceful impacts.

“After going through a number of development stages which included proving the basic integrity of the design concept and function, we needed to ensure that the VisiLok nut and stud was robust in all operating conditions, in which respect the adoption of Zytel® HTN for the housing of the safety mechanism was key,” explains Barry McGowan, managing director of VisiLok Limited (pictured above with the VisiLok nut). “We selected the component material combinations carefully to prevent corrosion from jamming the mechanism, while keeping the component count to a minimum, and ultimately produce a cost-effective, universally applicable solution which would put an end to the wheel loss problem once and for all.”