By now you will have heard about the safety warnings about drawbars and fifth wheel kingpins recently issued by the NZTA and you may be wondering what all the fuss is about, especially considering heavy vehicles undergo a CoF inspection every 6 months that you might expect to pick up any faults.
The first thing that must be realised is that a CoF inspector is not an engineer and is generally not qualified to make an assessment of the mechanical integrity of any structural component, nor is it their job. The CoF inspector can and should spot the obvious signs of a potential failure such as crack or major deformation as a reason for rejection but by the time these become visible it is often too late and the damage is done.
You may also have heard the comment; “these drawbars have been in service for 13 years, why should they fail now?”. A misconception. Heavy vehicle drawbars, drawbeams, fifth wheel kingpins and other structural components fail because they are subject to fatigue. Like all of us, a heavy vehicle also suffers from fatigue, the difference being that a heavy vehicle does not recover from fatigue, even after a good rest. For the working components of a vehicle fatigue is cumulative and, like rust, it never sleeps.
Drawbars and drawbeams are required to be designed for a twenty year life with recertification after ten years. The reasons for this midlife recertification are to determine if;
- The vehicle has had a change of use that would affect the fatigue life of the certified component such as transferring from line haul to quarry operation.
- The drawbar or drawbeam has been modified or damaged.
- The original design is fit for purpose.
- It still meets current requirements.
These decisions can only be made by a suitably qualified and appointed Heavy Vehicle Specialist (HVS) Certifying engineer with the appropriate category, in this case, ‘towing’, by calculation with reference to the relevant New Zealand standard (NZS5446) based on the original design, any modifications carried out, its previous use and what is known about its intended future use.
Fatigue is insidious and invisible, often until its too late. A poorly designed or overstressed drawbar, drawbeam or fifth wheel kingpin mount can look fine at CoF or even during a pre-trip walkaround inspection but have a sudden and catastrophic failure a few kilometres down the road because of the cumulative effect of fatigue, which changes the structure of the material and has finally resulted in a crack. This crack reduces the strength and integrity of the component causing further and rapid extension of the crack further reducing the strength of the structure ending with a sudden and catastrophic failure.
So, if you have a drawbar or fifth wheel kingpin that has been the subject of one of the recent NZTA safety alerts, you really do need to get it inspected and possibly recertified, by your local HVS Certifier as soon as possible – the risks are real and complacency or inaction puts you, your vehicle and the public at risk.