The issues found with drawbars and drawbeams are wide-reaching and not consistent. Please ensure any recertification undertaken is thorough and all aspects of the components are checked. Some examples of common issues found in the certification files so far include:
Mr Chu’s files rarely, if ever, state the material grades. Heavy Vehicle Specialist Certifiers (HVSCs) must satisfy themselves of the material grades used, and if in doubt, assume the unknown steel requirements as set out in AS 3990.
NZS 5446 requires that welding is carried out by welders qualified under AS/NZ 1554 in the appropriate position and technique. As set out in the VIRM, where a welder is unknown, the vehicle component should be rejected.
Mr Chu’s files rarely contain evidence of appropriate welder qualifications. To verify welder details, follow these steps:
If welder details cannot be located, vehicle owners may apply for an exemption from the Land Transport Rule for the weld to be accepted (see legislative details below). Exemptions are not guaranteed and are granted on a case-by-case basis.
It is anticipated that in most cases, the only basis where an exemption would be granted is where the HVSC can demonstrate that the drawbar/drawbeam is at least as safe as if the welding were carried out by a qualified welder. This could be achieved through a combination of:
If an exemption is granted, the HVSC can proceed with the recertification process.
If the vehicle owner is not applying for an exemption and there are no welder details, the HVSC may consider:
Vehicle owners may apply for individual exemptions from the requirements of the Heavy Vehicle Rule in respect of drawbars and drawbeams using the CA11 application form. Please note that for an exemption to be granted, the requirements of Section 166 of the Land Transport Act 1998 must be met. Section 166 requires the Agency to be satisfied in the circumstances of each case that the risk to safety will not be significantly increased by the granting of the exemption, and that one of the following criteria apply:
Following the revocation of Patrick Chu’s towbar certifications in December 2018, manufacturers or customers may engage you to recertify the towbar. Provided here are some guidelines for assessing and recertifying towbars.
The primary concern with the certification files that led to the revocation was consistent errors in the fatigue assessment due to the incorrect application of BS 7608, in particular the welds between the towbar beams and tongues were treated as a class E feature when they should have been treated as class G. This led to cyclic bending stresses in the towbar beams at the tongue weld location significantly exceeding the allowable value.
NZS 5467 requires that towbars rated over 1000kg MTM be designed for a minimum life of 400,000 load cycles, based on a load of 1.0 x MTM x g inclined at 20 degrees to the horizonal as defined in Section 7.3(d). A class G feature per BS 7608 will have an 85MPa allowable for 400,000 cycles. Note that the equivalent fatigue category if designed to AS 3990 would be category E.
The primary reason for revocation was the cyclic bending stress in the towbar beams at the tongue locations being consistently over the allowable value. However, when re-certifying the towing connection, it should not be assumed that this is the only issue – the certifier must take responsibility for the safe design of the full towing connection certification. Additional common issues with the towbar certification files include lack of information on the bolted joints; lack of welder qualification details and insufficient bolt group calculations
This advisory has been issued by the Transport Agency to ensure the recertification of these vehicles is carried out in a consistent manner in accordance with appropriate standards and guidelines
Any queries relating to these vehicles should be addressed to firstname.lastname@example.org, with Patrick Chu ‘Towing connection’ in the subject line