Technical
Johannesburg Centre
The IRTE SA’s Code of Practice for Tipper Gear Installation
Fatal Accidents resulting from inadequacies in tipping gear hydraulics & associated components
There are numerous incidents yearly of tipper vehicles hitting overhead objects because they are being driven with the bin raised, most, whilst not having the tragic human consequence of the accident investigated below, involving repair expenditure, and all having the potential of high expenditure, injuries and fatalities.
The acceleration of the Country’s economy, especially the construction industry, and the continuing poor performance record of rail transport, is seeing an unprecedented number of tipping vehicles entering service, all adding to a potential pro-rata increase in the above incidents.
It is believed that many, if not most, of the incidents that occur are put down to driver error, with investigation, if any, limited to the necessary statement taking, in order to satisfy the requirements of a disciplinary enquiry. (Laziness apart, the cost of carrying out the in-depth investigation below was far from insignificant, putting it beyond a smaller operator, especially when already bearing repair costs from the incident, and cash strapped authorities).
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It is therefore further believed that the failings found in the investigation were instrumental in many more of the incidents that have occurred, and that OEM’s, Dealers mandated to equip customer vehicles, Aftermarket suppliers and Operators not following the recommended IRTE Code Of Practice, drawn up on the basis of the findings of the investigation, are failing in their duties. The consequences of this particular incident are clear to see and the retrospective updating of existing equipment, where it does not meet the Code of Practice, is further strongly recommended. After joining a free way during the hours of darkness, a 2 axle truck tractor coupled to a tridem end tip trailer accelerated to a cruise speed of 95 kph for 10 kilometres, during which distance, |
unnoticed by the driver, the load bin was raised up to some 80% its full height, at which point the upper part of stage 4 of the 6 stage tipping ram hit the lower face of a road bridge, breaking away some concrete from the bridge. This initial impact bent stage 2 of the tipping ram and completely broke away the upper 2 stages of the ram at the base of stage 5.
The left front of the raised load bin then impacted the bridge with the impact force was absorbed by deformation of approximately three metres of the left front of the bin, and, due to the fact that the bridge was at an obtuse (+/- 130 degrees) angle to the roadway in direction of travel , by the angle of the bridge forcing the collapsing leading edge of the bin to the right, running it along the parapet of the bridge for some 4.5 metres, and causing a turning moment which forced the vehicle combination to finally fall on to its right side and thus clearing the bridge.
The vehicle combination, now on it’s side, but still with forward momentum and the load bin still raised, crossed the central reservation and presented itself as an open mouth ‘V’ shaped barrier to an oncoming, rigid/drawbar box van combination. This third party truck was severely damaged, fatally injuring its passenger and seriously injuring the driver, as it became wedged in the base of the ‘V’, between the tipper load bin and trailer chassis, with its drawbar trailer impacting on, and collapsing, the cab roof of the tipper truck tractor.
Findings
It was proved beyond reasonable doubt the PTO actuator/solenoid failed, in that its operating piston ‘stuck’ open, allowing air pressure through to engage the PTO without driver intervention or knowledge. The manufacturer’s liability is not in the failure of the actuator assembly, but in the lack of built in redundancy to protect against such failure. A lack of redundancy cover that means that even a small internal leak within the assembly, which could be expected as equipment ages (the component having no scheduled maintenance or replacement requirement), would result in pressure building up and engaging the PTO.
The operator had also changed the original pump for one of 60% greater capacity – but from a safety issue, adherence to item 2 below would have provided necessary protection against this kind of operator abuse.
The findings of the full report, as they affect the manufacture/aftermarket supplier, were:
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The vehicle manufacturer failed to provide the safest practical control system for the factory fitted PTO, and the electronic warnings of PTO engagement that were provided, were flawed in that they were sensed merely from the PTO engagement switch circuit, and not from the actual position of the PTO engagement gear. - Warning the driver of the actual gear being engaged would probably have avoided the accident, but what is irrevocably clear is, that had the air for PTO actuation been supplied through the handbrake circuit this accident, with its tragic and expensive consequences, would not, and could not, have occurred.
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The installer of the of tipping hydraulics fitted a single tipping valve with a return fluid return port and mating, return to tank hoses, of smaller size than either the supply (pressure) port/hoses, or the outlet port/hoses to (and from) the tipping ram. The use of a smaller return path (‘non-pressure’) than supply path (‘pressure’) goes against all hydraulic circuit conventions. The in-adequacy of its use in this particular application is further highlighted by the fact that even in the ‘normal’ circumstance of the bin being lowered with the PTO still in engagement; this smaller return port has to cope with oil flow coming from both the larger ports at the same time.
Whilst limiting the circumstances in which lift power (hydraulic pressure) can be generated must be considered the absolute solution, the fitment of a correctly sized, and conventionally ported, tipping valve (i.e. relief port/hose larger than supply) would have again ensured that, given the circumstances, this tragic and expensive accident could not have occurred – excess pump volume/pressure, for whatever reason, being ‘dealt with, prior to entry into the tipping valve and onward supply to the ram.
Recommendations
The IRTE SA’s Code of Practice for Tipper Gear Installation.
It is therefore the IRTE’s recommendation that manufacturers and equipment suppliers adhere to the following Code of Practice, and that all operators ensure that all new vehicles comply with the Code.
IRTE (SA) Code of Practice for safety of Vehicle Power Take Off & Hydraulic Pump Control Valve Installations
Power Take Off
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Power (air) for engagement will only be available when the vehicle brakes are applied. If tipping on the move is required, availability of air supply must only be permitted with the (low) gear engaged, or, at the (low) road speed, that the tipping operation is carried out at
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Driver warning of PTO engagement must be sensed from actual position of PTO gear/engagement fork.
Tipping Switch
Where a single spool tipping valve is being used with air/ electric actuation (that is non-direct mechanical) actuation; that energy source must be used in such a way that any failure results in the tipping valve going into tipping mode (to protect against failure of supply resulting in a raised bin falling) and it must be directly linked to the energy source engaging the PTO in such a way that any loss of energy supply effects both components equally.
Tipping Control Valve
Any return to tank port & return to tank hoses must have a flow capacity, inclusive of any filter restriction, greater than the pump port. Ideally the csa of the ‘to tank port’ should equal or exceed the sum of the csa of all incoming ports.
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