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July
2000
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The heavy traffic crawled and then stopped. My car halted alongside the tow-hitch of a freight carrier pulling a drawbar trailer. Snarled in gridlock, the traffic edged forward and, to my horror, I observed the complete tow-hitch assembly flexing upwards as the vehicle took up the inertia of the trailer mass. It was obvious that the rear cross member on the truck was twisting as it could not take the imposed stress. In addition the tow-hitch was not bolted but welded onto this flexing cross member. This was an accident in the making - a case of not 'if' but 'when' the trailer would break free and wander off into the on-coming traffic writes FleetWatch technical correspondent, Dave Scott. A recent audit of a bus operation revealed that a trailer coupling of the luggage trailer showed wear beyond acceptable tolerances. Firstly these 'luggage' trailers carry extra spare wheels - they are operationally not as light as they may appear to be. Secondly, a luggage trailer breaking free at speed on a highway is a chaotic incident in which the bus may very well be involved with other traffic trying to avoid the trailer. The trailer coupling of a bus luggage trailer can easily impact on the lives of the passengers.
Tow hitches are manufactured with specific design capacities to match the stresses imposed by the gross vehicle masses (GVM) of a truck and trailer within a vehicle combination. The ability of a tow hitch to deal with pivotal thrusts, occurring at the point where truck and trailer are joined, is denoted by it's D-value and is calculated in kilonewtons (kN). The danger in selecting a tow-hitch for a job is choosing the item on Rand value instead of D-value. The pitfall lies in selecting an under-rated tow-hitch for a task just to shave a few Rand off the purchase price. What operators overlook is that an overloaded combination of vehicles doesn't just impose excessive mass on the axles, tyres and suspension. Overloading also exceeds the safety critical D-value of a trailer coupling, whether a fifth wheel or drawbar tow-hitch. The D-Value of a trailer coupling is calculated in terms of a formula laid down in SABS 1648:1995.
This formula is only good for trailer couplings that do not have to transmit vertical bearing loads. A pup trailer with one axle unit and a rigid drawbar imposes vertical forces in addition to horizontal force onto the coupling - separate formulas exist for this type of trailer, as well as a D-value formula for fifth wheel couplings. SABS specifications - a benchmark Above 3 500 kg GVM SABS Specification 1648:1995 applies to mechanical coupling-devices for trucks and trailers. Homemade tow hitches, without a certified D-Value, just cannot be measured for capacity and are a waste of time. SABS 1648 also specifies that drawbar couplings should be automatic, the spring-loaded type that 'engage the coupling completely and properly, without any external intervention'. The SABS specification for mechanical trailer couplings is based on European standards. SABS is waiting for Revision 1 of ECE Regulation number 55 to enable the incorporation of these specifications into a single and updated document for both SABS 1505 and 1648. SABS specification 1505 will be phased out when this occurs.
A trailer coupling is a safety critical item - why can't these specifications be made compulsory? The longer it is left as optional the worse the problem becomes. The eye and pin - a matching couple
There appear to be two standard sizes of towing eyes used by South African operators with internal diameters of 40 and 50mm. The 40mm tow eye is used on couplings with a rating of around 130 kN, while the 50 mm is used in couplings with ratings of up to 190 kN. Towing cross members - integral component Many trucks have a gross combination mass (GCM) that allows towing of a trailer carrying at least 50% of the truck payload. The problem is that the manufacturer does not always specify a standard rear cross member that is designed for the GCM. The standard rearmost cross member of a truck is often just a bracing structure without any towing capacity. When towing, the rear cross member must have a material thickness of 18mm up to a maximum of 30mm, while the member must be braced with gussets at the corners. Non negotiable At present all of us have no protection against mild steel components being fitted to trailer couplings. Price seems to drive most of the road transport at the expense of quality, with SABS 1648 being 'nice-to-have' - if you feel like it. We would all feel differently if a trailer uncouples and destroys the lives of anyone we know. I insist - and it is time we all insisted - on compulsory standards for trailer couplings. I fail to see how safety critical components can be an optional or negotiable issue. The SABS also needs teeth. Who will ever enforce the fine ideas that the people at SABS create? Tow hitch safety critical random checklist
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Tortured
Tow-hitches
One
size does not fit all
Below
3 500 kg GVM and applicable to ball-joint towing attachments for
light vehicles, the local standard is SABS Specification 1505. Fleets
who make use of 'Ventertjies' and other small trailers towed behind
bakkies and buses should take note of this one.
The
SA Bureau of Standards is careful to point out that these SABS specifications
are not compulsory. This is quite evident out there when one sees
the thousands of 'Heath-Robinson' home-designed trailer couplings
on our roads. If one applies the simple specification of D-value
and automatic coupling to most truck trailer couplings they would
fail the test.
The
eye in a trailer drawbar is an item that must match the locking
pin of a trailer coupling. SABS 1648:1995 also provides load values
for standard drawbar eyes with D-values ranging from 130 up to 190
kN. The specification also points out that