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| Past Issues |
September 2009 |
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Trailer connector failure must be treated like the worst haemorrhagic
fever – one case can mean a wide-spread fleet epidemic. Root cause
analysis must be carried out to determine connector failure and build in
preventive maintenance forecasting as a countermeasure against future such
incidents. When a trailer accidently disconnects the consequential
disaster to both environment and other road users is, in the words of
erstwhile President John Vorster, ‘Too ghastly to contemplate!’
Dave Scott reports. But it all starts with selecting a correctly rated connector, whether 5th wheel, kingpin, automatic trailer coupling (tow hitch), or a towing eye. So, how do we compare these components in terms of the maximum horizontal force permitted between a towing vehicle and trailer? All of these safety critical components are designed with a D-Value in mind which is defined as the theoretical reference force for a horizontal force between towing vehicle and trailer. Expressed in kilo- Newtons (kN), D-value is taken as the basis for comparing horizontal loads in dynamic tests for all automatic coupling devices between a towing vehicle and trailer. All the modern trailer coupling manufacturers have detailed websites reflecting the above formulae for various types of articulated rigs and trailer combinations. When fifth wheels are involved the vertical load on the fifth wheel also has to be taken into account. A rigid-drawbar, centreaxle trailer will also impose a vertical load on a tow-hitch and this will be accounted for in terms of a V-value. 'V-value' is defined as the theoretical reference value for the vertical force between towing vehicle and centre axle trailer. The advantage for all transport operators is that trailer coupling websites contain user-friendly calculators – it’s really easy to compute kN D-values. For example, just go to http://www.jost.co.za/calculators.asp or to http://www.rockinger-catalogue.com/gb/ to select your rig.Worn connectors – measure and control Hardly a trailer fleet audit goes by without coming up against serious wear and deviation from trailer connector standard dimensions. Why do connectors show abnormal wear and damage?
Automatic tow hitches are at the rear of a chassis and whoever looks there for cleaning? Often overdesigned, heavy hinged- rawbars are difficult to handle, especially for a driver. During uncoupling, an over-weight drawbar crashes to the ground and is left in the dirt to accumulate wearing particles. This is not usually cleaned before the next coupling and abnormal wear sets in.
Lack of regular wear
checks – the essential tools of
trailer connector management are a tow-hitch gono- go gauge, as well as
king-pin and rubbing-plate gauges. Measurement is the only way to manage
wear and it will vary according to route and operating conditions.
King-pins do not wear evenly and the tolerance level is very fine
between new and worn levels – up to 2mm – as this Jost diagram
demonstrates. Wear occurs in the direction of travel and on opposing
faces of the king-pin areas.
Use of incorrect grease
– the Multipurpose grease will
just not work for fifth wheels and king-pins – it must be ‘Extreme
Pressure’ (EP) grease with graphite content. EP greases for trailer
couplings must withstand both pressure and heat – the grease heat range
capacity is an important factor.
Fitting mild steel
components – unbelievable,
but mild steel kingpins are being supplied to the SA road transport
industry. Only trailer connectors manufactured from the minimum an EN19T
or EN23T specification should be used. Any connector must be stamped and
traceable – the reputable brands also undergo X-ray crack detection
tests during manufacture.
Rubbing-plate
distortions – a king-pin and
fifth wheel are a matched set. Fixed and bolt-on king-pins must also be
correctly selected to suit the thickness of a rubbing-plate fitted to a
semi-trailer. A new 2-inch king-pin should protrude between 82,5 and
84mm while a 3,5-inch king-pin must protrude between 72 and 74mm below
the rubbing plate. New truck tractors very often fail to couple to old
semi- trailers because a new fifth wheel will struggle to couple to an
out-of-alignment, worn king-pin due to rubbingplate wear distortions.
Replace the semi-trailer king-pin and fix the rubbing-plate and a fifth
wheel then couples!
Driver abuse
– forgetting to open a fifth wheel
lock-jaw while trying to connect a trailer is a common and hidden reason
for connector failures. No driver will own up to this but the damage is
evident. Excessive painting – trailer couplings have been treated with a very abrasion resistant powder coating, applied over a uniquely powerful corrosion protection primer. Additional painting can, depending on the type of paint, weaken underlying layers that can have a negative effect on corrosion protection. Additional painting can have an adverse effect on functionality because the paint seeps in where it is not wanted. The signal pin can start to jam, and important information can be painted over. Towing-eyes – ignored
and overlooked A towing-eye is also kN rated. Here
are a few common problems we face in the market: Drawbar tow-eyes manufactured from mild steel,
conforming neither to size nor specifications, are surrendering safety
and standards for price alone. The replaceable bush in the toweye is often tack-welded
into the eye of a drawbar because the hole is elongated – this changes
the structure of molecular material making it brittle and prone to
failure. Cast iron lock-nuts sold into the market cannot take
shockloading with cracks forming prior to failure. Drawbar eye lock-nuts
must be manufactured from special grade-hardened EN30 steel and secured
with a split-pin. Replaceable drawbar eyes are being machined locally to
take a 43mm lock nut instead of the recommended 45mm nut. Lock washers or split pins through castellated nuts are
not used to secure the towing eye lock nut, resulting in the lock nut
working loose with disastrous results. This rubbing plate
gauge clearly shows that
there’s a dimensional fault in connecting to a fifth wheel – a
driver will damage the connectors in the process. The Regulations And if you think the law has nothing to do with this then
get hold of Government Gazette 15 August 2003 No. 25306. Under section 3.6
– Requirements for couplings on semi-trailers – the tolerances and use of
a rubbing plate gauge is set out, including the ‘Mechanical properties of
a kingpin.’ But don’t think that an accident destroys evidence. It’s
quite easy to prove whether a trailer coupling failed as the result of an
accident or whether abnormal wear and neglect was the cause of the
incident. The company management will have to appear in court to prove
that everything reasonable was done in terms of maintenance for
safetycritical trailer connectors to prevent an on- road failure – or be
damned. |
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2009 FleetWatch magazine and FleetWatch On-Line.
No part of this publication may be reproduced without the prior written
permission from the publishers.
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