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Tipping the

2nd August 2012, Page 29
2nd August 2012
Page 29
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Which of the following most accurately describes the problem?

balance

By identifying the main causes, it’s possible to largely eliminate the chances of a tipper roll-over

Words: Colin Sowman Thankfully, tipper roll-overs are now a rarity but, as regular CM readers know, they can still occur (CM 28 June). In many cases there might not be a single cause of an overturn, but a number of contributing factors which, if identiied and avoided or corrected, can prevent an accident happening.

The most comprehensive work on avoiding overturns has been done by the IRTE (Institute of Road Transport Engineers – now part of the SOE) following a spate of accidents in the late 80s and early 90s. In its revised Guide to tipper stability, the IRTE identiies three key elements in a roll-over: unsuitable ground conditions, sticking loads and the lexibility within the vehicle (tyres, suspensions and truck/trailer chassis).

It is impossible to restrict tipping operations to irm and level ground, and while elevating a tipping body on a signiicant cross-slope is self-evidently unwise, what might surprise some is just how close to level the ground needs to be to operate a tipper safely. The IRTE’s guide speciies two categories of vehicle: class A, which are capable of tipping safely on ground with a 7° cross-slope, and class B, which can cope with a maximum of 5°. Any vehicle unable to meet the 5° hurdle is classiied as unsuitable for tipping duties.

Detecting a slope

While many vehicle manufacturers and bodybuilders test and categorise their products under this system, it might not be easy for tipper drivers to detect a 5° or 7° slope on site amid uneven, undulating and irregular ground conditions. Where a slight incline can be detected, the IRTE says drivers should aim to place their vehicle facing up or down the gradient.

If this is not possible, the driver must determine if the slope is within acceptable limits. Carrying a spirit level as a portable horizon could seem like overkill, but they are small, cheap and reliable.

One of the main contributory factors of a roll-over is a sticking load. As the body tips, the lower part of the load is released by the tailboard and discharges while other material remains stuck to the body’s sides and loor. As the body rises with the stuck load, it also elevates the height of the centre of gravity and increases the vehicle’s instability. The effect of a sticking load is much worse if it has only stuck to one side of the body because this causes asymmetric loading. In its guide the IRTE says the effect of a load sticking asymmetrically in a tipping body can be the equivalent of tipping a vehicle on a 4° cross-slope.

If the load does become unequal or the vehicle has to tip on a slight cross-slope, the rigidity of the body/chassis becomes vitally important in minimising the effect of those weight transfer forces on the suspension, the tyres and the ground.

Lack of rigidity

However, according to Wilcox Commercial Vehicles director Chris Bartlett, the rigidity required has not always been available. “When we initially tested our tipper bodies and trailers on tilt tables, we found that, contrary to what you might expect, the trailers could be more stable than the eight-leggers,” he says.

He attributes this to the lexibility of some of the lightweight truck chassis, before quickly adding that the chassis on modern trucks and trailers are far more rigid than in the past.

While the IRTE’s categorisation can give drivers and operators an indication of a vehicle’s rigidity, it cannot quantify variables such as air suspension and tyres, which are addressed in the revised guide. The IRTE concludes that dumping the air prior to tipping removes one source of potential instability, but it does create another because with the suspension on the bump stops equal, loading on each of the rear axles cannot be guaranteed. If for whatever reason the side-to-side loading of a tipper becomes unequal, then a number of other factors become critically important if a roll-over is to be avoided, with the most important being the condition of the ground.

Raising a tipper body transfers the majority of the weight to the rear/trailer axles and any inequality of loading or cross-slope will see an even higher proportion of the weight imposed on the wheels on the ‘heavy’ or downhill side of the vehicle.

The unmade ground on many construction sites is likely to deform under such pressure and the deformation is likely to be greater beneath the heavy or downhill side of the vehicle. This will increase the sideways inclination of the vehicle and body towards the heavy (and by now certainly downhill) side, which increases the load and deformation still further. This vicious circle can magnify even small load inconsistency and cross-slopes into an eventual roll-over.

Tyres can compound uneven loading problems because those on the heavy side of the vehicle will deform more than on the light side and cause the elevated body to lean yet further from the vertical. Uneven tyre pressures have the potential to make this situation much worse – which is worrying in the light of Michelin’s indings that between 10% and 15% of the truck tyres it inspects are incorrectly inlated.

Weight transfer forces

Just how large and signiicant these weight transfer forces can be is highlighted in the

IRTE guide, which says loads of up to 16 tonnes can be exerted on the downhill side of a vehicle undergoing stability testing on a tilt table. As most tyres are rated for 9 tonnes or 11.5 tonnes, the IRTE recommends that tyres used during tilt testing are replaced before the vehicle goes back into service.

Driver training is key to identifying the contributing factors for a roll-over before the situation becomes critical and this subject is addressed by another IRTE publication entitled Safe working practice for open top tipping bodies.

The IRTE says tipper drivers must ensure that articulated vehicles are in a straight line before tipping begins and should remain in their cabs throughout the tipping operation. It also highlights the need for the driver to be aware of how their vehicle has been loaded and that they are not only responsible for their own safety, but also that of other site workers.

A driver concerned that a load might stick should ask another site worker to act as a banksman – some sites already insist on banksmen as a matter of procedure.

An inexperienced banksman should be told how to identify a sticking load and warned not to stand directly behind or to either side of the vehicle where they could be killed or injured by a swinging tailboard, discharged material or a roll-over. n

MICHELIN’S TYRE TIP

Michelin undertook tilt-table tests on a laden Kel-Berg tri-axle tipping semitrailer fitted with twinned 385/65 R 22.5 XTE3 trailer tyres and then with ‘super single’ X One MaxiTrailer tyres. Even without raising the body, the ‘uphill’ twinned tyres started to lose contact with the ground at 24.60, whereas with the ‘super single’ tyre the trailer could be tilted a further 6% (to 26.20) before contact was lost.

MOVING FLOORS

Moving floor vehicles discharge their loads without needing to raise the body, and provide an alternative to conventional tippers where headroom is restricted or the ground is sloping or otherwise unsuitable.

Bartlett says moving floor trailers can work safely on cross-slopes of up to 10° or even 12° and will discharge loads such as hot asphalt to the satisfaction of companies such as Tarmac and Aggregate Industries. By using high-tensile strength materials, the payload in a moving floor combination can still reach 27 or 28 tonnes.

OVERHEAD HAZARDS

Each year in the UK around five people are killed when their vehicle or machine comes into contact with live overhead power lines. Victims have included tipper drivers and those operating truck-mounted grabs and cranes.

Most of these incidents happen with cables supported on wooden poles, and the Health and Safety Executive (HSE) makes the point that the machinery need not actually come into contact with the cables because high-voltage electricity can arc across quite sizeable gaps.

In its information sheet Working safely near overhead power lines, the HSE says tipping (and

other) operations should not be carried out within 9m (30ft) of cables supported by wooden poles or 15m (50ft) of lines on steel towers and pylons.

If for some reason (freeing a sticking load or pulling the tailboard free from the deposited material), contact is made with a live power line, anybody touching the vehicle and the ground is likely to be electrocuted.

Drivers being asked to undertake tipping operations any closer to overhead cables should first satisfy themselves that the power has been turned off and the circuit isolated.

Should a vehicle or machine come into contact with overhead power cables and the driver/operator is unhurt, the HSE’s advice is to remain in the vehicle and lower the body/retract the boom to break the contact. If the driver/operator needs to leave the vehicle because of a fire or other hazard, they should jump as far away from the machine as possible.


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