Mar

Prevent the fatal bouncing BOMB

2011-03-01 13:27
BOMB

It is quite clear from FleetWatch’s Brake & Tyre Watch projects as well as from independent fleet audits that the state of wheel fastening on South African trucks leaves much to be desired. It’s all very well replacing safety-critical, missing nuts and broken studs but there is always a reason behind that lost nut or damaged stud and operators need to ask: What is that reason?

This needs to be known so as to prevent the wheel flying off on the road. A truck rim with a 12.00R22,5 running free in the traffic can cause one big mess, especially at night when one cannot see this coming and you have no clue what hit you.Take the case of spigot and stud mounted rims as an example. The Institute of Road Transport Engineers (IRTE), in conjunction with Bridgestone and Max T Solutions (formerly Maxiprest), worked in partnership with FleetWatch some time ago on compiling a wheel-fitting information poster with one of the main instructions being not to mix stud mounted and spigot mounted wheels and fittings. It seems, however, that operators are still doing so.When it comes to securing wheel rims to hubs, it’s not the number of studs that count but a fastening science that goes back to the original design – either spigot-mounted or stud-mounted rims.

These two photographs are typical of what FleetWatch encounters in the field. The drive-axle on a MAN TGA is missing two nuts - or 20% of the clamping force - while the other has four wheel nuts missing. If the other nuts are coming loose - as the missing nuts indicate - then disaster is not far off for these two rigs.

These two photographs are typical of what FleetWatch encounters in the field. The drive-axle on a MAN TGA is missing two nuts – or 20% of the clamping force – while the other has four wheel nuts missing. If the other nuts are coming loose – as the missing nuts indicate – then disaster is not far off for these two rigs.

Spigot-mounted wheels

There is hardly a new truck today that does not use spigot mounting for rims , the hub boss centres the rim on the machined inner edge of its nave. This design relieves the wheel studs and nuts of vertical load , studs and nuts only clamp the rim and absorb side forces. Friction at the clamping faces is thus basic to a spigot rim and using oil on the stud or nut is also not a good idea.The British IRTE, in a past report titled The Lost Wheels Mystery, stated that: “Some makers recommend lubricating the threads. But when this is done, the strain on the stud is increased by about 20 percent. Nut security needs friction, not lack of it, and lubrication does not seem advisable.’

These two photographs are typical of what FleetWatch encounters in the field. The drive-axle on a MAN TGA is missing two nuts - or 20% of the clamping force - while the other has four wheel nuts missing. If the other nuts are coming loose - as the missing nuts indicate - then disaster is not far off for these two rigs.

This means that spigot-mounted rims appear to suffer less from stud-breakage problems than other fixings but they are no guarantee of security.This application is only to be used on hub centering axles according to ISO 4107 (the wheel is centered and carries the load on the bore of the wheel/hub of the axle).The main dimensional characteristic of this wheel is the 26mm stud hole with no countersink and the tight tolerance bore 28mm (-0mm+0.2mm). The hub to support this wheel would be 280.8mm with a tolerance of- 0mm+0.2mm.

These rim stud holes are fretting because excessive paint on contact surfaces is preventing the surfaces from effective clamping and rim '˜chatter' will loosen the nuts.

Spherical and conical centred wheels

This design rim is found mainly on trailers and that’s where the rim mix-up occurs between prime-mover and trailers.These wheels can only be used on stud centering hubs. The wheel is centered and carries the load on the studs of the wheels. The main dimensional characteristics of the wheel are the countersunk holes of either a radius of 18 or a 40 degree angle. The bore also has an open tolerance of 280mm (-0mm+1.0 mm).

Certified-calibrated wheel torque wrenches , just not there

Table 1The standard method of wheel fastening in Africa seems to be a worn wheel-brace and extension pipe to provide excess leverage. The only thing you are assured of by using this system is that the studs will get stretched and the nuts/lugs cracked. And after that, no ordinary human will be able to unfasten the wheel in the event of a flat tyre.Bridgestone clearly recommends that a torque wrench is used to tighten wheel fixings, with the warning that power tools and long bars must not be used for final tightening. And there’s a difference too in the torque needed to fasten a stud-mounted rim compared a spigot/hub-mounted rim , around 540Nm versus 600-650Nm. Too little torque applied to the spigot/hubmounted rim results in ‘˜rim-chatter’ leading to tyre and rim damage and wheels falling off. The accompanying Table 1 shows what can result from insufficient or too much torque applied. Aim for the ideal torque.

 Truck and bus hubcaps may look smart but they invite sloppy maintenance and inspection. The effort required to remove the hubcap means that drivers overlook the important task of inspecting the nuts and hubs.

Truck and bus hubcaps may look smart but they invite sloppy maintenance and inspection. The effort required to remove the hubcap means that drivers overlook the important task of inspecting the nuts and hubs.

Training required

Stopping wheels from falling off is not an ad-hoc procedure. It should be a standard culture developed from a policy that requires training. There’s no doubt that working with tyres and wheels is wrongfully regarded as a low-level, unskilled task , it probably ranks alongside lubricating a truck which also , wrongfully , is regarded as requiring unrecognised skill. But it all starts with a signed job description that has detailed procedures attached as an annexure. This job description is the basis for training, discipline and accountability. Even tightening sequence procedures will be spelt out.Techies working with wheels are normally not trained to use a torque wrench, wheel-nut torque indicators and why contact surfaces must be paint-free. They don’t know the correct torque setting, when the torque wrench was calibrated , if they have one , and why wheels will fall off. Just watching them forcing a wheel-brace with a 2m pipe extension and dropping wheel nuts into the dirt around the wheel is enough to make one realise that techies must be trained to ‘˜work clean’ and not introduce grinding paste into wheel fastening.

 During a fleet audit, we found one '˜cheap' rim with two stud holes with conflicting machining , one with a countersink and the other without a countersink , on the same rim. This poor quality rim invites wheels to fall off.

During a fleet audit, we found one ‘˜cheap’ rim with two stud holes with conflicting machining , one with a countersink and the other without a countersink , on the same rim. This poor quality rim invites wheels to fall off.

Bomb (4)The driver’s role

What is a driver’s role in wheel fastening? Again this should be in the driver’s job description as a driver most certainly plays a role. Pre-trip driving inspections are not negotiable as well as on-road, walk-around safety checks , hub leaks, loose wheel nuts and rim/ tyre damage must be reported and recorded. The trip cannot start with missing nuts. While a driver cannot be expected to test every nut for fastness, visual inspection is essential and wheel nut torque indicators are a good aid.

 Wheel nut torque indicators are very useful for driver checks and quickly show if movement is occurring.

Wheel nut torque indicators are very useful for driver checks and quickly show if movement is occurring.

A preventive plan

Even if manufactured from the best-quality high-tensile steel, studs and nuts do not last forever and the problem here is that you cannot visually see fatigue. The prevailing attitude to trailer maintenance is ‘˜if it breaks, fix or replace’. Certainly studs and nuts get replaced when they fail but preventive maintenance would plan to replace studs and nuts after 10 wheel changes because of thread damage during handling and material fatigue under load on the road.It may be argued that stud and nut replacement after 10 wheel changes is too expensive but this is the benchmark. Where overloading occurs, 10 is a good number. The point is not to wait for failure but to set a fleet benchmark that goes around load, road, component quality and wheel fastening operational standards.

Africa's torque wrench. Note the long extension pipe on the ground waiting to be used.

Africa’s torque wrench. Note the long extension pipe on the ground waiting to be used.

Excessive paint leads to failure

Paint is often used to mask corrosion and damage but this is false vanity for cracks must be visible, not hidden under a well meaning coat of paint. Certainly the outside appearance of truck rims may look bland in their boring colours but leave them strictly in their original painted thickness, especially on the contact surfaces. This instruction has been a standard item in truck owner manuals , Mercedes-Benz in particular , for more than 30 years.Any additional paint thickness reduces fastening torque from clamping the mating surfaces of rims used in a dual set. Also, heat transferred to the rims under braking will soften paint in between the rims aggravating wheel looseness. That’s another spur to wheels falling off!This all assumes that mounting interfaces , wheel rims, nut/washer and hub mounting faces , are actually clean from dirt, corrosion or damage. Too often tyre and rim operations are conducted outside a workshop in the dirt and ordinary soap is used to lubricate the removal of tyres off rims which only aggravates the corrosion factor. There is no substitute for a tyre beading tool and beading lubricant in place of a chisel and hammer with soapy water.

 Why are wheel technicians so sloppy about using a torque wrench for wheel fastening? It requires more effort and care but the result is accurate and measurable and reduces the chance of failure.

Why are wheel technicians so sloppy about using a torque wrench for wheel fastening? It requires more effort and care but the result is accurate and measurable and reduces the chance of failure.

Sub-grade rims will fall off

Rim-junk is being dumped on the South African market. Container loads of cheap, low-quality, unmarked, untraceable truck rims are being sold to unsuspecting operators in Africa and no fastening procedure can make up for out-of-round rims with badly machined holes and surfaces. Wheels will fall off even before massive tyre damage is noticed. (See FleetWatch August 2006 article titled ‘˜Rim Robbers’)Given this as fact, it is time to measure what you’re getting. Rims must be traceable, dimensionally standard with details on the steel specifications and safety compliance. An unmarked rim could slip into a fleet without being noticed and after causing havoc with tyres, will then fall off.There are international wheel stamping norms , either SAE-J179 (Society of American Engineers), ISO -3911 (International Standard Organisation), or EUWA ES.08 (European Wheels Association) apply and are acceptable. Every one of these standards has obligatory information that provides dimension, manufacturer logo, date and part number plus additional info according to the standard. Look out for this information.

Insufficient torque can result in a broken stud such as seen here. One wonders how long this one , which is easily spotted - has been left unrepaired.

Insufficient torque can result in a broken stud such as seen here. One wonders how long this one , which is easily spotted – has been left unrepaired.

Aluminium rims need special attention

There is an increasing interest in aluminium rims to achieve weight savings but these rims require different wheel studs and nuts. Apart from following the truck manufacturer’s torque setting for the correct clamping force, single aluminium wheels require sleeved nuts with a short shank stud while for dual wheel mountings, long shank studs must be used. Alcoa supply short and long sleeve cap nuts in thread sizes for all European trucks.

Wheel nut/lug indicators

Physically checking for loose wheel nuts is a slow, exhausting procedure and the easiest way is a visual check by using wheel nut indicators such ‘˜Checkpoint’. The ‘˜Checkpoint’ indicator slips over the nut with each adjacent indicator pointing at the other point-to-point. Any movement in a nut is quickly detected in a vehicle walk-around.

Wheel hub leaks must ring alarm bells

Any vehicle inspection that reveals oil or grease oozing from a hub is a serious issue. It points to seal and bearing failure. The whole hub can catastrophically collapse. To suggest that this can be attended to ‘˜after the next trip’ is inviting the truck to never return. Drivers must be trained to look for this and understand the consequences of hub failure.Using mild steel parts for wheel studs and nuts should be classed as a road crime. Purchasing wheel studs and nuts is not where the lowest price rules. Safety critical parts must be requisitioned to a standard such as manufactured from EN 19 (709 M40) to B.S. 970(2) or to SABS 136 Grade 10.9 (min). And then there is also Grade 8.1 under SAE Standard J429, ‘˜Mechanical and Material Requirements for Externally Threaded Fasteners’.Every stud and nut must also carry distinct standard markings and be source-traceable , a valid batch certificate must be supplied on request. When a wheel falls off, the root-cause must be tracked to its origin as buyers must not be able to escape their accountability for safety through missing standards.

 For those operators using torque wrenches, don't forget that regular calibrating is necessary to ensure sustained accuracy. This photograph shows a torque trained specialist in the UD Trucks assembly plant at Rosslyn using specialised equipment to calibrate one of the torque wrenches used on the production line.

For those operators using torque wrenches, don’t forget that regular calibrating is necessary to ensure sustained accuracy. This photograph shows a torque trained specialist in the UD Trucks assembly plant at Rosslyn using specialised equipment to calibrate one of the torque wrenches used on the production line.

Last word

The last word goes to the Londonbased Institute of Road Transport Engineers research project , the Lost Wheels Mystery (Second Edition). It was published in 1986 but is as relevant now as it was then. Here’s an extract from the paper:”All that we can recommend maintenance men do until better equipment comes on the scene is to tighten nuts 20 percent beyond what manufacturers recommend, to avoid lubricants, not to be tempted by cheap components of dubious reputation, to use locking clips or compounds on nuts, not to slacken nuts before retightening, to check pitch circles of studs in hubs and holes in wheels and to beware of wheels with large ventilation holes , especially opposite stud holes.’There’s no option but to measure torque for that 20% extra and one cannot leave it to the personal judgment of tired arm and back muscles. If certain wheels on specific trucks are always loosening, don’t just fasten them. Root-cause analysis is essential to stop the wheels from falling off.

Acknowledgements & reference:

ITI Services , Wolfgang Lehmann for valuable insight and inputs.
Brake, the Road Safety Charity, UK

Case studies of bouncing bombs

While FleetWatch does not have on hand actual local accounts of wheels flying off trucks and killing people, there are many such cases documented around the world. To bring home to our readers the horror of the reality that can occur through lack of wheel maintenance, we have sourced four actual cases from ‘˜Brake, the road safety charity’ in the UK. These give an indication of the tragedies that can occur when the wheels fall off and truly do become bouncing bombs.

Truck loses two wheels and kills pensioner

Pensioner Hilda Weston, from East Sussex, died on 20 October 2006 when a wheel flew off a 32-ton-lorry and hit her as she waited at a bus stop. It was the second wheel to be lost from the tipper truck; less than a mile earlier an outer wheel flew off into a field but the driver, Mark Stansfield, said he hadn’t noticed because of the noise of the lorry. The inner wheel then worked its way off and hit 83-year-old Mrs Weston in the chest. An examination of the lorry after the incident revealed several of the remaining wheel nuts were loose. The crash investigator said the best way to ensure a tightened wheel nut was to use a torque wrench. Mr Stansfield said he had never used one in 20 years and had never been shown how to use one.Wheel crashes into car, killing woman Lesley Hadley, 39, from Walton-on- the-Hill, near Stafford, was killed on 7 June 2006 in a crash on the A50 when a skip lorry heading in the opposite direction lost a wheel, which then bounced across the central reservation into her car. Peter Ashworth, HM Coroner for Derby and South Derbyshire, who conducted the inquest into Lesley’s death, heard expert evidence that rust between two fixed rear nearside wheels on the skip lorry caused them to move and work loose. Both wheels came off and one caused Lesley’s death. He has since obtained information into other fatal cases and is calling for the Department of Transport to introduce regulations and promote the use of safety devices to prevent future tragedies.

Man killed when van collides with detached wheel

In February 2001, driver Kevin Jago was killed when a detached wheel from a truck collided with a van, sending the van veering out of control across the M2, where it struck Kevin’s van head on. Two days before the crash, the wheel had been removed to allow maintenance work to be carried out on the vehicle’s brakes. A forensic expert concluded that this was likely to have led to the wheel assemblies becoming detached.

Mother of five killed on pavement

In October 2004, Jeanette Bedford, a mother of five from Lostwithiel, in Cornwall, was killed when she was hit by a wheel that had come loose from a lorry as she walked along the pavement beside the A390. Traffic commissioner Philip Brown heard the firm Joyners Plants Ltd had failed “in almost every aspect’ to keep to the terms of its goods vehicle licence and drivers were not reporting defects on their vehicles.These are just four of many such incidents and to our readers, we plead for you not to let your company be involved in similar incidents on our roads. Pay attention to the wheels on your trucks , please!

Useful maintenance checklist from the Society of Operations Engineers:

1. Establish any causes of wear and damage on loose nuts before retightening.
2. Keep adjoining surfaces clean and preferably free of paint.
3. Ensure that nuts run freely over the whole length of the stud thread by hand action only.
4. Final tightening must be with a calibrated torque wrench set to the vehicle manufacturer’s torque value.
5. Power operated tools and extensions to wheel braces should not be used for final tightening.
6. All wheel nuts must be re-checked for tightness after 30 minutes whether the vehicle has moved or not OR after the vehicle has travelled about 80km.
7. When re-torquing, nuts should not be slackened and re-tightened but simply re-tightened to the recommended torque.
8. Commercial drivers should inspect tyres at the start of each shift for signs of damage, under inflation, cracked or distorted wheel rims, broken or loose fixings, signs of wheel looseness.
9. If drivers check for loose nuts, it should be with a socket and a bar no longer than 500mm to avoid over-tightening.

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