Schrödinger’s cat is a strange creature that inhabits the hazy borderline between quantum mechanics and scientific philosophy. Quantum physicist Erwin Schrödinger invented a ‘thought experiment’ where a cat was trapped in a sealed container with a vial of poison, and a small radioactive source with a Geiger-counter. If radioactive decay was detected, the Geiger-counter activated a trigger that released the poison and killed the cat.
Rather than advocating a bizarre form of animal cruelty, Schrödinger was trying to illustrate a key issue in quantum mechanics: that measuring a quantum wave causes its function to collapse, and the properties of a quantum particle such as position, velocity and rotation don’t exist until they are measured.
Measurement therefore causes disruption of whatever it is that the observer is trying to measure.
That’s a phenomenon that will be all too familiar to technicians and engineers charged with preventing wheels from becoming detached from heavy vehicles. The only way to be sure that the wheel is properly fixed to the hub is to remove it, clean and inspect the wheel, hub and fastenings, and reassemble. The trouble is, experience shows that a wheel is most likely to become detached shortly after it has been fitted, as the studs can relax and release the tension holding the wheel to the hub.
Regularly retightening the fastenings to ensure that they do not come undone can be problematic, too: the studs or bolts can be overstressed and fail.
IRTE’s own advice on this is clear. Before a wheel is fitted to a vehicle, the joining faces of the wheel and hub must be cleaned and inspected, including the stud holes in the wheel. So, too, the studs or bolts: the technician must look for signs of dirty or damaged threads, and try to ascertain if the stud itself has been over-stretched.
Switching nuts to different studs may help reveal damage to either component: it’s possible that a damaged nut might still conform to the damaged stud it originally came off, but its qualities will still be impaired.
If clean, and lubricated with the correct oil, the nut or bolt should run easily onto the thread. Performing this task by hand, rather than using an air-gun, helps the technician to detect any faults in the thread. Nuts/bolts should be tightened sequentially, starting at opposite points on the mounted wheel, with the final tightening completed with the wheel on the ground.
Even with all these precautions observed, when it comes to wheel detachments, the period of highest risk is immediately after the wheel has been disturbed. The normal protocol is to retorque the wheel after a short interval. Where the wheel has been removed during a routine workshop visit, it is easy to schedule this final task into the job. But it’s a different story after a puncture or other roadside breakdown; the vehicle may be in an exposed or dangerous position, and the driver will almost certainly feel under pressure to get back on the road as soon as possible to complete the delayed journey.
In these cases, Goodyear’s Truckforce technicians will follow the repaired vehicle for a suitable distance before carrying out the necessary retorque in a safe location.
Industry specialist Tructyre details its wheel fastening policy: 30 minutes after initial fitment and setting, the fasteners should be retorqued, irrespective of whether the vehicle has moved or not. Then the process should be repeated after the vehicle has been driven for 20 – 50 miles. Many operators also undertake a third check, after 24 hours.
Mid-market tyre manufacturer Giti’s technicians have a retorque app, which reminds them to undertake and record the recommended retorque 30 minutes after completing the initial refit.
The torque figure used should be the one specified by the manufacturer, or BS AU 50 Part 2 Section 7a 1995 (600Nm). On a single stud, this torque generates a clamping force of between 19 and 24 tonnes!
PART OF THE SOP
Retorquing should also be a part of every statutory maintenance inspection, with any issues recorded and acted upon to prevent recurrence.
In between PMIs, drivers are on the frontline of wheel-loss prevention. DVSA requires only a visual check of wheelnuts, but an experienced eye will spot tell-tale signs such as a bright metal ring around the offending nut, or a trail of powdered rust heading radially outward from the fastener. DVSA examiners themselves give each nut a tap with a light hammer: a variation in the note struck indicates a problem.
Continental warns that some wheels are more problematic than others. It finds 70% of detachment issues come from twin-wheel trailer fitments and suggests replacing these with super-singles. The more mating faces there are (as with twin wheels), the greater the area for contamination. Undersize wheels, such as 17.5s, are more problematic than their full-size counterparts, as they are more stressed.
Wheel nut indicator flags, as invented by Checkpoint, give a visual warning if a fastener has rotated since fitment. The company has since expanded its product range to include retainers that prevent nuts from rotating, as well as spin-on tools to check that threads are still free-running.
Former transport consultant Gary Thomas was inspired to develop the Wheely-Safe system in the wake of an incident where all procedures had been followed, but a detachment occurred because the torque-wrench used had gone out of calibration. Instead, the Wheely-Safe system uses two sensors mounted on each wheel that warn the driver directly of loose wheel nuts.
He explains that mechanical failures mean wheel loss can still occur even if the correct procedures are followed.
“Firstly, the whole truck wheel-hub interface is a really poor design. The wheel is a loose fit on the hub, and it is only held in place by the pressure generated by tensioning the studs. But the tension itself stretches the studs, and the pressure they generate varies with thermal load.
“Wheel loss appears to be a greater problem in colder climates where there is a big difference between the ambient temperature when the wheel is installed and the running temperature of the hub and wheel in use.
“Torque checks themselves can obviously only be done when the vehicle is parked, and depend on the accuracy of the wrench.”