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11 IV ith ever more vigour, law enforcers are blaming inadequate maintenance

2nd September 1999
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Page 38, 2nd September 1999 — 11 IV ith ever more vigour, law enforcers are blaming inadequate maintenance
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Which of the following most accurately describes the problem?

for cases of loose or lost wheels. Traffic commissioners and courts are distinctly less sympathetic to tales of the unexpected and are succumbing to the "it's just maintenance" propaganda inspired by government inspection agencies and defensive manufacturers.

It is not that simple. It helps to follow official recommendations on wheel fitting, but it does not ensure security.

To keep a spigot wheel secure with prolonged nut tightness requires the generation of enough preload in each bolt to provide 22 tonnes of clamping force, The British Standard (BSAU 5o Part 2) specifies between 19 and 24 tonnes clamp force.

The practical difficulty comes when decid

ing how much tightening torque will produce the required pressure. Small differences in the friction coefficient at the threads and between nut and washer bring big variations in the tightening torque needed to bring the clamping force to the safe level. But the friction coefficients vary from one bolt to another and from one tightening to another. The operator has no way of knowing how much friction occurs, and therefore to what extent the tightening torque should be altered.

The only palliative is to oil the nuts to reduce the variability in friction and to apply enough extra torque to achieve, without overstress, the required clamping if the friction is more than predicted. Data galore suggest that fulfilling that aim means a torque between 720 and gooNm with oiled nuts. The 600N m used for appraisals to British Standard apply to laboratory testing of a nut and bolt, not to an in-service assembly and certainly not to a twin-wheel assembly suffering the uncertainty of five joints from washer to bolt-head. Twin wheels are the most prone to looseness.

Friction coefficients

The Institution of Mechanical Engineers published a report in September 1996 by the Open University's Ray Morgan and Exeter University's Leslie Henshall recording nut and bolt frictions and the effects. They found friction coefficients to be between 4 and 75% greater than those in British Standard tests. They found an average coefficient of 0.4 and none less than o.ii. At 0.12 friction, 22 tonnes damp force required a tightening torque of 790Nm. When the threads and washer were dry the friction leapt so high that the clamp force plunged to only 7 tonnes. Looseness would be inevitable.

Morgan and Henshall produced a useful simplified formula for calculating how the tightening torque would be affected by fastener friction.

Multiply the friction coefficient by 279, add 2.39, then multiply the answer by the required clamp force in tonnes. The result gives you the required torque (in Nm).

The British Standard recommends retorqueing after 30 minutes or after 40 to gokm. When Bass road-tested this procedure in 1995 some nuts still slackened a little after 6 okm. For an operation with hilly routes when the brakes get hot, the slackening might be expected to be more pronounced because in laboratory experiments bolts can start to creep over 200°C. Some operators think the British Standard recommendations ought to be more demanding. The Institute of Road Transport Engineers plans more tests on this.

In March the institute published test results on manufacturers' wheel bolts and nuts. Nearly all conformed to the quality required by the British Standard, but the results also brought out the friction inconsistencies and consequent unpredictability of achieving the 22 tonnes damp force.

There was 20-35% scatter in clamp load when tightening too times at 600Nm. There was 47% variability with a Disc Lock nut, which has inbuilt extra friction. Most took several fittings to give the ideal 22 tonnes of damping.

The Mercedes-Benz and Volvo bolts took more than 30 fittings. The implied friction coefficients varied from o.o8 to 0.147.

Research by Perdeep Toora at Brunel University in 1997 found that a torque of at

least 72oNm was needed in a wheel attachment to be fairly confident of obtaining 22 tonnes clamp force.

Joint thickness

Two other telling facts came out of this research. One was that clamp force deteriorated as joint thickness increased—further confirmation of the effect of twin wheels. The other was that the quality of lubricant affected the clamp force. Compared with molybdenum disulphide grease, 3-in-I oil, WD-4o and used engine oil, the greatest clamp force (typi cally 30% more) was obtained consistently with new engine oil (a D4 15/40).

The work also examined several nut-locking methods, The broad conclusion was that, although they offered some comfort against complete loss, they neither gave assurance that enough clamp force was being generated nor precluded subsequent tightness checks.

So good maintenance practice is still no insurance against the many unpredictabilities inherent in the engineering. Can anything else be done? Well, tightening a bit more helps.

Unless using Rotabolts with built-in stretch indicators, the wheel-fitter does not know exactly whether the expected tension is really being generated. The closest practical approach to ideal is a torque-wrench set within a range that will give enough tension over the span of friction coefficents. The tension would be more predictable if the friction coefficient of nut, bolt and washer was part of the quality specification.

The tension per bolt—and therefore also the tightening torque—could be less if there were more bolts. That is why Prof Henshall wants to experiment with a rz-stud fixing at Brunel University. But that is looking ahead somewhat.

Meanwhile the only practi cal course is to tighten the nuts more. Mounting evidence means that's 720 to 800Nm, the torque used in a two-year trial after 1988 of nearly 25,000 vehicles in the fleets of IRTE members. There was no nut loosening, stud failure or stud stretch.

In 1992, after a series of tests, Roadway Express in the US ordered its workshops to tighten wheelnuts to 800-950Nm on its 16,000 vehicles and trailers. Wheel security is not just a British problem.


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