Loss of CV road-wheels
Page 18
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With regard to John Dickson-Simpson's letter (CM 24-30 July) there are a few facts which should be stated.
He is discussing 22mm nuts intended for spigotlocated wheels, the associated fixings of which could be damaged by his recommended tightening torque of 800 Nm. I realise that an extol sive series of vehicle trials was carried out with this high torque and no problems
were recorded 1 believe that security was actually improved. Also, a torque of 800 NM on dry fittings would probably not result in excessive clamping forces fie stud tension) and it could result in a suitable tension. However, it could cause permanent damage by scuffing the dry threads and the dry nut/washer interface.
More seriously, calculations based on practical measurements show that if 800Nm was applied to a nut with the threads and the nut/washer interface lubricated and fitted to a 10.9 grade stud, it would result in a stud tension 9% greater than the "yield point" and within 2°.0 of the ultimate strength.
This means either a stretched or a broken stud, and possibly distortion of
some components in the wheel assembly it should be noted that the IRTE recommends such lubrication and I have been recommending it for years to maintain the performance and quality of the threaded components.
It is agreed that torque values specified by some vehicle manufacturers are too low but an operator using very much higher torques mild find himself in trouble in the event of failure of the fixings.
It is not true to say that the performance figures in BS AU 50: Part 2: Section 3 are not attainable. During my period as a Department Head at MIRA, I witnessed tests both at MIRA and in Germany where torques of 600 Nm on 22mm lubricated fittings resulted in stud tensions of 250 kN or more which is in accordance with the standard.
It is agreed that it is currently not possible to buy nuts and studs marked indicating, compliance with the British Standard, but one of the UK's leading trailer manufacturers was ensuring that their wheel fixings met the requirements of the BS as long ago as 1994. So it can be done!
Finally, we should not overlook the advantages of the Disc-Lock nut, many thousands of which are in use worldwide. To the best of my knowledge not one wheel fitted with a Disc-Lock nut has been lost.
Don H Wright Eur lag, BSc (Eng), CEng, FIMechE, Chartered Automobile Engineer