Can be the Wheel Faster than a Car?

 
 
The question in the title could sound absurdly to the reader. How is it possible that the wheel can move faster than the automobile to which it belongs? But still, these cases really occur and they are not so unusual. Nearly 70% of the asked drivers during  their driving had some experiences with loosened fasteners, respectively with the wheel nuts of cars or lorries. Even some respondents stated that one or more nuts were missing after finishing the drive. Recorded were also worse cases, unfortunately, with tragic consequences. The rolling wheel cannot only destroy but also kill. Give the answer to the questions why can this happen and how to prevent it is the role of this article. 
 
Theory
 
The frequent cause of bolted joints fail is their spontaneous self-loosing during the operation including the car driving. The cause is on the side of vibrations and cyclical dynamic stressing of responsible structural nodes. If transversal force FT fulfils the condition: 
 
 FT > FV.μ,
 
when μ is a friction coefficient, then it comes to the relative movement in partition lines (Fig. 1), to the loss of the joint self-locking and progressively towards its total decay or fatigue fracture (Fig. 2) occurs. 
 
The relative movement may be in the axial or radial direction. Representative for automobiles is radial. 
 
The process of screw release occurs at three known stages:
 
I. Seating of material it means the microplastic deformation of roughness on the contact surfaces (s. Table Nr. 1)
 
II. Spontaneous unscrewing as a result of vibration or dynamic stressing (Fig. 1)
 
III. Decay of bolting as total unscrewing or fatigue fracture (Fig. 1 and 2)
 
The decrease of bolted joint clamp force to zero is the side effect which is, e.g. for automobiles accompanied by depletion of holes for fasteners on wheels disks (Fig. 3). This is very dangerous because there is an impulse of progressive self-loosening of all the nuts/screws and the wheel could fall off during the ride and besides such disks are no more usable.     
                              
Often the question of when spontaneous unscrewing and when of fatigue fracture occurs. The answer is in hand. Critical is what happens after the prestressing force decrease to zero. Either the free turning of the nut continues until complete disintegration of joint or protection against disintegration, as DIN 985 (Fig. 4) or alternatively cotter pin, stops the next turn of the nut. In the latter case, favorable conditions for fatigue fracture are created.   
                                                 
Explanation provides Fig. 5 - the DIN 985 stop nuts are not able to prevent the loss of the prestressing force. They serve as protection against disintegration only. The same applies to cotter pins.
 
There is one more dangerous phenomenon in the case of transport, namely the overloading of vehicles. All motor vehicles, especially lorries and trailers, have a limit of the acceptable loading determined by the producer. This limit is a result of difficult calculations and longitudinal tests. Mainly the axle is a critical construction component which transfers the vehicle weight and its external load onto the road. It also concerns screw connections which are deformed (Fig. 6) and with simultaneous rotation extremely complicated stressed.       
       
It would be clear if the drive continued that the wheel would fell off, either by the fatigue fracture or by the uncontrolled loosening of the nuts. Both cases would have the same  catastrophic consequences – the wheel slip. Preventive measures are simple, they consist in respecting prescribed loading capacity. 
 
Causes of wheel release
 
In the case of the automobile wheels the following specific factors have the influence on self-loosening of bolted joints:
 
  1. Vehicle overload
  2. Damaged threads and damaged and contaminated contact surfaces
  3. Wheel bearings overheating
  4. Poor road conditions
  5. Different heat expansion of the joint components
  6. Intensive wheel vibrations and dynamic stressing of bolted joints
  7. Low hardness of bolts and nuts
  8. Nuts or bolt threads out of specification
  9. Too much interfaces
10. Incorrect installation/assembly
 
All these cases are important, but extra attention deserves point 10 – tightening. The seemingly simple operation conceals many tricks. In the diagram in Fig. 7 is defined the so called force security rubikon (FSR). All force changes during tightening and during operation must take place within this FSR. Otherwise, the screw connection will occur. An important role is played by friction too. It does not matter whether they are tightened by the new or rusty screws (Fig. 8). Similarly, aluminum discs must be tightened differently than steel.
 
The accuracy of the tightening is also very important (Table Nr. 2). The worst is tightening per hand with factor αA  = 2,5 – 4 and scattering +/- 43 - +/- 60%.
 
Preventive measures
 
The automobiles producers try to solve the problems with loosening screws with the increase of the friction coefficient on the contact surfaces by the choice of ball or cone seating, or rather by nuts with integrated flange.
 
Such solutions are usually satisfying and the drivers do not have to be worried, that the wheel will fall off. However, it is worse after several tyre changes in the garage or amateurishly or provisionally on the road. The experience indicates that critical phase is after driving first ca. 50 to 100 km. That’s the impulse for the development of supplementary external insurance elements. The current market offers several such types as wedge washers, check points etc.  Respect also deserves the latest product from the Ferodom development workshop – the lock point TaTrim (s. FEM analysis on Fig. 9). In addition to reliable function, its big advantage is independence from the friction coefficient and from the tightening level. This independence of TaTrim lock points makes it possible to compensate for the failure of human factor by the wheel assembly.
 
Conclusion
 
The bolted joints of automobiles present significant safety risk. The incorrect assembly and disrespecting the principles of their behaviour during the operation could be dangerous not only for the crew of the incriminated vehicle but also for the unsuspecting surroundings. As shown, the vehicle’s wheel can be really faster than the car it belongs to. It will use every opportunity to think he’s in the F1 race. The task is to get the chimer out of his head. There is no need to be afraid. It is important to observe the recommended rules of the wheels assembly and do not regret the time for their continuous check. It will surely pay.
 
The author and the editorial staff wish in New Year all the readers a safe journey and in conclusion one appeal: Share your own experience with loosened automobile screws with Fastener Eurasia. Contribute this way to the increased safety on the roads because it will be the source of the instruction for the others.   
 
 

Date : 24.01.2018