Perhaps there is no industry that would experience such stormy development as cars. Thanks to Henry Ford, it has gradually shifted from manual to automated production. The automotive industry is today the most important industrial sector in many developed countries. Similarly, it is in Slovakia, which has dominated the world ranking for several years in the number of cars produced per 1000 inhabitants. For this reason it was often named as the “Detroit of Europe”.
Slovakia’s attractiveness lies primarily in the strategic position in the heart of Europe (Fig. 1) at the crossroads of major transport routes between Krakow - Budapest and Berlin – Moscow, further to the relative sufficiency of skilled labor force and last but not least in a solid infrastructure, either road or rail. It is noteworthy that all Slovak carmakers are located in the northwestern part of the country, which is already starting to activate national labor migration.
As can be seen from Fig. 2 approximately 44% of the total industrial production of Slovakia consists of the production of cars. Brands such as VW, Peugeot, Citroën, KIA have been established for a long time here and now Jaguar Land Rover. Overall, more than 106
cars are produced in Slovakia a year. There is plenty of reason to pay adequate attention to this industry generally.
1. Specifications of automotive production in terms of assembly
The characteristic features of automotive production are:
1. High series
2. High degree of automation of production
3. High demands on quality and safety
Considering that the dominant assembly technology of car manufacturing is welding and screwing and that one car contains as many as 2000 different screws, nuts and other fasteners, then it is globally a gigantic consumption of fasteners that must meet the conditions of automatic assembly. In the next text, therefore, attention will be paid to the specificities of vehicle assembly in terms of the screw connections used.
As can be seen from Fig. 3, the mounting robot screws must have at least 3 freedom degrees. It is important to supply components that have to be properly oriented. The necessity of orientation has caused that wherever there is no need for a demountable joint, welding dominates automotive production.
However, there are a number of cases where the screws are indispensable. For example engine surroundings, gearbox, chassis, brake system, wheel attachment and many other construction nodes require detachable joints to service and maintain. According to VDI 2862 “Minimum restrictions for the application of fastening systems and tools in the automotive industry” there are three categories of screw connections in cars (Fig.4).
The most dangerous category is „A“ because it is responsible for car accidents. This includes, for example, fastening wheels. It is the most critical operation, not from the manufacturer’s point of view, but from the point of view of the user, because he is sometimes forced to do it himself, amateur in the field, without the use of the appropriate professional tools. Statistics show that wheel fastening bolts are most likely to be released after 50 to 100 kilometers of ride from repeated assembly. In such cases, it is advisable to visit an authorized service center as soon as possible and have the tightening torque checked. Figure 5 shows a cautionary example of the amateur mounting of the wheel bolt. The broken thread indicates that the bolt has been tightened by a too high tightening torque. The consequences are not hard to guess.
Drivers mostly ignore it, but it is enough to see statistics of motor vehicle crashes as a result of relaxed driving wheels. Released wheel threatens not only the crew of the vehicle but also the unsuspecting surroundings.
Dangers can also be caused by overloading the vehicle. Around 7.5% of trucks checked by VOSA (Vehicle Operators Services Agency) were found to be overloaded. Excessive load of one (Fig. 6) or both axles causes deformation of the screws (Fig. 7), to dislodge the mounting holes on the discs and gradually to disengage the nuts and, in the extreme case, also the driving wheels. This type of stress is defined as a rotating bending, and it is one of the most complicated and most dangerous combined stresses in elasticity and strength.
The only effective measure is to strictly adhere to the load prescribed by the manufacturer and to ensure uniform loading. Often the weight of the crew, which may exceed the weight of the luggage in passenger cars, is often ignored.
It should be remembered that even for incorrect assembly of wheels after their replacement or overloading the vehicle, the manufacturer is not responsible. Likewise, a garage can not be responsible for what it can not influence. However, driving schools should include at least an montage minimum in their syllabus, focusing in particular on wheel assemblies. The reason is simple - this is the most common operation that a driver must sometimes perform. Such training would, for example, learn about the importance of friction when tightening. It does not matter if an oiled or dry, new or rusty screw connection is tightened. Impact of surface condition of screws and nuts is good confirmed in Fig. 8.
Therefore, it is important to entrust this work to a specialist workshop that not only controls the theory and practice of wheel assembly, but also has professional tools.
In the world, the number of cars and the number of crashes are rising. Central Europe is no exception. As the statistics suggest, the causes of the crash are not just the non-compliance with traffic regulations, but the state of the vehicle is also important. As can be seen from the text, the driver is sometimes forced to supply the manufacturer or garage, with amateurs being a huge contrast in knowledge. A separate chapter is overloading of vehicles. Except for the danger of an accident, there is a potential risk of loss of balance for driving.