1. Opole Univ. of Technology, Dept. of Mechanics and Machine Design, Mikołajczyka 5 Street, Opole 45-271, PL (Corresponding author), e-mail: a.nieslony@po.opole.pl
2. Opole Univ. of Technology, Dept. of Mechanics and Machine Design, Mikołajczyka 5 Street, Opole 45-271, PL