1. A. S. Velichkovich and S. V. Velichkovich, “Bottom-hole shock absorber with a shell-like elastic element of variable stiffness,”Khim. Neft. Mashinostr., No. 8, 10–11 (1994).
2. S. V. Velichkovich, “Improving efficiency in the drilling of wells,”Ekspress-Informatsiya. Ser. Burenie, No. 9, 5–7 (1986) (published by VNIIOENG in Moscow).
3. Author's Certificate No. 1059319 USSR, MKI E 16 F 15/02.Elastic Spindle for a Bottom-Hole Engine.
4. Author's Certificate No. 842189 USSR, MKI B21 B 17/06.Drill-Rig Shock Absorber.
5. Author's Certificate No. 607943 USSR, MKI E 21 B 17/06.Shock Absorber.