Amended Calculation Procedure for Involute Marine Gears With Parallel Axis

Calculation of geometry, surface durability (pitting) and tooth root bending strength of gear pairs implemented in marine propulsion and auxiliary machinery has been prescribed by the International Association of Classification Societies (IACS) Unified Requirement UR M56. The calculation procedure described in this document has been based upon international standards of ISO 6336 series. Previous edition of IACS UR M56 (Rev. 1/1994, Corr. 1996) originated from the requirements of edition 1996 of these standards. Since that time the standards have undergone important amendments in 2006 and corrected 2008. In certain cases, especially in helical gear pairs with higher helix angles, these amendments would have a significant influence to the decision whether a gear pair is acceptable or not. For these reasons, Croatian Register of Shipping (CRS), member of the IACS, initiated the task PM 11918 within the IACS Machinery Panel, aiming to amend and update the UR M56 to reflect these new requirements and prepared the draft of the new UR edition. Within the scope of IACS task CRS experts developed a computer program in MS Excel/VBA (Visual Basic for Applications), based upon the newly amended UR M56, in order to enable verification of the new calculation procedure on real marine gear pairs. This task took almost 20 months to be completed within the IACS bodies, including industry hearing. In December 2013 the IACS Council finally adopted the UR M56 (Rev. 2/Oct 2013) and made it publicly available on the IACS web site. The UR enters into force on January, 1 st 2015. The aim of this paper is to present the background for the task performed, together with its results, final outcome and the benefits gained. The most important benefit is a common explicit approach to calculation of load capacity of marine spur and helical gears with involute toothing and parallel axes, entirely in accordance with the actual presently valid international ISO 6336 standards, with certain specifics related to marine machinery implementation. The additional aim of the paper is to promote programming in MS Excel/VBA, within marine and mechanical engineering community, being a powerful tool for development of engineering calculations computer programs.