Colloquia

Summary

Over the years the loads used in the gear calculations of open gear transmissions in movable bridges increased. However, the gear calculations itself used in NEN-6786 hardly changed and remained very simplistic. Therefore, gear calculations became more challenging. This led to the usage of larger open gear transmissions with higher quality steels. The change in size and material led to unexpected complications, such as manufacturing mistakes and contact pattern issues. Therefore, improvements on the gear calculations according to NEN-6786 became the topic of this study.

The standard for movable bridges (NEN-6786) provides calculations and guidelines for the design of all gears used in a movable bridge. These calculations test four different scenarios: bending overload, bending fatigue of the tooth root, contact fatigue and additional contact fatigue. The same calculations are also allowed to be performed using the international standard  NEN-ISO 6336, which provides a more extensive calculation method. It is desired that both standards can be used and provide similar results. 

Firstly, a comparison of both standards led to improvements regarding the approach of certain stress increasing factors in particular the form and load factor. Secondly, it also provided the knowledge that the additional contact fatigue test appears not to be adequate for the problems it represents. Thirdly, it showed that for some materials a comparison is rather difficult between both standards. Finally, it showed that in some cases the safety factors that are asked within the calculation are not reasonable. 

Besides the needed improvements, insight is provided in the sensitivity of multiple effects covered by factors in the calculations, to draw conclusions on when not to use the simplified method proposed in the NEN-6786. Quality and load per unit length are often the parameters, where the most sensitivity was found. However, the amount of sensitivity is dependent on the material and configuration of the gears as well. 

The conclusions from the comparison led to new improvements of NEN-6786, which led to more similar results compared to NEN-ISO 6336, with a small amount of conservatism for NEN-6786, due to the simplified calculation approach. Also, proposed simplification proved that within boundaries a simplified method still can be used. It also provided the possibility to use both standards interchangeably, when more accuracy is desired. Less conservatism can be reached by using the actual amount of load cycles improvement on the form factors and lowering the unreasonable safety asked for NEN-ISO 6336.