Autor: Thomas Münzing

Sizing of ball screws according to ISO-standard is strongly related to the sizing of roller bearings and focuses on their (until now) main usage in the feed axis of machine tools. Due to their high efficiency, the field of applications of ball screws extends into classical hydraulic piston applications. In combination with an electric motor and a gearbox the ball screws of these so called electromechanical actuators (EMAs) are faced with load and motion profiles, that are not yet considered by standard sizing approaches. In particular, these conditions are oscillating or intermitting motions instead of continuous strokes or revolutions, highly frequent applied loads at (nearly) standstill and extreme temperature ranges. A challenging application in this context is the primary flight control system of a helicopter, represented by the swash plate actuation system. A failure of this system is life threatening for the passengers. Therefore, a reliable sizing of the used ball screws is mandatory. The comparison of the required sizing tools with those delivered by given standards and available literature shows a lack concerning a rating life or modified rating life calculation, the general wear behaviour and the evaluation of special risks like the occurrence of False Brinelling. The longitudinal section of a ball screw shows the similarity of its internal geometry to a complete bearing represented by a set of angular contact ball bearings or four point contact ball bearings. For that reason, on the one hand approaches of bearing sizing focusing on oscillating motions or the wear behaviour along the raceway can easily be adopted. On the other hand, the inevitable necessary recirculation system and the helix geometry of the ball chain results in an internal load distribution that cannot be described by analytical approaches, yet. Also, the used materials can vary from common bearing steel. Based on bearing literature and standards the established sizing of ball screws can be modified into a new sizing approach. After checking criterions for minimum load, maximum load and the avoidance of wear (especially False Brinelling), a modified rating life calculation can be executed. Since real motion profiles are only seldom describable by a single sinusoidal signal, a more general approach is introduced considering an equivalent load and a stochastic positioning signal. Furthermore, a method is shown that enables a modified rating life calculation taking lubrication conditions into account. Finally, the special motion behaviour and load distribution within the ball screw is the starting point for advanced calculation approaches considering strokes, recirculation systems and single loads on balls. In contrast to established sizing approaches the last aspect requires load distribution information of a FEM simulation. For proving the derived approaches, tests focusing on endurance were performed to validate fatigue life. Furthermore, the wear behaviour of different recirculation systems and ball screw materials was investigated. Finally, False Brinelling tests varying the choice of material and lubricant were performed to prove the resistance of the ball screw at standstill. During endurance testing seven out of seven specimens reached the calculated number of oscillations. This means that the calculation approach is valid for 90 % of a large number of identical ball screws at the same loading, which is commonly known as L10-life. Wear tests performed with similar ball screws at very small oscillating motions showed that tube recirculations withstand longer severe wear conditions than internal recirculations realized by crossover inlays. Within the tests the impact of the recirculation design seems to be stronger than the raceway material which was represented by a highly wear resistant nitrogen bearing steel and a common bearing steel. False Brinelling tests showed that aeronautic standard grease is more capable to avoid standstill marks than a common „bad reference“ industrial grease. The usage of ceramic balls instead of steel balls can make a positive contribution as well. However, highly wear resistant nitrogen raceway-steel showed no benefits. Based on the new approaches and the validation tests a ball screw can be designed that fulfils the requirements of the primary flight control system of a helicopter.