The roller screw, an efficient and reliable mechanical component of electro-mechanical actuators

Abstract

The roller screw exists in three different designs which may be used in very different applications: the three designs are respectively known as planetary, differential planetary and recirculating roller screws. Most EMAs which are using roller screws use the planetary design because of its inherent capacity to withstand very high accelerations and speeds and its ability to handle heavy to very heavy loads for a long time. This capability places the planetary roller screw between large ball screws and hydraulic cylinders. The roller screw compares favorably with both other designs because its resistance to adverse environment is higher than that of ball screws and because it is not prone to leaks and other unpredictable failures of hydraulic cylinders. This paper discusses the efficiency of the planetary roller screw and the means to improve it and maintain it for the life of the component. It also discusses the noise level of roller screws and the way it can be controlled and maintained at a low decibel level (below 72 dB-A). We define a relation between temperature level and necessary oil rate and give a formula based on past experience. Then, we present different modes of failure of the planetary roller screw. We discuss their most common order of precedence; cases of failure and analysis of those failures are presented and analyzed. We establish a tentative reliability formula linking the B10 fatigue life to experimental MTBF. An ongoing application in the aerospace field is used as a basis to calculate this relationship. Reliability levels of 95%, 99%, 99.9% and even 99.95% are achievable with the roller screws.