Development, Pre-qualification and Application of an  Active Bearing Preload System  Simon Lewis* and Martin Humphries**  Abstract  This paper describes the development to pre-qualification status of a novel and widely applicable  technology development known as a Bearing Active Preload System (BAPS). The BAPS can be thought of  as a “smart bearing housing” which replaces a conventional bearing or mechanism housing and whose  function is to permit ball bearing preload variation on command. An overview of typical BAPS  requirements, its design, the range of actuation options and some performance data are provided. A  number of historical and recent bearing applications are reviewed and the benefits realizable by a  capability to vary preload are highlighted. These can include order of magnitude improvements in lubricant  lifetime, reductions in bearing torque, mechanism mass, cost and complexity.  Introduction  Ball bearings are preloaded to provide adequate rotor location and bearing stiffness, as well as to protect  the bearings themselves from damage due to “hammering” during launch. However, increased preload  also has some undesirable effects discussed below.  Grease- or oil-lubricated bearings have essentially two main torque components, namely a speed and  temperature dependent “Viscous” component and a load dependent “Coulombic” torque component. At  low- to moderate-speeds, the Coulombic component can be dominant, such that mean bearing torque is  approximately proportional to preload4/“. This relationship is also true at all speeds for solid- and self-  lubricated bearings. Furthermore, peak Hertzian ball-raceway contact stress is proportional to preload’I3  [1 I.  In ESTL much work has been done in the past concerned with characterization of bearing torque and  lifetime, particularly for self-lubricating (e.g., Duroid or PGM-HT) and solid-lubricated (e.g., thin films of  MoS2 or lead) bearings. A review of some of the highlights of this experimental work [2-51 demonstrates  that preload (or peak Hertzian contact stress) has a very significant effect, both on film lifetime for solid  lubricated bearings, and on separator (cage) wear for self-lubricating bearings as shown in Figure 1. For  liquid lubricated bearings too (especially PFPE-based oils and greases), though the lifetime may be often  be longer, ultimately lubricant degradation due to shear or chemical reaction at balVraceway asperities and  separator stability/wear issues are aggravated by high bearing preload.  In summary, the use of a high preload throughout life not only increases the Coulombic torque and  therefore motor mass/power requirements for the bearing, but also decreases the potential operational  lifetime significantly. Furthermore, since in most applications the high bearing stiffness required for launch  is no-longer essential once in-flight, the capability to operate in-flight at relatively low preload is highly  desirable, particularly in long-lifetime applications and those with challenging thermal requirements (e.g.,  those having a particularly wide operational temperature range, large or adverse thermal gradients).  Historically bearing “off-load devices” or “launch-locks” have been used to protect bearings during launch  by providing an alternative load path in those relatively few applications where the launch loads  necessarily exceed the capacity of the bearings, or more often where the low-torque or long-life  requirement dictated a moderate to low preload should be selected (e.g., Giotto de-spin bearings [6]).  However, bearing off-load devices are relatively mass inefficient and need to be tailored to the application.  * ESTL (European Space Tribology Laboratory) - ESR Technology Ltd., Warrington, Cheshire, U.K. ** Sula Systems Ltd., Wotton-under-Edge, Gloucestershire, U.K.  Proceedings of the 3dh Aerospace Mechanisms Symposium, Langley Research Center, May 17- 19,2006  133