Bearing lifetime of linear PM machines

With the ever advancement and increased force density of slotted brushless linear permanent magnet (PM) actuators the attraction forces become more dominating. In the early years this attraction force was advantageous, although that it now becomes a problem due to the reduced bearing lifetime. This lifetime is influenced by the lubrication, rolling contact fatigue, time of operation, metal selection and pre-loading. As such, the attraction force of slotted linear permanent magnet machines has to be compensated to extend the maintenance free operating time of contactless Pick-and-Place robots. This has been achieved by counteracting the attraction force with passive permanent magnets placed underneath a soft-magnetic plate. Various ways to calculate the attraction force will be explained in this paper, where it will be shown that 2D finite element analysis suffices and that implementing this passive attraction force compensation considerably extends the bearing life.