The bearings may take one of two forms; ball/roller or sleeve type.
Ball races will be fitted at the compressor end to locate the shaft, and thereby fix clearance between the casing and the blades of the compressor impeller (the most critical of clearances where performance is concerned, as any increase in clearance there would result in a rapid fall off in compressor performance, and too small a clearance would result in rotor to casing contact).
The shaft is able to slide through the bearing at the turbine end of the unit where the relative expansion is accommodated in a roller bearing. Expansion between the casing and rotor occurs because of the elevated temperatures, and difference in materials of casing and shaft. The bearings are enclosed in resilient mountings to protect them from damage by vibration. The mountings is a housing fitted with small plate type springs that dampen vibration transmission, so that the races receive some protection from impact damage by the balls or rollers (sometimes referred to as brinelling or false brinelling).