A vertically mountable shaft and bearing assembly includes a thermally insulated housing which protects the shaft, bearing(s), seal(s), and bearing lubricant from overheating due to a surrounding process fluid at a temperature exceeding 65° C. The thermal insulation, which in some embodiments includes vacuum, air, or an insulating material, enables a rolling element bearing to be located near the shaft's distal end, thereby reducing shaft and bearing size requirements. Lubricant is circulated past the bearing(s) by a circulation and cooling system having a capacity typical of systems used with process fluids at much lower temperatures. Additional roller and/or journal bearings can be included. The lubricant circulation system can be actuated by the shaft or driven separately. Lubricant within the shaft housing can be pressurized, for example by pressurizing gas or by filling the lubricant above the surrounding process fluid level. The shaft can operate a process fluid pump impeller.