In an experiment, a flexible shaft(stainless steel) of size 1.5 mm, is placed inside a 180 degree curved cylindrical conduit (a reinforced polymer tube of ID of 1.8 mm). The shaft is designed to transmit power inside the tube at over 45,000 rpm, driven by a motor located outside the tube. This experimental setup was introduced to determine the critical speed at which the flexible shaft becomes prone to failure. However, during experimentation, the shaft experiences significant heating due to friction between the shaft and the tube, and vibration leading to the melting of the tube, particularly at the initial bend of 135/45 degree. Also it causes the shaft broken at same location.

Can anyone suggest how to improve the flexible shaft to prevent failure and avoid melting of the tube?