TMT M3 system tilt axis bearing method

The tertiary mirror pointing assembly(M3PA) of the thirty meters telescope(TMT) is the largest tertiary mirror in the world. The tracking and pointing performance of M3PA is better than any other telescopes which have been built, and the working condition is even worse. So the designers face an enormous challenge. The tracking system includes the bottom rotator axes and the tilt axes. The study of this paper focused on the tilt axes. There were two bearing arrangement methods for the tilt axes. The first one used two single angular contact bearing to stand the shaft. The second used two double rows angular contact bearings. They both met the requirement when the M3PA was vertical. But the first method became invalid when the M3PA was horizontal, while the second can provide a better support about 15.1 Hz of the first modal frequency. Then some FEA were done to find out the carrying capacity of the bearings. The results show that the maximum load applied on the rollers is up to 5 000 N, while the maximum stress is up to 2 300 MPa. Otherwise, the load distribution just likes a cosine curve. So it can be found that the analysis is believable. In addition, there are evident load applied on the four rows rollers, which proves that the bearing arrangement can disperse the load effectively. It can be concluded that the selection of the bearing arrangement is a good choice for TMT tilt axes design.