backlash and jamming in an environment of high torque and low rotational rate, may preclude meeting requirements. The basic types of gears include spur gears, helical gears, worm gears, harmonic drive gears and bevel gears. The features of each of these options are presented in Fig. 3.2-3. The "no backlash" feature of the harmonic drive, in addition to the potential to achieve high gearing ratios with minimal packaging difficulty, would lead to selection of this approach. The major problem with the harmonic drive, however, is the poor life inhibited in limited tests of these gears. The worm gear approach, particularly for the elevation drive, is not recommended because of alignment difficulties, high friction and the inability to drive the gears backwards. A spur gear drive would provide a simple positive traction for transfer of torque; however, the design of gear teeth would have to provide a significant positive safety margin to preclude tooth breakage. Wear is not considered a problem due to the low speed environment. 3. 2. 2.2 Bearings Bearings for the MPTS interface control system should be rolling-element types to provide the lowest friction possible. Options for selection include ball bearings, roller bearings or individual rollers. The individual roller approach was used in the SSPS design shown in Fig. 3.2-2. This approach results in high friction and has questionable fatigue life. A ball-bearing approach would minimize friction and provide better fatigue life. The large diameter (65 meters) of the azimuth interface would cause problems in design and assembly of conventional, machined-race, low-friction bearings. Because of the ball bearing design problem, it is recommended that the individual roller and track bearing arrange- ment be retained. Further study and definition should include assessment of the following: • Static load capacity • Dynamic load capacity • Fatigue life • Stiffness • Friction • Tolerance to thermal gradients • Lubrication • Materials • Maintainability
RkJQdWJsaXNoZXIy MTU5NjU0Mg==