tailieunhanh - McGraw-Hill - Robot Mechanisms and Mechanical Devices Illustrated - 2003 Part 7

Tham khảo tài liệu 'mcgraw-hill - robot mechanisms and mechanical devices illustrated - 2003 part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 114 Chapter 3 Direct Power Transfer Devices TEN UNIVERSAL SHAFT COUPLINGS Hooke s Joints The commonest form of a universal coupling is a Hooke s joint. It can transmit torque efficiently up to a maximum shaft alignment angle of about 36 . At slow speeds on hand-operated mechanisms the permissible angle can reach 45 . The simplest arrangement for a Hooke s joint is two forked shaft-ends coupled by a cross-shaped piece. There are many variations and a few of them are included here. Figure 3-20 The Hooke s joint can transmit heavy loads. Antifriction bearings are a refinement often used. Figure 3-21 A pinned sphere shaft coupling replaces a crosspiece. The result is a more compact joint. Figure 3-22 A grooved-sphere joint is a modification of a pinned sphere. Torques on fastening sleeves are bent over the sphere on the assembly. Greater sliding contact of the torques in grooves makes simple lubrication essential at high torques and alignment angles. Chapter 3 Direct Power Transfer Devices 115 Figure 3-23 A pinned-sleeve shaft-coupling is fastened to one saft that engages the forked spherical end on the other shaft to provide a joint which also allows for axial shaft movement. In this example however the angle between shafts must be small. Also the joint is only suitable for low torques. Constant-Velocity Couplings The disadvantages of a single Hooke s joint is that the velocity of the driven shaft varies. Its maximum velocity can be found by multiplying driving-shaft speed by the secant of the shaft angle for minimum speed multiply by the cosine. An example of speed variation a driving shaft rotates at 100 rpm the angle between the shafts is 20 . The minimum output is 100 X which equals rpm the maximum output is X 100 or rpm. Thus the difference is rpm. When output speed is high output torque is low and vice versa. This is an objectionable feature in some mechanisms. However two universal joints connected by an intermediate shaft solve .