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Mechanical Engineers Data Handbook Episode 4

Tham khảo tài liệu 'mechanical engineers data handbook episode 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | APPLIED MECHANICS 65 2.2 Belt drives 2.2.1 Flat vee and timing belt drives Formulae are given for the power transmitted by a belt drive and for the tensions in the belt. The effect of centrifugal force is included. A table of information on timing belt drives is included. Symbols used Fj belt tension tight side F2 belt tension slack side ra radius of pulley a rb radius of pulley b Na speed of pulley a Nb speed of pulley b m mass of belt per unit length p power transmitted 4 coefficient of friction between belt and pulley Fo initial belt tension ớa arc of belt contact pulley a ớb arc of belt contact pulley b L distance between pulley centres s percentage slip 47 belt velocity c. . Nb rb 100 s Speed ratio 7 Nb ra 100 when pulley b is the driver Arc of contact r rb g. 18O 2sin 1 r 2r-eb 18O -2sin 1 r 2r Tension ratio for belt about to slip f For pulley a For pulley b e 8b where e base of natural logarithms 2.718 . Power capacity P vịFì-Fỉ where belt velocity v 2nrtN 2nrbNb no slip . Pulley torque r. ra F1-F2 Tb rb Fi F2 F 4- F. Initial tension F - 2 0 2 Effect of centrifugal force the belt tensions are reduced by mv2 so that F2 mv2 Vee belt The wedge action of the vee belt produces a higher effective coefficient of friction n p sin a where a the half angle of the vee 4 2.9 4 for a 20 . 66 MECHANICAL engineer s data handbook Timing belts Timing belts have teeth which mate with grooves on the pulleys. They are reinforced with high strength polymer strands to give power capacity up to three times that of conventional belts at three times the speed. There is no slip so a constant ratio is maintained. A large number of speed ratios is available. Belts are made in several strengths and widths. Timing belt sizes BS 4548 1970 Type Meaning Pitch mm Widths mm Constant K XL Extra light 5.08 6.4 7.9 9.6 L Light 9.53 12.7 19.1 25.4 1.53 H Heavy 12.70 19.1 25.4 38.1 50.8 76.2 5.19 XH Extra heavy 22.23 50.8 76.2 101.6 12.60 XXH Double extra heavy 31.75 50.8 76.2 101.6 127.0 .