khả năng tăng tốc là không thực tế trong khoảng thời gian. Tuy nhiên, tại biên giới của khoảng thời gian, nơi mà tăng đáp ứng thấp ở một bên và cao ở trên lưu ý, rằng các chức năng vận tốc là multivalued. Có gián đoạn tại những ranh giới này. Ảnh hưởng của những gián đoạn là để tạo ra một phần của đường cong vận tốc có độ dốc vô hạn và không thời gian. | GEAR TRAINS 449 volute were still in place after the base circle s enlargement to an infinite radius the string would be pivoted at infinity and would generate an involute that is a straight line. This linear gear is called a rack. Its teeth are trapezoids yet are true involutes. This fact makes it easy to create a cutting tool to generate involute teeth on circular gears by accurately machining a rack and hardening it to cut teeth in other gears. Rotating the gear blank with respect to the rack cutter while moving the cutter axially back and forth across the gear blank will shape or develop a true involute tooth on the circular gear. Figure 9-19 shows a rack and pinion. The most common application of this device is in rotary to linear motion conversion or vice versa. It can be backdriven so it requứes a brake if used to hold a load. An example of its use is in rack-and-pinion steering in automobiles. The pinion is attached to the bottom end of the steering column and turns with the steering wheel. The rack meshes with the pinion and is free to move left and right in response to your angular input at the steering wheel. The rack is also one link in a multibar linkage which converts the linear translation of the rack to the proper amount of angular motion of a rocker link attached to the front wheel assembly to steer the car. Bevel and Hypoid Gears Bevel Gears For right-angle drives crossed helical gears or a wormset can be used. For any angle between the shafts including 90 bevel gears may be the solution. Just as spur gears are based on rolling cylinders bevel gears are based on rolling cones as shown in Figure 9-20. The angle between the axes of the cones and the included angles of the cones can be any compatible values as long as the apices of the cones intersect. If they did not intersect there would be a mismatch of velocity at the interface. The apex of each cone has zero radius thus zero velocity. All other points on the cone surface will have nonzero .
