Tham khảo tài liệu 'advanced vehicle technology episode 2 part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | movement ratios. A small input effort applied to the end of a perpendicular lever fixed to the screw is capable of moving a much larger load axially along the screw provided that the nut is prevented from rotating. If the screw is prevented from moving longitudinally and it revolves once within its nut the nut advances or retracts a distance equal to the axial length of one complete spiral groove loop. This distance is known as the thread pitch or lead p . The inclination of the spiral thread to the perpendicular of the screw axis is known as the helix angle a . The smaller the helix angle the greater the load the nut is able to displace in an axial direction. This is contrasted by the reduced distance the nut moves forwards or backwards for one complete revolution of the screw. The engaged or meshing external and internal spiral threads may be considered as a pair of infinitely long inclined planes Fig. a and b . When the nut is prevented from turning and the screw is rotated the inclined plane of the screw slides relative to that of the nut. Consequently a continuous wedge action takes place between the two members in contact which compels the nut to move along the screw. Because of the comparatively large surface areas in contact between the male and female threads and the difficulty of maintaining an adequate supply of lubricant between the rubbing faces friction in this mechanism is relatively high with the result that mechanical efficiency is low and the rate of wear is very high. A major improvement in reducing the friction force generated between the rubbing faces of the threads has been to introduce a series of balls Fig. which roll between the inclined planes as the screw is rotated relatively to the nut. The overall gear ratio is achieved in a screw and nut steering gearbox in two stages. The first stage occurs by the nut moving a pitch length for every one complete revolution of the steering wheel. The second stage takes place by converting the
