tailieunhanh - Mechanical Engineering-Tribology In Machine Design Episode 7

Tham khảo tài liệu 'mechanical engineering-tribology in machine design episode 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Friction lubrication and wear in tower kinematic pairs 137 Again suppose X and y are the perpendicular distances of the fulcrum F from the lines of action of T2 and Tị respectively. It is assumed that the brake is used in such a manner as to prevent the rotation of the drum when the crane is carrying a load Q attached to a rope passing round the circumference of the barrel. If a force p at leverage d is necessary to support this load then Pd Tiy T2x. The relation between the effective tensions T and T2 is given by T1 T2 ef where is the coefficient of friction for the contact surfaces and 0 is the angle of wrap of the band round the drum. Hence combining the above expressions i Tla and so that M ef y x p ad e tì 1 To study the effect of varying the ratio x y on the brake action now consider the following cases Case 1. X 0 Here the line of action of T2 passes through F and a downward movement of the force p produces a tightening effect of the band on the drum. Case 2. x y p ad In this case there is no tightening effect since the displacements of A and B in the directions of T2 and Tị are equal in magnitude. Hence to maintain the load the band would have to be in a state of initial tension. Case 3. x y efe . p o and x y Tl T2. For this ratio a small movement of the lever in the negative direction of p 138 Tribology in machine design would have the effect of tightening the band and the brake would be selflocking. Case 4. y 0 Mx 1 ad ef 0 1 Here the direction of p must be reversed to tighten the band on the drum. From the above conclusions it follows that if e tì x y 1 downward movement of the force p would tend to slacken the band. Hence for successful action X must be less than y. When the brake is used in the manner indicated above there is no relative sliding between the friction surfaces so that is the limiting coefficient of friction for static conditions. The differential tightening effect of the band brake is used in the design .