tailieunhanh - Text Book of Machine Design Part 15

Levers | CONTENTS c H A P T E R 15 Levers 1. Introduction. 2. Application of Levers in Engineering Practice. 3. Design of a Lever 4. Hand Lever. 5. Foot Lever. 6. Cranked Lever. 7. Lever for a Lever Safety Valve. 8. Bell Crank Lever. 9. Rocker Arm for Exhaust Valve. 10. Miscellaneous Levers. Introduction A lever is a rigid rod or bar capable of turning about a fixed point called fulcrum. It is used as a machine to lift a load by the application of a small effort. The ratio of load lifted to the effort applied is called mechanical advantage. Sometimes a lever is merely used to facilitate the application of force in a desired direction. A lever may be straight or curved and the forces applied on the lever or by the lever may be parallel or inclined to one another. The principle on which the lever works is same as that of moments. Consider a straight lever with parallel forces acting in the same plane as shown in Fig . The points A and B through which the load and effort is applied are known as load and effort points respectively. F is the fulcrum about which the lever is capable of turning. The perpendicular distance between the load point and fulcrum l1 is known as load arm and the perpendicular distance between the 558 CONTENTS Levers 559 effort point and fulcrum 12 is called effort arm. According to the principle of moments W x 11 P x 12 or W _12 p 71 . Mechanical advantage W 12 . - P 11 Fig. . Straight lever. The ratio of the effort arm to the load arm . 12 11 is called leverage. A little consideration will show that if a large load is to be lifted by a small effort then the effort arm should be much greater than the load arm. In some cases it may not be possible to provide a lever with large effort arm due to space limitations. Therefore in order to obtain a great leverage compound levers may be used. The compound levers may be made of straight pieces which may be attached to one another with pin joints. The bell cranked levers may be used instead .