Lecture13-14-MEC223 - Copy.pptxgffffffffffff

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1. MEC223 Design of Machine Element -I

2. GIVE ME A PLACE TO STAND AND I WILL MOVE THE EARTH 2

3. 3 “Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.”

4. levers A lever is defined as mechanical device in form of a rigid bar pivoted about the fulcrum to multiply or transfer the force.

5. Terms defined Taking moment of forces about fulcrum F = Load P = Effort l 1 = Length of Effort Arm l 2 = Length of Load arm or resistance Arm or

6. Mechanical Advantage and Leverage The Ratio F/P is called the ‘mechanical advantage’ The Ratio l 1 /l 2 is called the ‘leverage’ 6

7. Types of levers

8. 2 ND CLASS LEVER 8

9. 3 RD CLASS LEVER 9

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11. Types of levers When l 1 <l 2 (mechanical advantage<1) When l 1 =l 2 ( mechanical advantage=1) When l 1 >l 2 ( mechanical advantage>1 ) The Ratio l 1 /l 2 is called the ‘leverage’

12. Design of levers Lever is easy compared with the design of other machine elements. The length of the lever is decided on the basis of leverage required to exert a given load F by means of an effort P. The cross section of lever is designed on the basis of bending stress .

13. Design of levers Step 1: Force analysis:- The effort required to produce the force is calculated by taking moments about the fulcrum R is the reaction at the fulcrum pin. Since the sum of vertical forces acting on the lever must be equal to zero R=F+P

14. Design of levers Second type :-In this type of lever force F(Load) and P(Effort) act in opposite direction.

15. Design of levers If forces F and P act along lines that are inclined to one another. In such cases, l 1 is the perpendicular distance from the fulcrum to line of action of the force P. Similarly, l 2 is the perpendicular distance from fulcrum to the line of action of force F.

16. Design of levers Rule from static apply to the reaction R at the fulcrum: Magnitude of reaction R is equal to the resultant of load F and effort P. It is determined by the parallelogram law of forces. The line of action of the reaction R passes through intersection of P and F., i.e the point O and also through fulcrum

17. Design of levers A bell crank lever with the arms that are inclined at angle Ө with one another When the arms of the bell crank lever are inclined at right angle to one another then

18. Design of levers Step2:- Design of lever arm The cross section at which bending moment is maximum can be determined by constructing a bending moment diagram

19. Design of levers Step2:- Design of lever arm The cross section of the lever can be rectangular, elliptical or I section For a rectangular cross section For an elliptical cross section

20. Design of levers Step2:- Design of lever arm Usually d = 2b for rectangular cross section and a = 2b for elliptical cross section Using the above mentioned proportions, the dimensions of the cross section of the lever can be determined by

21. Design of fulcrum Pin The fulcrum pin is subjected to reaction R as shown The dimensions of the pin are determined by bearing consideration and then checked for shear consideration . Where p is the permissible bearing pressure The ratio l/d 1 is usually taken from 1 to 2 P (l X d 1 )  

22. Design of fulcrum Pin A phosphor bronze bush , usually 3 mm thick, is fitted inside the boss to reduce the friction. The permissible bearing pressure for a phosphor bronze bush is 5 to 10 N/mm 2 . Expression for bearing pressure p  

23. Problem A lever loaded safety valve is mounted on the boiler to blow off at a pressure of 1.5 MPa gauge. The effective diameter of the opening of the valve is 50 mm. The distance between the fulcrum and the dead weights on the lever is 1000mm. The distance between the fulcrum and the pin connecting the valve spindle to the lever is 100 mm. The lever and the pin are made of plain carbon steel 30C8 ( Syt = 400 N/mm 2 ) and factor of safety is 5 the permissible bearing pressure at the pins in the lever is 25 N/mm 2 . The lever has a rectangular cross section and the ratio of width to thickness is 3:1. Design a suitable lever for the safety valve. 23

24. Problem 24

25. Problem 25 Given:- S yt = 400 N/mm 2 f s = 5 Valve dia = 50 mm Lever lengths l 1 = 1000 mm, l 2 = 100 mm d/b = 3 Bearing pressure p = 25 N/mm 2 Blow of pressure = 1.5 Mpa Calculation of permissible stresses Calculation of forces acting on lever Diameter and length of the pin Width and thickness of the lever

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