DESIGN OF RIVETED JOINTS.pptx

A presentation on how to design Riveted Joints.

1. DESIGN OF RIVETED JOINTS DR.KROVVIDI SRINIVAS DEPARTMENT OF MECHANICAL ENGINEERING

2. A rivet is a short cylindrical bar with a head integral to it. The cylindrical portion of the rivet is called shank or body and lower portion of shank is known as tail , as shown in Fig.1. The rivets are used to make permanent fastening between the plates such as in structural work,ship building, bridges, tanks and boiler shells. The riveted joints are widely used for joining light metals. The fastenings ( i.e. joints) may be classified into the following two groups : 1. Permanent fastenings, and 2 . Temporary or detachable fastenings.

3. METHODS OF RIVETING INITIAL POSITION FINAL POSITION HOT AND COLD RIVETTING ARE TWO METHODS OF RIVETTING

4. MATERIAL OF RIVETS The material of the rivets must be tough and ductile. They are usually made of steel (low carbon steel or nickel steel), brass, aluminium or copper, but when strength and a fluid tight joint is the main consideration, then the steel rivets are used. The rivets for general purposes shall be manufactured from steel conforming to the following Indian Standards : ( a ) IS : 1148–1982 (Reaffirmed 1992) – Specification for hot rolled rivet bars (up to 40 mm diameter) for structural purposes; or ( b ) IS : 1149–1982 (Reaffirmed 1992) – Specification for high tensile steel rivet bars for structural purposes.

5. ESSENTIAL QUALITIES OF A RIVET According to Indian standard, IS : 2998 – 1982 (Reaffirmed 1992), the material of a rivet must have a tensile strength not less than 40 N/mm2 and elongation not less than 26 percent. The material must be of such quality that when in cold condition, the shank shall be bent on itself through 180°without cracking and after being heated to 650°C and quenched, it must pass the same test. The rivet when hot must flatten without cracking to a diameter 2.5 times the diameter of shank.

6. MANUFACTURE OF RIVETS According to Indian standard specifications, the rivets may be made either by cold heading or by hot forging. If rivets are made by the cold heading process, they shall subsequently be adequately heat treated so that the stresses set up in the cold heading process are eliminated. If they are made by hot forging process, care shall be taken to see that the finished rivets cool gradually .

7. RIVET HEADS FOR GENERAL PURPOSES (BELOW 12 MM DIAMETER) AS SHOWN IN FIG. BELOW , ACCORDING TO IS : 2155 – 1982 (REAFFIRMED 1996).

8. RIVET HEADS FOR GENERAL PURPOSES (FROM 12 MM TO 48 MM DIAMETER) AS SHOWN IN FIG. BELOW ACCORDING TO IS : 1929 – 1982 (REAFFIRMED 1996).

9. RIVET HEADS FOR BOILER WORK (FROM 12 MM TO 48 MM DIAMETER, AS SHOWN IN FIG. BELOW , ACCORDING TO IS : 1928 – 1961 (REAFFIRMED 1996).

10. TYPES OF RIVETED JOINTS

11. SINGLE RIVETED DOUBLE STRAP BUTT JOINT

12. EQUAL STRAP BUTT JOINT

13. UNEQUAL DOUBLE STRAP JOINT

14. 1. Pitch . It is the distance from the centre of one rivet to the centre of the next rivet measured parallel to the seam . It is usually denoted by p . 2. Back pitch . It is the perpendicular distance between the centre lines of the successive rows It is usually denoted by pb . 3. Diagonal pitch . It is the distance between the centres of the rivets in adjacent rows of zig-zag riveted joint . It is usually denoted by pd . 4. Margin or marginal pitch . It is the distance between the centre of rivet hole to the nearest edge of the plate. It is usually denoted by m .

15. CAULKING AND FULLERING

16. FAILURES OF A RIVETED JOINT TEARING OF RIVET ACROSS EDGE TEARING ACROSS ROW OF RIVETS

17. SHEARING OF THE RIVETS LAP JOINT IN SINGLE SHEAR SINGLE STRAP BUTT JOINT IN SINGLE SHEAR

18. DOUBLE STRAP BUTT JOINT IN DOUBLE SHEAR

19. CRUSHING FAILURE

20. STRENGTH OF A RIVETED JOINT The strength of a joint may be defined as the maximum force, which it can transmit, without causing it to fail. The least value of Pt , Ps and Pc which are the pulls required to tear off the plate,shearing off the rivet and crushing off the rivet. If the joint is continuous as in case of boilers, the strength is calculated per pitch length . But if the joint is small , the strength is calculated for the whole length of the plate.

21. EFFICIENCY OF A RIVETED JOINT The efficiency of a riveted joint is defined as the ratio of the strength of riveted joint to the strength of the un-riveted or solid plate. We have already discussed that strength of the riveted joint = Least of Pt , Ps and Pc Strength of the un-riveted or solid plate per pitch length, P = p × t × σ t

22. Efficiency of the riveted joint, η = Least of P t , P s and P c/ Strength of the solid plate t Least of Pt Ps Pc/p × t × σ where p = Pitch of the rivets, t = Thickness of the plate, and σ t = Permissible tensile stress of the plate material. σ c = Permissible Compressive strength of the plate τ = Permissible shear stress.

23. DESIGN OF BOILER JOINTS There are two joints in the boiler. 1. Longitudinal Joint: A flat plate made in the form of a cylindrical object by bringing the ends of the plates together and riveting the plates. A butt joint is used for this purpose. 2. Circumferential Joint: For increasing the length of the boiler, two cylindrical surfaces are brought together and riveted.A lap joint is used for this purpose.

24. ASSUMPTIONS IN DESIGNING BOILER JOINTS 1 . The load on the joint is equally shared by all the rivets. The assumption implies that the shell and plate are rigid and that all the deformation of the joint takes place in the rivets themselves. 2. The tensile stress is equally distributed over the section of metal between the rivets. 3. The shearing stress in all the rivets is uniform. 4. The crushing stress is uniform. 5. There is no bending stress in the rivets. 6. The holes into which the rivets are driven do not weaken the member. 7. The rivet fills the hole after it is driven. 8. The friction between the surfaces of the plate is neglected

25. DESIGN OF LONGITUDINAL BUTT JOINT FOR A BOILER T he thickness of the boiler shell is determined by using the thin cylindrical formula, i.e . t = P D/2 σ t* ɳ l + 1 in mm where t = Thickness of the boiler shell, P = Steam pressure in boiler, D = Internal diameter of boiler shell, σ t = Permissible tensile stress, and ɳ l = Efficiency of the longitudinal joint.

26. EFFICIENCY OF THE BOILER Indian Boiler Regulations (I.B.R.) allow a maximum efficiency of 85% for the best joint .

27. FACTOR OF SAFETY OF THE BOILER JOINT According to I.B.R., the factor of safety should not be less than 4.

28. DIAMETER OF THE RIVET d = 6 √t UNWIN’S FORMULA Where “d” is the diameter of the rivet and “t” is the thickness of the plate. Valid when the thickness of the plate is 8 mm are more. If the thickness is lesser than 8 mm, then equate the shear strength with the crushing strength .

29. PITCH OF RIVETS . Pitch of the rivet is obtained by equating the Tearing strength with Shear Strength of the rivet. Pitch of the rivet can’t be less than “2d” where d is the diameter of the rivet.

30. VALUE OF C

31. PREFERRED LENGTH TO DIAMETER AS PER IBR

32. DISTANCE BETWEEN ROWS OF RIVETS

33. THICKNESS OF THE BUTT STRAP MARGIN M IS 1.5d

34. DESIGN OF CIRCUMFERENTIAL LAP JOINT FOR A BOILER Thickness of the shell and diameter of rivets . The thickness of the boiler shell and the diameter of the rivet will be same as for longitudinal joint. 2. Number of rivets . Since it is a lap joint, therefore the rivets will be in single shear. ∴ Shearing resistance of the rivets, Ps = n × π × d 2 × τ /4 n = Total number of rivets

35. Knowing the inner diameter of the boiler shell ( D ), and the pressure of steam ( P ), the total shearing load acting on the circumferential joint ,

36. 3. PITCH OF RIVETS. If the efficiency of the longitudinal joint is known, then the efficiency of the circumferential joint may be obtained. It is generally taken as 50% of tearing efficiency in longitudinal joint, but if more than one circumferential joints is used, then it is 62% for the intermediate joints. Knowing the efficiency of the circumferential lap joint ( η c ), the pitch of the rivets for the lap joint ( p 1) may be obtained by using the relation : η c = [ p – d]/p

37. Number of rows = Total number of rivets Number of rivets in one row and the number of rivets in one row π [ D+ t]/P1 After finding out the number of rows, the type of the joint ( i.e. single riveted or double riveted etc.) may be decided. Then the number of rivets in a row and pitch may be re-adjusted. In order to have a leak-proof joint, the pitch for the joint should be checked from Indian Boiler Regulations.

38. After knowing the distance between the rows of rivets ( pb ), the overlap of the plate may be fixed by using the relation, Overlap = (No. of rows of rivets – 1) pb + m Where m = Margin.