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Steam & Excursion > Whazzit? (16)Date: 01/20/20 07:52 Whazzit? (16) Author: LarryDoyle Date: 01/20/20 07:53 Re: Whazzit? (16) Author: tomstp Lapp seam?
Date: 01/20/20 09:56 Re: Whazzit? (16) Author: LarryDoyle tomstp Wrote:
------------------------------------------------------- > Lapp seam? I see two lap seams. LD Date: 01/20/20 09:58 Re: Whazzit? (16) Author: choodude I don't know the correct practice.
My guess is the rivets where the horizontal lap seam meet the vertical lap seam appear too close to the edge and appear to provide a perfect place to start a crack. Brian Date: 01/20/20 13:03 Re: Whazzit? (16) Author: Elesco The big problem with lap seams is secondary bending loads caused by the offset between the planes of the two plates being joined. See the figures below illustrating how it works. Particularly with longitudinal seams, there a great deal of tensile load across the joint due to boiler pressure pushing outward on the shell.
The bending loads cause high localized stresses in the joint, often resulting in fatigue cracks and risk of catastrophic failure if not caught in time. They also cause bending and tensile forces in the rivets, although that doesn't seem to be the critical issue. Much preferred is the butt joint design with inner and outer splice plates or straps. There is no offset between the plates, no secondary bending, and the rivets are loaded in double shear. Date: 01/20/20 13:15 Re: Whazzit? (16) Author: Kimball Three over-driven rivets, the ones without conical heads?
Date: 01/20/20 14:45 Re: Whazzit? (16) Author: LarryDoyle Elesco Wrote:
------------------------------------------------------- > The big problem with lap seams is secondary > bending loads caused by the offset between the > planes of the two plates being joined. See the > figures below illustrating how it works. > Particularly with longitudinal seams, there a > great deal of tensile load across the joint due to > boiler pressure pushing outward on the shell. > > The bending loads cause high localized stresses in > the joint, often resulting in fatigue cracks and > risk of catastrophic failure if not caught in > time. They also cause bending and tensile forces > in the rivets, although that doesn't seem to be > the critical issue. > > Much preferred is the butt joint design with inner > and outer splice plates or straps. There is no > offset between the plates, no secondary bending, > and the rivets are loaded in double shear. Exactly! While circumfrential seams are not subject to the stresses described and thus are used liberally in all riveted boilers, the longitudinal lap seam was the major cause of boiler failure prior to 1900. -LD |