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    Discussion in 'Calculations' started by binulagos, Jan 31, 2013.

    1. binulagos

      binulagos Member

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      awaiting for a reply
       
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    3. Lochnagar

      Lochnagar Well-Known Member

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      Thanks for the photographs Binulagos.

      The exact proportions of your trailer are not shown - but I sense you have quite a large overhang from the middle axle to the back of the semi trailer - and so if you draw a bending moment diagram for your semi trailer - there will be two peaks - one will be above the center axle - where the I beam has cracked (hogging) - and the other will be roughly mid way between the center of the king pin and the middle axle (sagging). The maximum stress occurs in the I beam at the point of the maximum bending moment. Additionally the stress will vary due to the oscillatory motion of the trailer on the road. Additionally, these I beams see a lot of stress from flexing - where the tractor unit and trailer are on uneven ground.

      I have also noticed from your photographs - that your semi trailer has folding sides - which I sense are quite high too. I suspect that the semi trailer is used for carrying different materials - one of which might be corn - and the other which you have alluded to is cement. So - the problem quite often is that the people that load these trailers - quite often do not appreciate that different materials have different densities - and so the philosophy is - load the semi trailer up - until the cargo is level with the top of the sides of the trailer. As cement is quite a dense material - and certainly more dense than many materials (and certainly corn) - then the load that the semi trailer will be carrying - will be more than it should be carrying.

      So I think the cause of the problem - is the trailer is being loaded with too much material when you are carrying cement.
       
      Last edited: Feb 5, 2013
    4. binulagos

      binulagos Member

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      thanks Mr Lochnagar.......Is there any method to avoid the flexibilty while in motion
       
    5. ChrisW

      ChrisW Well-Known Member

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      Binulagos, There are a few issues you may be experiencing here.
      1) As I suggested earlier, the most common cause of fractures of this kind is a sudden change in section stiffness. Is there a cross-member above the location of the crack? Is this the location of a welded cross-member? For this reason, cross-members etc are often bolted or riveted through the web to prevent changing the chassis rail stiffness. Chassis runners (rails placed on top of the chassis as part of a body) require extremely low angle taper to the end cut, again to prevent a sudden change in stiffness.
      2) The mechanical load compensator linkage used has relatively small range of axle travel. If used on very rough roads they can bottom out and transmit a very severe shock load to the chassis.
      3) Is the compensator linkage set up correctly for the height of the tractor which is towing it? Flat bed trailers often have a front high towing position, not level. If the linkage has been set for a level attitude, the range of articulation is greatly reduced. On very uneven ground this can lead to the full load suddenly being applied to the rear axle only, the front two actually leaving the road or carrying very little weight. Check clearance front and back on the linkages to ensure they can move equally in both directions.
       
    6. Lochnagar

      Lochnagar Well-Known Member

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      Binulagos - you posed the question - why are these semi trailers cracking when they carry cement - but not any other material.
      As you are the man on location where these semi trailer chassis are cracking I reckon you have a few ideas of your own as to what is different - between a semi trailer carrying cement - and a semi trailer carrying some other material.
      So it would be good to hear your own ideas as to what is different
      - between a semi trailer carrying cement - and a semi trailer carrying some other material.
      1)Have you actually counted the bags of cement - to compute the actual load the trailer is carrying - when it is carrying cement?
      2)Have you computed the weight of the other material (maybe corn) - that these trailers carry - when they are carrying other material (maybe corn or whatever)?
      3)How do the weights in 1) and 2) above compare
      4)How does the type of road journey (roughness of the road) compare between a semi trailer carrying cement - and a semi trailer carrying some other material?
       

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