Crane : Boom Bracket

Original Design

The original design was tested with 37.812kN of force in a downward direction with Armstrong 240 as the material of choice. Right of the bat we see that it is clearly unsuitable, with a maximum stress of 999.5MPa located primarily along the vertical members of the design spreading outward from that. Note that in this case the metal thickness is 4.55mm throughout. We can note that for the parts below the upper flange the material and design performed fairly well, especially considering the small hole geometry, which still comes in with stresses at of below 140.MPa, which is very close to being within the safety factor.

Stress at underside edge

It is still important to note that the stresses are still concentrated toward the areas where the flange joined the lower members and extends to the first bolt hole.

Stress at join

The easiest option would be simply change the material selection. If the design is kept the same then to maintain the safety factor then the material yield point would need to be 2000.0MPa and higher. On a small part like this this could be feasible however for the sake of design we will look to modify the geometry.

Given the magnitude of the stresses simply changing the material is not enough and should be done in conjunction with modifying the design. By the area that is visibly affected by the stresses this is best place to start simply by increasing the thickness.

Upper flange of 5.95mm

We can see two outcomes, firstly the area of stress has shrunk, and more importantly the maximum stress has gone down by almost 50% of the original value to 580.0MPa. This points to how modifying the geometry can affect the stresses.

The stresses are also reduces in the underside even around the boltholes.

Reduction in underside stresses.

The upper flange was increased to 7.938mm and this too further decreased the stresses, down to a maximum of 345.5MPa.

This increase in thickness was also applied to the lower members as well. This further decreased the stresses however, it also decreased the stress along the very centre. This shows the very relationship geometry and stress have and that changing more than one characteristic can have extensive results.

7.938mm Upper flange
7.938mm Upper and lower members

The increase in the thickness of the lower members has shown a decrease in the stresses along the edges, so much so that the stresses are largely confined to the upper flange, little extending to the lower members.

For the edges the best way to reduce the stresses would be to add a rounded fillet.

If said fillet of too small though then it may not serve the purpose, it will just be another point of stress concentration.

The metal thickness was further increased to 10.32mm all around resulting in a maximum stress of 241.5MPa, a significant reduction but well above the yield point let alone the safety factor.

Lower stresses in lower members w/ fillet & 7.938mm

Even with 10.32mm all around the max stress is too high, further work is required specifically to the geometry. The spacing between the 2 member was increased to 10.32mm which got the stress down to below the yield point of 240MPa to 215.6MPa. Still though this high stress existed close to the edge of the upper and lower members, modifying the fillet was a way forward.

10.32mm all around

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