Crane : Boom (Pt. 1 of 2)

Now the boom has been adjusted, with the overall design having a cross-member, this introduces new forces and geometries that have to be better taken into consideration. The initial design while demonstrating the basics when applying “the math” (number & formulae) is rather simple not taking into account all the forces at work. The modified free-body diagram is still accurate and forms a template to which your design will be based around.

The first model is based around the original freebody and assumptions with no cross member. For the most part the simulation proved what was already calculated. It showed that for these circumstances the original material selection choice was a good one the Armstrong 240 with a yield point of 240-320Mpa can satisfy the criteria. However it exposed a potential flaw in the calculation and model.

Initial Design w/o bolt holes

The initial design took moments about the point D, in this case the model works well with all forces coming in as calculated. However this in reality is not the case at point D the boom is fixed to the turn table by four 15.0mm bolt holes. This geometry means that stress concentration may occur in this place. This was not accounted for in the original design.

The original design had no bolt holes and the fixed end was the entire face not the individual bolt holes. In the subsequent designs bolt holes were added as such these became the fixed points thus introducing stress concentrations. The bolt holes for the cross-member also added bolt holes and more points for stress concentrations.

The simulation was able to easily accommodate these things and account for it.

The initial design which had no holes had a calculated stress of 45.11MPa, however the simulation proved this to be 68.83MPa at its maximum. This is fairly close however this could also be due to the fact that the placement of the force is isolated to a very small edge, to which the force is applied. The maximum stress also occurs very close to the applied edge again this could be due to where the force in applied.

However if we look at the plot we see that the stress at the fixed end is roughly within the the range of 45.0MPa. This lends credibility to the manually calculated results. As far as design and appropriateness this is good however we will see as the model begins to better reflect real world conditions changes a have to be made.


The other aspect of the model and the calculations are the deflection that is incurred by the force applied. The calculated amount for the 10.477kN force was 0.00827m or 8.23mm, the simulation proved it to be 8.610mm at its maximum. The image shows the point of maximum deflection is the load bearing end as expected. This instance firmly highlights the accuracy of the manual calculation, to the more complex computer model.

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