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Monday, July 27, 2015

Stiffness of a welded straight square tube

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To modify or improve process of the existing machine in a production line may have some constraints regarding available spaces. There may be other moving objects or fixed parts which obstruct mounting of newly design parts. The part that comes later may be more complex since it has a limited space for mounting. The green support, design C from earlier post, is the example.
complex shape of welded part to avoid interference with existing object
Part with other object occupied spaces
This is the side view of the above picture. The moving object occupies the space above the horizontal square tube and the design is in the L shape as shown below in order to avoid interference.
Projected view of support design C
Side view of design C
This green support takes both loads in x and z directions. The load in z will try to bend the part and the load in x will try to twist the part. The dimension L is quite long from the center of the horizontal tube and the torque which is product of Fx and L will easily twist the horizontal tube. Since L is long, only a few twisting degrees of the tube will result in having larger displacement at the end (at the top flange where the loads applied).
Loads that bend and twist the square tube
Loads on welded part (design C)
If there is no obstruction above the horizontal tube, we can then change the design not to form the L shape in order to avoid twisting of the tube as shown below.
New design with no torque on the tube
Straight tube design when space is available
The force in z is still trying to bend the straight tube up. However, the force in x will not try to twist the tube anymore. It's trying to bend the tube to the left instead. And with this design we can reduce the mass from 0.92 kg of design C to 0.71 kg which is about 23% reduction. And here is the finite element analysis result using the same fixations and loads.

FEA result: displacement of straight tube design
Finite element analysis result: displacement of straight tube design
Not only mass reduction, but also the stiffness increases. The displacement reduces from 9.7 mm of design C to 6.98 mm which is about 28% reduction. This is because we eliminate the torque from the tube. But of course, this is when there is available spaces above the horizontal tube of design C.

We will apply this torque reduction design to other applications in later posts. Please stay tuned!

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