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Tips & Tricks - machine bracket redesign

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A cast bracket, made from Gray Cast Iron, was redesigned as a welded mild steel bracket. To check that this is as strong or stronger than the original, a comparative analysis was run. A vertical load of 10kN was applied to the bracket face. Making use of symmetry this was halved to 5kN. The bottom face of the bracket was restrained vertically, and a side restraint added at the corner to completely restrain the model. Note that a comparative analysis avoids the need to determine the effect of the supporting mounting face and contact conditions etc.

Original Cast Bracket

Von Mises Stress 25 (MPa)	Deflection 0.0666 (mm)
Click on the images to download the eDrawings

Welded Bracket Design 1

Von Mises Stress 55 (MPa)	Deflection 0.0287 (mm)
Click on the images to download the eDrawings

Welded Bracket Design 2

Von Mises Stress 61 (MPa)	Deflection 0.0266 (mm)
Click on the images to download the eDrawings

Results Summary

	PEAK STRESS (MPa)	DEFLECTION (mm)	YIELD STRENGTH (MPa)	SAFETY FACTOR
ORIGINAL CAST	25	0.0666	100	4
WELDED 1	55	0.0287	250	4.5
WELDED 2	61	0.0266	250	4

The stress plots show that the highest stresses in both the cast and first welded design occur at the gusset transition into the bottom plate. The stresses at these internal corners are re-entrant stresses, and therefore do not converge (as they are dependant on the mesh size). So analysing these would be a challenge. This could potentially cause problems, as weld quality at the weld toe is often inconsistent and a cause of stress concentrations. Simple redesign however minimises the need to analyse these areas.

The second welded bracket design moves the maximum weld stress from the weld toe into the length of the fillet weld, removing the peak stress at the gusset transition. It also gives a comparable factor of safety, with no tooling or patterns required and it is easily fabricated from two parts. The deflections are also smaller.

Note that the weld stresses in the above analyses will need checking with hand calculations to ensure they won't be a problem, but the second welded bracket is clearly better than the first for weld stresses.