Sunday, February 12, 2017

More on the battery box calculations

We have been doing some structural calculations.
The boxes each have 11 modules @ 8lbs ea.  therefore ~110 lb per battery box
The DMOC645 is ~ 60lb
Therefore ~300lb for the system.
We have looked online and consulted others with our best standard maximum loading guess as:
3g V+ 2g Lat + 1g Long
2g Lat + 2g brake on 20% grade  We live more than 50miles from ANY grade therefore we are not worried about the 2g down 20% grade...But it was surprising to us that the brakes do not exert more than a 1g load at full capability.
The frame will be made from A36 steel:
minimum yield strength of 36,000 psi (250 MPa)
ultimate tensile strength of 58,000–80,000 psi (400–550 MPa).
Based on these numbers we will design daily usage to  3-2-1.  Our safety factor will therefore be 1.8 to failure and likely due to impact of some sort.
EVERYONE likes pretty pictures:  The frame was designed in CREO 2, cleaned in OnShapeCad, and analyzed in SimScale.  The tubes are hollow 0.065" thick and 1" square.  The I-beam is 3"x2" and the smallest we can get but cheaper than C-Channel and Square tube of the same modulus.  The mesh is quad and tet.

1g Vertical Stress Field

1g Vertical Displacement Field
Nothing to see here, load is well under the 250MPa yield stress.

Unfortunately the scale is in black and does not show.  I set the max to 250MPA.  The max displacement is 2mm in the 3g loads.

Interesting only until you look at the actual value of the red section.  The displacement is a fraction of a millimeter.  The system does not react symmetrically because the area where the near tube connects to the right plate is in front of the exhaust tunnel and therefore not supported as rigidly.  I did do a check on the system where I fixed the tunnel area and the system did behave symmetrically.

3g Vertical, 2g Lateral, 1g Long. Stress Field

3g Vertical, 2g Lateral, 1g Long. Displacement Field

5g Vertical, 5g Lateral, 5g Long. Stress Field

From the analysis, there will be local yielding under a 5g load, but the maximum tensile strength will not be exceeded.  A crash would likely apply more than 5g to the system, but then significant other deformation of the frame has occurred at that point too.

5g Vertical, 5g Lateral, 5g Long. Displacement Field

My hand calculations are on the same order of magnitude, SO please point out my errors if you see them.

  The thickness of the rear plate was 0.25", I have rerun the results, there are not surprises.  I will run a 0.125" plate instead.