Bracket design -load expectations

I?m adding some equipment that requires mounting, and I?m analyzing existing structures and adding some new brackets, mostly under the panel. The AC is an RV-9A, and I?m stuck hand forming without most of my shop tools at the moment (moved, in temp housing, shop in storage) so some simplicity is required. Right now I?m working with 20g loading to yield of the material, mostly for robustness with respect to vibration, which I can?t really quantify at the moment. I would appreciate some guidance from the experts and those whom have gone before. We?re talking boxes of two and three pounds, nothing big. I have a BSME, so the basics aren?t a problem. Thanks.

rjcthree said:

I?m adding some equipment that requires mounting, and I?m analyzing existing structures and adding some new brackets, mostly under the panel. The AC is an RV-9A, and I?m stuck hand forming without most of my shop tools at the moment (moved, in temp housing, shop in storage) so some simplicity is required. Right now I?m working with 20g loading to yield of the material, mostly for robustness with respect to vibration, which I can?t really quantify at the moment. I would appreciate some guidance from the experts and those whom have gone before. We?re talking boxes of two and three pounds, nothing big. I have a BSME, so the basics aren?t a problem. Thanks.

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I used 12g, but it was arbitrary for a 7. I used 2024 where possible and it was mainly for stiffness, minimal deflection. More work went into the rivet loads.

Handling Loads

In addition to G loading, you need to consider "handling" loads for light objects. 20 G's on a small bracket that weighs about 0.01 lbm (1" X 3" X 0.032" aluminum bracket) is next to nothing. However, if you accidently bump into it or grab it, the loads will be much, much higher. Small brackets that hold on wiring and other lightweight items are usually under-designed for handling loads and end up causing problems in service because of this lack of forethought.

A good rule of thumb for handling loads are:

1) If you can hook a finger on it = 20 lbf
2) If you can grab it with your hand = 50 lbf
3) If you can step on it = 300 lbf

Adding small flanges onto brackets, or adding an "out of plane" support on a bracket adds very little weight, but makes them orders of magnitude stronger and stiffer. It saves a bunch of grief later on when you accidentally bump a bracket and break it. Especially if that bracket is buried deep in a hard to get to place that makes it next to impossible to repair.

I hope this helps.

Jeff

Loads

Bill and Jeff, thanks for your replies. For clarification, Use as a handhold or bumping is not likely, due to position. I?m a little worried about natural frequency and damping, but wiring on the primary bracket in question - for an electronic ignition ECU - will provide more damping than situation - I hope. Rivet loads are the easy part for this one. I?m pretty sure I?m overthinking this.

A handy way to estimate the natural frequency is to use this equation:

f = (1 / (2 * pi)) * sqrt (g / y)

where

f = natural frequency, Hz.,
g = 386.1 in/sec^2,
y = the one-g static deflection of the system.

It's important to include the effects of every contributor, including the supporting structure, to the flexibility of the system when calculating the deflection. Note that this equation can be used in any direction, and should be, since the deflection will probably differ between them.

Don't worry about damping. It's virtually impossible to quantify for things like this. Assume zero damping.

Dave

BillL said:

....I used 2024 where possible and it was mainly for stiffness, minimal deflection....

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It should be noted that the stiffness of aluminum is within a few percent, alloy to alloy, until it reaches the proportional limit. During yield, the alloys behave differently, and the stress for the onset of yielding (one measure of it's strength) definitely varies from one alloy to another.

Dave