Weight Distribution (RTU Corner Weights, Know Center of Gravity)
Weight Distribution (RTU Corner Weights, Know Center of Gravity)
(OP)
I have an RTU (roof top air conditioning unit) that has a total weight of 8444lbs. I know that the center of gravity is and the overall dimension. I need to figure out what the weight at each corner is. Could anyone tell me how to figure this out? I have attached a document showing the data. Let me know if there is anything else you need. Thanks Alot.
B.
B.





RE: Weight Distribution (RTU Corner Weights, Know Center of Gravity)
the closer the corner is to the CG, the more load it reacts.
P = k/L
P1+P2+P3+P4 = W
k/L1+k/L2+k/L3+k/L4 = W
k = W/(1/L1+1/L2+1/L3+1/L4)
P1 = k/L1, ...
RE: Weight Distribution (RTU Corner Weights, Know Center of Gravity)
rb1957's solution assumes that the RTU is perfectly rigid, which it is not. The method might be "close enought" though.
RE: Weight Distribution (RTU Corner Weights, Know Center of Gravity)
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Weight Distribution (RTU Corner Weights, Know Center of Gravity)
The problem as rb well knows, is indeterminate and dependent on the flexibility of the mounting points and the understructure of the roof.
RE: Weight Distribution (RTU Corner Weights, Know Center of Gravity)
how stiff is the roof structure under each corner ?
how stiff is the body, providing the loadpath from the CG to each corners) ?
how flat is the roof ?
how different is the correct answer from the rough one ?
how big is the safety factor ? (so is the difference significant ?)
RE: Weight Distribution (RTU Corner Weights, Know Center of Gravity)
To be better though you really should consider line loads at the sleepers.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Weight Distribution (RTU Corner Weights, Know Center of Gravity)
RE: Weight Distribution (RTU Corner Weights, Know Center of Gravity)
You need a program that accounts for the mass distribution , the forcing frequencies and their locations.
If I did this , I would put springs at the designated mounting points, write the 3 dynamic equations. Then I would try to eliminate the rocking modes by judicious spring selection. A big problem but depends on the driving forces.
Elimination of the rocking modes would allow one to treat this as a single degree of freedom linear problem and an isolation damper could be found for a single frequency input.
RE: Weight Distribution (RTU Corner Weights, Know Center of Gravity)
if this means the sring stiffness is selected so that the basic rigid assumption (plane sections remain plane) still holds, then i'd've thought that the rigid assumption would still be pretty good.
dynamic response is a whole different problem. maybe people who mount these things for a living could help you better. how will the support springs react to imbalance loads ? to oscillating loads ??
since we've only just heard of springs being involved (maybe they were assumed from the description), maybe we'll also hear "well of course they're damped".
how did you derive your reactions ? is weight the critical load ??
RE: Weight Distribution (RTU Corner Weights, Know Center of Gravity)
Elimination of the rocking modes would allow one to treat this as a single degree of freedom linear problem and an isolation damper could be found for a single frequency input"
Disregard this statement; you can't eliminate the rocking mode since the forcing function is rotational at the motor/compressor.
It is fairly simple to write the linear and rocking equations.
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RE: Weight Distribution (RTU Corner Weights, Know Center of Gravity)
RE: Weight Distribution (RTU Corner Weights, Know Center of Gravity)
If you don't know, don't assume the risk.
RE: Weight Distribution (RTU Corner Weights, Know Center of Gravity)
Then what do you think he's looking for?