Lift Analysis of a Cabinet 2

[kbujuru]

Hello All,

I am trying to determine if the cabinet top plate will fail/rupture when lifted using the available 4 lifting eye-bolts. The weight of the cabinet is 1200 lbs and vendor supplied eye bolt has a load capacity of 500 lbs each.

I am not sure how to proceed with the analysis? Should I be considered this as a "rectangular plate simply supported on 4 edges with 4 point loads acting normal to the plate"? please advise

thanks

 

[kbujuru]

here is another picture of the sectioned cabinet.

 

[kbujuru]

Also, please note that the top plate is stiffened along the edges with brackets. I am not sure how to consider this bracket's effect on the analysis? please advise

 

[desertfox]

Hi

Were the lifting lug positions recommended by the cabinet manufacturer?

If you assume the lifting cables are vertical then the top plate will be in bending with the mass of the cabinet acting as a udl between the reactions of the lifting eyes, however if the lifting cables are not vertical then the top plate will be subject to a compressive load in addition to bending loads.
Bear in mind this is highly theoretical because if the centre of mass of the cabinet is not equally spaced between the lifting eyes then the forces in each lifting cable will be different and the problem becomes statically indeterminate.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[kbujuru]

Desertfox,

thank you for your time in responding to my query.

Yes, the lifting lug positions are recommended by the cabinet manufacturer. Also, the lifting cables will be not vertical leading to compression as well as bending of the plate as you have identified. Assuming that the center of mass of the cabinet falls at the center equidistant from all lifting points, what should be my boundary conditions of the plate?

Also, if you could direct me to a reference where these type of analyses are addressed that would be really helpful.

thank you

 

[drawoh]

kbujuru,

Your second picture is the same as the first.

Work up a list of failure points.

[ul]
[li]One of more lugs fail. Assume your load is not equally distributed.[/li]
[li]The sheet metal fails in shear around one or more lugs. Again, assume the load is not equally distributed. [/li]
[li]The sheet metal top fails around the edges. [/li]
[li]How are things attached inside? Could the sheet metal bottom fail around the edges?[/li]
[li]???[/li]
[/ul]

Your lifting chains, lugs, your box and whatever is inside are a chain that will fail at the weakest link.

--
JHG

 

[desertfox]

Hi kbjuru

Have a look at this link:-

http://www.lift-all.com/calculations/four_leg

Now if these are recommended lifting points by the cabinet manufacturer then you maybe worrying needlessly because they are part of the OEM design.
Does the manufacturer give a recommended lifting method for this cabinet with a safe working load? If so follow that procedure.

If however you simply bought the cabinet empty and filled it with your equipment then I guess that's a different matter however the cabinet maker may still be able to advise you on lifting.

Without seeing the cabinet in much greater detail it's hard to advise, that said I would start by looking at the amount of thread engagement and making sure that there was enough to cover the amount of thread on the lifting eye, also if your slings are at 45 degrees to the lifting eyes then the tension in the sling will be of the order 424 pounds which doesn't seem much of a margin to me for lifting eyes rated around the 500 pound mark.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[gruntguru]

A 500lb lifting eye should be OK to use at up to 500lb. The safety margin is built into the 500lb rating.

The eyes are pretty close to the corners of the cabinet. If the thickness of the lid is adequate to prevent failure at the eye itself, it will no doubt be sufficient to avoid a bending failure considering the short distance to the load (walls of the cabinet.) I am assuming the walls are fixed to the entire perimeter of the lid.

A conservative simplified analysis might consider one long edge as a beam made of 90 deg folded sheet metal with 50% of the total weight as a distributed load.

je suis charlie

 

[kbujuru]

Thank you all for your responses

Drawoh,

I have added a better picture that explains how the lifting lug interfaces with the cabinet. Please see below my responses to your comments
1."One of more lugs fail. Assume your load is not equally distributed."- By the way how this cabinet is going to be handled, I believe it is safe to assume that the lifting lug doesnt fail given its load bearing capacity is 500 lbs ( -lug has intrinsic conservative factor of safety)
2."The sheet metal fails in shear around one or more lugs. Again, assume the load is not equally distributed"- With regards to this, the question is what should be the shear area that needs to be considered for determining the shear tearout strength of the sheet metal around the lifting lug
3."The sheet metal top fails around the edges"- This is my idea regarding how this engineering problem should be constructed. Considering the top plate as a beam, the left most edge of the top plate is fixed, with a transverse loads acting at a distances x (first row of lugs) and y (Second row of lugs). Determine the max. bending stress in the plate using Euler beam theory, and compare with the max. yield stress of the steel to determine if it fails under given load. Please let me know if my idea is flawed
4.The picture attached will give you a better idea of how the lifting lugs are attached to the cabinet
5. My objective to just determine if the sheet metal fail/ rupture under the given load.

thank you

 

[kbujuru]

Desertfox,

Thank you for the response.

This design is not completely OEM and hence an attempt to validate our design, specifically this lifting problem.

Hopefully the linked pictures in the above post and the one in this will provide you a better picture of the problem. Assuming that the thread shear stresses in the lifting bolts are under the allowable limit,how should the problem of sheet metal tear out be formulated? Also, please see my comments in the post above to provide your input on my design methodology.

thanks

 

[kbujuru]

Gruntguru,

thanks for the response. I was wondering about how should I formulate the engineering problem to determine whether the lifting lug will tear out of the cabinet or otherwise? Your feedback is sincerely appreciated

thanks

 

[desertfox]

Hi

Have a look at this link on page 25 it gives load ratings for different angles for lifting eyes, at 45 degrees the load rating for the eye bolt should have its vertical load rating reduced by 75% which means the 500lb rated eye bolts for your lift aren't really man enough.

On looking at the pictures you posted I would analyse just the angle bracket on one side of the cabinet assuming that it's a cantilever beam with the loads imparted on the horizontal leg of the angle, I must confess I believe those lifting points are purely for lifting an empty cabinet and not a kitted out one, another option might be get the cabinet on a pallet and fork lift it to where you want it.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[gruntguru]

From a quick glance at your first sectioned drawing I would guess the tear out strength of the lifting bolt (correctly tightened) at significantly more than 5,000 lb. Not bad for a 500 lb lifting eye.

Sorry for the lack of accurate calculations, but I don't think tear out will be the failure mode.

je suis charlie

 

[desertfox]

Here's the link that I should of posted earlier page 25:-

http://energy.gov/sites/prod/files/2014/01/f6/HoistingRigging_Fundamentals.pdf

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[boo1]

consider using a spreader bar, to keep all the lifts vertical

 

[BUGGAR]

Actually that would be a spreader frame since the cables can tilt in two planes.
If the load comes from the cabinet hanging below why not just attaché the eyes to the side skins that actually support the load and leave the top plate alone (assuming the cabinet loads come from the cabinet floor and not just from the top of the cabinet)?

 

[boo1]

spreader frame..

 

[tbuelna]

A dedicated lift fixture that is fastened to the cabinet upper corners and does not produce any off-axis loads on the cabinet attachment locations is a good idea. But this requires the technicians to always use the lift fixture when removing/replacing the cabinet.