modelling slings in lifting analysis

[Syahzada]

Suppose i want to analyze on a finite element software, a steel frame lifting with 4-sling configuration using 3 in. width synthetic slings without me having the specific elastic modulus of the material. One of the output that i want from the analysis is to check the tension load of the slings have met the sling capacity or not. My question is:
1. For the sling model, i've modelled it as a rod/tube with the diameter is exactly at 3 in. as per specification. In anyway does it affects the result of the load if i model the sling with any other dimension value?
2. Should i specifically use the spesific elastic modulus for the sling member properties such as Nylon, Polyester, HDMPE, (about 200-600 ksi) or can i treat it the same as steel (29000 ksi)?

 

[rb1957]

why use FEA (when a napkin/serviette will do) ?

All 4 slings are the same, so stiffness isn't an issue.
Deflection isn't an issue (too small to affect the angle of the sling).

No?

sling load is W/4/(sin(theta))*SF ... no?

 

[Syahzada]

but if the object geometry is not as simple as a "box", so the length of the slings is differ between each (+- 3 meter). How do i calculate the skew or the uneven distribution of the slings?

 

[3DDave]

The elasticity of the sling depends on the weave of the material and the way it is assembled and is a complex thing to analyze. This is mostly handled by test measurements to determine the spring rate of the finished item rather than any prediction that would require modeling 10,000 different fibers in complex paths including friction and restrictions from interactions between the fibers.

It is generally the policy to assemble slings so that there are no options where elasticity makes a difference.

Here's a document covering spring rate; note that it is not a constant for synthetic fiber slings: https://apps.dtic.mil/sti/tr/pdf/AD0747814.pdf There are several charts for different materials and specific constructions.

The following is in that document:

Spring Rate
Spring rate for some textile materials are given in Figures 34 through 40.
Consult the material manufacturer for spring rates of other textile materials.

I would add that slings degrade from use and modeling textile damage is even more difficult.

 

[Ron247]

The Factor of Safety [FOS or FS) on lift equipment is much higher than most other common structures. Hand calcs are probably accurate enough unless you are just practicing FEA modeling.

First, always model as close to the real thing as possible. If the E is somewhere between 200 and 600, why jump to 29,000 when you know it is not remotely possible. Check it at 200 and 600. Magnitude of E affects elongation. Elongation affects angle, angle affects strap load. But is a longer strap length good or bad? Think out your deformations. The ends of the 4 slings are not going to change much unless your "box" is a wobbly box but your length can change.

I am assuming your lifted load is somewhat consistent in how it is dispersed or is one end of the box a lot heavier than the other. If so, model it that way.

Suppose i want to analyze on a finite element software, a steel frame lifting with 4-sling configuration using 3 in. width synthetic slings

You don't lift using "width", you lift using Area. What is your true area?

1. For the sling model, i've modelled it as a rod/tube with the diameter is exactly at 3 in. as per specification.

What do you mean "per specification"? 3" diameter rod is a lot more Area than a 3" wide sling if that is where the 3" number came from.

 

[MintJulep]

You absolutely need to know the elasticity of the sling (not the material it's made of), or at the very least have reasonable upper and lower bounds for it.

It's fine to model the slings as rods using the sling elasticity as e. You can probably get away with setting the diameter as needed to keep the stress well inside the elastic limit. On the other hand, "unit area" could make subsequent steps easier.

A few trial and error iterations will tell you if your geometry is overly sensitive to the sling elasticity or not, and what design changes you need to make if it is. For the reasons hinted at by @Ron247.

Then you can directly compare the force in the rods to the rated capacity of the sling.

Pay attention to where factors of safety are already built into whatever information you're using.

 

[Syahzada]

The elasticity of the sling depends on the weave of the material and the way it is assembled and is a complex thing to analyze. This is mostly handled by test measurements to determine the spring rate of the finished item rather than any prediction that would require modeling 10,000 different fibers in complex paths including friction and restrictions from interactions between the fibers.

It is generally the policy to assemble slings so that there are no options where elasticity makes a difference.

Here's a document covering spring rate; note that it is not a constant for synthetic fiber slings: https://apps.dtic.mil/sti/tr/pdf/AD0747814.pdf There are several charts for different materials and specific constructions.

The following is in that document:



I would add that slings degrade from use and modeling textile damage is even more difficult.

thanks for the reference Dave! appreciate it

 

[Syahzada]

The Factor of Safety [FOS or FS) on lift equipment is much higher than most other common structures. Hand calcs are probably accurate enough unless you are just practicing FEA modeling.

First, always model as close to the real thing as possible. If the E is somewhere between 200 and 600, why jump to 29,000 when you know it is not remotely possible. Check it at 200 and 600. Magnitude of E affects elongation. Elongation affects angle, angle affects strap load. But is a longer strap length good or bad? Think out your deformations. The ends of the 4 slings are not going to change much unless your "box" is a wobbly box but your length can change.

I am assuming your lifted load is somewhat consistent in how it is dispersed or is one end of the box a lot heavier than the other. If so, model it that way.

You don't lift using "width", you lift using Area. What is your true area?

What do you mean "per specification"? 3" diameter rod is a lot more Area than a 3" wide sling if that is where the 3" number came from.

1. The actual sling is made of synthetic webbing, which comes in plies. Because of this, I'm unsure how to determine or calculate its true area. However, I want to understand how to model such slings, especially for more complex lifting configurations that utilize synthetic web slings.

2. The actual specification as mentioned in No. 1, Which is 1-ply 3" width synthetic webbing. I do thought it maybe somewhat acceptable to model the rod diameter as the width of the 3" sling as per specification. But yeah as you've said i think i got it wrong.

 

[Ron247]

1. The actual sling is made of synthetic webbing, which comes in plies. Because of this, I'm unsure how to determine or calculate its true area. However, I want to understand how to model such slings, especially for more complex lifting configurations that utilize synthetic web slings.

If you have access to it, you can measure it. If you have data for the same material in a different shape such as a solid round, you could estimate from there. If you had load data for a 3/4" diameter solid for example, you could determine its unit stress under load. Then ratio the 3" strap load to the 3/4" diameter load and apply that to the area of the 3/4" round to obtain a reasonable estimate of area.

 

[rb1957]

This is not my "wheelhouse" so I'll defer to other's opinions. I think FEA is overkill for this, the Safety Factor required will swamp any small load variation due to strap stiffness (IMHO). I assume it is a single point lift, so the lift and the load are a two force member, the load CG will be under the load line of action. If this means the load will shift (not a good thing, IMHO) then it will. It would be a good idea to include some means of adjusting the position of the lift so you can align with the CG.

 

[Syahzada]

You absolutely need to know the elasticity of the sling (not the material it's made of), or at the very least have reasonable upper and lower bounds for it.

It's fine to model the slings as rods using the sling elasticity as e. You can probably get away with setting the diameter as needed to keep the stress well inside the elastic limit. On the other hand, "unit area" could make subsequent steps easier.

A few trial and error iterations will tell you if your geometry is overly sensitive to the sling elasticity or not, and what design changes you need to make if it is. For the reasons hinted at by @Ron247.

Then you can directly compare the force in the rods to the rated capacity of the sling.

Pay attention to where factors of safety are already built into whatever information you're using.

got it, thanks for the input MintJulep!

 

[Syahzada]

If you have access to it, you can measure it. If you have data for the same material in a different shape such as a solid round, you could estimate from there. If you had load data for a 3/4" diameter solid for example, you could determine its unit stress under load. Then ratio the 3" strap load to the 3/4" diameter load and apply that to the area of the 3/4" round to obtain a reasonable estimate of area.

Ok i think its clear enough for now, thanks for the input Ron247, very helpful!

 

[Syahzada]

This is not my "wheelhouse" so I'll defer to other's opinions. I think FEA is overkill for this, the Safety Factor required will swamp any small load variation due to strap stiffness (IMHO). I assume it is a single point lift, so the lift and the load are a two force member, the load CG will be under the load line of action. If this means the load will shift (not a good thing, IMHO) then it will. It would be a good idea to include some means of adjusting the position of the lift so you can align with the CG.

Alright rb1957, I just want to understand the basic theory and practical aspects of modeling lifting analysis using FEA, but i do think maybe this problem should have been made more simple using said method, anyway thank you for the input.

 

[Syahzada]

The elasticity of the sling depends on the weave of the material and the way it is assembled and is a complex thing to analyze. This is mostly handled by test measurements to determine the spring rate of the finished item rather than any prediction that would require modeling 10,000 different fibers in complex paths including friction and restrictions from interactions between the fibers.

It is generally the policy to assemble slings so that there are no options where elasticity makes a difference.

Here's a document covering spring rate; note that it is not a constant for synthetic fiber slings: https://apps.dtic.mil/sti/tr/pdf/AD0747814.pdf There are several charts for different materials and specific constructions.

The following is in that document:



I would add that slings degrade from use and modeling textile damage is even more difficult.

oh and btw Dave, in some of the figures it said on the curve there are differences in CPS, what does the CPS mean? i have been examining the reference but i can't find it. Is it the amount of plies?

 

[Ron247]

CPS could possibly mean "Coated Polyester Sling" or "Continuous Polyester Sling" but like any acronym, you are at the mercy of the creator of it. With my luck, it could also mean "Can't Predict Safety" AKA "Don't Get Under It"

 

[Ron247]

I just want to understand the basic theory and practical aspects of modeling lifting analysis using FEA

It generally helps when everyone knows what your end goal is in terms of the responses you get. The advice for actually lifting the frame is different than the advice for learning to more accurately use FEA.

Also, there is a huge difference in someone telling you all the technical parameters to input into a FEA and reviewing a real situation or existing situation and determining how to most accurately model it. FEA scares me when I feel someone is relying on FEA for their answers and not their own ability and judgment. FEA aids you in looking at something in more detail PROVIDED you have an instinct about what is going on in general.