Eyebolt Strength 3

[T_Mech]

Hello,

This is my first time specifying an eyebolt for lifting. Imagine a plate with a through hole. The eyebolt is inserted into the through hole on one side of the plate and is tightened with a nut on the other side of the plate. Given that we know the thread size and material of the eyebolt and nut, how do we go about determining the eyebolt's strength?

If you torque down the eyebolt, this develops a tension preload on it. It would appear to me that any lifting force on the eyebolt would then increase the tension in the eyebolt in addition to the initial preload. Is the strength of an eyebolt then determined by how large of a preload you initially develop?

 

[JohnRBaker]

First off, does the eye-bolt have a shoulder on it, like this?

If not, you can't really apply any meaningful preload.

But speaking of preload, as long and the nut is tight and if you use a lockwasher or a shakeproof nut:

I don't think the amount of preload is going make all that much difference unless the the preload is really high. In fact, if there was a way to assure that the nut was not going to loosen, either by using lockwashers, shakeproof nut or a second jam nut, I wouldn't put hardly any preload on it. Just tight should be enough.

In the end, if you don't exceed the rating of the eye-bolt you should be OK, but there are some issues with how the load is applied. If your rigging looks like this:

Then the angle that the force is applied comes into play, keeping in mind that rating of the eye-bolt assumes that the load is being applied vertical, or in-line with the axis of the threaded section. As the angle moves from 90˚ (relative to the surface of the plate) the effective strength of the eye-bolt is reduced. And it should never be less than 45˚, as shown in these diagram:

John R. Baker, P.E. (ret)
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[Jboggs]

RULE #1 in selecting eye bolts for lifting: The item description should specifically state that it's rated "FOR LIFTING". That specification will also include the "RATED LOAD", thus eliminating the need for you to make those calculations yourself. If the line of tension will not be parallel to the centerline make sure you understand the details in JohnRBaker's very fine post above.

 

[mfgenggear]

great explanation guys

 

[dik]

Thanks... I've only ever spec'd a single washer; is there a reason for using multiple washers...

A little slow this AM... just realised the threaded portion of the shank may protrude below the bottom of the connection...

If the load, is critical, I use Loctite Red on the threads...

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[T_Mech]

Thank you everyone for your input. Very helpful insight.

One thing I'm still confused by is how the rated loads are determined by manufacturers. For example, take a look at these 1/4"-20 eyebolts:

https://www.mcmaster.com/eyebolts/thread-size~1-4-20/eyebolts-for-lifting-8/

The rated vertical lifting capacity is 500 lbs. For a 1/4"-20 thread with a tensile stress area of .0318 in2 and 42 ksi yield strength (316 SS), the load required to cause yielding would be (.0318)*(42,000) = 1335 lbs. If the preload on the eyebolt is minimal, as has been mentioned above, then the factor of safety would be 1335/500 = 2.6

Is this normally how eyebolt lift capacities are determined? I'd like to understand how the manufacturers arrive at their numbers. Thanks for bearing with me, everyone. I really appreciate all the discussion!

 

[JohnRBaker]

When it comes to lifting anything, I would never consider going with a safety fact of less than two. In fact, I would feel much better at around three or four.

John R. Baker, P.E. (ret)
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[dik]

Generally 3 minimum and often 5 and for mining (down the shaft stuff) 10 is generally used.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[EdStainless]

First of all the min yield strength for 316 is 30ksi, I don't see any spec referred to for the 316 eye bolts.
Second, a safety factor of 3 or 4 is very reasonable, even when personnel safety isn't the issue.
Lifting is hazardous.
I guess third is why SS? Are these outdoors? Near the coast?
If so you will need to be very warry to keep then clean, such as rinse with potable water often.
Salt from the air, rain, fog reaches a few miles inland and can lead to both pitting corrosion and stress corrosion cracking of 316.
We used to use a thick hard washer (not hardware store grade) under the shoulder and on the back side.
We always used self locking fasteners and tightened snug by hand wrench.
There are also swivels that you can use if angles are an issue.

https://www.mcmaster.com/hoist-rings/

These are really rated for lifting.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[dik]

Good link, Ed... thanks.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Jboggs]

Random thoughts:
1. A 2:1 safety factor in situations that might affect human life or limb is, in my opinion, completely unacceptable. You would have to convince me to go with 3:1. I like 4:1 a lot better.
2. Why? Because the real world is not an engineering laboratory. S___ happens. Parts are misassembled, or allowed to corrode. Or Bubba thinks "this extra load will be no problem!" Or the manufacturer (probably in China) received a bad batch of material and used it anyway (I have a particularly painful personal experience involving that scenario). Or the installer screwed up. I was on a work platform suspended beneath a bridge 200 ft above a river when an improperly installed anchor bolt let loose and the whole platform suddenly began swinging back and forth with a dozen guys on it. Strangely, the world really does not care what the engineering textbooks might tell you.
3. Plaintiffs lawyers make a killing convincing average citizens on a jury that "the engineer should have known to expect" this or that completely unanticipated condition.

I could go on. Any questions?

To answer your question about how manufacturers end up with their actual ratings. First, engineering calculations have little, if anything, to do with it. It is purely a matter of test, and retest, and retest, and damage it and then retest, and damage it again and retest just to be safe. Then they hand the results over to their corporate attorneys, and THEY are the ones that decide what ratings will be published.

 

[Joe591]

Normally a eye bolt that is rated for lifting will have the rated load printed on it in some way. something like WLL 1/4 or something like that where 1/4 means quarter tonne. If it doesn't have a clear rating like that you shouldn't use it for lifting and most safety officers will chase you off site if they catch you using them.

the picture shows a eyebolt that is rated for 1/2 tonne

 

[T_Mech]

JohnRBaker & dik: Thank you for recommendations on the safety factor and general eyebolt design insight!

Ed: I was just using SS 316 as an example. There was no other reasons than just for illustrative purposes. But thank you for the information on the corrosion impacts on SS316. I'm just a few months into my Mechanical Engineering career so this is all very useful information that I'll keep in mind and learn from.

JBoggs: Thank you for the personal insight. You were in some gnarly situations, but glad you're ok! I guess on the bright side, you got some stories to tell us youngins . I appreciate the info on how eyebolts are rated. I'm still in the very theoretical/analytical mindset from university, so I'm trying to understand everything from an academic standpoint but real world engineering is quite different. In hindsight, it makes complete sense that you'd want to conduct testing for lifting, which is as you mentioned hazardous, and continually do this until you get a number you feel is safe for the eyebolt.

Joe: Very useful visual. Thanks! This definitely will help with identifying eyebolt specs!

Thank you all, I feel like I've learned a lot from this thread!

 

[geesaman.d]

A lot of wheels being reinvented. Leverage industry standard shouldered eyebolts and spend your time (and legal exposure) on higher value problems.

Use shouldered eyebolts unless circumstances require a straight shank.
For metric, buy a copy of DIN 580 and see if it works for you. Overload factors, deratings for angled loading, and testing regimen are included in the standard.
For inch, buy a copy of ASME A489 for the material and proof testing requirements and ASME B18.15 for the geometric and loading details.

"If you torque down the eyebolt, this develops a tension preload on it. It would appear to me that any lifting force on the eyebolt would then increase the tension in the eyebolt in addition to the initial preload. Is the strength of an eyebolt then determined by how large of a preload you initially develop?"

The tension is present in the bolt whether it's coming from external tensile load or screw preload. As you add external working load, it's subtracted from the screw preload. Just like (almost*) any other bolted connection, you should have more preload than working load in all cases, or else the shoulder loses contact and the joint is outside of the design limits. If you observe the documented load limits and torque the lifting eyebolt to a reasonable spec, there will be a significant margin between max working load and overcoming the preload.

*The only place I've seen where bolt tension is additive to the tensile load is embedded / adhesive anchors installed without a nut supporting the anchored component.

David

 

[kjoiner]

Tmech wrote:
"If you torque down the eyebolt, this develops a tension preload on it. It would appear to me that any lifting force on the eyebolt would then increase the tension in the eyebolt in addition to the initial preload. Is the strength of an eyebolt then determined by how large of a preload you initially develop?"

geesman.d wrote:
"The tension is present in the bolt whether it's coming from external tensile load or screw preload. As you add external working load, it's subtracted from the screw preload. Just like (almost*) any other bolted connection, you should have more preload than working load in all cases, or else the shoulder loses contact and the joint is outside of the design limits. If you observe the documented load limits and torque the lifting eyebolt to a reasonable spec, there will be a significant margin between max working load and overcoming the preload."


Screw preload and external load seems counterintuitive at first. This is one of the best examples I've seen to explain preload vs external load. Someone posted this in another thread. I don't recall who, but they deserve the credit.

 

[MintJulep]

For your equipment and your application calculate your load at each lifting location.

Calculate the loads that will occur if one of the lift points fails.

Multiply that by wherever safety factor makes you comfortable.

Select a lifting eye from a reputable manufacturer with a WLL greater than that, derated for angle if needed.

Lifting eye are not that expensive. No reason for price being a deciding factor.