×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Bolt shear in overhead lifting application

Bolt shear in overhead lifting application

Bolt shear in overhead lifting application

(OP)
Hi,

I am currently designing a hook that is going to be attached to a lifting beam we just purchased. The beam is rated for 2000 pounds, and we have to lift up to 600 pounds total (2 hooks, therefore 300 pounds per hook).

Attached is a drawing of the current design.

Because of the thickness of the hook, there is a gap between the bracket that attaches the hook and the bracket that is welded at the end of the beam.

I know how to calculate the shear in the 3-bolts section of the system, but how do i take into consideration the 1/8'' gap there is on both ends of the beam bracket.

The hole in the beam's bracket allows a 3/8'' screw to go thru. I was thinking of using a 3/8'' shoulder bolt as shown on the drawing, and two 1/8 (or a little less, it has to be able to swivel a bit) steel spacers to fill the gaps.

**Please note that you do not see the full beam in the drawing. Only the end plate of the beam which the hook is attached to is shown.

Thank you for your time.

RE: Bolt shear in overhead lifting application

I think that the old-timers referred to that as combined stress loading.  Your spacers will not do much to alleviate bending moments. I would increase the thickness of the middle eye plate to reduce the bending moment.  You might also look at the combined stress on the bolts to see if it is acceptable.

RE: Bolt shear in overhead lifting application

(OP)
Thanks for the reply,

i can't modify the center bracket, and i am aware that the spacers won't help.

i rapidly calculated using Beam 2D 69 MPa bending and 10 MPa Shear, then using Von-mises (sigma_bending^2-sigma_bending*sigma_shear+sigma_shear^2)^1/2 gives 75 MPa

do you believe this approch is conservative enough?

(used E=210 GPa for the calculation)

Thanks,

 

RE: Bolt shear in overhead lifting application

Hi, several comments...

1) What did you assume for your bending stress? Have you assumed a fixed-fixed beam? I wouldn't because unless the bolt is an interference fit into the side plates it will provide little to no moment restraint for the very small bending deflections the bolt will be seeing.  I would assume as a first pass, a pinned-pinned beam with end supports mid-way through the side plates, and a central point load. If that is no good then start working your way back to remove conservatism.

2) Remember to include a dynamic magnification factor on your lifted load unless you can demonstrate otherwise (this would involve consideration of your crane lifting torque, flexibility in the ropes, etc, to calculate an acceleration). As a starting point I would suggest you use a factor of 2, and if that is no good then try to prove it is lower.

3) Have you thought about using a standard to guide you and give some confidence in your design substantiation? BS2573 (British crane design code) is quite good (it is in process of being replaced by a new Eurocode, but hasn't been fully replaced yet, and is a lot briefer than the Eurocode!).

Hope this is some help,
Pete.

RE: Bolt shear in overhead lifting application

Hi, a couple of other comments...

4) Remember to check bearing stresses on the inner beam, outer plates, and the bolt. As a quick check I usually assume half the projected areas.

5) Finally, don't forget fatigue!

Cheers,
Pete.

RE: Bolt shear in overhead lifting application

Typical, I just remembered one other thing as I hit reply :)

6) I'm not sure what country/industry you are in, but in my experience any lifting equipment must be proof tested with recorded evidence of this. Usually we try to aim for 2x maximum design load. So in this case, with an impact factor of 2, that would be to 4 x 300lb. This may be too high for your component, in which case set a lower limit that is sensible. Could be 20% above, 50% above, whatever makes sense for proving the integrity.

Pete.

RE: Bolt shear in overhead lifting application

Isn't this just a typical lug/clevis problem? When evaluating such a problem, the gap often causes significant bending in the pin. The methods for this sort of problem are well documented, though do not always agree amongst the various sources. ESDU has a pretty good solution.

Some simple conservative assumptions could be used by combining pin bending and shear stresses along with pinned-pinned assumption.

To relax some conservatism, you can consider:

- plastic bending of the pin at the ultimate load.
- something "better" than pinned-pinned depending on the member stiffnesses and contact distribution (see the standard methods)
- the location of max bending may not occur at the location of max shear and therefore the combination of the max's may conservative.

Brian
www.espcomposites.com

RE: Bolt shear in overhead lifting application

I'd be careful about stating combined pinned-pinned bending and shear as conservative. These problems have very complex stress fields and are very sensitive to the geometry and material properties. The peak stresses are likely to be significantly higher than those predicted by the hand calc. I think Roarks has some discussion about how the hand calcs for shear become increasingly unconservative for shorter deeper beams. However, I'd say that the accepted methods of combined bending and shear are reasonable provided you can demonstrate decent elastic margin, if not you are into the territory of FE analysis. Also if you go down that route you would be slightly on your own in terms of defining stress limits, because codes/standards have not really caught up with the use of FE. Also if you do FE you will likely find some results which are hard to ratify against the elastic intent of a lot of codes/standards, ie you are likely to predict localised plasticity which the codes/standards don't allow for really. Pete.

RE: Bolt shear in overhead lifting application

I am sure it is different for every industry, but the aircraft industry is well aware of how to treat the problem. Since I have not heard of any failures, I assume it works pretty well.

Of the public resources, ESDU 91008 can be considered.
http://www.esdu.com/graphics/dataitem/di_91008.htm

"The consideration of the effects of pin bending in a double shear joint is also included."

Brian
www.espcomposites.com

RE: Bolt shear in overhead lifting application

Another reference is Niu's "Airframe Stress Analysis and Sizing", p.327.  

Brian
www.espcomposites.com

RE: Bolt shear in overhead lifting application

From the wording in that ESDU web page 'failure by rupture' is it coming at this from a limit load perspective? (I've not got access to the document). If so, it gets tricky trying to use this philosopy when the code/standard/industry accepted targets are based on significant margins against yield, and S-N based fatigue.

Also I don't agree with your logic that because you haven't heard of any issues that the methods we are using are working well. I think in many cases the benign plastic behaviour of materials like steel and aluminium protect us from design mistakes and lack of understanding. I think we should always recognise that our methods are mainly based on empirical observations, that we don't understand fully the underlying physics, and act with appropriate caution.

Cheers,Pete.

RE: Bolt shear in overhead lifting application

(OP)
Hi, thanks for all the replies.

badbunny:

1)  Attached is my FBD. It is pretty much what you recommended.

2)  Security factor used will be 4 so that it includes impact.

3)  We have cranes built exactly like this one (products that come from Italy), they are used to lift 9000 pounds (lot bigger than what i am lifting). The gap between the center bracket and the two side brackets is 2.5" (1.25 per side approx. but this has to be done because they have to adjust the position of the crane). That being said, the pin used on those is 1.5" diameter.

4)  I did check bearing stress on the top part of the outer plate as it is the most critical part of the design. The center plate is part of the certified 1 ton beam so i don't have to calculate it.

5)  I do not know what i could do about fatigue in this situation. I might consider it in the 4-5 S.F. The number of cycles is not huge.

6)  I am in Canada. To make sure everything is safe, i am doing the calculation 1- Manually, 2- Mathcad, 3- Beam 2D and then 4- ANSYS.

ESPComposites:

Thanks for the reply. so Von-mises is appropriate to combine both just like i mentionned in my previous post?

badbunny :

Your suggestion is exactly what i had in mind, but i am doing the calculation with the gap between the brackets to be conservative as i do not want to have to take 2 thickness and the radius/chamfer in the calculation.

Thanks for your help.

 

RE: Bolt shear in overhead lifting application

HI, von mises is fine to combine the shear and bending, just have in the back of your mind that the average bending and shear calculations are a gross simplification of the actual stress field which is complicated, and that if you are getting close to the limit you should probably redesign or move to a more complex analysis (ie FE). With that in mind though, if you can proof test to 4x then that test and subsequent inspection should give plenty of confidence.  Cheers, Pete.

RE: Bolt shear in overhead lifting application

Pete,

I am sure we could debate this forever, but the aircraft industry has developed many tried and true methods. The approach is to develop a method, with a physical basis, as demonstrated in the references provided. The method is then usually compared with lab test data. Once implemented onto the aircraft, it is continually monitored and inspected to make sure the behavior is as expected. If there is a failure, then it is investigated to determine where the breakdown may be. Due the nature of the safety requirements and passenger expectations (i.e. don't kill them), the process is required to be highly reliable. I can't give you an exact date, but this particular method has been around for decades. I suspect you won't find anything more comprehensive than this, in any industry.

While you are correct in saying it is not "exact", very few problems in engineering are ever exact. But then again, that is the point of the engineer...to solve the problem without knowing every inconsequential detail.

Again, being from different industries, the approaches may be different. But this is not "my" logic, it is the logic of the aircraft companies.  

Brian
www.espcomposites.com

RE: Bolt shear in overhead lifting application

Hi Brian, my comment wasn't about the logic of this method (which I use and don't have a problem with), but the optimistic/unrealistic view that because we've not seen any failures to date means there is no problem with our methods. Maybe I'm just becoming cynical with old age though! :) Pete.

RE: Bolt shear in overhead lifting application

Your condition is double shear with bending.  The 1/4" gap causes the pin to have bending moments.  Depending on the amount of clearance at the pin holes, it can be modeled as pinned or fixed.

Check both shear and bending, then use a unity check for the two of them.

RE: Bolt shear in overhead lifting application

Pete,

I understand what you are saying. There have times when I have wanted to change methods because they could be "better" - at least in theory. But the problem is then you may have to start the test cycle over again, which could take years/decades. So even if it may not be as good as it could be, is it worth the redo? Usually we just say "if ain't broke, don't fix it".

Being cynical is OK too. But if we look at failures, I think we will find that they are far more often due to misinterpretation of the method, rather than shortcomings of method itself (provided it is tried).

Sorry to get off track! I just wanted to point out that this is a relatively common problem, with known solution approaches. You can make some conservative assumptions, which are fine. If you want to relax some of the conservatisms, then there are ways to go about that, as cited early.

Brian
www.espcomposites.com

RE: Bolt shear in overhead lifting application

Chronik,

If you haven't already read through ASME BTH-1 and your provincial OH&S code.  For example Alberta requires a 5:1 design on ultimate breaking strength.

RE: Bolt shear in overhead lifting application

ASME B30.20 requiring a minimum design factor of three of the yield strength of the material.  ASME BTH-1–2005, Design of Below-the-Hook Lifting Devices, includes design criteria, including a lower design factor, and the necessity to address other failure modes such as fracture, shear and buckling, and design topics, such as impact and fasteners. your safety factor or 4 is within the guidelines of ASME BTH-1–2005.

There are several design elements to check, including bearing, tearout, plate strength.  It appears from you model there is not much material depth in the single plate, radially from the pin.  Consider the check:

3-3.3 Pinned Connections gives Static Strength of the Plates
Pt = (Fu/1.20Nd)2tbeff

 

RE: Bolt shear in overhead lifting application

An interesting problem, and a classical structural engineering problem.  There are textbooks that deal with combined loading on bolted connections, I would be surprised if you couldn't find one very similar to yours.

You need to look at the bolting as a patterned system and find the loading through each bolt, they do not carry identical loads.  That is obvious from just the statics.  You also need to determine "maximum lifting acceleration" in order to do the analysis under worst case.  Remember that acceleration is like gravity, the division of the two numbers would give you a weight multiplier for your analysis.

As far as the gap, I would over bore the holes slightly and sleeve the bolts to the required length.  Remember to leave a little short so that when doing the make-up torque, the bolts will squeeze the piece slightly and you can achieve the recommended make-up.

An interesting design, the lack of geometric detail leaves much to the imagination and impossible to complete the mathematical model that details your situation.  But good luck with it.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

RE: Bolt shear in overhead lifting application

While spacers will not alter the bending stress in the shoulder bolt, they will keep the load centered.  Without the spacers the load WILL shift to one side or the other. This is not the problem that is being analyzed.
I would consider reaming the holes in the side plates if you are very concerned about localized stresses.  
Consider a larger diameter bolt... I realize that the fun is in the details... but a larger diameter bolt will usually solve the problem.

RE: Bolt shear in overhead lifting application

"The hole in the beam's bracket allows a 3/8'' screw to go thru...."  With that statement, you are flirting with mishap.  

You have three (3/8)bolts on the end of the lifting hook and you want to use only one 3/8" bolt on the link bolted to the hook as shown on your original drawing.  Something does not jive.

RE: Bolt shear in overhead lifting application

Not only is there 3 bolts vs 1 to take the same load, but the design with the one bolt wil also introduce bending, which avoid in the 3 bolted connection. Can you increase the lug on the crane end to give room for more bolts?

Also, be careful with swithcing to a more refined methods like FEM. I can not see how a FE analysis would give you more information than you can get with a simple hand calculation. This is a simple geometry. FEM will help you if you have a complex 3D geometry. The challenge of interpreting complex FE results on a simple geometry is not straight forward.

(Unless you are in research and not in the business of solving practical problems.)

RE: Bolt shear in overhead lifting application

Can anyone point me to a European counterpart of the ASME BTH-1 spec? Preferably British, but other EU will also do.

I'm looking into a lifting eye that is bolted to a machine, where the bolts provide a friction transfer of the load.

In ASME BTH-1 the maximum allowable load is n*0.26*A*Fu / (1.2*Nd) where n is number of bolts, A is bolt section area, Fu is ultimate allowable stress of the clamped parts, and Nd is the safety factor. I don't understand where the 0.26 originates from. (I expected it to be 0.24/0.36/0.48, from a friction coefficient of 0.1/0.15/0.2 and a bearing allowable of 2.4*Fu).

Background:
According to ASME we need 4 bolts to transfer the load, but the engineer says there is no room (always the same excuse) and "we have always used 3 bolts". I told him no deal, so now the project manager wants to know if the American standard is applicable <sigh>.

 

RE: Bolt shear in overhead lifting application

iceblink,you should start a new thread as you are on a different subject and get rid of this post.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources