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.
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
RE: Bolt shear in overhead lifting application
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
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
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
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
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
RE: Bolt shear in overhead lifting application
RE: Bolt shear in overhead lifting application
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
Brian
www.espcomposites.com
RE: Bolt shear in overhead lifting application
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
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
RE: Bolt shear in overhead lifting application
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
RE: Bolt shear in overhead lifting application
Check both shear and bending, then use a unity check for the two of them.
RE: Bolt shear in overhead lifting application
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
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
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
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
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
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
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
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
RE: Bolt shear in overhead lifting application
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