Tapped Hole Capacity 1

[TeslaApp]

Hi Everyone,

I am after some help please.

I am currently designing a boltable lifting point which consists of a removable padeye bolting to a frame using 6x M16 grade 8.8 bolts. The bolts go into a 25mm thick S355 steel tapped plate.

Could anyone please clarify the pull out capacity of an M16 thread in a 25mm thick S355 plate? I have read multiple resources and have come across 2 opinions:

1. that as a rule of thumb a 1.5D deep tapped hole will be stronger than the bolt. Or similarly that the thread depth can be increased to ensure the bolt stips before the plate.

2. The first thread sees 37% of the load and anything thicker than around 6 threads makes no difference to the strength.

I have used the formulas within Bickford's book to calculate the required thread depth based on comparing the UTS of the bolt and the strength of the plate. The formulas show the 25mm plate to be strong enough but this falls down when considering the 2nd theory (which Bickford mentions a few pages before the formulas but doesn't go into anymore detail)

Applying the 2nd theory,my calculations show that achieving full preload of 220Nm (75% proof strength, bolt tensile load around 72kN) would strip the threads as one thread cannot support 37% of the preload.

I have come across a commercially available lifting point which uses 4x M16 bolts for a larger load, specifying 240Nm torque and a similar plate thickness.

https://www.liftingsafety.co.uk/product/rud-bolt-on-lifting-points-4730.html

Any help would be greatly appreciated.

 

[jdonville]

If this is to be used for lifting application, then while it might have satisfactory capacity from a structural point of view, there are requirements, at lest in USA, for the design of "below the hook" devices for lifting applications. You can get by with smaller reserve capacity for one-off situations, but if you are intending the system to be used for more general use - even if you don't intend to sell/share the design - then you are in murkier waters AFAIK.

 

[dauwerda]

1. that as a rule of thumb a 1.5D deep tapped hole will be stronger than the bolt.

As long as the the steel plate yield strength is similar to that of the bolt, this sounds like a reasonable rule of thumb.

2. The first thread sees 37% of the load and anything thicker than around 6 threads makes no difference to the strength.

If the first thread is overloaded, it will just yield locally so that load sharing happens between the threads.

Here is a thread length calculator I just came across - it does have the caveat that values are theoretical and should be tested, but it may be a good gut check:

https://www.bossard.com/en/assembly...nverters/thread-engagement-length-calculator/

 

[Tmoose]

What is the nominal load your padeye will support ?

What is your pad eye's thickness and overall size compared to the commercial 4 holer ?

Have you thought about the actual load distribution of the fasteners for each padeye based on thickness of the padeye, and the 25 mm thick plate and its surroundings? Many times plate deflection and prying effects modify the actual individual fastener loading in surprising ways.

As a first cut regarding gussets and etc on machinery baseplates, if fasteners are further away from gussets than the minimum necessary for wrench clearance, my ears perk up and I decide right then the baseplate design needs detailed analysis RE: thickness.

I think using UTS in practically any "strength" calculation of components made of ductile materials will "yield" results that must be kind of suspect.

I also believe that a few tests with a dozen or two of the actual bolts you will be using, and a hunk of the 25 mm thick S355 plate replete with tapped holes, and a beam or dial type torque wrench (NOT clicker) will quickly prove your design, and identify which of the "opinions" is a little too ivory tower for practical design work.

 

[TeslaApp]

Thank you for the replies everyone.

The padeye is being used as a one off to lift a frame into position and won't be used for general lifting use once the frame is installed, however, I want to ensure it is safe to lift at any point.

The loading on the padeye is minimal and includes load factors as well as factors accounting for uneven loading of the lifting points, which there are 4 of. The total load is 40kN

My design is coincidentally very similar to the commercial 4 holed unit in terms of size/dimensions however, my design utilises an additional 2 central bolts for an increased safety factor. In terms of loading on the bolts, I have taken into account the loadings from prying etc.

Taking all of your advise on board I am leaning towards hiding a permanent lifting point within the structure which will be fully welded to code. I'm really safety conscious and don't feel comfortable utilising threaded holes without 100% confidence from a theoretical and practical stand point.

Thanks again.