Bronze thread strength .

[Bill Perkins]

I need to know the withdrawal strength of 5/8th bronze coarse all thread rod per inch of insertion in a threaded female fitting .Any ideas on where to look ?

[John E Hardiman]

5 thread engagement will give the full strength of the fastener provided the internal thread is the appropriate class for the external thread. Deeper engagement get you nothing except more resistance to vibration back out.

[mmd]

I agree with John's comments. As for the load-bearing strength of the rod, there are probably tables somewhere that provide this info, but from first principles, via Machinery’s Handbook, Ed 26 …

Determine:
Major (nominal) diameter (d)

Calculate:
Pitch (P) = 1/n
Where n = threads per inch (5/8” Unified National Coarse = 11 TPI)

Pitch diameter (dp) = d - 0.649519 x P
Minor diameter (dm) = d – 1.299038 x P
Thread tensile-stress area (As) = (Pi / 4) x ((dm + dp) / 2)^2

Determine:
Metal tensile strength (T)
Silicon bronze = 90,000 psi

Calculate:
Maximum allowable load (Lmax)
Lmax = As x T

Safe working load (SWL)
Apply safety factor (Sf) Varies from 1 for unimportant fastenings (coat-hook screws) to 2.5+ for important things (keel bolts, safety harness fittings).
SWL = Lmax / Sf

NOTE: the above calculations are only for loads in line with the rod axis. They do not allow for side (bending & shear) loads nor cyclical loads, which will reduce the load-bearing strength of the rod by an amount relative to the severity of these loads. If your or somebody else's safety depends on the results of your calculations performed from these formulae, please have your calculations checked by a qualified mechanical engineer or technologist! I can't take responsibility for any errors or omissions in these formulae, or the results derived from them.

[Bill Perkins]

Thanks for your reply John [Edited to add: And MMD! ]. I got a secondhand fitting for a lifting eye from an old CrissCraft . This is a 5/8th in. female fittting that bolts to the top of the keel with 2 3/8th in. through bolts . The all thread I bought fits well ,but I don't know the "class" of thread .What should I ask for ? My hope is to match the strength of the two 3/8ths bolts .I need about a thousand lb. working load .

[ 11-10-2004, 02:34 PM: Message edited by: Bill Perkins ]

[John E Hardiman]

Most threaded rod you buy is class 2A (1 is the loosest fit, 2 is a running fit, 3 is tight, 4 is interference, A is external, B is internal...so normaly you would use a 2A bolt in a 3B nut for "normal disassembly" applications, and 3A in 3B for "machinery" and "structural" applications where you will not be taking it apart often
). But I don't know what the internal threads would be. The more it has been used, the more wear and the looser the fit. The rod should screw into the eye freely, but should not "rattle". If it is too loose, things start to get iffy as the threads now fail in shear before the body fails in tension.

As mmd pointed out, you need to consider both the axial and shear loads. Tables in the Machinists Handbook and other places give safe load for a 5/8-11 80K tensile as ~1800 lbs in any direction which is conserative.

[Bill Perkins]

That sounds great . I won't need to disasemble this ,so I'm thinking I'll dip the rod in epoxy before screwing it home , not that the fit is loose.This setup will connect to a lifting eye on the fordeck ; the transom will be lifted by 2 big eyebolts .

Since the CrissCraft had an inboard engine to lift and I'll just have an outboard perched on the transom ,I figured this forward fitting would be adequate for my 24 foot open boat, but I did want to run some numbers .At Dave's suggestion on this forum I actually purchased an old Machinery's Handbook ( 15th edition ), but I was unable to extract the formula MMD has shown .Again thanks for all the help .I'll do a test lift with significant extra weight on board .

[ 11-10-2004, 03:38 PM: Message edited by: Bill Perkins ]

[Stiletto]

John, does the five thread engagement apply to all sizes?

[John E Hardiman]

Originally posted by Stiletto:
John, does the five thread engagement apply to all sizes?Generally yes for coarse threaded bolt/nut materials of "normal" sizes and classes with similiar yield strength/modulus (there... is that couched enough? ;) ). Most heavy hex nuts are about 5 threads thick which is about the diameter of the bolt. If using steel in Al or CI then you would need to work out the engagement. The reason for fine threads is not a strength issue, but rather a vibration one....they are harder to install (more torque)which makes them more likely to self lock and less likely to back out.

[Gary E]

I would not agree on 5 threads.. use 1 and 1/2 body Dia as a Minimum. You are lifting dead weight, extra thread length is insurance it wont fall. Take a look at machinery Eye Bolts, for an idea of how much thread is used.

For a 5/8 Dia screw that would be .94 in.. make sure you have a hole tapped at least 1 inch deep.

If you use 5 threads on a 5/8-11 std screw you have a lot less than 1/2 inch of thread holding.

(5 threads /11 threads per inch) = 0.45 inches engagement...EQUAL NOT strong enough.

[ 11-10-2004, 05:17 PM: Message edited by: Gary E ]

[John E Hardiman]

Gary E, that's way longer than necessary.

Let look at this the engineering way. In order to generate enough force to fail the bolt in tension, there must be enough thread engagement to prevent shearing the threads off. Now the tension to fail a bolt is pi*D^2*T/4 where D is the minor diameter and T is the tensile strength. Now the length of engegement needed to carry the shear is pi*D*S*L where L is the engagement and S is the shear strength. Now for most ductile material we can relate S to T by von Mises theory so S~.577T and we can subistute L=aD to relate the thread engagement to diameter.

We now have:
pi*D^2*T/4 = pi*D*.577T*aD
solving
a =1/(4*.577) or a=.433 so L=.433D

So now a 5/8" bolt with a minor diameter of 0.5135 we would need a thread engagement of .433*.5135 or 0.222" which would be 3 threads (11*.222).

Of course you would like a safey factor so....5! :D

PS edit;

I just re-read your post and wondered about the eye-bolt length (BTW most standard "bolts" are always threaded the same, full length or 2D+1/4” for bolts less than 6” in length, and 2D+1/2” for bolts longer than 6” long...has to do with recommended bolt sizes used to join structures of a given thickness vs minimum # of standard lengths, likewise most standard "screws" are fully threaded...just ANSI B18.2.1 not a strength issue). A quick look into several handbooks shows that for cast iron only longer thread engagement is recommended (15/16 thread depth for 5/8-11 bolt). This is because the threads less ductile in the cast iron and therefore the load needs to be spread out more.

[ 11-10-2004, 06:22 PM: Message edited by: John E Hardiman ]

[Gary E]

Yes John I am not in any way arguing aginst the calculations you showed.

But, there's always a but isnt there. A fast glance in my Machinery Handbook, ok so it's the 16th Edition, 1959 is a date inside and there may be "new and improved" and I will admit to making everything stronger than you think you need. Well anyway I looked at the page that shows Drop Forged Carbon Steel Eye Nuts, and the 5/8 Dia shows a thread lenght of 1 1/4 in. Then I turned the page and looked at T Nuts that would be used in a milling machine table, 5/8 Dia has a thread length of 25/32.

A little note about over engineering that I agree to every time...
The job was to design a feed gearbox for a traveling column horizontal NC profiler, for an idea of it's size, a full size Ford Van would fit on it's table with plenty of room for more (CNC had not even been thought of at this time)

Well after we got finish with the design which was really just taking a smaller unit and making it bigger to fit the larger machine with larger loads, an older engineer was now checking all this work that this new group of freshly minted just out of school boys came up with. He said.. ok..fellows, looks fine, but you see that 2 in Dia shaft, it's gona look a lot better at 2 7/8 and that gear set is 1 1/2 wide, it will look better at 2 in. Not that you guys are wrong, this is something that we just dont ever want breaking, and the materials are just not that much more money.

I learnd a lot from the older grayer engineers that had built these machine tools for a long time and many of them are still running today.

And yes, I admit to working with cast iron a lot, maybe that's where I get my ideas.

[ 11-10-2004, 09:17 PM: Message edited by: Gary E ]

[Stiletto]

I knew nothing of the theory but thought that the thickness of any given nut must be about right.

Eyebolts-----I remember working on a construction site back in the seventies, we had large shutters for forms for pouring concrete into . These were moved about the site by crane hooked onto a large eyebolt. One day the wind caught a shutter in mid air and spun it around and undid the nut, there were yellow hatted guys running everywhere! Fortunately no injuries and a very stern reminder to use two eye bolts in future.

[John E Hardiman]

Originally posted by Gary E:
Yes John I am not in any way arguing aginst the calculations you showed.

But, there's always a but isnt there. Gary, I agree with your story (as a grey hair myself smile.gif ), and often beef stuff up, especially to the next stock/standard size up (we had a new guy figure out he needed a .9-something shaft and bearing once and was going to have them custom made :rolleyes: ) or to standardize nut sizes; i.e. who wants to be up on a ships mast and have to carry 6 wrenchs...or go back down and get the one you forgot...make all the bolts the same size.

Machine tools are a whole different matter as far a sizing goes. You NEVER want to break the machine. Not only does it cost money to fix it, the owner is now late on his schedules and the machine brand gets a bad reputation. And for a company that may only sell 50 big machines a year, a bad reputation will soon put you out of business.

[George Roberts]

Bill Perkins ---

I feel 5 threads is not enough engagement (I like 1 bolt diameter) but ...

There are lots of interesting stories with regard to lifting heavy objects. I hope your story is not interesting.

[maa. melee]

One note: ALWAYS use a torque wrench, a washer (lock washer or flat) and some lock-tite (that red goop that stops unthreading). You might even think about using a split nut with some wire or nylon thread nut with your setup. Is your threading rod machine threaded or are the threads rolled on?

Check this out. (http://www.rockcrawler.com/techreports/fasteners/index.asp)

((edited to add link))

[ 11-16-2004, 11:32 PM: Message edited by: maa. melee ]

[John E Hardiman]

Originally posted by George Roberts:
There are lots of interesting stories with regard to lifting heavy objects. I hope your story is not interesting.Yeah...well... it has always been said by riggers that to depend on a threaded connection in tension during a lift is just a matter of how big your nuts are....