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Neil Jones
03-04-2009, 07:50 PM
I remember reading something, somewhere that said you don't gain any advantage in strength no mater how deep you go with the thread after something like three times the diameter of your thread. Is this "law" written anywhere so it can be reviewed? :) If the thread fails do all the threads pull out no matter how deep you have gone?

Michael Moore
03-04-2009, 07:58 PM
If you look in Machinerys Handbook it says that increases over 60% do "not significantly increase the strength of the thread."

cheers,
Michael

Your Old Dog
03-04-2009, 08:09 PM
I can't answer your question directly but can add this.

It is likely the rule holds because the diameter of the bolt at the head equals the mass of all the threads combined. Using large numbers to make it easier to grasp, once the 1cubic inch of diameter of the bolt equals the total of all threads up to 1 cubic inch in total the bolt head would simply break off. That the thread was in the hole 15 feet wouldn't make the head any stronger.

Probably a poor explanation but you might get might get my drift.

Stepside
03-04-2009, 08:11 PM
After three diameters of depth the bolt will shear or break off rather than strip the threads out. The 60% is a percent of the full depth of thread when dealing with the thread profile. So tap full depth threads at least 3 times the diameter of the fastener and tap drill with the drill that gives 60% of thread depth. Any higher percent just increases the chance of breaking a tap and is more work.
That said, if the tap is dull get rid of it and drill through holes whenever possible.

lane
03-04-2009, 08:13 PM
1 1/2 times the dia of the thread is all a thread will hold . Such as tap a 1/4-20 thread 3/8 deep full thread. It written some where read it before.

dfw5914
03-04-2009, 08:24 PM
Odd how you rarely see nuts more than about 2X thick.

It also has something to do with uneven loading of the threads as the fastener stretches, concentrating more of the stress on the first few threads.

JDF
03-04-2009, 08:37 PM
The general rules don't always apply, particularity if tapped holes are in a material which is softer than the bolt. Steel bolt in aluminum for example. Strength of materials play a part, but it is how the threads of the bolt and part interact that really control how many threads are engaged. If the tapped material is more elastic than the bolt, then the upper threads will deflect a bit, allowing more threads below to engage the bolt.

Same thing dfw is talking about really.

Boucher
03-04-2009, 08:43 PM
There is or was a Military specification for threads that will answer most questions if you can stand to read it. Thread tolerances do take into account the length of engagement. Exceeding the recommended length of engagement is going to result in inteferences ie tightness and gauling.

Mcgyver
03-04-2009, 11:03 PM
I remember reading something, somewhere that said you don't gain any advantage in strength no mater how deep you go with the thread after something like three times the diameter of your thread. Is this "law" written anywhere so it can be reviewed? :) If the thread fails do all the threads pull out no matter how deep you have gone?

you can work it out if you want, the only reason the thread doesn't get stronger the deeper you go is because the area of the minor diameter of the bolt becomes less the the area of the threads (a cylinder going through the threads). the material is only as strong as its shear strength and will fail first where the section is least. There'll be 47 posts explaining why that's too simplified, :D but its the basic reason; if the area of the bolt is less than the area of the cylinder of engaged threads after a certain thread depth.

Rich Carlstedt
03-04-2009, 11:17 PM
You have three variables.
1. Size or diameter of thread.
2. Strength of screw material ( Tensile)
3. Strength of hole (Nut) material ( Tensile)

Now Yield strength is a better measure, but lets keep it simple:
Maximum strength in a joint is achieved when thread engagement is the same length as the diameter of the thread AND THEY ARE THE SAME material. (Mild steel is 1018)
So a (1/4-20) 1018 screw in a 1018 nut or 1018 plate will have maximum strength of the fastener with 1/4" of thread engagement.

Now, If the screw was a American Made Allen Socket Head Cap Screw which has a tensile 3 times that of 1018, then the hole must be 3 times diameter in depth to match the screw in strength.
If the screw was 1018 and the plate was 3 times the tensile, then any engagement of more than one diameter is futile, as the screw will fail.
Technically, Aluminum is only 1/6 Th of steel in tensile and that would mean 6 times the diameter.

PRACTICAL PRACTICES.
As a practical matter , there is more thread material on the nut side as you go larger than the pitch diameter of the thread, than the screw.
The screw also has a smaller crossection due to the root diameter being the limit in strength
Therefore "when the engagement matches the strength ratios(outlined above), the screw will fail."
Because of this "phenomina" nuts can be made shorter.
Years ago the nut heights were equal to their thread size.
Today if they are , they are called "Heavy duty nuts"
Most are 75 % of diameter in height or length to "match" the root diameter more closely.

What a previous poster stated is correct, most thread engagements today are made at 1.5 times the diameter in machinery, this is equilvilent to about 2 x diameter because of the nut phenomina.
Since most plates are 1018, a Grade 5 bolt will be pretty close to max using that formula.
Also as pointed out , tap depths become impractical after 3 x depth and so for steel/Aluminum engagements, if max forces are required, inserts are used.

Rich

Jpfalt
03-05-2009, 10:27 AM
When you load up a screw thread, the screw stretches under load. That means the first couple of threads in the nut carry the bulk of the load. The only way to even it out would be to have the thread pitch slightly longer in the nut and get progressively shorter the deeper you go, but such is not readily manufacturable and would require quite a bit of dimensional control to put the thread pitches in the right place to do the most good.

One way to even out the loading is to use helicoils. These wire inserts allow easier deflection in the threads with the highest load and transfers the load to threads that are deeper in the nut.

Long lengths of thread engagement are usually used to increase wear area on the thread face under low loadings where fastener stretch isn't an issue, such as a cross slide screw and nut.

RPM
03-05-2009, 12:50 PM
I may have got this wrong from somebody, but I remember reading that you only need the first three threads engaged for maximum strength, after that the rest of the threads were just insurance.

Is this correct?
Richard in Los Angeles

2ManyHobbies
03-06-2009, 12:35 AM
This is one of those really neat random encounters type threads. It all makes logical sense when you see the math on it, but is something you can eyeball in about 2 seconds to determine if your bolt or threaded material even have a chance. You know if the material is too thin, that is easy. You also never choose to thread far more than you need for a given application.

This is something I would have never imagined looking up, but will probably remain a random fact floating around in my brain for a number of years. Thanks for taking the time to share the knowledge all.

knedvecki
03-06-2009, 01:35 AM
Neil Jones - I sent you a PM about the H-28 Thread Book

lazlo
03-06-2009, 10:33 AM
You have three variables.
...
So a (1/4-20) 1018 screw in a 1018 nut or 1018 plate will have maximum strength of the fastener with 1/4" of thread engagement.

Great post Rich!

I was getting a little worried with all the different answers, but you're saying the maximum strength is at 1 - 3 (or so) diameters, depending on the material of the screw, and the nut. That makes a lot of sense.


Also as pointed out, tap depths become impractical after 3 x depth and so for steel/Aluminum engagements, if max forces are required, inserts are used.

I used steel thread inserts on that servo (with a little 's' :) ) motor mount I made for the Bridgeport J-Head (the through-hole needed to be 8mm to match the motor tabs, but I needed a smaller hole at the end for 8mm threads).

So at first I thought that was going to make the thread stronger, but when you think about it, the steel threaded insert is also threaded, so you're just replacing one aluminum to steel interface with another. Now, the insert is 1/2-13, so it's substantially bigger than the original 8mm thread, but if I hadn't used a converter insert, wouldn't it have been the same strength?

Jpfalt: that's a really interesting point about Helicoils stretching and allowing more thread engagement than a solid insert. I've been using the solid thread inserts because they don't require special insertion tools.

oldtiffie
03-06-2009, 05:58 PM
Have a look through these sites to take some of the guess-work out of it:
http://www.boltscience.com/pages/info.htm?

http://www.tribology-abc.com/calculators/e3_6a.htm

bruce.heron
03-06-2009, 09:38 PM
A bolted connection should be designed so that the tensile stress area of the screw will fail in tension before the threads fail in shear. This practice will prevent someone from ripping the threads out by overtightening. If the screw does fail the connection can be repaired by removing the broken screw.

If the material of the screw (male thread) has nearly the same tensile strength as that of the nut (female thread) then a little over .5 D thread engagement is adequate. Anything more than this is wasted. Compare the diameter of a alloy steel screw to the thickness of a standard alloy steel nut.

If the tensile strength of the screw is considerably greater than that of the nut a greater thread engagement is required. A common situation is to use grade 8 alloy steel screws (150 ksi TS) into aluminum alloy having a tensile strength of 30-50 ksi. In this case an engagement length of approximately 2.3 is required. If the tensile strengh of the nut is even lower than that of alloy aluminum, as often occurs with wood or most plastics an even greater engagement length is required. Look at the engagement length of the longer lag screws.

ASME Spec B 2118 is the specification for standard grade 8 screws. A appendix to this specification gives the formula for calculating the required thread engagement which is:
Le = 2ATs/ASs For the first case above in which the screw threads fail.

Le = 2 TSs/ TSn ( ATs/ASn) For the case where the nut threads fail.

In these formulas:
Le = Engagement Length (inches)
ATs = Axial Tension Area of screw thread (square inches)
ASs = Axial Shear Area of screw thread (square inch/inch of length)
TSs = Tensile strength of screw material (lb/square inch)
TSn = Tensile strength of nut material (lb/square inch)
ASn = Axial Shear Area of nut thread (square inch/inch of length)


Above thread parameters are tabulated in thread handbook H28 and its current ASME replacement, as are the formulas for calculating them for ACME, Witworth, buttress, NPS, multiple start threads. and other nonstandard threads.