thread strength vs. percent thread vs. depth

Cal Haines · Jan 30, 2009

I'm trying to find a good reference that relates the strength of a thread to percent thread depth and the length of the thread. I intuitively know that if I increase the tap drill size beyond some point the threads get weaker, but if I increase the number of threads that are engaged the joint will be stronger and at some point the larger tap drill won't matter. But I have not found a good reference to back me up.

On page 1924 of my Machinery's Handbook (27th ed.), under the heading "Diameter of Tap Drill", they make the statement:

"Tests have shown that any increase in the percentage of full thread over 60 per cent does not significantly increase the strength of the thread. Often, a 55 to 60 per cent thread is satisfactory, although 75 per cent threads are commonly used to provide an extra margin of safety."

However, the table that immediately follows, Recommended Hole Size Limits Before Tapping Unified Threads, lists min and max hole sizes for various lengths of thread engagements. For 1/4-20 threads less than 1/3 diameter long (< .083"), the table calls for 87 to 100% thread depth. For thread lengths from 1½ to 3 diameters long (.375 to .750"), they call for 75 to 85%. I haven't checked any other sizes, but they don't seem to be following their own 60% rule.

For 1/4-20, my handy-dandy Starrett reference card calls for a #7 (0.201") tap drill, which is 91% thread depth. A #2 (0.221”) tap drill gives a 54% thread depth, with a lot less effort required to tap and less chance of tap breakage.

At present, I'm dealing with 1/4-20 by 0.50" thread depth hole in 6061-T6. How big can I make the tap drill without sacrificing strength, assuming 0.50" thread engagement? Anyone know of any good references or on-line calculators to answer this sort of question?

Cal

mrainey · Jan 30, 2009

If you want an online calculator, this looks promising.
http://www.boltplanet.com/Un/un.asp?txtseries_page=UN

Software For Metalworking
http://closetolerancesoftware.com

Holescreek · Jan 30, 2009

One of my jobs here at work is to evaluate thread strength on cast iron brake calipers. I use a long breaker bar connected to a torque transducer that measures torque and angle and graphs it out. The object of coarse is to reach or surpass the maximum assembly torque without pulling the threads out of the hole. There are a lot of variables in what you are asking including depth of hole, tap size, number of threads engaged, bolt dimensions etc. I have never seen a chart like you are referring to but it would be interesting to look at past data and see if it jives with the calculation. -Mike

SAG 180 · Jan 30, 2009

I've seen an old Loctite booklet which had information about the total tensile support each thread turn contributed to the total strength. I can't find it on their website but I'll see if I can photocopy the pages and take em home to scan if you are interested.

From memory the first turn was over 30% and it dropped off exponentially from there, so the first six turns took all the load and any threads engaged after that were wasted..

USMCPOP · Jan 30, 2009

I found this interesting:

http://www.spiralock.com/technology/about_technology/torque.html

Holescreek · Jan 30, 2009

I found the spiralock thread data very interesting at first, then it hit me. If you are only contacting at one point on each thread (vs. an entire side of the thread) the load would naturally redistribute as the thread "settled in" under a load. It would seem that as the load increased and the thread began to deform that it would very quickly take the shape of the standard 60 degree V thread. Still cool. I saved the link into my "Work" folder. Thanks! -Mike

Mcgyver · Jan 30, 2009

Isn't it just a question on how much metal would have to shear and what the shear strength is, expressed as an area. ie if you had 100% engagement, and a 1" dia bolt and 1" long threads, you've got 3.14 sq inches that have to shear for the thread to fail.... multiply that by the material's shear strength and that's when it should fail. less that 100%, if thread depth is 1", amount of shear will be less that 1" as there is a bunch of air along the shear line. theoretically pitch won't matter in the thread failure, (but will to the core size of the fastener and when it fails). if that isn't totally out to lunch, is should be easy to have a calculator or or chart to show how strength is reduced as a percentage for each % reduction in engagement (i'm too damn tired on a Friday night to do it)

Armenius · Jan 30, 2009

Ye olde rule of thumb and erring on the safe side:

Steel part thread engagement 1x diameter of screw.

Aluminum part thread engagement 3x diameters of screw.

By and large this will ensure that the screw breaks before the thread strips out of the hole.

If it is critical work then the relative hardness of the nut and bolt are important... The nut should be appreciably softer than the bolt.

Arminius

John Madarasz · Jan 30, 2009

here Is a little resource too...

http://www.gizmology.net/nutsbolts.htm

fwiw this is the tap/drill selector chart I've been using for many years with good success...simple, quick and easy reference. You can highlight the columns in blue and red for easy reading

http://www.physics.arizona.edu/~haar/ADV_LAB/Tapdrill_2.pdf

Cal Haines · Jan 31, 2009

Guys, Thanks for all the links. I had heard that 3 full threads gave full strength. I didn't realize that more than 6 threads was useless.

John, I like that tap drill chart. Thanks!

Anyone have any problems with 55 to 60% threads in aluminum?

Cal

SAG 180 · Jan 31, 2009

Cal Haines said:
Guys, Thanks for all the links. I had heard that 3 full threads gave full strength. I didn't realize that more than 6 threads was useless.

John, I like that tap drill chart. Thanks!

Anyone have any problems with 55 to 60% threads in aluminum?

Cal

One of those links mentioned you can't trust the first two thread turns on a bolt as the threads may not be fully formed to engage properly. You may need the 75% thread for the extra safety margin.

PixMan · Jan 31, 2009

Another important consideration in the strength of threads is the method of creating them. Typically, a formed thread will be stronger than a cut thread of the same percentage thread height.

EPAIII · Jan 31, 2009

Cal Haines said:
Guys, Thanks for all the links. I had heard that 3 full threads gave full strength. I didn't realize that more than 6 threads was useless.

John, I like that tap drill chart. Thanks!

Anyone have any problems with 55 to 60% threads in aluminum?

Cal

These "number of full threads" strength figures are working approximations, not absolutes. EVERY extra engaged thread will add some amount of extra strength. It is just that the amount of added strength rapidly decreases with each additional one. By the time you are up to 6 threads or so, the next one (7th) would only add an additional 1% or less and the 8th one would add far less than the 7th. Three threads may be 50 to 80 percent of the max. I am using approximate numbers because the actual numbers will depend on many factors like the materials, method of forming the threads, percent of full depth, etc. It is just a question of where you choose to stop.

Original question had to do with thread depth. One thing to remember is that both the male and female threads can be reduced depth. Also I have seen male threads that had greatly reduced pitch diameter and this can contribute a lot to loss of strength. Some cheap clamps I have had so much play that they easily stripped even though the male threads looked fully formed and the female were at least 75%. They had at least 10 "fully" engaged threads.

jim rozen · Jan 31, 2009

In the case where the mating parts are of the same material, the male threaded
part will always fail before the tapped hole will fail. Only for badly mis-matched
materials will the threads in the tapped hole fail first.

When a bolt in a threaded holes fails under tension, it will always do so where
the first thread crosses the surface of the part with the tapped hole in it, do to
stress concentrations at the root of the thread form at that point.

Jim

Metalcutter · Jan 31, 2009

This may be bad manners, but I'd like to raise another question.

Considering a Unified class 2a/2b fit, and the outside diameter of the "post" where the threads are, and the strength of internal threaded portion (post and threads), and the strength of the screw. I'm trying to balance the strengths so I don't go to thin on the post diameter and I don't want to too much overkill the diameter of the post.

I hope you can make sense from what I've written. smile

My example is a 1/4-20 screw bolting into a 5/16 diameter shaft.

Is there a direction you can point me?

Thank you,

Stan-

Jim Monte · Sep 30, 2021

I know this thread is over a decade old as of now, but it is placed highly in related web searches. So it is still relevant in some sense. A bit of useful information I found is at

Tapping Torque Vs Thread Strength - Tapmatic Corporation

or

Tapping Torque Vs Thread Strength - Tapmatic Corporation

if the first one should disappear someday. I would expect that what Tapmatic writes on this topic is accurate.

Jim

rons · Oct 1, 2021

Cal Haines said:
I'm trying to find a good reference that relates the strength of a thread to percent thread depth and the length of the thread. I intuitively know that if I increase the tap drill size beyond some point the threads get weaker, but if I increase the number of threads that are engaged the joint will be stronger and at some point the larger tap drill won't matter. But I have not found a good reference to back me up.

I would think that a certified mechanical engineer would come up with the integral calculus equation that allows strength computation based on length and depth.

4GSR · Oct 1, 2021

rons said:
I would think that a certified mechanical engineer would come up with the integral calculus equation that allows strength computation based on length and depth.

It's already been done.

Look at Shingley's book on mechanical design. I have the formulas setup in a excel file. Been using it for nearly 30 years now.

4GSR · Oct 2, 2021

There's also some thread stress calculations in the ASME/AISI B1.1 or 1.2 specifications, too.

It's not so much percent of thread left in the hole. It's more of how much shear stress can the thread handle without failure, which is based more on the shear plane of the threaded hole. This is determined by the tensile yield strength of the materials of the screw thread and or the nut thread. The weaker material will determine the shear plane.

My rule of thumb for tap drill size, go by what the charts say to use, but don't let the hole size go bigger than the pitch diameter minimum of the screw thread.

All of my threads I deal with are off the charts that most of us carry around in our shirt pocket! ANSI Standards for Vee threads have formulas for calculating the minimum/maximum bore diameters for any screw threads. Bore diameters calculated will allow for any of the percentages up to around 55% as called out in the charts.

There's a lot to learn about threads, even myself, I still don't know all there is to know about threads.

Ken

JS · Oct 2, 2021

Tensile stress area vs root area

Difference Between Root Area and Tensile Stress Area of a Threaded Fastener

thread strength vs. percent thread vs. depth

Diamond

Stainless

Titanium

Titanium

Titanium

Titanium

Diamond

Cast Iron

Stainless

Diamond

Titanium

Diamond

Diamond

Diamond

Titanium

Plastic

Diamond

Diamond

Diamond

Stainless

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