Do non-full (partial) threads add to the thread engagement length? 1

[Jojo5000]

Hello,

I have an interesting challenge that I'm facing. I would like to understand the strength that partial threads add to the total strength of a threaded joint. I've included links to pictures trying to explain this conundrum.

I'm beginning to understand the thread engagement length equations that are used to calculate the stripping strength of a threaded joint, but am looking for any previous testing or anecdotal references for screws used in non-parallel plane internal threads. I'm wanting to convert the angular length of the fully formed thread and add it to total thread engagement length, but was hoping someone else already experimentally verified this.

I'm using a thread engagement length that is well below the "required" length for the fastener shaft to break before the threads strip, so need every bit of strength that I can get.

Your file's link is:

http://files.engineering.com/getfil...-6566189a1cb5&file=Partial_Threads_Angled.png

Your file's link is:

http://files.engineering.com/getfil...9df8592bcb&file=Partial_Threads_Side_View.png

Thanks,

Joseph

 

[Sparweb]

This is a pretty unconventional joint, so I don't understand much of your question, except this:

"I would like to understand the strength that partial threads add to the total strength of a threaded joint. "

Simple answer: None.
The fastener's own threads can slide away from the crescent of threads on one side before taking up any significant load at all.



STF

 

[SwinnyGG]

This is a pretty unconventional joint

I'd argue that it's extremely common in certain situations- namely the back side of a threaded thru-hold through the lip of a casting, designed to attach a cover or bracket or whatever.

The strength added by partial threads will certainly be >0, but not significant enough that I, personally, would attempt to use partial threads to increase load capacity.

SparWeb is correct that the threads will slip, and impart a bending load into the bolt. On the back side of a thru-hole, this is no big deal. If for some very odd reason the partial threads are not on the back side, this could be a major issue if your fasteners are seeing high loading.

 

[MikeHalloran]

That joint will likely break a tap before you ever get a bolt in it.
Add a boss where you need more thread engagement,
or put a partial c'bore where the ramp is, and use a finer thread pitch,
OR
throw out the whole damn thing and start over.
Your sketches do not provide enough detail to allow constructive suggestions.


Mike Halloran
Pembroke Pines, FL, USA

 

[swertel]

Take a look at these quick-install knobs.

https://www.mcmaster.com/#machine-component-knobs/=19wbxye

The webpage doesn't provide the notice that are included with the knobs - not to be used for load bearing.
In other words, whoever designed these knobs knew that partial threads were not sufficient to include in load bearing calculations.

I'm going to have to join Mike in saying that if you need to include the partial threads because your design is so close to its limits, then you probably should rethink the design.

--Scott
www.aerornd.com

 

[Jojo5000]

Hi,

Thanks for the fast responses! Unfortunately, my hands are tied with some of the design that I can place around these "joints", so I'm trying to establish why these work like I've experienced them working. Here's a more clear image of what I'm dealing with. Side View With Threads and With Fastener

There's a counterbore that allows that head of the screw to seat on one seat over the majority of the hole, resisting the screwing of the axis that SparWeb describes or the quick-install threads from Swertel (That's a really sweet concept though, sort of like an interrupted thread in a breech bore)The first thread is very hard to describe in terms of how much of a full thread is actually being cut into the plate, the second thread might be 2rds of a full circle, and after that the threads are less than 50% of a "whole" thread.

There's a difference between a the stripping force required to shear the threads in a flat plate with the minimum thread thickness and a partially threaded hole like this, so there is some strength that this provides. I realize that the design is flawed, so I don't need more comments suggesting that. I'm more asking if there's any standard or research work that addresses this topic. I really appreciate the feedback regarding the mechanics of what's going on.

Thanks,
Joseph

 

[Sparweb]

Hi Jojo,
And Welcome to Eng-Tips.
Please forgive my little test. Too often a new member asks an off-the-wall question, gets answers, but is never heard from again. The rest of us keep coming back for the off-the-wall questions that DO get some engagement from the new member.
...and forgive me again... bad pun...

So it looks like you are dealing with holes tapped in the material at an oblique angle. This can happen in certain "repair" scenarios. I also appreciate that you are doing your best not to reveal information that you shouldn't, but the more you can give the more we can give. There are some crucial details that are important before we can go very far. Answer as many as you can...
What can you tell us about the second component being fastened?
Is this the only fastener between the components?
Is there a shear-resistant bond between the two (or more) components?
Can you describe how the joint is typically loaded?
Is the fastener installed with a significant amount of torque?
Is installation torque limited by the fastener or the substrate(s)?

I honestly cannot recall ever seeing a study of oblique fastener installations... but I will look again because I think your last diagram gives me enough to think up some ideas.
With your membership associated with "bioengineering" it is tempting to jump to conclusions about just what material you would endeavor to repair.

STF

 

[swertel]

If you have a copy of Bickford's and Nassar's Handbook of Bolts and Bolted Joints there is a section on Length of Engagement relative to thread stripping and defines the eccentricity that is allowable for minimum depth of engagement.

The section on Thread Shear Area is 6 pages long, so I can't exactly explain everything here.

https://smile.amazon.com/Handbook-B...&keywords=handbook+of+bolts+and+bolted+joints

--Scott
www.aerornd.com

 

[dhengr]

Jojo5000:
I doubt very much that you will find any all-encompassing testing or design methods which cover your problem. Anyone with this type of problem is pretty much on their own, and must do some of their own testing as they hone in on a solution. Get yourself some good Engineering Mechanics, Strength of Materials, and Machine Design textbooks, along with a copy of Machinery’s Handbook. There are a number of texts and handbooks on bolts, bolted joints, various threaded fasteners, and start reading. Develop a good fundamental understanding of how threaded fasteners work. Obviously, a complete thread, or number of male threads, works better, and stronger, when confined by complete matching female threads. The relative fit or thread class comes into play, as do the mechanical properties of the two mating parts. A soft nut may fail in thread shear, or visa-versa, a softer bolt might shear at the threads, or fail in tension when mated with a stronger female part. The fact that your threads are not well confined laterally allows the mating part to move laterally w.r.t. each other weakening the thread shear cap’y. If the mating parts are both very stiff laterally and well confined or supported laterally, this relative movement will be minimized and well improve the joint strength. For example, a small dia. bolt will just flex and move laterally w.r.t. the majority of the threads on the left, and reduce the shear cap’y. of the threads on the left. A larger dia. bolt will be stiffer and might be the only way to increase the joint strength. If you can keep the threads properly mated, a big if, the strength of the joint boils down to the circumferential length of the threads in contact. You’ve kept a lot of important design info. pretty close to the vest so it’s tough to know what you are really trying to do. Remember, we can’t see it or all the associated parts from here.

 

[Screwman1]

I agree with DH on the lack of testing for this situation. There is not enough testing done on regular joints, let alone a situation like you have. I am wondering if the bending moment and resulting high friction and forces on one side of the head are creating a significant amount of braking on your fastener. Remember that approximately 50% of the applied torque goes into overcoming underhead friction and this situation will certainly increase that proportion significantly. There are a ton of variables that go into a situation like this one and I think that you are going to have to just "make 'em and break 'em" to get an idea of what contributes what to the performance.

 

[Jojo5000]

Thanks for the great discussion around this. It helps to calm my desire to grab a solution and run with it. SparWeb, yes, I would definitely be that guys if I didn't feel like I was out of my comfort zone with this and needed some sanity check for what I'm doing. Here's some replies to your questions:

What can you tell us about the second component being fastened? The screw will be locked into the obliquely threaded plate and used to support bone fragments that need to be fixed in place. Effectively, the "joint" will be the screw being locked into the plate. Here's an example of a typical bone plate.
[ul]
[li] Is this the only fastener between the components? I'm not sure what you're asking here. I suppose the mechanics of my question can be separated into two risks, shearing of the threads by stripping or shearing of the threads do to a bending moment of a transverse load on the screw (effectively a cantilever bend) My primary concern right not is the stripping, as multiple screws will lower the risk of a failure due to a cantilever load.[/li]
[li] Is there a shear-resistant bond between the two (or more) components? There is no chemical bond securing the screw in plate.[/li]
[li] Can you describe how the joint is typically loaded? I'm more concerned with the mechanics of the stripping, so there will be a torque and the resulting shear, axial, force.[/li]
[li] Is the fastener installed with a significant amount of torque? No, the torque is relatively low, due to the risk of stripping. I would like to optimize the insertion torque with the thread engagement length.[/li]
[li] Is installation torque limited by the fastener or the substrate(s)? The fastener limits the installation torque.[/li]
[/ul]

Swertel, I'm working on getting the tome from my local public library...or I'll see if my boss wants to buy it. The eccentric loading notes are pretty fascinating.

From the looks of this it will be some lucky guessing for what amount of partial thread should make a difference and some testing to verify this. The main point of my thread was to pool the body of knowledge that ya'll have for other standards and what not are out there, so I think I've accomplished that. That said, if you want to discuss this more with me, I'd be happy to share hear more of your thoughts. Unfortunately, I don't think I'll be able to share many of my results, but will try to synthesize the testing down to some general principles, if possible.

Thanks,

Jojo

 

[Sparweb]

Hi Jojo,
No directly useful info is coming to hand. Of course I know very little about arthroscopic surgery, though my wife has three such screws in her ankle.
I do have a few general thoughts to share, though they may not be very helpful:

When thin metallic or plastic parts must be tapped for a fastener, and doing so limits the strength of the joint, sometimes it is advantageous to install an "insert" into the substrate first, allowing the fastener to go into stronger complete threads. Helicoils and Keenserts are examples of these kinds of inserts. Inserts in some cases can be designed specially to interlock the components even before the fastener is installed, and in other cases to protrude out the opposite side allowing the fastener head to pull up extra clamping force.

An inclined contact surface will cause a shear pre-load in the joint. The shear will be proportional to the installation torque of the fastener. The shear pre-load will exacerbate the tendency of the fastener to deflect away from the oblique threading. This may be mitigated by abrasion or knurling of the two contact surfaces, but no controllable result could be expected.

Common UN threads are just one style of many, and this may be an application for a more structural thread such as Acme or Buttress. While I have no technical experience with these threads, your concern about installation torque and rigidity of the joint make me reach for Machinery's Handbook and look at thread profiles with different contact angles.


STF

 

[Screwman1]

For bone screws there are a number of special thread forms with modified thread geometry that is designed to work best with different types of bone. If I remember correctly, there are actually ASME standards for the thread forms. If I recall correctly, they tend towards asymmetrical with a very healthy radius root design. Many of them also have a single or double thread cutting flute to 'tap' into the bone. It is a very specialized area and it seems that the doctors must have done some development work on this product in the past.